Down syndrome is a genetic condition caused by the presence of an extra copy of chromosome 21, typically apparent from birth, resulting in characteristic facial features, intellectual disability of variable degree, and associated congenital anomalies such as cardiac defects.

Exam domain	One-glance essentials
Definition / diagnosis	Chromosome 21 dosage disorder, usually free trisomy 21 ^[1] Translocation and mosaic Down syndrome are less common ^[1]
Epidemiology / risks	Risk increases with maternal age, but most affected births occur to younger mothers ^[1] A parental Robertsonian translocation changes recurrence risk ^[1]
Core mechanism	Extra chromosome 21 alters neurodevelopment and organogenesis ^[1] AV septal defects reflect endocardial-cushion disruption ^[1]
Clinical picture	Hypotonia, characteristic facies, single palmar crease and developmental delay ^[1] Look for congenital heart disease, duodenal atresia and hearing/vision problems ^[1]
Investigations	Confirm with postnatal karyotype; antenatal screening is not diagnostic ^[2] Echocardiography, hearing, vision, thyroid and blood-count surveillance are essential ^[2]
Management	Multidisciplinary developmental, cardiac and educational support ^[3] Treat associated thyroid, hearing, visual and gastrointestinal disease ^[3]
Complications	Leukaemia, hypothyroidism, OSA and atlanto-axial instability ^[4] Early Alzheimer-type dementia and recurrent infection are important later risks ^[4]
Prevention / follow-up	Offer prenatal screening and diagnostic counselling without coercion ^[3] Follow syndrome-specific health-surveillance schedules ^[3]
Exam traps	*A positive cell-free DNA test requires diagnostic confirmation* ^[2] Avoid routine cervical radiographs in asymptomatic children; assess symptoms clinically ^[3]

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p503, Section 15.2.1 — Down Syndrome)

^[2] Senior notes: Adrian Lui Pediatrics Notes.pdf (p503, Section 15.2.1 — Down Syndrome mechanisms, screening, recurrence risks)

^[3] Senior notes: Adrian Lui Pediatrics Notes.pdf (p503 — Down Syndrome mechanisms, clinical features)

^[4] Senior notes: Adrian Lui Pediatrics Notes.pdf (p503 — Down Syndrome, mosaicism and milder phenotype)

Down Syndrome (Trisomy 21) — Paediatrics

Down syndrome (DS; 唐氏綜合症) is a chromosomal disorder caused by the presence of an extra chromosome 21 or its material, resulting in a constellation of phenotypic features including characteristic dysmorphic facies, intellectual disability (the most frequent form of intellectual disability caused by chromosomal aberration), hypotonia, and multi-system medical comorbidities ^[1]^[2].

The name "trisomy 21" literally tells you the problem: "tri-" = three, "-somy" = bodies (chromosomes) — three copies of chromosome 21 instead of the normal two.

Down syndrome is the most common chromosome abnormality among newborns and the most common genetic cause of severe learning difficulty ^[1]^[2].

Why Chromosome 21?

Chromosome 21 is the smallest human autosome (~200 genes). Despite being the smallest, an extra copy is still lethal in many conceptions — about 80% of trisomy 21 conceptions are spontaneously lost. The liveborn survivors represent those who tolerate the gene dosage imbalance. The relatively small gene content of chr21 is precisely why trisomy 21 is the most common viable autosomal trisomy — trisomies of larger chromosomes (e.g., chr1) are almost universally lethal in utero.

Parameter	Detail
Overall incidence	~1 in 700 live births (internationally); ~1.5 per 1,000 births across all ages in Hong Kong ^[1]^[2]
Sex ratio	Slight male predominance (~1.3:1 M:F)
Chromosomal abnormality prevalence	DS is the most common autosomal trisomy compatible with postnatal survival
Survival	With modern paediatric cardiac surgery and healthcare, > 90% survive to adulthood; median life expectancy now ~60 years (previously <25 years in the 1980s)

Hong Kong context: The Hospital Authority (HA) Trisomy 21 Registry and the universal prenatal screening programme (including NIPT offered since 2019 at public hospitals for high-risk mothers) have made DS a commonly encountered and well-screened condition. Despite screening, DS remains the most common chromosomal cause of intellectual disability seen in paediatric clinics in HK.

Risk Factors

Advanced maternal age is defined as ≥35 years old on the estimated date of confinement (EDC) ^[2].

The single most important risk factor. The mechanism relates to the prolonged arrest of oocytes in meiosis I (from fetal life until ovulation — potentially 40+ years). During this extended arrest, the cohesin protein complex that holds sister chromatids and homologous chromosomes together gradually degrades ("cohesin fatigue hypothesis"), leading to increased rates of non-disjunction errors.

Maternal Age at Delivery	Incidence (per 1,000 births)	Risk
All ages	1.5	1 in 650
30	1.4	1 in 700
35	2.2	1 in 450
38	5.0	1 in 200
40	10.0	1 in 100
42	16.5	1 in 60
44	25	1 in 40

^[2]

Common Exam Trap

Although the risk per pregnancy increases with maternal age, the majority of DS babies are actually born to mothers < 35 years — simply because younger women have far more pregnancies overall. This is why population-based screening (not just age-based) is essential.

Contributes modestly (↑ paternal age is a risk factor for meiotic non-disjunction) ^[1]
Less well-established than maternal age; effect is smaller

For a mother < 35y who has had one child with trisomy 21 due to non-disjunction: recurrence risk is ~1 in 200 (or 2× age-specific risk if > 35y) ^[1]

Robertsonian translocation carriers have significantly elevated recurrence risk (see Etiology below)

Chromosome 21 is the smallest autosome (21q22.1–22.3 is the "Down syndrome critical region" or DSCR). Key genes in this region whose overexpression contributes to the DS phenotype include:

Gene	Normal Function	Consequence of 1.5× Dosage
DYRK1A (Dual-specificity tyrosine-phosphorylation-regulated kinase 1A)	Neuronal development, synaptogenesis	Intellectual disability, altered brain development
APP (Amyloid Precursor Protein)	Neuronal membrane protein	Overproduction of amyloid-β → virtually all DS individuals develop Alzheimer disease neuropathology by mid-40s
SOD1 (Superoxide Dismutase 1)	Antioxidant enzyme	Oxidative stress imbalance
GATA1	Megakaryocyte and erythroid transcription factor	Transient abnormal myelopoiesis (TAM) and myeloid leukaemia related to Down syndrome
RUNX1	Haematopoietic transcription factor	Altered haematopoiesis
COL6A1/COL6A2	Collagen VI (connective tissue)	Joint laxity, hypotonia
GART	Purine synthesis	Altered folate metabolism

The fundamental pathophysiological principle is gene dosage effect: having 3 copies instead of 2 → 1.5× expression of chromosome 21 genes → downstream protein overproduction → multi-system developmental and functional consequences.

Etiology and Pathophysiology

A. Mechanisms of Trisomy 21

Chromosome 21 pair fails to separate during meiosis (meiotic error) → one gamete has two copies of chr21 (the other has none) → fertilisation with a normal gamete gives rise to a zygote with trisomy 21 (47,XX,+21 or 47,XY,+21) ^[1]

When does it occur?
- Meiosis I error (most common, ~75% of non-disjunction cases): Homologous chromosomes fail to separate → both copies go to one daughter cell
- Meiosis II error (~25%): Sister chromatids fail to separate
- Overwhelmingly maternal in origin (> 90%); paternal meiotic errors account for ~5–10%
Why maternal age matters (mechanistic explanation):
- Female oocytes enter meiosis I during fetal development (by 20 weeks gestation) and arrest at prophase I (dictyotene stage)
- They remain arrested for years to decades until ovulation
- The cohesin complexes (Rec8, SMC1β) that hold bivalents together degrade over time ("cohesin fatigue")
- Additionally, spindle checkpoint function declines with age
- Result: older oocytes have more non-disjunction errors
Karyotype: 47,XX,+21 or 47,XY,+21 (all cells have three copies of chromosome 21)
Risk factors: ↑ maternal age and ↑ paternal age ^[1]
Recurrence risk: For mother < 35y with one affected child — 1 in 200 (or 2× age-specific risk if > 35y) ^[1]

Chromosome 21 fuses with another acrocentric chromosome (usually chromosome 14, occasionally 15/22/21) → results in phenotypically normal carrier with 45 chromosomes (two being 'joined together') or Down syndrome with 46 chromosomes but with three copies of chr21 material ^[1]

What is a Robertsonian translocation?
- "Robertson" + "translocation" — named after W.R.B. Robertson; a type of translocation where two acrocentric chromosomes (13, 14, 15, 21, 22) fuse at their centromeres
- The short arms (containing only ribosomal RNA genes, which exist in multiple copies on other chromosomes) are lost — this is phenotypically inconsequential
- The carrier has only 45 chromosomes but all essential genetic material
Most common: rob(14;21)(q10;q10) — fusion of chromosomes 14 and 21
Why it matters clinically:
- The translocation carrier is phenotypically normal
- But their gametes may carry the translocated chromosome PLUS a free chromosome 21 → offspring effectively has trisomy 21 with only 46 chromosomes on karyotype
- This form is NOT related to maternal age — can occur in young mothers
Recurrence risks: ^[1]
- 10–15% if mother is the translocation carrier
- ~2.5% if father is the carrier (lower because trisomic sperm are at a competitive disadvantage)
- 100% if a parent carries a 21;21 translocation (every viable gamete will have either two copies or zero copies of chr21 — monosomy 21 is lethal, so all surviving offspring have DS) — this is rare
- < 1% if neither parent carries the translocation (75% of translocation DS cases are de novo) ^[1]

Clinical Pearl — When to Suspect Translocation DS

Suspect translocation DS when:

The child with DS has a young mother (< 30y)
There is a family history of DS or recurrent miscarriages
Always perform parental karyotyping when translocation DS is identified in a child — it changes recurrence risk and genetic counselling dramatically.

Usually arises post-zygotically by non-disjunction at mitosis, or by late mitotic non-disjunction in a trisomy 21 conception (trisomic rescue) ^[1]

Result: Two cell populations — some cells have 47 chromosomes (trisomy 21) and some have 46 (normal)
- Karyotype: 47,XX,+21/46,XX (or XY equivalent)
Can result in a milder phenotype ^[1] — the proportion and tissue distribution of trisomic cells determines severity
- Some mosaic individuals have near-normal intelligence
- The clinical phenotype is highly variable
Trisomic rescue mechanism: A trisomy 21 zygote may lose the extra chromosome in some cells during early mitotic divisions → mosaic outcome

The fundamental mechanism underlying ALL features of DS is gene dosage imbalance:

3 copies of chr21 genes → 1.5× mRNA → 1.5× protein production
→ Disruption of developmental pathways
→ Multi-system consequences

This is NOT simply about "too much" of individual proteins. The imbalance disrupts:

Stoichiometric relationships between protein complexes (e.g., transcription factor complexes require precise ratios)
Signalling pathway thresholds that are dose-sensitive
Epigenetic regulation — trisomy 21 causes widespread DNA methylation changes across the entire genome, not just chr21

Classification

Type	Frequency	Karyotype	Maternal Age Association	Recurrence Risk
Free trisomy 21 (non-disjunction)	94%	47,XX(XY),+21	Yes	~1% or age-specific (whichever higher)
Robertsonian translocation	5%	46,XX(XY),rob(14;21),+21 (typical)	No	Depends on carrier parent (see above)
Mosaic	1%	47,XX(XY),+21/46,XX(XY)	Variable	Low (~1%)

Full trisomy 21 — complete phenotype (vast majority)
Partial trisomy 21 — rare; only part of chr21 is triplicated (helps identify DSCR)
Mosaic DS — variable, often milder phenotype

Clinical Features

The clinical features of DS are best understood by organ system, with each feature traced back to the underlying gene dosage effect on developmental pathways. The classic teaching approach is "head-to-toe."

Mnemonic for DS Features

A helpful mnemonic for exam recall: "PALMS SHUT"

Protruding tongue, Prominent epicanthic folds
Atlanto-axial instability
Low-set ears, Leukemia risk
Mental retardation (intellectual disability), Muscle hypotonia
Single palmar crease, Sandal gap
Short stature, Squint
Heart defects (AVSD)
Upslanting palpebral fissures
Third fontanelle, Thyroid disease

A. SYMPTOMS (What the parent/patient reports)

Presentation: Parents typically notice delayed motor milestones (sitting, walking) and delayed speech/language development
Pathophysiology: Overexpression of DYRK1A (involved in neuronal proliferation, dendritic spine morphology) and other chr21 genes → reduced dendritic arborisation, reduced synapse density, abnormal neuronal migration → intellectual disability (the most common genetic cause) ^[1]^[2]
Severity: Variable; IQ typically 25–75 (mean ~50); mosaic DS may have milder cognitive impairment ^[1]
Trajectory: Development continues to progress but at a slower rate than peers; peak functioning typically reached in 3rd–4th decade, followed by cognitive decline (early Alzheimer disease)

Presentation: Poor suck, difficulty latching, slow feeding, frequent choking
Pathophysiology: Generalised hypotonia (reduced muscle tone throughout including oropharyngeal muscles) + protruding tongue (relative macroglossia vs. small oral cavity) + poor coordination of suck-swallow-breathe reflex

Presentation: Frequent upper respiratory tract infections, otitis media (very common — up to 50–70%), pneumonia
Pathophysiology:
- Immune dysfunction: T-cell deficiency (thymic hypoplasia), reduced antibody responses, impaired neutrophil chemotaxis
- Anatomical factors: narrow ear canals → impaired drainage → recurrent otitis media; narrow nasopharynx; adenoidal hypertrophy
- Midface hypoplasia → sinus drainage impairment

Presentation: Common complaint throughout childhood
Pathophysiology: Hypotonia of GI smooth muscle + reduced physical activity; must always exclude Hirschsprung disease (present in ~2% of DS children vs. 0.02% general population — a 100× increase)

Presentation: Snoring, restless sleep, daytime somnolence, behavioural problems
Pathophysiology: Midface hypoplasia + relative macroglossia + adenotonsillar hypertrophy + hypotonia of pharyngeal muscles + narrowed upper airway → airway obstruction during sleep. Affects 50–75% of DS children.

Presentation: Delayed speech/language (may be attributed solely to cognitive delay if hearing not formally tested)
Pathophysiology:
- Conductive (most common): narrow ear canals, recurrent otitis media with effusion (glue ear), ossicular abnormalities
- Sensorineural (less common): cochlear developmental abnormalities, progressive SNHL in adulthood

Presentation: Squinting (strabismus), not tracking objects, rubbing eyes
Pathophysiology: Refractive errors (myopia, hypermetropia, astigmatism) in > 50%; strabismus (up to 45%); congenital cataracts (syndromal association) ^[3]; nystagmus; Brushfield spots (see Signs)

B. SIGNS (What the examiner finds)

Sign	Pathophysiological Basis
Brachycephaly (short, flat occiput)	Abnormal skull base development from altered endochondral ossification (chr21 gene dosage effect on chondrocyte differentiation)
Flat facial profile / midface hypoplasia	Reduced midface skeletal development; maxillary hypoplasia
Upslanting palpebral fissures	Orbital bone morphology — shallow orbits with lateral upward tilt (characteristic dysmorphic feature due to altered craniofacial skeletal development)
Epicanthic folds	Flat nasal bridge + midface hypoplasia → prominent medial canthal skin fold (not true epicanthus but gives appearance of pseudostrabismus)
Brushfield spots	White/pale speckled iris spots; aggregation of connective tissue in iris stroma; more visible in blue/light eyes; present in ~50–80%
Flat nasal bridge	Midface hypoplasia; underdevelopment of nasal bones
Small, low-set ears	Abnormal cartilage development; may have overfolded helices
Open mouth with protruding tongue	Small oral cavity (reduced maxillary and mandibular growth) relative to normal-sized tongue (relative macroglossia) + hypotonia of orofacial muscles → tongue protrusion; mouth breathing
Third fontanelle	Delayed skull ossification; palpable fontanelle between anterior and posterior fontanelles along the sagittal suture
Short neck with excess nuchal skin	Redundant nuchal fold tissue (the same finding detected on prenatal ultrasound as "increased nuchal translucency/thickening")

Sign	Pathophysiological Basis
Generalised hypotonia	Fundamental finding; present from birth. Reduced muscle tone due to altered CNS motor pathway development + possible peripheral nerve conduction abnormalities. Contributes to feeding difficulties, motor delay, joint laxity
Joint hypermobility/laxity	Connective tissue abnormality from overexpression of COL6A1/COL6A2 (collagen VI genes on chr21) → ligamentous laxity
Atlanto-axial instability (AAI)	Ligamentous laxity at C1-C2 → increased atlanto-dens interval (> 5 mm). Present in 10–30% of DS children radiographically, but symptomatic in only 1–2%. Risk of spinal cord compression with neck flexion (relevant for sports clearance, anaesthesia)
Short stature	Multi-factorial: growth hormone axis abnormalities, skeletal dysplasia (short limbs, brachydactyly), hypothyroidism, poor nutrition, cardiac disease. DS-specific growth charts should be used
Short broad hands with clinodactyly	Brachydactyly (short fingers) + single palmar crease (also called "simian crease," present in ~45% of DS vs. ~4% general population); clinodactyly of 5th finger (incurving due to hypoplasia of middle phalanx of 5th digit)
Sandal gap	Wide gap between 1st and 2nd toes — due to altered toe morphogenesis

~40–50% of children with DS have congenital heart disease (CHD) — this is the leading cause of morbidity and mortality in DS in the first year of life.

