Noonan Syndrome
Noonan syndrome is an autosomal dominant genetic disorder of the RAS-MAPK pathway, typically presenting in childhood with short stature, characteristic facial features (hypertelorism, low-set ears, webbed neck), congenital heart defects (most commonly pulmonary valve stenosis), and developmental delays of variable severity.
Noonan Syndrome — Paediatric Clinical Notes
Noonan syndrome (NS) is an autosomal dominant (AD) genetic disorder characterised by a constellation of short stature, distinctive craniofacial dysmorphology, and congenital heart disease (typically right-sided lesions, most commonly pulmonary stenosis) [1][2][3].
The name "Noonan" comes from Dr Jacqueline Noonan, a paediatric cardiologist who first described the syndrome in 1963 in a series of patients with pulmonary stenosis and characteristic facial features.
Key Conceptual Distinction — Noonan vs Turner
Noonan syndrome is phenotypically similar to Turner syndrome but has NO aneuploidy of chromosomes, is inherited in an autosomal dominant (AD) fashion, and affects both sexes [2]. Turner syndrome (45,X) is a chromosomal disorder affecting only phenotypic females, primarily associated with left-sided cardiac lesions (e.g. bicuspid aortic valve, coarctation of the aorta). Noonan syndrome involves right-sided cardiac lesions (pulmonary stenosis, ASD) and has a normal karyotype. The old term "male Turner syndrome" is outdated and should not be used in exams — it is a misnomer because NS affects both sexes and the genetic basis is completely different [1][2].
| Parameter | Detail |
|---|---|
| Incidence | ~1 in 1,000 – 2,500 live births [1][2] |
| Sex ratio | Affects both sexes; no true gender predominance, but may be more recognisable in boys since they can present with cryptorchidism [2] |
| Inheritance | Nearly always AD; ~2/3 of patients are the first affected person due to de novo mutation [2] |
| Ethnic distribution | Worldwide, no known ethnic predilection |
| Hong Kong context | One of the more common genetic syndromes encountered in paediatric cardiology clinics; all ethnicities affected |
Noonan syndrome is among the most common syndromic causes of congenital heart disease (second only to Down syndrome in some series). Because the phenotype can be subtle, particularly in infancy and in females, NS is underdiagnosed — the true incidence may be higher than quoted figures.
3. Anatomy and Function (Relevant Normal Developmental Biology)
To understand NS from first principles, you need to know about the RAS-MAPK pathway (also called the RAS-mitogen-activated protein kinase pathway):
- RAS = a family of small GTPase proteins (H-RAS, K-RAS, N-RAS) that act as molecular switches
- MAPK = mitogen-activated protein kinase — a cascade of kinases (RAF → MEK → ERK) that relay growth signals from the cell surface to the nucleus
- The pathway is activated when a growth factor (e.g. EGF, FGF, PDGF) binds a receptor tyrosine kinase (RTK) on the cell membrane → adaptor proteins (GRB2, SOS) activate RAS → RAS activates RAF → RAF phosphorylates MEK → MEK phosphorylates ERK → ERK translocates to the nucleus → activates transcription factors controlling cell proliferation, differentiation, survival, and migration
This pathway is critical during embryogenesis for:
- Cardiac development (semilunar valve formation, myocardial growth, septation)
- Craniofacial morphogenesis (neural crest cell migration, bone/cartilage formation)
- Skeletal growth (growth plate chondrocyte proliferation)
- Gonadal development (testicular descent)
- Lymphatic vessel development
- Neurodevelopment (neuronal proliferation, synaptic plasticity)
When gain-of-function mutations in this pathway cause constitutive or excessive activation, the result is dysregulated signalling during these developmental windows — producing the multi-system phenotype of NS.
Since short stature is a cardinal feature of NS, it is important to recall normal growth:
- Normal birth length: ~50 cm; normal birth weight: ~3.0–3.5 kg
- Growth velocity is highest in infancy (~25 cm/year in year 1), slows through childhood (~5–7 cm/year), then accelerates in puberty (~8–12 cm/year)
- Final adult height is influenced by genetic potential (mid-parental height), nutrition, hormonal milieu (GH, IGF-1, thyroid, sex steroids)
- In NS, growth failure begins postnatally; the growth curve diverges from the 50th centile during infancy and childhood. Specific NS growth charts exist for clinical use.
4. Aetiology
Noonan syndrome is caused by mutations in genes of the RAS-MAPK pathway [1][2][3].
| Gene | Chromosome | Protein Encoded | Frequency | Notes |
|---|---|---|---|---|
| PTPN11 | 12q24.1 | SHP-2 (a protein tyrosine phosphatase) | ~50% | Most common; SHP-2 has diverse roles in signal transduction [1][2] |
| SOS1