CHD	Frequency in DS	Notes
Atrioventricular septal defect (AVSD)	~40–45% of DS-CHD	STRONG association between AV canal defects and trisomy 21; 40–50% risk of Down syndrome in a fetus in whom an AV canal defect is detected ^[4]. Complete AVSD → common AV valve, large ASD + VSD → L-to-R shunt → pulmonary hypertension (develops earlier in DS due to upper airway obstruction/hypoventilation)
VSD	~30%	Perimembranous most common
Secundum ASD	~10%
PDA	~5%
TOF	~5%	DS is one of the syndromal conditions associated with TOF ^[5]

Why AVSD is so common in DS: Chromosome 21 genes (particularly DSCAM and COL6A1/COL6A2) are involved in endocardial cushion development. The endocardial cushions form the central core of the heart — the AV septum, mitral and tricuspid valve leaflets. Gene dosage imbalance → defective endocardial cushion fusion → AVSD.

Clinical consequence: All newborns with DS must have an echocardiogram in the neonatal period, even if asymptomatic — some defects (e.g., AVSD) may present with heart failure only after pulmonary vascular resistance drops in the first weeks of life.

Pulmonary Hypertension in DS

Children with DS and large left-to-right shunts develop irreversible pulmonary vascular disease (Eisenmenger syndrome) earlier than chromosomally normal children with the same defect. This is because DS children have:

Upper airway obstruction (hypotonia, macroglossia, adenoidal hypertrophy) → chronic hypoventilation → hypoxic pulmonary vasoconstriction
Possible intrinsic pulmonary vascular abnormalities (reduced alveolar number)

This means the window for surgical repair is narrower — early surgical referral is critical ^[4].

Feature	Details
Duodenal atresia/stenosis	Present in ~5% of DS neonates; the "double bubble sign" on abdominal X-ray is classic. ~30% of all duodenal atresia cases are associated with DS
Hirschsprung disease	~2% of DS (vs. 0.02% in general population); absent ganglion cells in distal bowel → functional obstruction
Tracheo-oesophageal fistula (TOF/OA)	Increased incidence
Imperforate anus	Increased incidence of anorectal malformations
Coeliac disease	5–16% prevalence in DS (vs. ~1% general population); screen with anti-tTG antibodies

Feature	Details
Transient abnormal myelopoiesis (TAM)	Unique to DS neonates (~10%); clonal proliferation of megakaryoblasts with GATA1 mutation; presents with leukocytosis, hepatosplenomegaly, and sometimes liver fibrosis. Usually self-resolves within 3 months but ~20% progress to myeloid leukaemia of Down syndrome (ML-DS) within 4 years ^[6]
Myeloid leukaemia of DS (ML-DS)	AML (particularly acute megakaryoblastic leukaemia, previously FAB M7) occurs at 500× the rate in DS children < 5 years; responds well to chemotherapy (better prognosis than AML in non-DS children) ^[6]^[7]
ALL	DS children have a 15–20× increased risk of ALL as well ^[7]^[8]
Polycythaemia	Common in DS neonates; possibly related to chronic fetal hypoxia
Macrocytosis	Mean corpuscular volume is typically higher in DS individuals

Down syndrome is a risk factor for both AML and ALL (15–20× increase in risk) ^[7]^[8]

Feature	Details
Hypothyroidism	Most common endocrine disorder in DS; congenital hypothyroidism in ~1% (vs. 0.03% general population); acquired autoimmune (Hashimoto) thyroiditis develops in 15–30% by adolescence. Screen with TSH annually
Type 1 diabetes mellitus	Increased risk (autoimmune predisposition)
Type 2 diabetes mellitus	Increased risk (obesity, reduced physical activity). DS is listed as a genetic syndrome associated with DM ^[9]
Short stature	GH-IGF1 axis abnormalities contribute; DS-specific growth charts must be used
Obesity	Reduced basal metabolic rate + hypotonia (reduced physical activity) + hypothyroidism
Delayed/incomplete puberty	Variable; fertility reduced (males almost always infertile; females have reduced fertility but can conceive)

Feature	Details
Intellectual disability	Universal (IQ 25–75, mean ~50); severity variable; mosaic DS may be milder
Seizures / Epilepsy	10% of DS individuals develop epilepsy ^[10]; infantile spasms (West syndrome) more common in DS; later-onset seizures may herald Alzheimer disease
Early-onset Alzheimer disease	Overexpression of APP gene on chr21 → amyloid-β accumulation. Virtually all DS individuals develop sufficient AD neuropathology by their mid-40s ^[11]. Clinical dementia typically appears in mid-50s
Hypotonia	See musculoskeletal above
Atlanto-axial instability	See musculoskeletal above

Feature	Details
Autism spectrum disorder (ASD)	Higher prevalence (~5–10%) in DS than general population; DS is listed as a genetic disorder associated with ASD ^[12]^[13]
Dermatological	Dry skin (xerosis), folliculitis, alopecia areata, cutis marmorata, palmoplantar hyperkeratosis
Ophthalmological	Congenital cataracts (syndromal association) ^[3], Brushfield spots, refractive errors, strabismus, nasolacrimal duct obstruction, keratoconus (adolescence/adulthood)
Dental	Delayed eruption, small teeth (microdontia), missing teeth, periodontal disease
Genitourinary	Cryptorchidism in males (~5%); renal malformations (uncommon)
Immune	Thymic hypoplasia → T-cell dysfunction; increased autoimmune disease (thyroiditis, coeliac disease, T1DM, alopecia areata)

Growth and Development in Down Syndrome

Milestone	Typical Child	Child with DS
Social smile	1–2 months	2–4 months
Rolling over	4–5 months	6–12 months
Sitting independently	6–7 months	9–12 months
First words	12 months	18–24 months
Walking independently	12–15 months	24–36 months
Toilet training	2–3 years	3–6 years

Standard growth charts are NOT appropriate for DS children
DS-specific growth charts (CDC DS charts or the Palermo charts) should be used to avoid misclassifying normal DS growth as pathological or missing true growth faltering

Family-Centred Care and Communication

Breaking the news: Ideally both parents present; in a private, quiet setting; with senior clinician; be compassionate, honest, and balanced (discuss both challenges and positive outcomes)
Avoid outdated terminology: Use "intellectual disability" not "mental retardation"; "Down syndrome" not "Down's child" (person-first language: "a child with Down syndrome")
Provide resources: Connect with support groups (e.g., Hong Kong Down Syndrome Association 香港唐氏綜合症協會)
Multidisciplinary team early: Paediatrician, geneticist, cardiologist, early intervention therapists (OT, PT, SLP)

For prenatal screening decisions: Fully informed consent from both parents; non-directive genetic counselling
For the child: As they grow, involve them in healthcare decisions age-appropriately; assent from ~7 years for simple procedures; capacity for consent varies individually

Method	Type	Timing	Details
Combined first trimester screening	Screening	11–13+6 weeks	Nuchal translucency (US) + maternal serum PAPP-A + free β-hCG. Detection rate ~85–90%
Quadruple test	Screening	15–20 weeks	AFP ↓, unconjugated oestriol ↓, hCG ↑, inhibin A ↑
Non-invasive prenatal testing (NIPT)	Screening	≥10 weeks	Cell-free DNA from maternal blood ^[1]; detection rate > 99%; false positive rate < 0.1%; but is a SCREENING test — positive results require confirmatory invasive testing
Chorionic villus sampling (CVS)	Diagnostic	11–14 weeks	Karyotype from placental tissue; ~1% miscarriage risk
Amniocentesis	Diagnostic	≥15 weeks	Karyotype from amniocytes; ~0.5% miscarriage risk

Screening tools: biochemical markers, ultrasound (for nuchal thickening), and NIPT (e.g., cell-free DNA from maternal blood) ^[1]

High Yield Summary

Definition: Trisomy 21 — most common autosomal trisomy, most common genetic cause of intellectual disability
Epidemiology: ~1/700 live births; HK incidence 1.5/1000 births
Mechanisms: Non-disjunction (94%, maternal age-related), Robertsonian translocation (5%, NOT age-related, check parental karyotype), Mosaicism (1%, milder phenotype)
Cardinal features: Characteristic facies (upslanting palpebral fissures, epicanthic folds, flat nasal bridge, brachycephaly), hypotonia, intellectual disability, single palmar crease, sandal gap
Cardiac (40-50%): AVSD is the most characteristic (strong bidirectional association with DS); also VSD, ASD, PDA, TOF. All DS neonates need echo
GI: Duodenal atresia (double bubble), Hirschsprung disease (100× increase), coeliac disease
Haem: TAM (self-resolving but 20% → ML-DS), 15-20× risk of leukaemia (AML and ALL)
Endocrine: Hypothyroidism (screen annually with TSH), DM, obesity
Neuro: Epilepsy (10%), early-onset Alzheimer disease (virtually all by mid-40s due to APP overexpression)
Screening: Combined 1st trimester screen, quad test, NIPT (cell-free DNA) → confirmatory CVS/amniocentesis
Recurrence: Non-disjunction ~1/200 if mother < 35y; translocation depends on carrier status (10-15% if mother, 100% if 21;21)

Active Recall - Down Syndrome (Trisomy 21)

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p503, Section 15.2.1 — Down Syndrome) ^[2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p818 — Down syndrome) ^[3] Senior notes: Ryan Ho Opthalmology.pdf (p122 — Congenital Cataract, syndromal associations including Down) ^[4] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p561–563 — AVSD and Down syndrome association) ^[5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p581–583 — TOF and Down syndrome) ^[6] Senior notes: Ryan Ho Haemtology.pdf (p53 — Myeloid proliferation related to Down syndrome) ^[7] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p1380 — AML risk factors including DS) ^[8] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p1391 — ALL risk factors including DS) ^[9] Senior notes: Maksim Medicine Notes.pdf (p80 — Genetic syndromes associated with DM including Down) ^[10] Senior notes: Ryan Ho Neurology.pdf (p102 — Genetic aetiology of epilepsy including Down syndrome 10%) ^[11] GC 241. Reference (1) - Alzheimers Dementia - Revised criteria for diagnosis and staging of Alzheimer's disease.pdf (p6 — DS and AD neuropathology by mid-40s) ^[12] Senior notes: Adrian Lui Pediatrics Notes.pdf (p86 — ASD biological risk factors including Down syndrome) ^[13] Senior notes: Ryan Ho Psychiatry.pdf (p256 — ASD aetiology, genetic disorders including DS) ^[14] Lecture slides: CFB (PSY04) Aetiology of Psychiatric Disorders.pdf (p8 — Chromosomal abnormalities including Down syndrome as example of aneuploidy) ^[15] Lecture slides: Block C - The malformed child: hereditary syndromes and anomalies.pdf (p1 — Genetic vs environmental risk factors, chromosomal disorders)

Differential Diagnosis of Down Syndrome (Trisomy 21)

The differential diagnosis of Down syndrome arises in two distinct clinical contexts:

Prenatal context — An abnormal screening test (e.g., increased nuchal translucency, abnormal first-trimester combined screen, positive NIPT) raises the possibility of DS, but other conditions can produce similar screening abnormalities.
Postnatal context — A neonate or infant presents with features suggestive of DS (hypotonia, characteristic facies, congenital heart disease, etc.), and the clinician must distinguish DS from other conditions that share overlapping dysmorphic features, hypotonia, intellectual disability, or congenital heart defects.

The key principle: DS is confirmed by karyotype (or chromosomal microarray/FISH), so the DDx is really about what else to consider while awaiting cytogenetic confirmation, or when the karyotype is normal but the phenotype is suggestive.

A. Differential Diagnosis by Presenting Feature

Hypotonia is the most consistent neonatal finding in DS — present in virtually 100% at birth. But many other conditions present with a floppy newborn ± dysmorphic features:

Condition	Key Distinguishing Features	Mechanism
Down syndrome (Trisomy 21)	Characteristic facies (upslanting palpebral fissures, flat nasal bridge, epicanthic folds, brachycephaly), single palmar crease, sandal gap, CHD (~45%)	Gene dosage effect from 3 copies of chr21
Edwards syndrome (Trisomy 18)	Clenched fist with overlapping fingers (index over 3rd, 5th over 4th), rocker-bottom feet, prominent occiput, micrognathia, CHD (VSD, PDA), severe ID; much more lethal (50% die within 1 week)	Gene dosage effect from 3 copies of chr18
Patau syndrome (Trisomy 13)	Midline defects: holoprosencephaly, cleft lip/palate, polydactyly, microphthalmia/cyclopia, scalp defects (cutis aplasia), CHD; most lethal autosomal trisomy (median survival ~10 days)	Gene dosage effect from 3 copies of chr13
Prader-Willi syndrome (PWS)	Severe neonatal hypotonia with poor feeding (in contrast, DS has mild-moderate hypotonia); later develops hyperphagia/obesity; hypogonadism; almond-shaped eyes; narrow bifrontal diameter. NO characteristic DS facies	Deletion or loss of paternal 15q11-13 (imprinting disorder) — loss of paternally expressed genes in the PWS critical region
Congenital hypothyroidism	Hypotonia, large fontanelles, macroglossia, prolonged jaundice, constipation, coarse features — can superficially resemble DS. Key difference: detected on newborn screening (elevated TSH), and features are reversible with early T4 replacement	Inadequate thyroid hormone → impaired CNS and somatic development
Congenital myotonic dystrophy	Severe neonatal hypotonia, facial diplegia ("tented" upper lip), respiratory failure; mother often affected (check for myotonia, grip difficulty). Triangular facies, NOT DS facies	Autosomal dominant CTG trinucleotide repeat expansion in DMPK; anticipation → neonatal form is severe
Spinal muscular atrophy (SMA) Type 1	Severe hypotonia with preserved intellect and alertness, tongue fasciculations, absent deep tendon reflexes, bell-shaped chest. NO dysmorphic features	AR; homozygous deletion/mutation in SMN1 gene → loss of anterior horn motor neurons
Zellweger spectrum disorders	Severe hypotonia, high forehead, large fontanelles, hepatomegaly, seizures, stippled epiphyses; lethal. Flat facial profile can overlap with DS	Peroxisome biogenesis disorder → unable to form functional peroxisomes → accumulation of very long chain fatty acids

High Yield Exam Point — Prader-Willi vs Down Syndrome in a Floppy Neonate

Both present with neonatal hypotonia, but the quality differs:

DS: Moderate hypotonia, but baby can still feed (though slowly); characteristic facies present
PWS: Profound hypotonia with extremely poor/absent suck — often needs NG/tube feeding; almond-shaped eyes, thin upper lip, narrow bifrontal diameter — facies are different from DS
Karyotype will distinguish: DS = trisomy 21; PWS = normal karyotype but abnormal methylation at 15q11-13

Since ~40–50% of DS children have CHD, and CHD is a common neonatal presentation, consider:

Condition	Characteristic CHD	Distinguishing Features
Down syndrome	AVSD (most characteristic), VSD, ASD, PDA, TOF ^[1]^[16]	Upslanting palpebral fissures, hypotonia, single palmar crease
Turner syndrome (45,X)	Left-sided cardiac lesions: coarctation of aorta, bicuspid aortic valve, hypoplastic left heart ^[16]	Female only; short stature, webbed neck, low hairline, widely spaced nipples, cubitus valgus, lymphoedema of hands/feet at birth
Noonan syndrome	Right-sided cardiac lesions: pulmonary valve stenosis with dysplastic valve cusps, ASD, HCM ^[16]	"Turner-like" but occurs in BOTH sexes; normal karyotype; ptosis, downslanting palpebral fissures (opposite to DS!), low-set ears, short stature, cryptorchidism in males
DiGeorge syndrome (22q11.2 deletion)	Conotruncal abnormalities: interrupted aortic arch, truncus arteriosus, TOF, VSD ^[5]^[16]	CATCH22: Cardiac, Abnormal facies, Thymic hypoplasia (T-cell deficit), Cleft palate, Hypocalcaemia ^[5]. Facies: tubular nose, small mouth, low-set ears
Williams syndrome (7q11.23 deletion)	Supravalvular aortic stenosis, peripheral pulmonary artery stenosis ^[16]	Elfin facies, hypercalcaemia, "cocktail party" personality, intellectual disability ^[16]
Edwards syndrome (Trisomy 18)	VSD, PDA, polyvalvular disease	Clenched fists with overlapping fingers, rocker-bottom feet, prominent occiput
CHARGE syndrome	Conotruncal defects, AV canal defects, aortic arch anomalies	Coloboma, Heart defect, Atresia choanae, Restricted growth/development, Genital anomalies, Ear anomalies
Alagille syndrome	Peripheral pulmonary stenosis, TOF	Butterfly vertebrae, cholestasis, posterior embryotoxon (eye), characteristic facies (broad forehead, deep-set eyes, pointed chin)

Common syndromes associated with congenital heart diseases: Down syndrome (45%) — AVSD, VSD, ASD, PDA, TOF; Turner syndrome (20%) — left-sided lesions; Williams syndrome (75%) — supravalvular AS; Noonan syndrome (50–80%) — right-sided lesions, PS with dysplastic valve, HCM; DiGeorge syndrome (80%) — conotruncal abnormalities ^[16]

Pattern Recognition — Sidedness of CHD by Syndrome

A quick mental model for exams:

Down → "Down the middle" → AV canal/septal defects
Turner → "Left" → Left-sided lesions (CoA, bicuspid AoV)
Noonan → "Right" → Right-sided lesions (PS, HCM)
DiGeorge → "Outflow" → Conotruncal (truncus, interrupted arch, TOF)
Williams → "Supra" → Supravalvular AS

When a child presents with intellectual disability (ID) and dysmorphism, the differential is broad. Syndromic ID means ID associated with recognisable physical features ^[17]:

Condition	Key Features	Inheritance/Mechanism
Down syndrome	As described	Trisomy 21
Fragile X syndrome	Most common inherited cause of ID (1/4000 males); long face, large ears, macro-orchidism (post-pubertal), joint laxity, ASD features; may lack obvious dysmorphism in young children	X-linked; CGG trinucleotide repeat expansion in FMR1 gene
Angelman syndrome	Severe ID, absent speech, ataxic gait ("puppet-like"), frequent laughter, seizures (up to 90%), microcephaly	Deletion of maternal 15q11-13 (same region as PWS but on maternal allele — imprinting)
Rett syndrome	Almost exclusively females; normal development for 6–18 months → regression with loss of hand skills (stereotypic hand-wringing), loss of speech, acquired microcephaly, seizures	X-linked dominant; MECP2 mutation
Smith-Magenis syndrome	Brachycephaly (like DS), flat midface, broad nasal bridge, sleep disturbance, self-injurious behaviour, "self-hug" behaviour	Deletion 17p11.2 or RAI1 mutation
Neurofibromatosis type 1 (NF1)	Café-au-lait macules, axillary freckling, neurofibromas; Noonan syndrome may have café-au-lait macules ^[18]	AD; NF1 gene on chr17
Cornelia de Lange syndrome	Microcephaly, synophrys (joined eyebrows), long eyelashes, thin downturned lips, limb reduction defects, ID, GORD	NIPBL, SMC1A, SMC3 mutations
Kabuki syndrome	Long palpebral fissures (can be confused with DS upslanting), arched eyebrows, short columella, large ears, fetal fingertip pads, ID, CHD	KMT2D or KDM6A mutations

Fragile X vs Down Syndrome

Both are common genetic causes of ID and may coexist. However:

DS is cytogenetically visible (karyotype/FISH); Fragile X requires specific molecular testing (PCR/Southern blot for CGG repeats)
In clinical exams, if a child has ID but a normal karyotype, always consider Fragile X testing — it is the most common inherited cause of ID and is missed by standard karyotype

Finding	Possible Diagnoses
Increased nuchal translucency (NT)	Trisomy 21, trisomy 18, trisomy 13, Turner syndrome (45,X — cystic hygroma), Noonan syndrome, congenital heart defects (without aneuploidy), skeletal dysplasias, congenital diaphragmatic hernia, congenital infections
Positive NIPT for trisomy 21	True positive (DS); false positive (confined placental mosaicism, maternal copy number variants, vanishing twin with trisomy 21, maternal malignancy); must be confirmed by CVS/amniocentesis
Abnormal quad screen (↓ AFP, ↓ uE3, ↑ hCG, ↑ inhibin A)	Trisomy 21 (classic pattern); but abnormal values can also occur with incorrect gestational dating, multiple pregnancy, molar pregnancy

DS Feature	DDx to Consider
Transient abnormal myelopoiesis (TAM) in DS neonate with leukocytosis	Neonatal sepsis, congenital leukaemia (non-DS), reactive leukocytosis, congenital CMV/toxoplasmosis with extramedullary haematopoiesis
Duodenal atresia ("double bubble")	Duodenal stenosis, annular pancreas, midgut volvulus, jejunal atresia; note ~30% of duodenal atresia is associated with DS
Hirschsprung disease in DS toddler	Functional constipation (very common in DS due to hypotonia), hypothyroidism (also common in DS and causes constipation), coeliac disease
Early-onset dementia in adult with DS	Hypothyroidism (common in DS), depression, sensory loss (hearing/vision), sleep apnoea, medication side effects — all treatable causes of cognitive decline that must be excluded before attributing to Alzheimer disease

Feature	Down Syndrome	Congenital Hypothyroidism	Prader-Willi	Noonan
Palpebral fissures	Upslanting	Normal	Normal	Downslanting
Hypotonia	Moderate	Moderate	Profound	Mild
Feeding	Slow but possible	Poor, prolonged jaundice	Very poor, often needs tube	Variable
CHD	AVSD, VSD	Usually none	Usually none	PS, HCM
Karyotype	47,+21	Normal	Normal	Normal
Confirmatory test	Karyotype/FISH	TSH/T4	Methylation 15q11	RAS/MAPK gene panel
Reversible?	No	Yes, with levothyroxine	No	No

Newborn screening programme (HK): Congenital hypothyroidism is screened for on the routine newborn heel-prick test (dried blood spot TSH) — this should help exclude hypothyroidism early as a mimic of DS features
NIPT availability: Since 2019, NIPT has been offered at public hospitals in HK for high-risk pregnancies; however, it screens for trisomies 21/18/13 and sex chromosome aneuploidies — does NOT detect DiGeorge/Williams/Noonan (these require CMA or targeted testing)
Chromosomal microarray (CMA) has largely replaced conventional karyotyping as first-line cytogenetic investigation for unexplained ID/developmental delay in many HK paediatric genetics services — it detects both aneuploidies AND submicroscopic copy number variants (e.g., 22q11.2 deletion for DiGeorge)

High Yield Summary — DDx of Down Syndrome

DDx context: Arises prenatally (abnormal screening) or postnatally (dysmorphic neonate with hypotonia ± CHD)
Key mimics of the "floppy baby with dysmorphism": Prader-Willi (profound hypotonia), congenital hypothyroidism (reversible!), congenital myotonic dystrophy, other trisomies (18, 13)
CHD pattern helps narrow DDx: AVSD = think DS; left-sided = Turner; right-sided = Noonan; conotruncal = DiGeorge; supravalvular = Williams
ID without obvious DS facies but normal karyotype → test for Fragile X (most common inherited cause of ID)
Confirmation: Karyotype/FISH/CMA distinguishes DS from all other conditions. If karyotype normal, consider CMA → gene panels → WES/WGS
Always exclude congenital hypothyroidism — it is treatable and screened for on newborn screening in HK
Prenatal false positives: Positive NIPT must be confirmed by CVS/amniocentesis; increased NT has a broad DDx beyond trisomy 21

Active Recall - DDx of Down Syndrome

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p503, Section 15.2.1 — Down Syndrome mechanisms and features) ^[5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p581–583 — DiGeorge syndrome/CATCH22, TOF syndromal associations) ^[16] Senior notes: Ryan Ho Cardiology.pdf (p185 — Common syndromes associated with congenital heart diseases table) ^[17] Senior notes: Ryan Ho Psychiatry.pdf (p243 — Intellectual Disability definition and classification) ^[18] Senior notes: Ryan Ho Rheumatology.pdf (p172 — NF1 and Noonan syndrome DDx with café-au-lait macules)

Diagnostic Criteria, Diagnostic Algorithm, and Investigation Modalities for Down Syndrome (Trisomy 21)

Down syndrome is fundamentally a cytogenetic diagnosis — the definitive answer comes from demonstrating an extra copy of chromosome 21 (or its material) through laboratory testing. There are no "clinical diagnostic criteria" in the way that, say, rheumatic fever has the Jones criteria. However, clinical recognition of the phenotype drives the decision to test, and the distinction between screening (identifying those at increased risk) and diagnosis (confirming the condition) is critical.

The cardinal rule: A screening test (including NIPT) is NOT a diagnostic test. A positive screen must always be followed by confirmatory cytogenetic analysis. ^[2]^[19]

Diagnostic Criteria

Down syndrome is confirmed when cytogenetic analysis demonstrates one of the following:

Cytogenetic Finding	Karyotype Notation	Frequency
Free trisomy 21	47,XX,+21 or 47,XY,+21	94%
Robertsonian translocation	46,XX,rob(14;21)(q10;q10),+21 (most common)	5%
Mosaicism	47,XX,+21[x]/46,XX[y] (proportion of trisomic cells reported)	1%
Partial trisomy 21	46,XX,dup(21)(q22.1q22.3) (rare; duplication of DSCR)	Very rare

The method of cytogenetic analysis determines what can and cannot be detected:

Method	What It Detects	Limitations	Turnaround
Conventional karyotyping (G-banding)	All chromosomes examined; detects numerical abnormalities (trisomy), translocations, large structural rearrangements	Cannot exclude small deletions, duplications, rearrangements, or mosaicism (low-level) ^[19]	~3 weeks ^[19]
FISH (Fluorescent In Situ Hybridisation)	Rapid detection of extra copy of chr21 using a specific probe	Can only indicate that an extra copy of chromosome 21 is present but cannot detect translocation ^[19]; cannot determine mosaicism accurately	24–48 hours
Rapid aneuploidy testing (QF-PCR)	Selected chromosomes examined (chr 13, 18, 21, X, Y); takes ~3 working days ^[19]	Cannot exclude small deletions, duplications, rearrangement or mosaicism; cannot exclude genetic diseases or other congenital abnormalities ^[19]	~3 working days ^[19]
Chromosomal microarray (CMA)	Detects copy number variants across entire genome; confirms trisomy 21 and identifies concurrent microdeletions/duplications	Cannot detect balanced translocations or low-level mosaicism; more expensive	2–4 weeks

Critical Exam Point — FISH vs Karyotype

A positive FISH result should be confirmed by a complete chromosome analysis to identify translocation that has implications for further reproductive counselling for the parents ^[19]. Why? Because FISH uses a chr21-specific probe — it will light up three signals for ANY form of trisomy 21 (free trisomy, translocation, or partial trisomy) but CANNOT tell you the mechanism. If it's a Robertsonian translocation, parental karyotyping is essential because recurrence risk is dramatically different (up to 10–15% if mother is carrier, vs ~1% for free trisomy).

While there are no formal clinical diagnostic criteria, the following constellation of features should trigger urgent cytogenetic testing:

In the neonate:

Generalised hypotonia (present in virtually all)
Characteristic craniofacial features: upslanting palpebral fissures, epicanthic folds, flat nasal bridge, brachycephaly, small ears
Single transverse palmar crease, clinodactyly of 5th finger
Sandal gap (wide space between 1st and 2nd toes)
Congenital heart disease (especially AVSD)

No single feature is pathognomonic. Many DS features (e.g., epicanthic folds, single palmar crease) can occur in isolation in the normal population. It is the cluster of features together that makes DS clinically recognisable.

In practice, experienced neonatologists and midwives recognise DS "at the door" — the gestalt of the face + hypotonia. But always confirm with cytogenetics before informing parents of a definitive diagnosis.

Prenatal Screening and Diagnosis — Complete Algorithm

Prenatal detection follows a two-tier screening pathway in Hong Kong (TWH/QMH model) ^[20]:

Component	What It Measures	Expected Pattern in DS
Nuchal translucency (NT) on ultrasound	Subcutaneous fluid collection at fetal neck	Increased (≥3.0 mm or ≥95th centile) — reflects impaired lymphatic drainage due to cardiac defects, altered lymphatic development
Maternal serum PAPP-A (Pregnancy-Associated Plasma Protein A)	Placental protein	Decreased (< 0.4 MoM) — reduced placental function
Maternal serum free β-hCG	Placental hormone	Increased (> 2.0 MoM) — altered placental trophoblast function
Maternal age	Risk factor	Integrated into algorithm

Detection rate: ~85–90% for a 5% false positive rate
Combined with maternal age to generate a risk score (e.g., "1 in 150")
Positive screen = risk ≥ agreed cut-off (varies by institution, typically 1 in 250 to 1 in 300 in HK)

Additional first-trimester ultrasound markers (used in some centres to refine risk):

Absent/hypoplastic nasal bone
Tricuspid regurgitation
Reversed ductus venosus flow
Abnormal facial angle

If the first tier screening test is positive, we counsel about second tier screening: (1) non-invasive prenatal test (NIPT), or (2) diagnostic testing in the form of chorionic villus sampling or amniocentesis ^[20]

Option A: Non-Invasive Prenatal Testing (NIPT)

NIPT — screening blood test based on the identification and counting of cell-free fetal DNA fragments in maternal plasma originating from the placenta ^[20]

Detects 99.7% of trisomy 21 with a ~0.1% false positive rate ^[20]

Feature	Detail
Basis	Cell-free DNA (cfDNA) in maternal blood — ~10–20% is fetal in origin (from placental trophoblast apoptosis)
Timing	≥10 weeks gestation (sufficient fetal fraction needed; unreliable if fetal fraction < 4%)
Detection rate for T21	99.7% ^[20]
False positive rate	~0.1% ^[20]
Key limitation	Still a screening test, NOT diagnostic. False positives occur due to: confined placental mosaicism (placenta trisomic but fetus normal), vanishing twin, maternal copy number variants, maternal malignancy
If positive	Must be confirmed by invasive diagnostic testing (CVS or amniocentesis)

Option B: Direct Diagnostic Testing

Chorionic Villus Sampling (CVS) ^[19]

Feature	Detail
Timing	1st trimester (11–13⁺⁶ weeks of gestation) ^[19]
Procedure	Needle through maternal abdominal wall, uterus and into the placenta to aspirate the chorionic villi. USG will be used to guide needle insertion and aspiration. Local anaesthesia injection in maternal abdominal wall is required ^[19]
Sample	Chorionic villi (placental tissue — shares fetal genome)
Available tests	Karyotype (~3 weeks), FISH (24–48 hours), QF-PCR (~3 working days), CMA (2–4 weeks)
Miscarriage risk	~0.5–1% (procedure-related)
Limitation	Risk of confined placental mosaicism (discrepancy between placental and fetal karyotype); may require amniocentesis for confirmation

Amniocentesis ^[19]

Feature	Detail
Timing	≥15 weeks gestation (2nd trimester)
Procedure	Ultrasound-guided needle through maternal abdomen into amniotic cavity; aspiration of ~15–20 mL amniotic fluid containing fetal cells (amniocytes shed from fetal skin, urinary tract, respiratory tract)
Available tests	Karyotype, FISH, QF-PCR, CMA
Miscarriage risk	~0.5%
Advantage over CVS	No confined placental mosaicism issue (directly tests fetal cells); can also measure amniotic fluid AFP for neural tube defect screening

Quadruple test (15–20 weeks):

Marker	Direction in DS	Mnemonic
AFP (alpha-fetoprotein)	↓
Unconjugated oestriol (uE3)	↓
hCG	↑
Inhibin A	↑

Detection rate: ~80% for 5% FPR
Mnemonic: "In DS, the baby makes less of what it should (AFP, uE3) but the placenta makes more signals (hCG, inhibin A)"

Second-trimester ultrasound soft markers for DS:

Increased nuchal fold thickness (≥6 mm at 15–20 weeks)
Short femur / short humerus
Echogenic bowel
Mild pyelectasis (renal pelvis dilatation)
Echogenic intracardiac focus
Absent/hypoplastic nasal bone
None are diagnostic alone; they modify the background risk

Postnatal Investigation Modalities — Baseline Workup for Confirmed DS

Once DS is confirmed (or strongly suspected clinically), a comprehensive baseline workup is performed. The rationale for each investigation connects to the known multi-system complications:

Investigation	Rationale	Key Findings
Echocardiogram	~40–50% have CHD; must be done in ALL DS neonates even if asymptomatic (some defects become symptomatic only after PVR drops) ^[16]	AVSD (common AV valve, primum ASD + inlet VSD, AV valve regurgitation); VSD; ASD; PDA; TOF
ECG	Baseline cardiac rhythm; evaluate axis	Superior axis deviation (left axis) in AVSD; RVH or biventricular hypertrophy
CXR	Assess heart size and pulmonary vascularity	Cardiomegaly, increased pulmonary vascular markings (in large L-to-R shunt)

All DS neonates should have an echocardiogram regardless of the presence or absence of a murmur. Some AVSDs present without an obvious murmur in the first days when PVR is still high.

Investigation	Rationale	Key Findings
TSH (± free T4)	Congenital hypothyroidism in ~1% of DS neonates (vs 0.03% general population); acquired hypothyroidism in 15–30% over childhood	Elevated TSH ± low fT4 in hypothyroidism; check at birth (routine newborn screen), 6 months, 12 months, then annually

Investigation	Rationale	Key Findings
Complete blood count (CBC) with differential	Screen for TAM, polycythaemia, and later for leukaemia ^[6]^[7]	TAM: leukocytosis with circulating blast cells (megakaryoblasts); polycythaemia in neonatal period; macrocytosis common in DS
Peripheral blood film	Morphology of blast cells if TAM suspected	Megakaryoblastic blasts; confirm with immunophenotyping (flow cytometry)
GATA1 mutation testing	If TAM confirmed, identifies the somatic mutation driving the clonal proliferation	Mutation present in TAM blast cells; helps predict risk of progression to ML-DS

Investigation	Rationale	Key Findings
Newborn hearing screen (OAE/ABR)	Conductive and sensorineural hearing loss both common; early detection essential for speech/language development	Refer for formal audiological assessment if screen failed; repeat 6-monthly in first 3 years
Formal audiometry (when age-appropriate)	Ongoing surveillance	Conductive loss (glue ear, narrow canals); sensorineural loss (less common); mixed

Investigation	Rationale	Key Findings
Red reflex test (neonatal)	Screen for congenital cataract (syndromal association) ^[3]	Absent or white reflex = leukocoria → urgent ophthalmology referral
Ophthalmological review	Refractive errors (> 50%), strabismus (up to 45%), nystagmus, Brushfield spots, nasolacrimal duct obstruction	Formal assessment by 6 months; then annually

Investigation	Rationale	Key Findings
Abdominal X-ray (if bilious vomiting/failure to pass meconium)	Screen for duodenal atresia, Hirschsprung disease	"Double bubble" sign (duodenal atresia); distended loops with absent distal gas (Hirschsprung)
Contrast enema / rectal biopsy (if suspected Hirschsprung)	Confirm aganglionosis	Transition zone on contrast; absent ganglion cells on biopsy (full-thickness or suction rectal biopsy)
Anti-tTG antibodies (IgA)	Screen for coeliac disease (~5–16% prevalence)	Check total IgA concurrently (IgA deficiency → false negative anti-tTG); screen from age ~2 years or earlier if symptomatic

Investigation	Rationale	Key Findings
Lateral cervical spine X-ray (flexion/extension views)	Atlanto-axial instability (AAI) in 10–30%; symptomatic in 1–2%	Atlanto-dens interval > 5 mm = abnormal; used for sports clearance (avoid contact sports, trampolining, gymnastics if unstable); required before anaesthesia
MRI cervical spine	If symptomatic AAI (neck pain, gait changes, hyperreflexia, bowel/bladder dysfunction)	Cord compression, signal change

Timing: Cervical spine screening is typically performed around age 3–5 years (when child can cooperate for flexion/extension views), and before participation in sports or before general anaesthesia.

Assessment	Rationale	Details
Developmental assessment	Global developmental delay is universal; early intervention improves outcomes	Use DS-specific developmental milestones; refer to early intervention programme (physiotherapy, occupational therapy, speech-language pathology) as early as possible
ASD screening	Higher prevalence of ASD in DS (~5–10%) ^[12]^[13]	Modified Checklist for Autism in Toddlers (M-CHAT) at 18 and 24 months
Behavioural/psychiatric assessment	Increased rates of ADHD, depression, anxiety, OCD in DS	Ongoing surveillance throughout childhood and adolescence

Component	Detail
Parental karyotyping	Essential if translocation DS is identified in the child — determines carrier status and recurrence risk ^[1]^[19]
Recurrence risk counselling	Free trisomy: ~1% or age-specific risk (whichever higher); translocation: depends on carrier parent (10–15% if mother, ~2.5% if father, 100% if 21;21, < 1% if de novo) ^[1]
Offer genetic counselling to extended family	If parental translocation identified, siblings of the carrier parent may also carry the balanced translocation

Age	Key Investigations/Assessments
Birth	Clinical assessment, urgent FISH/QF-PCR, echocardiogram, CBC, TSH (newborn screen), hearing screen (OAE/ABR), red reflex, feeding assessment
1 month	Full karyotype result; early intervention referral; lactation support
6 months	Ophthalmology review; hearing reassessment; repeat TSH; developmental assessment
12 months	Annual TSH; vision and hearing review; developmental milestone check
2 years	Coeliac screen (anti-tTG + total IgA); ASD screening (M-CHAT)
3–5 years	Cervical spine X-ray (for AAI screening); dental review; sleep study if snoring/OSA symptoms
Annually ongoing	TSH, vision, hearing, growth (DS-specific charts), developmental assessment, dental review
Adolescence	Discuss transition to adult services; screen for obesity, mental health, thyroid, reproductive counselling

Interpretation of Key Investigations — What Each Result Means

Result	Interpretation	Action
47,XX,+21	Free trisomy 21 (non-disjunction) — most common	Recurrence risk counselling (~1%); no parental karyotype usually needed
46,XX,der(14;21)(q10;q10),+21	Robertsonian translocation DS	Parental karyotyping is ESSENTIAL ^[19]
47,XX,+21^[15]/46,XX^[5]	Mosaic trisomy 21 (75% trisomic cells in this example)	Phenotype may be milder; percentage of trisomic cells reported but does not perfectly predict severity (tissue distribution matters)
46,XX (normal) but clinical suspicion remains	Consider: mosaic DS with low proportion of trisomic cells (may need skin fibroblast karyotype), or alternative diagnosis (CMA, gene panels)	Refer to clinical genetics

Result	Interpretation	Caveat
3 signals for chr21 probe	Extra copy of chr21 present	Cannot distinguish free trisomy from translocation ^[19] — must do full karyotype
2 signals	Normal chr21 copy number	Does not exclude very low-level mosaicism

Pattern	Most Likely Diagnosis
↓ PAPP-A, ↑ free β-hCG, ↑ NT (1st trimester)	Trisomy 21 (DS)
↓ PAPP-A, ↓ free β-hCG, ↑ NT	Trisomy 18 (Edwards) or Trisomy 13 (Patau)
↑ NT alone with normal biochemistry	Consider cardiac defect (without aneuploidy), Noonan syndrome, Turner syndrome
↓ AFP, ↓ uE3, ↑ hCG, ↑ inhibin A (quad test)	Trisomy 21 (DS)
↓ AFP, ↓ uE3, ↓ hCG, ↓ inhibin A	Trisomy 18

Exam Mnemonic — Quad Test in DS

Remember: "DS pregnancies are HIGH on hCG and inhibin A, LOW on AFP and uE3" Think of it as: the placenta in DS is overactive (↑ hCG, ↑ inhibin A) but the fetus is underproducing (↓ AFP, ↓ uE3) — this reflects altered placental and fetal hepatic function.

Candidates for diagnosis include: (1) Fetal trisomy in previous pregnancy, (2) At least 1 major or 2 minor fetal structural anomalies in current pregnancy, (3) Chromosomal translocation, inversion or aneuploidy in the pregnant woman or her partner ^[19]

These criteria identify pregnancies at sufficiently high risk to justify the procedural risk of invasive testing (CVS ~1% miscarriage; amniocentesis ~0.5%).

Consent for postnatal karyotyping: In HK, parental consent is required for blood sampling from the neonate. Explain clearly: "We want to do a blood test to check the baby's chromosomes — this will tell us definitively whether your baby has Down syndrome and which type, which is important for planning care and understanding the chance of it happening again in future pregnancies."
Communication when karyotype is pending: Be honest with parents that clinical features are suggestive; avoid giving a definitive label until karyotype is confirmed. Use sensitive language: "Your baby has some features that can be associated with Down syndrome. We have sent a blood test to confirm this, and we should have a preliminary result within 1–3 days."
Newborn screening (HK heel-prick test): Screens for congenital hypothyroidism (TSH), G6PD deficiency, and selected IEMs. This is complementary to (not a substitute for) DS-specific testing.

High Yield Summary — Diagnostics of Down Syndrome

DS is a cytogenetic diagnosis — confirmed by karyotype, FISH, QF-PCR, or CMA showing trisomy 21
FISH is rapid (24–48h) but cannot detect translocation — always confirm with full karyotype ^[19]
Prenatal screening is two-tier in HK: 1st trimester combined screen → if positive → NIPT or invasive testing (CVS/amniocentesis) ^[20]
NIPT detects 99.7% of T21 with ~0.1% FPR — but is STILL a screening test; positive NIPT needs confirmatory CVS/amniocentesis ^[20]
Quad test pattern in DS: ↓ AFP, ↓ uE3, ↑ hCG, ↑ inhibin A
If translocation DS: parental karyotyping is ESSENTIAL for recurrence risk counselling ^[19]
Postnatal baseline workup: Echo (all neonates), CBC (TAM), TSH, hearing, red reflex, developmental referral
Ongoing surveillance: Annual TSH, hearing, vision, growth (DS-specific charts), coeliac screen, cervical spine X-ray at 3–5y, dental, ASD screening

Active Recall - Diagnostics of Down Syndrome

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p503, Section 15.2.1 — Down Syndrome mechanisms, screening, recurrence risks) ^[2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p818 — Down syndrome epidemiology and risk factors) ^[3] Senior notes: Ryan Ho Opthalmology.pdf (p122 — Congenital Cataract, syndromal associations including Down) ^[6] Senior notes: Ryan Ho Haemtology.pdf (p53 — Myeloid proliferation related to Down syndrome: TAM and ML-DS) ^[7] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p1380 — AML risk factors including DS 15-20× increase) ^[12] Senior notes: Adrian Lui Pediatrics Notes.pdf (p86 — ASD biological risk factors including Down syndrome) ^[13] Senior notes: Ryan Ho Psychiatry.pdf (p256 — ASD aetiology, genetic disorders including DS) ^[16] Senior notes: Ryan Ho Cardiology.pdf (p185 — Common syndromes associated with congenital heart diseases) ^[19] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p828 — Diagnostic tests for DS: CVS, karyotype, FISH, QF-PCR, limitations) ^[20] Lecture slides: CFB (OGPAE01-1) Perinatal Medicine, Antenatal Care and Pre-pregnant Counselling (Part I).pdf (p27 — Down syndrome screening in TWH/QMH, NIPT details)

Management of Down Syndrome (Trisomy 21)

Down syndrome is not a disease to be "cured" — it is a lifelong chromosomal condition. Management is therefore fundamentally about:

Anticipatory care — proactively screening for and treating the known medical comorbidities before they cause harm
Early intervention — maximising developmental potential through therapies started as early as possible
Family-centred care — supporting the whole family unit (parents, siblings, extended family) with information, psychological support, and practical resources
Health surveillance — structured, age-specific screening protocols that continue from birth through adulthood
Management of specific comorbidities — treating each complication on its own merits with standard paediatric approaches, but with awareness of DS-specific nuances

There is no medication that "treats" trisomy 21 itself. All management targets the downstream consequences of the gene dosage imbalance.

A. Immediate Neonatal Management (Birth to 1 Month)

This is one of the most important moments in the family's life. How the diagnosis is communicated has lasting impact on the parents' adjustment and bonding with their child.

Principle	Rationale
Both parents present if possible	Ensures both receive the same information; supports each other
Private, quiet setting	Respects dignity; allows emotional processing
Senior clinician delivers	Demonstrates importance; authority reassures
Baby present with parents	Reinforces that this is their baby first; a baby with DS, not "a DS baby"
Balanced, honest information	Acknowledge challenges but also positive outcomes (improved life expectancy, quality of life, educational achievement in current era)
Use person-first language	"Your baby has Down syndrome" NOT "Your baby is a Down's"
Avoid outdated terms	"Intellectual disability" not "mental retardation"; "Down syndrome" not "mongolism"
Provide written resources and support contacts	Hong Kong Down Syndrome Association (香港唐氏綜合症協會); parent-to-parent support networks
Arrange early follow-up	Parents will have questions over the following days; scheduled follow-up within 1–2 weeks

All should be initiated within the first week of life:

Karyotype (to confirm and determine mechanism — essential for genetic counselling) ^[1]^[19]
Echocardiogram — regardless of murmur presence ^[16]
CBC with differential — screen for TAM ^[6]
TFTs (TSH ± fT4) — newborn screening + DS-specific check
Hearing screen (OAE/ABR)
Red reflex examination — screen for congenital cataract ^[3]

Issue	Intervention	Rationale
Poor suck due to hypotonia and relative macroglossia	Lactation consultant; specialised nipple/teat; positioning support	Hypotonic orofacial muscles impair the suck-swallow-breathe coordination
Breast or bottle feeding	Encourage breastfeeding (immunological benefits particularly important given DS immune dysfunction)	Breast milk provides IgA, reduces otitis media risk
NG tube feeding	May be needed transiently if oral feeding is inadequate	Ensure adequate caloric intake while oral feeding skills develop
Assess for GI obstruction	If bilious vomiting / failure to pass meconium → AXR	Duodenal atresia (~5%), Hirschsprung disease (~2%)

Early intervention is the single most impactful management strategy for DS. It should begin as soon as possible — ideally within the first month of life.

Therapy	Target	Mechanism of Benefit
Physiotherapy (PT)	Gross motor development (head control, sitting, walking)	Compensates for hypotonia; strengthens muscles; prevents secondary orthopaedic complications (flat feet, scoliosis)
Occupational therapy (OT)	Fine motor skills, feeding skills, self-care, sensory integration	Addresses hypotonia-related hand function difficulties; facilitates independence
Speech-language pathology (SLP)	Communication, oro-motor function, feeding	DS children have particular difficulty with expressive language (receptive often better); early signing/augmentative communication bridges the gap

In Hong Kong, referral pathways include:

Hospital Authority early intervention programmes
Child Assessment Service (CAS) referral for formal developmental assessment
Non-governmental organisations (e.g., Heep Hong Society 協康會, HK Down Syndrome Association)

B. Management of Specific Comorbidities

CHD	Management	Timing	Special DS Considerations
Complete AVSD	Surgical repair (patch closure of ASD + VSD components, AV valve reconstruction)	Typically at 3–6 months of age — earlier than in non-DS children because of accelerated pulmonary hypertension risk	DS children develop irreversible pulmonary vascular disease (Eisenmenger) earlier due to chronic hypoventilation (hypotonia, airway obstruction) → narrower surgical window ^[4]
VSD	Small: observe (many close spontaneously). Large: surgical closure	Within first year if haemodynamically significant	Medical management of heart failure (diuretics, ACE inhibitors) as bridge to surgery
ASD	Secundum ASD: device closure or surgical repair if significant	Usually at 3–5 years	Monitor for pulmonary hypertension
PDA	Pharmacological closure (ibuprofen/indomethacin in premature neonates); surgical/percutaneous closure if persistent	As needed
TOF	Complete surgical repair	Typically 3–6 months

Pre-operative and peri-operative considerations in DS:

Atlanto-axial instability — must assess cervical spine before intubation for anaesthesia; fibre-optic intubation or in-line stabilisation if AAI present
Subglottic stenosis — DS children often need a smaller endotracheal tube than predicted by age (subglottic diameter ~2 sizes smaller)
Airway management — relative macroglossia, midface hypoplasia, short neck → may be a difficult airway
Post-operative recovery — hypotonia affects respiratory function; higher risk of atelectasis; may need prolonged ventilatory support

Medical management of heart failure in DS infants (bridge to surgery):

Drug	Dose (Paediatric)	Mechanism	Notes
Furosemide	1–2 mg/kg/day PO (divided BD-TDS)	Loop diuretic → reduces preload/pulmonary congestion	Monitor electrolytes (K⁺, Na⁺); supplement potassium
Captopril	0.1–0.5 mg/kg/dose TDS (start low, uptitrate)	ACE inhibitor → reduces afterload, decreases L-to-R shunting	Monitor renal function, K⁺; avoid in bilateral renal artery stenosis
Spironolactone	1–3 mg/kg/day PO (divided BD)	K⁺-sparing diuretic + aldosterone antagonist	Adjunct to furosemide; reduces K⁺ wasting
Digoxin	5–10 μg/kg/day PO (maintenance)	Positive inotrope + rate control	Rarely used now; narrow therapeutic index; monitor levels

High Yield — Timing of AVSD Repair in DS

Do NOT delay surgical repair of complete AVSD in DS. Unlike non-DS children (where repair can sometimes wait until 6–12 months), DS children need earlier repair (3–6 months) because pulmonary hypertension develops faster. If Eisenmenger physiology develops (irreversible), surgery becomes contraindicated.

Condition	Management	Monitoring
Congenital hypothyroidism	Levothyroxine 10–15 μg/kg/day PO (neonatal dose); titrate to normalise TSH and fT4	TSH + fT4 at 2 weeks after starting, then every 1–2 months in infancy, then every 3–6 months
Acquired hypothyroidism (autoimmune/Hashimoto)	Levothyroxine at age-appropriate doses (typically 2–4 μg/kg/day in children)	Annual TSH screening in all DS children; more frequently if abnormal
Subclinical hypothyroidism (raised TSH, normal fT4)	Controversial; many paediatric endocrinologists treat if TSH > 10 or if symptomatic	Repeat in 3–6 months; treat if persistent

Why annual TSH screening is essential: Hypothyroidism in DS can be insidious — symptoms (fatigue, weight gain, constipation, developmental slowing) overlap with baseline DS features and can be easily missed clinically. Only biochemical screening reliably detects it.

Paediatric formulation: Levothyroxine is available as oral solution (25 μg/mL) for infants who cannot swallow tablets; tablets can be crushed and mixed with a small amount of water/breast milk.

Transient Abnormal Myelopoiesis (TAM) — Neonatal Period

Scenario	Management
Asymptomatic TAM (incidental finding on CBC)	Observation; serial CBCs every 1–2 weeks until resolution (usually within 3 months)
Symptomatic TAM (hepatomegaly, liver dysfunction, hydrops, DIC, respiratory distress)	Low-dose cytarabine (1–1.5 mg/kg/day IV for 5–7 days); supportive care (exchange transfusion if hydrops, platelet transfusion if bleeding)
Post-TAM surveillance	CBC every 3 months for first 4 years → screen for progression to ML-DS (~20% risk) ^[6]

Myeloid Leukaemia of Down Syndrome (ML-DS) — Typically age 1–4 years

Feature	Management
Diagnosis confirmed (bone marrow blast ≥ 20% with megakaryoblastic features + GATA1 mutation)	Reduced-intensity chemotherapy (DS children have increased sensitivity to cytarabine and methotrexate — standard AML protocols cause excessive toxicity)
Prognosis	Better than AML in non-DS children — ~80% event-free survival with reduced-intensity protocols
Relapsed ML-DS	Standard-intensity AML chemotherapy ± HSCT

ALL in DS children

Feature	Management
DS children with ALL	Standard ALL chemotherapy protocols but with modifications — DS children have increased methotrexate toxicity (reduced dihydrofolate reductase activity from chr21 gene dosage) and increased infection risk
Prognosis	Slightly worse than non-DS ALL (higher treatment-related mortality, higher relapse rate for some subtypes)

DS-Specific Chemotherapy Sensitivity

DS children have altered pharmacokinetics/pharmacodynamics for several cytotoxic agents due to gene dosage effects:

Methotrexate: Increased toxicity — chr21 encodes genes involved in folate metabolism (GART, CBS). Dose reduction needed.
Cytarabine: Increased sensitivity — DS blasts have higher expression of cytarabine-activating enzymes. Beneficial for ML-DS but requires dose awareness. Always use DS-specific oncology protocols.

Problem	Management
Otitis media with effusion (glue ear)	Conservative initially (watchful waiting 3 months); if persistent → grommet insertion (ventilation tubes); hearing aids if not surgical candidate
Recurrent acute otitis media	Appropriate antibiotics (amoxicillin first-line in paediatrics); consider prophylactic antibiotics or grommets if ≥ 3 episodes in 6 months
Sensorineural hearing loss	Hearing aids; cochlear implant assessment if severe/profound bilateral SNHL
Obstructive sleep apnoea (OSA)	First-line: Adenotonsillectomy (but residual OSA common in DS — ~50% still have OSA post-surgery); CPAP/BiPAP if persistent; weight management; positional therapy
Narrow ear canals	Regular cleaning by ENT; ear care education for parents

Hearing surveillance: Formal audiology every 6 months in the first 3 years, then annually. This is critical because undetected hearing loss compounds the existing speech/language delay.

Problem	Management
Refractive errors	Spectacle correction (very common — > 50% need glasses)
Strabismus	Glasses, patching (for amblyopia prevention), surgical correction if needed
Congenital cataract	Cataract extraction if visually significant; treat amblyopia before and after surgery ^[3]
Nasolacrimal duct obstruction	Conservative (massage, warm compresses) initially; probing if persistent beyond 12 months
Keratoconus	Develop in adolescence/young adulthood; spectacles → contact lenses → corneal cross-linking or transplant

Ophthalmology surveillance: First assessment by 6 months of age, then annually.

Problem	Management
Duodenal atresia	Surgical repair (duodenoduodenostomy) — usually within first days of life. Pre-op: NG decompression, IV fluids, correct electrolytes
Hirschsprung disease	Diagnostic confirmation by rectal suction biopsy (absent ganglion cells); definitive: pull-through surgery (e.g., Soave, Duhamel, Swenson procedures). DS is a risk factor for Hirschsprung-associated enterocolitis (HAEC) — the most severe and lethal complication ^[21]
Coeliac disease	Lifelong gluten-free diet; screen from age ~2 years with anti-tTG IgA (+ total IgA to exclude IgA deficiency)
Constipation	Common due to hypotonia; dietary fibre, adequate fluids, polyethylene glycol (Movicol paediatric — available as powder sachets, dose by age); always exclude Hirschsprung and hypothyroidism first
GORD	Common in infancy; feed thickeners, positioning, PPI (omeprazole 0.7–1 mg/kg/day) if severe

Hirschsprung-associated enterocolitis (HAEC) risk factors include: delayed diagnosis of HD > 1 week, increased length of aganglionic segment, presence of Trisomy 21, and presence of other associated anomalies ^[21]

Problem	Management
Atlanto-axial instability (AAI)	Asymptomatic: cervical spine X-ray screening at 3–5 years; avoid high-risk activities (contact sports, trampolining, diving, gymnastics, rugby) if atlanto-dens interval > 5 mm. Symptomatic (myelopathy signs): urgent neurosurgical referral for C1-C2 stabilisation/fusion
Flat feet (pes planus)	Supportive footwear; orthotics; physiotherapy for strengthening
Hip instability	Clinical surveillance; orthopaedic referral if subluxation
Scoliosis	Annual spinal examination; physiotherapy; bracing or surgery if progressive

Anaesthesia considerations for AAI:

Pre-anaesthetic cervical spine assessment is mandatory
If AAI present or cannot be excluded: maintain neutral neck position during intubation; consider fibre-optic intubation
Document AAI status in medical notes prominently

Domain	Management
Intellectual disability	Early intervention (PT, OT, SLP); special education support; individualised education plan (IEP); social skills training
Epilepsy (~10%) ^[10]	Standard AED management based on seizure type; infantile spasms in DS respond well to ACTH/vigabatrin; later seizures — levetiracetam or valproate commonly used
ASD (~5–10%) ^[12]^[13]	Applied behaviour analysis (ABA); speech therapy; social skills groups; referral to developmental paediatrician
ADHD	Behavioural management first; methylphenidate if medication needed (careful dose titration — DS children may be more sensitive to side effects)
Behavioural difficulties	Structured environment; positive reinforcement; psychological support for caregivers
Depression/anxiety	Common in adolescents and young adults with DS; cognitive behavioural therapy (adapted); SSRIs if needed (start low, go slow)

Problem	Management
Delayed eruption, microdontia, missing teeth	Regular dental review from age 2; preventive care (fluoride, sealants)
Periodontal disease	More common in DS (immune dysfunction, poor oral hygiene); regular dental hygienist visits
Bruxism	Night guard if severe

DS children should receive all routine childhood immunisations per the Hong Kong Childhood Immunisation Programme
Additional recommendations:
- Annual influenza vaccine (from 6 months of age) — DS children are at higher risk of respiratory complications
- Pneumococcal vaccine — both PCV13 (routine) and PPSV23 (consider if not in routine schedule)
- RSV prophylaxis (palivizumab) — consider for DS infants with haemodynamically significant CHD in the first 2 years
- COVID-19 vaccination — per age-appropriate schedule
No live vaccine contraindications specifically from DS, unless the child has a co-existing severe immunodeficiency

Age	Assessments
Birth	Clinical examination, karyotype, echo, CBC, TFTs, hearing screen, red reflex, feeding assessment, early intervention referral
1 month	Karyotype result; genetic counselling; parental support
2–6 months	Ophthalmology assessment; hearing reassessment; repeat TFTs at 6 months; developmental monitoring
6–12 months	Annual TSH; growth (DS-specific charts); developmental assessment; feeding review
12–24 months	Vision and hearing; TSH; dental review; M-CHAT at 18 months
2–3 years	Coeliac screen (anti-tTG + total IgA); ASD screening; sleep assessment for OSA
3–5 years	Cervical spine X-ray (flexion/extension) for AAI; pre-school developmental assessment
School age	Annual: TFTs, hearing, vision, growth, development, dental, behavioural screening; IEP support
Adolescence	Puberty assessment; obesity prevention; mental health screening; discuss fertility and reproductive counselling; begin transition planning to adult services
Adulthood	Continue annual thyroid, hearing, vision, dental; screen for early-onset Alzheimer disease (from age 40); cardiac reassessment

Begin planning from age 14–16
Identify adult physicians who will take over medical care (GP, adult cardiologist, endocrinologist, psychiatrist)
Ensure the young person (with capacity-appropriate involvement) understands their own medical history
Discuss vocational training, supported employment, independent living skills
Address legal capacity, guardianship if needed (in HK, Mental Health Ordinance provisions may apply for those without capacity to manage affairs)
Reproductive counselling:
- Males: Almost universally infertile (impaired spermatogenesis)
- Females: Reduced but possible fertility; 50% risk of having a child with DS if conceive naturally; genetic counselling essential

Therapy	Status	Rationale
DYRK1A inhibitors (e.g., leucettine, epigallocatechin gallate/EGCG)	Clinical trials ongoing	Targeting the overexpressed kinase that contributes to intellectual disability
Memantine	Some trials in adults with DS for Alzheimer prevention	NMDA receptor antagonist used in Alzheimer disease
Gene silencing (XIST-mediated chr21 silencing)	Pre-clinical only	Inserting XIST gene into the extra chr21 to transcriptionally silence the entire extra chromosome — proof of concept in cell lines
Anti-amyloid therapies	Trials in DS adults	Targeting APP/amyloid-β pathway for Alzheimer prevention

None of these are currently standard of care. Management remains supportive and anticipatory.

Parameter	Current Data
Life expectancy	Median ~60 years (dramatic improvement from < 25 years in the 1980s, largely due to cardiac surgery advances)
Quality of life	Generally reported as high by individuals with DS and their families; many achieve semi-independent living, meaningful employment, and social participation
Leading causes of death	Congenital heart disease (infancy); respiratory infections (childhood); early-onset Alzheimer disease (adulthood); leukaemia

High Yield Summary — Management of Down Syndrome

No cure exists — management is anticipatory surveillance + early intervention + comorbidity treatment
Breaking the news: Senior clinician, both parents, baby present, person-first language, balanced information, support resources
Neonatal baseline: Echo (ALL neonates), CBC (TAM), TFTs, hearing, red reflex, karyotype
Early intervention (PT/OT/SLP) is the single most impactful management strategy — start within first month
Cardiac surgery for CHD: AVSD repair at 3–6 months (earlier in DS due to accelerated pulmonary hypertension risk); medical HF bridge with furosemide + captopril
Thyroid: Annual TSH screening; levothyroxine if hypothyroid (10–15 μg/kg/day neonatal dose)
Haem: Observe asymptomatic TAM; low-dose cytarabine for symptomatic TAM; DS-specific reduced-intensity chemo for ML-DS
GI: Surgical repair of duodenal atresia/Hirschsprung; coeliac screening from age 2; HAEC surveillance post-Hirschsprung repair
Cervical spine: AAI screening at 3–5y; avoid high-risk sports if unstable; pre-anaesthetic assessment mandatory
Hearing + vision: 6-monthly audiology in first 3 years, then annual; annual ophthalmology
DS children need smaller ETT for intubation and are at higher risk of difficult airway — important for anaesthesia
Immunisations: All routine + annual influenza; consider RSV prophylaxis for those with significant CHD

Active Recall - Management of Down Syndrome

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p503 — Down Syndrome mechanisms, clinical features) ^[3] Senior notes: Ryan Ho Opthalmology.pdf (p122 — Congenital Cataract management, syndromal associations) ^[4] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p561–563 — AVSD pathophysiology, pulmonary hypertension in DS) ^[6] Senior notes: Ryan Ho Haemtology.pdf (p53 — Myeloid proliferation related to Down syndrome: TAM, ML-DS) ^[10] Senior notes: Ryan Ho Neurology.pdf (p102 — Genetic aetiology of epilepsy, Down syndrome 10%) ^[12] Senior notes: Adrian Lui Pediatrics Notes.pdf (p86 — ASD and Down syndrome association) ^[13] Senior notes: Ryan Ho Psychiatry.pdf (p256 — ASD aetiology, genetic disorders) ^[16] Senior notes: Ryan Ho Cardiology.pdf (p185 — Syndromes associated with CHD, DS cardiac defects) ^[19] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p828 — Diagnostic tests, FISH limitations, karyotype) ^[21] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (p1065 — Hirschsprung-associated enterocolitis, DS as risk factor)

Complications of Down Syndrome (Trisomy 21)

The complications of Down syndrome are best understood as the downstream consequences of the 1.5× gene dosage imbalance across multiple organ systems. They are not random — each complication traces back to specific pathophysiological mechanisms involving overexpression of chromosome 21 genes and their effects on development, immunity, connective tissue, and haematopoiesis.

Complications can be organised by:

Timing — neonatal, childhood, adolescent/adult
Organ system — cardiovascular, haematological, endocrine, neurological, GI, respiratory, musculoskeletal, immune, ophthalmological, ENT, psychiatric
Severity — life-threatening vs chronic morbidity

A. Cardiovascular Complications

This is the leading cause of morbidity and mortality in the first year of life in DS.

Complication	Mechanism	Clinical Consequence
Atrioventricular septal defect (AVSD)	Overexpression of chr21 genes (DSCAM, COL6A1/A2) → defective endocardial cushion fusion → common AV valve with primum ASD and inlet VSD	Left-to-right shunting → volume overload of both ventricles → pulmonary overcirculation → heart failure in early infancy (as PVR drops) ^[4]
VSD, ASD, PDA, TOF	Various mechanisms of cardiac septation/outflow tract development affected by gene dosage	Range from asymptomatic to severe haemodynamic compromise

DS children with large left-to-right shunts develop irreversible pulmonary vascular disease (Eisenmenger syndrome) EARLIER than chromosomally normal children ^[4]

Factor	Why It Accelerates Pulmonary Hypertension in DS
Upper airway obstruction	Hypotonia of pharyngeal muscles + relative macroglossia + adenotonsillar hypertrophy → chronic hypoventilation → hypoxic pulmonary vasoconstriction
Reduced alveolar number	DS lungs may have fewer alveoli and a reduced pulmonary vascular bed — intrinsic developmental abnormality
Chronic aspiration	GORD + oropharyngeal incoordination → recurrent micro-aspiration → airway inflammation → pulmonary vascular remodelling

Why this matters clinically: Once Eisenmenger physiology develops (irreversible elevation of PVR with reversal of shunt direction to right-to-left → cyanosis), cardiac surgery is contraindicated because removing the shunt in the presence of fixed elevated PVR would cause right heart failure. The window for surgical repair is therefore narrower in DS → repair should occur at 3–6 months rather than waiting longer.

Signs of Eisenmenger syndrome: Central cyanosis (not correctable with supplemental O₂), clubbing, loud P2, RV heave, polycythaemia, paradoxical emboli (stroke, brain abscess).

B. Haematological Complications

Feature	Detail
Incidence	~10% of DS neonates (clinically apparent); up to 30% on sensitive flow cytometry
Mechanism	Somatic GATA1 mutation in the context of trisomy 21 → clonal proliferation of megakaryoblasts. GATA1 is a transcription factor essential for megakaryocyte and erythroid differentiation; the truncated mutant form (GATA1s) cooperates with chr21 gene dosage to drive abnormal megakaryoblast proliferation
Presentation	Leukocytosis with circulating blasts (may resemble acute leukaemia); hepatomegaly; occasionally liver fibrosis, hydrops, effusions, DIC
Natural history	Usually self-resolves within 3 months — because the fetal haematopoietic environment that supports the GATA1-mutant clone transitions to adult haematopoiesis ^[6]
Complications of TAM	Hepatic fibrosis (can be fatal); DIC; hydrops; ~20% progress to myeloid leukaemia of Down syndrome (ML-DS) within 1–4 years

Feature	Detail
Incidence	~20% of TAM survivors develop ML-DS; occurs almost exclusively before age 5
Mechanism	The initial GATA1 mutation persists or is reacquired in a progenitor cell → acquires additional mutations (e.g., in cohesin complex genes, CTCF, EZH2) → frank leukaemia
Subtype	Acute megakaryoblastic leukaemia (previously FAB M7) — virtually unique to DS in this age group; DS children have a 500× increased risk compared to non-DS
Treatment response	Better prognosis than AML in non-DS children — ~80% event-free survival with reduced-intensity chemotherapy. DS blasts are hypersensitive to cytarabine (increased expression of cytarabine-activating enzyme cystathionine-β-synthase on chr21) ^[6]^[7]
Complication of treatment	Increased treatment-related toxicity (mucositis, infections, cardiotoxicity) — DS children have impaired immune function and altered drug metabolism

Down syndrome is a risk factor for both AML and ALL (15–20× increase in risk) ^[7]^[8]

Feature	Detail
Incidence	15–20× increased risk compared to non-DS population ^[8]
DS-ALL characteristics	Predominantly B-cell ALL; higher frequency of CRLF2 rearrangements and JAK2 mutations (DS-ALL specific biology); fewer favourable cytogenetics (e.g., ETV6-RUNX1 less common)
Prognosis	Worse than non-DS ALL — higher treatment-related mortality (infections, methotrexate toxicity) and higher relapse rate for some subtypes
Treatment caveat	Increased methotrexate toxicity — chr21 encodes genes involved in folate metabolism (GART, CBS); dose adjustments required

Common in DS neonates (possibly due to chronic fetal hypoxia from cardiac defects and placental dysfunction)
May cause hyperviscosity → poor feeding, lethargy, cyanosis, thrombosis
Management: partial exchange transfusion if symptomatic and haematocrit > 70%

Elevated MCV is a common finding throughout life in DS
Mechanism not fully understood — possibly related to altered folate metabolism (chr21 gene dosage) or intrinsic erythropoietic abnormality
Important: do not reflexively investigate macrocytosis in DS as you would in the general population unless there is concurrent anaemia or other cytopaenias

C. Endocrine Complications

Feature	Detail
Congenital hypothyroidism	~1% of DS neonates (vs 0.03% general population) — thyroid dysgenesis or dyshormonogenesis
Acquired hypothyroidism	15–30% by adolescence — predominantly autoimmune (Hashimoto thyroiditis). DS is associated with immune dysregulation favouring autoimmunity
Why it is insidious	Symptoms (fatigue, constipation, weight gain, developmental slowing) overlap with baseline DS features → easily missed without biochemical screening
Consequence if untreated	Worsens intellectual disability, impairs growth, causes obesity, constipation, and cardiac complications

Thyroid function test should be checked in the newborn period and annually thereafter ^[22]

Feature	Detail
Prevalence	> 50% of DS adolescents and adults are overweight/obese
Mechanism	Reduced basal metabolic rate + hypotonia (decreased physical activity) + hypothyroidism (if untreated) + possible dysregulation of satiety hormones
Consequences	Type 2 diabetes, OSA worsening, orthopaedic complications, reduced quality of life
Prevention	Lower caloric intake than recommended for age in typical individuals; promotion of physical activity; supplemental vitamins and minerals ^[22]

Both Type 1 DM (autoimmune predisposition — same immune dysregulation that causes thyroiditis) and Type 2 DM (obesity, reduced activity) are more common
DS is listed as a genetic syndrome associated with DM ^[9]

Multi-factorial: GH-IGF1 axis abnormalities, skeletal dysplasia (short limbs), hypothyroidism, poor nutrition, cardiac disease
Must use DS-specific growth charts — plotting DS children on standard charts will either falsely alarm clinicians or miss true growth faltering ^[22]

Males: Almost universally infertile (impaired spermatogenesis — reduced testicular volume, Sertoli cell dysfunction); cryptorchidism in ~5%
Females: Reduced but possible fertility; normal menarche usually (may be slightly early); ~50% chance of having a child with DS if conceive

D. Neurological Complications

Universal in DS; IQ typically 25–75 (mean ~50)
Mechanism: DYRK1A overexpression → reduced dendritic arborisation and synapse density → impaired higher cortical function
Mosaic DS may have milder cognitive impairment ^[1]
Lifelong; peak cognitive functioning typically in 3rd–4th decade, followed by decline due to early-onset AD

10% of DS individuals develop epilepsy ^[10]

Age of Onset	Seizure Type	Notes
Infancy	Infantile spasms (West syndrome)	More common in DS; good response to ACTH/vigabatrin
Childhood	Generalised tonic-clonic, myoclonic, absence	Standard AED management
Adulthood (> 40 years)	Late-onset myoclonic epilepsy	Often heralds Alzheimer disease; associated with progressive cognitive decline

Feature	Detail
Mechanism	Overexpression of APP (Amyloid Precursor Protein) gene on chr21 → lifelong overproduction of amyloid-β peptide → amyloid plaque accumulation + neurofibrillary tangles
Neuropathology	Virtually all DS individuals develop sufficient AD neuropathology to meet diagnostic criteria by their mid-40s ^[11]
Clinical dementia	Typically manifests in mid-50s (some earlier); presenting features include personality change, loss of daily living skills, new-onset seizures, gait deterioration
Diagnostic challenge	Declining function must be distinguished from treatable causes: hypothyroidism, depression, sensory loss (hearing/vision), OSA, medication effects — all common in DS and all reversible
Management	Cholinesterase inhibitors (donepezil) used in practice though evidence base in DS is limited; ongoing clinical trials of anti-amyloid therapies

High Yield — Why DS = Early Alzheimer

The APP gene is on chromosome 21. Three copies → 1.5× amyloid-β production from birth. This is essentially a lifelong "amyloid dose" — analogous to autosomal dominant familial Alzheimer disease with APP mutations, which also causes early-onset AD. In DS, the mechanism is gene dosage rather than a point mutation, but the downstream pathology is identical.

Feature	Detail
Prevalence	Radiographic instability in 10–30%; symptomatic in 1–2%
Mechanism	Ligamentous laxity at C1-C2 (overexpression of COL6A1/A2 on chr21 → abnormal collagen VI → loose transverse ligament of atlas) → increased atlanto-dens interval
Consequence if symptomatic	Spinal cord compression → progressive myelopathy (hyperreflexia, gait disturbance, urinary incontinence, neck pain, torticollis) — can be catastrophic with neck flexion (e.g., during intubation, contact sports, diving)
Screening	Lateral cervical spine X-ray with odontoid and AP views to examine evidence of AAI or subluxation ^[22]; screening at 3–5 years
Management	Asymptomatic AAI: activity restriction (avoid contact sports, trampolining, gymnastics, diving); symptomatic: urgent neurosurgical C1-C2 stabilisation/fusion

E. Respiratory Complications

Feature	Detail
Prevalence	50–75% of DS children (one of the most common complications)
Mechanism	Midface hypoplasia + relative macroglossia + adenotonsillar hypertrophy + pharyngeal muscle hypotonia + narrow upper airway + obesity → airway collapse during sleep
Consequences	Neurocognitive impairment (worsens existing ID), behavioural problems, pulmonary hypertension (compounds cardiac issues), failure to thrive, nocturnal hypoxaemia
Diagnosis	Polysomnogram and pulse oximetry monitoring during sleep is recommended in all children with DS after 4 y/o ^[22]; earlier if symptomatic (snoring, apnoeic pauses, restless sleep)
Management	Adenotonsillectomy (first-line but residual OSA in ~50% post-surgery); CPAP/BiPAP for persistent OSA; weight management; positional therapy; lingual tonsil reduction if contributing; rarely tracheostomy for severe refractory cases

Immune dysfunction (T-cell deficiency, impaired neutrophil chemotaxis) + aspiration risk + anatomical airway abnormalities → increased susceptibility
Aspiration pneumonia: Particularly in those with severe GORD, oropharyngeal incoordination, or neurological impairment
RSV bronchiolitis: DS infants with haemodynamically significant CHD are at high risk → consider palivizumab prophylaxis

DS children frequently have a narrower subglottic diameter (~2 sizes smaller than age-predicted ETT)
Important complication in the peri-operative setting: recurrent/failed intubation, post-extubation stridor
Can also cause chronic croup-like presentations

F. Gastrointestinal Complications

Presents in first 24–48 hours of life with bilious vomiting, upper abdominal distension
Classic "double bubble" sign on AXR (dilated stomach + proximal duodenum with no distal gas)
~30% of ALL duodenal atresia cases are associated with DS
Requires surgical repair (duodenoduodenostomy)

100× increased risk compared to general population
Presents with delayed passage of meconium ( > 48 hours), abdominal distension, constipation
Risk factor for the most severe and lethal complication — Hirschsprung-associated enterocolitis (HAEC): features include explosive foul-smelling diarrhoea, abdominal distension, fever, sepsis. Risk factors include delayed diagnosis > 1 week, increased length of aganglionic segment, presence of Trisomy 21, and presence of other associated anomalies ^[21]
Definitive management: pull-through surgery (Soave/Duhamel/Swenson)

Autoimmune enteropathy triggered by gluten in genetically predisposed individuals
DS immune dysregulation → higher prevalence of autoimmune conditions including coeliac
Presents with chronic diarrhoea, failure to thrive, abdominal distension, irritability, iron deficiency anaemia
Screen with anti-tTG IgA + total IgA from age ~2 years; lifelong gluten-free diet if confirmed

Very common in DS infants — hypotonia of LOS + delayed gastric emptying + oropharyngeal incoordination
Complications: aspiration pneumonia, oesophagitis, feeding refusal, failure to thrive

DS (Trisomy 21) is listed as an associated syndrome for hepatoblastoma ^[23]
Rare but important to recognise; most hepatoblastomas occur in first 2 years of life
Presents as abdominal mass, hepatomegaly, elevated AFP
Treatment: surgical resection ± chemotherapy (cisplatin-based regimens)

Feature	Detail
Thymic hypoplasia	Reduced T-cell numbers and function → impaired cell-mediated immunity
Impaired neutrophil chemotaxis	Reduced ability of neutrophils to migrate to sites of infection
Reduced antibody responses	Impaired B-cell function → suboptimal vaccine responses
Autoimmune predisposition	Paradoxical: immune deficiency PLUS autoimmunity (thyroiditis, coeliac, T1DM, alopecia areata) → reflects immune dysregulation rather than simple immunodeficiency
Clinical consequence	Recurrent respiratory infections, otitis media, pneumonia, skin infections; higher hospitalisation rates for RSV, influenza, and COVID-19

Complication	Prevalence	Mechanism	Consequence
Otitis media with effusion (glue ear)	50–70%	Narrow ear canals + Eustachian tube dysfunction (midface hypoplasia) + adenoidal hypertrophy + immune dysfunction	Conductive hearing loss → compounds speech/language delay
Sensorineural hearing loss	10–20%	Cochlear developmental abnormalities; progressive with age	May be missed if conductive loss is assumed to be the only cause
Narrow ear canals	Very common	Craniofacial skeletal hypoplasia	Difficult otoscopic examination; impacted cerumen; recurrent otitis externa

Newborn hearing screen with brainstem auditory evoked response (BAER); repeat hearing evaluation regularly — every 6 months until 4–5 y/o and yearly after 4–5 y/o ^[22]

Complication	Prevalence	Mechanism
Refractive errors	> 50%	Abnormal globe/corneal development
Strabismus	Up to 45%	Hypotonia of extraocular muscles + refractive errors
Congenital cataract	~3%	Syndromal lens opacification ^[3]
Nasolacrimal duct obstruction	~20%	Midface hypoplasia → narrow nasolacrimal canal
Keratoconus	Develops in adolescence/adulthood	Abnormal collagen in corneal stroma (chr21 collagen gene dosage)
Brushfield spots	50–80%	Aggregation of connective tissue in iris stroma; benign; no visual consequence

Ophthalmological assessment in newborn or at least before 6 months; affected children should have annual assessment of vision ^[22]

Complication	Mechanism	Management
Joint hypermobility	COL6A1/A2 overexpression → ligamentous laxity	PT for strengthening; joint protection strategies
Pes planus (flat feet)	Ligamentous laxity + hypotonia	Supportive footwear; orthotics
Patellar instability	Ligamentous laxity + muscle hypotonia around knee	PT; bracing; surgery if recurrent dislocation
Hip subluxation/dislocation	Ligamentous laxity + shallow acetabulum	Clinical and radiological surveillance; orthopaedic management
Scoliosis	Hypotonia + ligamentous laxity → spinal instability	Annual spinal examination; bracing/surgery if progressive
Osteoporosis	Reduced physical activity + hypothyroidism + altered bone metabolism	Weight-bearing exercise; calcium/vitamin D supplementation; monitor calcium and vitamin D intake to minimize bone loss ^[22]

Complication	Mechanism
Xerosis (dry skin)	Abnormal epidermal lipid composition
Folliculitis	Immune dysfunction predisposes to bacterial skin infections
Alopecia areata	Autoimmune hair loss — part of the autoimmune predisposition in DS
Palmoplantar hyperkeratosis	Altered keratinisation
Cutis marmorata	Vasomotor instability (mottled skin pattern in cold)
Acrocyanosis	Peripheral vasomotor instability — common in DS neonates

Complication	Prevalence	Features
Depression	10–15% of DS adolescents/adults	May present as withdrawal, loss of skills, appetite change, sleep disturbance — can mimic early dementia
Anxiety disorders	Common	Generalised anxiety, separation anxiety, specific phobias
OCD	More common than general population	Rituals, insistence on sameness
ADHD	~5–10%	Inattentive type more common than hyperactive
Autism spectrum disorder (ASD)	~5–10%	DS is listed as a genetic disorder associated with ASD ^[12]^[13]
Regression/catatonia	Rare but recognised	Acute loss of skills, psychomotor retardation, mutism — must exclude medical causes (hypothyroidism, coeliac, depression, seizures)
Behavioural challenges	Variable	Stubbornness, task avoidance, attention-seeking; often respond to positive behavioural approaches

Complication	Detail
Delayed tooth eruption	Both deciduous and permanent teeth; ~6–12 months behind typical schedule
Microdontia	Small, abnormally shaped teeth
Congenitally missing teeth	Particularly lateral incisors and second premolars
Periodontal disease	Very common — immune dysfunction + poor oral hygiene + altered salivary composition → early and severe periodontitis
Malocclusion	Midface hypoplasia → relative mandibular prognathism → anterior open bite, crossbite
Bruxism	Common; may cause tooth wear

Sex	Detail
Males	Almost universally infertile; reduced testicular volume, impaired spermatogenesis; cryptorchidism in ~5% (requires surgical orchidopexy before 12 months to optimise residual fertility potential and reduce malignancy risk)
Females	Reduced but possible fertility; regular menses; ~50% risk of having a child with DS; contraception counselling needed; genetic counselling if pregnancy desired

Timing	Key Complications
Neonatal	CHD (AVSD), TAM, duodenal atresia, Hirschsprung disease, congenital hypothyroidism, congenital cataract, polycythaemia, feeding difficulties
Infancy/Early childhood	Heart failure from CHD, recurrent infections (otitis media, LRTI), hearing loss, OSA, GORD, constipation, developmental delay, coeliac disease, ML-DS
School age	Intellectual disability, learning difficulties, obesity, thyroid disease, dental problems, AAI, behavioural/psychiatric issues
Adolescence	Obesity, hypothyroidism, keratoconus, scoliosis, psychiatric disorders (depression, anxiety), transition planning
Adulthood	Early-onset Alzheimer disease (virtually universal neuropathology by mid-40s ^[11]), late-onset epilepsy, osteoporosis, cervical spondylotic myelopathy, cataracts

High Yield Summary — Complications of Down Syndrome

Leading cause of death in infancy: CHD (especially AVSD) — repair early (3–6 months) because Eisenmenger develops faster in DS
Haematological: TAM (10% neonates, self-resolving, but 20% → ML-DS); 15–20× increased leukaemia risk (both AML and ALL); increased methotrexate toxicity in ALL treatment
Endocrine: Hypothyroidism (15–30% by adolescence, screen annually with TSH); obesity ( > 50%); DM (both types)
Neurological: Universal ID; epilepsy (10%); early-onset Alzheimer disease (virtually all by mid-40s due to APP overexpression)
Respiratory: OSA (50–75%, screen with polysomnography after age 4); recurrent LRTI; subglottic stenosis
GI: Duodenal atresia (~5%, "double bubble"), Hirschsprung (~2%, HAEC risk), coeliac disease (5–16%), GORD
MSK: AAI (10–30% radiographic, 1–2% symptomatic; screen at 3–5y; restrict contact sports; pre-anaesthetic assessment)
ENT: Glue ear/hearing loss (50–70%; screen every 6 months until 4–5y then annually)
Ophthalmology: Refractive errors ( > 50%), strabismus (45%), congenital cataract, keratoconus
Immune: Paradoxical immunodeficiency + autoimmunity (thyroiditis, coeliac, T1DM, alopecia areata)
Psychiatric: Depression, anxiety, ASD (5–10%), ADHD — can all be mistaken for "just DS"

Active Recall - Complications of Down Syndrome

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p503 — Down Syndrome, mosaicism and milder phenotype) ^[3] Senior notes: Ryan Ho Opthalmology.pdf (p122 — Congenital cataract, syndromal associations) ^[4] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p561–563 — AVSD pathophysiology, pulmonary hypertension in DS) ^[6] Senior notes: Ryan Ho Haemtology.pdf (p53 — Myeloid proliferation related to Down syndrome: TAM, ML-DS) ^[7] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p1380 — AML risk factors including DS 15–20×) ^[8] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p1391 — ALL risk factors including DS 15–20×) ^[9] Senior notes: Maksim Medicine Notes.pdf (p80 — Genetic syndromes associated with DM including Down) ^[10] Senior notes: Ryan Ho Neurology.pdf (p102 — Genetic aetiology of epilepsy, Down syndrome 10%) ^[11] Lecture slides: GC 241. Reference (1) - Alzheimers Dementia - Revised criteria for diagnosis and staging of Alzheimer's disease.pdf (p6 — DS and AD neuropathology by mid-40s) ^[12] Senior notes: Adrian Lui Pediatrics Notes.pdf (p86 — ASD and DS association) ^[13] Senior notes: Ryan Ho Psychiatry.pdf (p256 — ASD aetiology, genetic disorders) ^[21] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (p1065 — HAEC risk factors including Trisomy 21) ^[22] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p834 — DS follow-up plan: OSA screening, thyroid, AAI, hearing, ophthalmology, CBC, growth, obesity) ^[23] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (p1077 — Hepatoblastoma associated syndromes including Trisomy 21)

High Yield Summary

Definition: Trisomy 21 — most common autosomal trisomy, most common genetic cause of intellectual disability
Epidemiology: ~1/700 live births; HK incidence 1.5/1000 births
Mechanisms: Non-disjunction (94%, maternal age-related), Robertsonian translocation (5%, NOT age-related, check parental karyotype), Mosaicism (1%, milder phenotype)
Cardinal features: Characteristic facies (upslanting palpebral fissures, epicanthic folds, flat nasal bridge, brachycephaly), hypotonia, intellectual disability, single palmar crease, sandal gap
Cardiac (40-50%): AVSD is the most characteristic (strong bidirectional association with DS); also VSD, ASD, PDA, TOF. All DS neonates need echo
GI: Duodenal atresia (double bubble), Hirschsprung disease (100× increase), coeliac disease
Haem: TAM (self-resolving but 20% → ML-DS), 15-20× risk of leukaemia (AML and ALL)
Endocrine: Hypothyroidism (screen annually with TSH), DM, obesity
Neuro: Epilepsy (10%), early-onset Alzheimer disease (virtually all by mid-40s due to APP overexpression)
Screening: Combined 1st trimester screen, quad test, NIPT (cell-free DNA) → confirmatory CVS/amniocentesis
Recurrence: Non-disjunction ~1/200 if mother < 35y; translocation depends on carrier status (10-15% if mother, 100% if 21;21)

High Yield Summary — DDx of Down Syndrome

DDx context: Arises prenatally (abnormal screening) or postnatally (dysmorphic neonate with hypotonia ± CHD)
Key mimics of the "floppy baby with dysmorphism": Prader-Willi (profound hypotonia), congenital hypothyroidism (reversible!), congenital myotonic dystrophy, other trisomies (18, 13)
CHD pattern helps narrow DDx: AVSD = think DS; left-sided = Turner; right-sided = Noonan; conotruncal = DiGeorge; supravalvular = Williams
ID without obvious DS facies but normal karyotype → test for Fragile X (most common inherited cause of ID)
Confirmation: Karyotype/FISH/CMA distinguishes DS from all other conditions. If karyotype normal, consider CMA → gene panels → WES/WGS
Always exclude congenital hypothyroidism — it is treatable and screened for on newborn screening in HK
Prenatal false positives: Positive NIPT must be confirmed by CVS/amniocentesis; increased NT has a broad DDx beyond trisomy 21

High Yield Summary — Diagnostics of Down Syndrome

DS is a cytogenetic diagnosis — confirmed by karyotype, FISH, QF-PCR, or CMA showing trisomy 21
FISH is rapid (24–48h) but cannot detect translocation — always confirm with full karyotype ^[19]
Prenatal screening is two-tier in HK: 1st trimester combined screen → if positive → NIPT or invasive testing (CVS/amniocentesis) ^[20]
NIPT detects 99.7% of T21 with ~0.1% FPR — but is STILL a screening test; positive NIPT needs confirmatory CVS/amniocentesis ^[20]
Quad test pattern in DS: ↓ AFP, ↓ uE3, ↑ hCG, ↑ inhibin A
If translocation DS: parental karyotyping is ESSENTIAL for recurrence risk counselling ^[19]
Postnatal baseline workup: Echo (all neonates), CBC (TAM), TSH, hearing, red reflex, developmental referral
Ongoing surveillance: Annual TSH, hearing, vision, growth (DS-specific charts), coeliac screen, cervical spine X-ray at 3–5y, dental, ASD screening

High Yield Summary — Management of Down Syndrome

No cure exists — management is anticipatory surveillance + early intervention + comorbidity treatment
Breaking the news: Senior clinician, both parents, baby present, person-first language, balanced information, support resources
Neonatal baseline: Echo (ALL neonates), CBC (TAM), TFTs, hearing, red reflex, karyotype
Early intervention (PT/OT/SLP) is the single most impactful management strategy — start within first month
Cardiac surgery for CHD: AVSD repair at 3–6 months (earlier in DS due to accelerated pulmonary hypertension risk); medical HF bridge with furosemide + captopril
Thyroid: Annual TSH screening; levothyroxine if hypothyroid (10–15 μg/kg/day neonatal dose)
Haem: Observe asymptomatic TAM; low-dose cytarabine for symptomatic TAM; DS-specific reduced-intensity chemo for ML-DS
GI: Surgical repair of duodenal atresia/Hirschsprung; coeliac screening from age 2; HAEC surveillance post-Hirschsprung repair
Cervical spine: AAI screening at 3–5y; avoid high-risk sports if unstable; pre-anaesthetic assessment mandatory
Hearing + vision: 6-monthly audiology in first 3 years, then annual; annual ophthalmology
DS children need smaller ETT for intubation and are at higher risk of difficult airway — important for anaesthesia
Immunisations: All routine + annual influenza; consider RSV prophylaxis for those with significant CHD

High Yield Summary — Complications of Down Syndrome

Leading cause of death in infancy: CHD (especially AVSD) — repair early (3–6 months) because Eisenmenger develops faster in DS
Haematological: TAM (10% neonates, self-resolving, but 20% → ML-DS); 15–20× increased leukaemia risk (both AML and ALL); increased methotrexate toxicity in ALL treatment
Endocrine: Hypothyroidism (15–30% by adolescence, screen annually with TSH); obesity ( > 50%); DM (both types)
Neurological: Universal ID; epilepsy (10%); early-onset Alzheimer disease (virtually all by mid-40s due to APP overexpression)
Respiratory: OSA (50–75%, screen with polysomnography after age 4); recurrent LRTI; subglottic stenosis
GI: Duodenal atresia (~5%, "double bubble"), Hirschsprung (~2%, HAEC risk), coeliac disease (5–16%), GORD
MSK: AAI (10–30% radiographic, 1–2% symptomatic; screen at 3–5y; restrict contact sports; pre-anaesthetic assessment)
ENT: Glue ear/hearing loss (50–70%; screen every 6 months until 4–5y then annually)
Ophthalmology: Refractive errors ( > 50%), strabismus (45%), congenital cataract, keratoconus
Immune: Paradoxical immunodeficiency + autoimmunity (thyroiditis, coeliac, T1DM, alopecia areata)
Psychiatric: Depression, anxiety, ASD (5–10%), ADHD — can all be mistaken for "just DS"

Memory palace for Down Syndrome (Trisomy 21)

Memory palace hooks for Down Syndrome (Trisomy 21)

How to Use This Memory Palace

Each numbered symbol is a recall hook mapped back to this page's own notes. The Note concept column is the source of truth; the symbol logic explains why the visual cue should trigger that concept.

This first pass maps the symbols supplied for labels 1-68. Any additional labels visible in the image should be added only after they are tied back to the MBBSPedia note sections.

Source Guardrails

This page follows the local MBBSPedia notes where they differ slightly from the Sketchy script: Robertsonian translocation is listed as about 5%, CVS timing is 11-13+6 weeks in the HK screening pathway, amniocentesis is from 15 weeks, and hearing surveillance is 6-monthly in early childhood before annual checks.

No.	Symbol	Source tab	Note concept	Etymology / symbol logic
1	DOWNTown sign	Etiology / Summary	Down syndrome is extra chromosome 21 material causing characteristic facies, hypotonia, intellectual disability of variable severity, and multi-system comorbidities.	"Down" in DOWNTown anchors the diagnosis; the city scene cues a broad syndrome rather than one isolated anomaly.
2	"Junction 21 21 21" bar	Etiology / Dx	Free trisomy 21 from meiotic non-disjunction is the most common mechanism; karyotype is 47,XX,+21 or 47,XY,+21.	Three 21s at the junction cue three chromosome 21 copies after a non-disjunction error.
3	"Robertson" plus transit location	Etiology / Dx	Robertsonian translocation Down syndrome occurs when chromosome 21 fuses with another acrocentric chromosome, most often chromosome 14.	Robertson plus transit location sounds like Robertsonian translocation.
4	Extra acrocentric track	Etiology / Dx	Acrocentric chromosomes 13, 14, 15, 21, and 22 can be involved in Robertsonian translocations; extra chromosome 21 material produces the phenotype.	The extra track cues an acrocentric chromosome route carrying extra 21 material.
5	Mosaic sign	Etiology / Dx	Mosaic Down syndrome results from post-zygotic mitotic error or trisomic rescue, creating both trisomic and normal cell populations.	A mosaic sign is made of different pieces, just like mixed cell lines.
6	Number 1 finger	Etiology / Summary	Down syndrome is the most common viable autosomal trisomy and the most common chromosomal cause of intellectual disability.	The raised first finger cues "number one" among liveborn chromosomal abnormalities.
7	Advanced-age mother	Etiology / Summary	Advanced maternal age, classically 35 years or older at delivery, increases the risk of meiotic non-disjunction and Down syndrome.	The older mother cues oocyte ageing and non-disjunction risk.

No.	Symbol	Source tab	Note concept	Etymology / symbol logic
8	Screen on first of three windows	Dx / Summary	First-trimester combined screening uses nuchal translucency plus maternal serum PAPP-A and free beta-hCG with maternal age.	The first window cues first-trimester screening.
9	Screen on second of three windows	Dx / Summary	Second-trimester quadruple testing suggests Down syndrome with low AFP, low uE3, high hCG, and high inhibin A.	The second window cues second-trimester screening.
10	"NIP" taking "FREE" DNA samples	Dx / Summary	NIPT analyses cell-free fetal DNA in maternal plasma; it is highly sensitive for trisomy 21 but remains a screening test.	"NIP" cues NIPT, and free samples cue cell-free DNA.
11	Pregnant figure with ultrasound bullhorn	Dx	Prenatal ultrasound can show increased nuchal translucency and other soft markers, but ultrasound findings are not diagnostic alone.	The bullhorn announces sonographic warning signs rather than confirmation.
12	Corn sampling	Dx	Chorionic villus sampling obtains placental tissue in the first trimester for fetal cytogenetic testing.	Corn sounds like chorion, and sampling cues CVS.
13	Apple juice sample	Dx	Amniocentesis samples amniotic fluid from 15 weeks onward to test fetal cells and confirm the karyotype.	Apple juice stands in for amniotic fluid being sampled.
14	Cookie types	Dx / Mx	Karyotype confirms Down syndrome and distinguishes free trisomy 21, Robertsonian translocation, and mosaicism.	Sorting cookie types cues sorting chromosomes by number, size, and shape.

No.	Symbol	Source tab	Note concept	Etymology / symbol logic
15	Dysmorphic motorcycle face	Etiology / Dx	Newborns with Down syndrome often have characteristic dysmorphic craniofacial features that should trigger cytogenetic testing.	The stylised face cues the recognisable dysmorphic gestalt.
16	Floppy soldier	Etiology / Dx	Generalised hypotonia is a fundamental neonatal finding and contributes to feeding and motor delay.	The floppy soldier lacks tone, like a hypotonic infant.
17	Broken "eat" sign with dribbling milk	Etiology / Mx	Poor suck and feeding difficulty can occur from hypotonia, relative macroglossia, and poor suck-swallow-breathe coordination.	The broken eating sign and milk dribble cue feeding dysfunction.
18	Upslanting headlights	Etiology / Dx	Upslanting palpebral fissures are a characteristic craniofacial feature.	Headlights angled upward cue the upward slant of the palpebral fissures.
19	Epicanthal shield	Etiology / Dx	Epicanthic folds arise from flat nasal bridge and midface hypoplasia and can mimic pseudostrabismus.	A shield at the inner eye corner cues medial canthal folds.
20	Brachycephaly helmet	Etiology / Dx	Brachycephaly with a short, flat occiput is a common craniofacial sign.	The short helmet cues a shortened head shape.
21	Small, low-set side mirrors	Etiology / Dx	Small, low-set ears with abnormal cartilage development are part of the facies.	Low side mirrors cue low-set ears.
22	Protruding furrowed tongue tire	Etiology / Dx	Open mouth and protruding tongue reflect a small oral cavity, relative macroglossia, and orofacial hypotonia.	The tongue-like tire protruding from the face cues tongue protrusion.
23	Excessive fabric folds	Etiology / Dx	Short neck with excess nuchal skin is a typical physical finding and relates to prenatal nuchal thickening.	Folded fabric at the back of the neck cues redundant nuchal skin.
24	Hand crease	Etiology / Dx	A single transverse palmar crease is a classic hand finding, often with short broad hands.	One line across the hand cues the single palmar crease.
25	Toe gap	Etiology / Dx	Sandal gap is a wide space between the first and second toes.	The visible gap between toes cues the named sandal-gap sign.
26	Spotted iris spotlight	Etiology / Complications	Brushfield spots are pale speckled iris spots and are usually benign.	The spotted spotlight cues speckles around the iris.
27	Anomalous floppy glove	Etiology / Complications	Extremity findings include short broad hands, clinodactyly of the fifth finger, and joint hypermobility.	The floppy glove cues lax joints and abnormal hand shape.
28	"Order cookie types now!" sign	Dx / Mx	Order karyotyping when Down syndrome is suspected to confirm the diagnosis and determine the cytogenetic mechanism.	The urgent cookie-type order cues sending a chromosome-sorting test.

No.	Symbol	Source tab	Note concept	Etymology / symbol logic
29	Down syndrome differential sign	DDx	Consider other trisomies and syndromes with hypotonia, dysmorphism, congenital heart disease, or intellectual disability while awaiting cytogenetics.	The DDx sign cues the "what else could this be?" framework.
30	"Congrats!" banner	Mx	When speaking with parents after birth, first congratulate them on their baby and use person-first, balanced language.	The congratulations banner keeps the baby-first communication priority visible.
31	"Go team!" sign	Mx	Diagnosis disclosure and ongoing care should involve a multidisciplinary team including paediatrics, genetics, cardiology, therapists, and social support.	The team sign cues coordinated care rather than a solo disclosure.
32	Counsellor figure	Mx	Counselling should occur in a private setting with both parents when possible, empathy, active listening, and clear explanation of the findings.	The counsellor figure cues structured, compassionate communication.
33	"Recurrence" with "1%"	Dx / Etiology	Free trisomy 21 recurrence risk is about 1% or the maternal age-specific risk, whichever is higher.	The recurrence sign plus 1% anchors the standard free-trisomy counselling figure.
34	"Recurrence" with more than 1	Dx / Etiology	Robertsonian translocation recurrence risk depends on carrier status and can be much higher, especially when the mother is the carrier.	More-than-1 recurrence cues the higher translocation risk.
35	Community sign	Mx	Families should be offered written resources, parent groups, and local community support such as Down syndrome associations.	The community sign cues connecting parents beyond the hospital.
36	Guidelines sign	Mx / Summary	Down syndrome requires structured lifelong health surveillance for cardiac, thyroid, hearing, vision, growth, development, haematology, GI, and musculoskeletal issues.	The guideline sign cues anticipatory care protocols.

No.	Symbol	Source tab	Note concept	Etymology / symbol logic
37	Heart bullhorn	Dx / Mx	Every neonate with Down syndrome needs an echocardiogram because 40-50% have congenital heart disease.	The bullhorn over the heart cues actively looking for hidden CHD.
38	Equal sign connecting left and right heart	Etiology / Complications	AVSD is the most characteristic heart defect; failed endocardial cushion fusion creates ASD and VSD components with a common AV valve.	The equal sign bridging heart sides cues an AV canal connection.
39	"Obstructed" tied-off duodenum	Etiology / Complications	Duodenal atresia or stenosis is associated with Down syndrome and may present with bilious vomiting and double-bubble sign.	The tied-off duodenum cues mechanical obstruction.
40	Hirsch-"spring" antenna	Etiology / Complications	Hirschsprung disease is increased in Down syndrome and presents with delayed meconium, distension, constipation, or bilious vomiting.	"Hirsch-spring" cues Hirschsprung disease.
41	Evaluating the broken "EAT" sign	Mx	Feeding assessment and support are needed when infants have poor suck, choking, slow feeds, or growth concerns.	Inspecting the broken eat sign cues a feeding evaluation.
42	Elevated H with polycythaemic flower dress	Dx / Complications	CBC with differential screens neonates for polycythaemia, transient abnormal myelopoiesis, and other haematological abnormalities.	The high H cues high haematocrit and the blood-count screen.
43	Elevated white birds with "blasts"	Dx / Complications	Transient abnormal myelopoiesis causes leukocytosis with circulating blasts in Down syndrome neonates and often self-resolves.	White birds cue white cells; blasts cue blast cells on smear.
44	Hearing-loss earmuffs	Dx / Complications	Down syndrome increases congenital and acquired hearing loss, so newborn screening and repeated audiology are essential.	Earmuffs cue reduced hearing.
45	"Ophtho" cloudy eye	Dx / Complications	Red reflex and ophthalmology assessment screen for congenital cataract and other eye problems.	The cloudy eye cues cataract.
46	Undone bowtie with stimulating bowtie and T4 bomb	Dx / Complications	Screen thyroid function with TSH and free T4 because congenital and acquired hypothyroidism are common.	The thyroid bowtie plus T4 cues thyroid testing.
47	"Continuous coverage" sign	Mx	Ongoing follow-up with primary care and specialists is central to Down syndrome management.	Continuous coverage cues surveillance that continues beyond the newborn period.

No.	Symbol	Source tab	Note concept	Etymology / symbol logic
48	"Expect delays" sign	Etiology / Mx	Developmental delay is expected; early PT, OT, and speech-language therapy should start as early as possible.	The delay sign cues slower milestones and early intervention.
49	Falling IQ textbook	Etiology / Complications	Intellectual disability is universal but variable, and mosaic Down syndrome may be milder.	The falling textbook cues cognitive impairment and learning support needs.
50	Shaking figure with brain hat	Complications / Mx	Epilepsy risk is increased, including infantile spasms; neurological surveillance should continue across childhood and adulthood.	Shaking cues seizures; the brain hat cues neurological complications.
51	Broken cardiac mic	Complications / Mx	Children with CHD require cardiology and primary care follow-up, with monitoring for heart failure signs such as tachypnoea, poor growth, and feeding intolerance.	The broken cardiac mic cues a heart that cannot keep up.
52	Broken valves	Complications / Mx	Later cardiac follow-up matters because valve problems and regurgitation can occur even beyond congenital defects.	Broken valves cue adolescent or adult valvular disease.

No.	Symbol	Source tab	Note concept	Etymology / symbol logic
53	Navy SEALiac	Complications / Mx	Coeliac disease is more common in Down syndrome; screen from about age 2 or earlier if symptomatic.	SEALiac sounds like coeliac.
54	Blood cancer crab, bacterium lantern, and bruised camo	Complications	Down syndrome increases leukaemia risk; screen symptoms such as unexplained fever, bone pain, night sweats, weight loss, infections, or bruising.	The blood cancer crab and bruises cue haematological malignancy warning signs.
55	Falling iron tools and H	Complications / Mx	Anaemia and macrocytosis can occur; routine blood surveillance and diet review help detect iron deficiency or other cytopaenias.	Falling iron tools cue iron deficiency, and H cues haemoglobin.
56	Checking headphones	Mx / Complications	Hearing should be reassessed regularly: 6-monthly in early childhood, then annually.	Checking headphones cues scheduled audiology surveillance.
57	Finger in infected ear	Complications / Mx	Otitis media with effusion and recurrent otitis media are common causes of conductive hearing loss.	The infected ear cues recurrent middle-ear disease.
58	Eye logo	Complications / Mx	Annual ophthalmology screens for refractive errors, strabismus, cataracts, nystagmus, keratoconus, and other eye disease.	The eye logo cues ongoing vision surveillance.
59	Missing tooth	Complications / Mx	Dental problems include delayed eruption, small or missing teeth, malocclusion, and periodontal disease; regular dental care is needed.	The missing tooth directly cues dental surveillance.
60	Undone thyroid bowtie	Complications / Mx	Thyroid disease is common; TSH is checked at birth, 6 months, 12 months, then annually.	The undone bowtie cues thyroid dysfunction and repeated TSH checks.
61	Elevated candy	Complications	Diabetes mellitus is more prevalent because of autoimmune risk and obesity; monitor symptoms such as polyuria and polydipsia.	Elevated candy cues high glucose.
62	Unstable head warning	Complications / Mx	Atlanto-axial instability can cause cervical subluxation and cord compression; avoid high-risk neck flexion/extension and restrict unsafe sports when unstable.	The unstable head warning cues C1-C2 instability danger.
63	Neuron wires	Dx / Complications	Well checks should include neurological examination for spinal cord compression signs such as gait change, weakness, bowel or bladder change, and neck pain or tilt.	Neuron wires cue spinal cord and neurologic monitoring.
64	Snoring figure	Complications / Mx	Obstructive sleep apnoea is common; screen from birth for symptoms and obtain polysomnography around age 4 or earlier if symptomatic.	Snoring cues sleep-disordered breathing.
65	Obesity and short-stature dots	Complications / Mx	Short stature and obesity are common; plot growth on Down syndrome-specific growth charts and encourage healthy diet and activity.	Body-size dots cue tracking growth pattern and weight.
66	Immunocompromised crutch	Complications	Down syndrome causes immune dysfunction with recurrent infections plus autoimmune disease risk.	The crutch cues impaired immune defence.
67	Confused scientist	Complications	Adults with Down syndrome have early Alzheimer disease risk from APP overexpression on chromosome 21, with dementia often emerging in midlife.	The confused scientist cues Alzheimer-like cognitive decline.
68	Falling reproduction folder	Complications / Mx	Fertility is reduced, especially in males; adolescents need sexuality, contraception, and genetic counselling when appropriate.	The falling reproduction folder cues reduced fertility and reproductive counselling.

Down Syndrome (trisomy 21)

References

Down Syndrome (Trisomy 21) — Paediatrics

Risk Factors

Etiology and Pathophysiology

A. Mechanisms of Trisomy 21

Classification

Clinical Features

A. SYMPTOMS (What the parent/patient reports)

B. SIGNS (What the examiner finds)

Growth and Development in Down Syndrome

Family-Centred Care and Communication

Active Recall - Down Syndrome (Trisomy 21)

Differential Diagnosis of Down Syndrome (Trisomy 21)

A. Differential Diagnosis by Presenting Feature

Active Recall - DDx of Down Syndrome

References

Diagnostic Criteria, Diagnostic Algorithm, and Investigation Modalities for Down Syndrome (Trisomy 21)

Diagnostic Criteria

Prenatal Screening and Diagnosis — Complete Algorithm

Postnatal Investigation Modalities — Baseline Workup for Confirmed DS

Interpretation of Key Investigations — What Each Result Means

Active Recall - Diagnostics of Down Syndrome

References

Management of Down Syndrome (Trisomy 21)

A. Immediate Neonatal Management (Birth to 1 Month)

B. Management of Specific Comorbidities

Active Recall - Management of Down Syndrome

References

Complications of Down Syndrome (Trisomy 21)

A. Cardiovascular Complications

B. Haematological Complications

C. Endocrine Complications

D. Neurological Complications

E. Respiratory Complications

F. Gastrointestinal Complications

Active Recall - Complications of Down Syndrome

References

On this page

Down Syndrome (trisomy 21)

Definition

Epidemiology

Risk Factors

1. Advanced Maternal Age (AMA)

2. Paternal Age

3. Previous Affected Pregnancy

4. Parental Translocation Carrier Status

Anatomy and Function — Chromosome 21

Etiology and Pathophysiology

A. Mechanisms of Trisomy 21

1. Meiotic Non-disjunction (94%) [1]

2. Robertsonian Translocation (5%) [1]

3. Mosaicism (1%) [1]

B. Gene Dosage Effect — The Unifying Pathophysiology

Classification

By Cytogenetic Mechanism (as above)

By Phenotypic Severity

Clinical Features

Approach to Clinical Features in Down Syndrome

A. SYMPTOMS (What the parent/patient reports)

1. Developmental Delay and Intellectual Disability

2. Feeding Difficulties in the Newborn Period

3. Recurrent Infections

4. Constipation

5. Obstructive Sleep Apnoea (OSA)

6. Hearing Impairment

7. Visual Impairment

B. SIGNS (What the examiner finds)

Craniofacial Features

Musculoskeletal Features

Cardiovascular Features

Gastrointestinal Features

Haematological Features

Endocrine Features

Neurological Features

Other Associated Features

Growth and Development in Down Syndrome

Developmental Milestones (Approximate Ranges)

DS-Specific Growth Charts

Family-Centred Care and Communication

Communication with Caregivers at Diagnosis

Consent/Assent

Prenatal Screening and Diagnosis (Brief Overview — will be expanded in Diagnostic section)

Summary of Key Pathophysiology Connections

Active Recall - Down Syndrome (Trisomy 21)

References

Framework: When Do We Consider the DDx?

A. Differential Diagnosis by Presenting Feature

1. DDx of the "Floppy Baby" (Neonatal Hypotonia + Dysmorphic Features)

2. DDx of Dysmorphic Neonate with Congenital Heart Disease

3. DDx of Intellectual Disability with Dysmorphic Features

4. DDx of Abnormal Prenatal Screening (Increased Nuchal Translucency / Positive NIPT)

5. DDx of Specific DS-Associated Complications

B. Diagnostic Approach — Algorithmic DDx

C. Key Distinguishing Points — DS vs Most Commonly Confused Conditions

D. Special Considerations in Hong Kong

Active Recall - DDx of Down Syndrome

Overview of the Diagnostic Framework

Diagnostic Criteria

1. Definitive Diagnosis — Cytogenetic Confirmation

2. Clinical Recognition (Not "Criteria" but Recognition Triggers)

Prenatal Screening and Diagnosis — Complete Algorithm

Tier 1: First-Trimester Combined Screening (11–13⁺⁶ weeks)

Tier 2: If First-Tier Screen is Positive [20]

Second-Trimester Screening (If No First-Trimester Screening Performed)

Postnatal Diagnostic Algorithm

Postnatal Investigation Modalities — Baseline Workup for Confirmed DS

A. Cardiac Assessment

B. Thyroid Function

C. Haematological Assessment

D. Hearing Assessment

E. Ophthalmological Assessment

F. Gastrointestinal Assessment (as indicated)

G. Cervical Spine Assessment

H. Developmental and Behavioural Assessment

I. Genetic Counselling

Down Syndrome Health Surveillance Schedule (Summary)

Interpretation of Key Investigations — What Each Result Means

Karyotype Interpretation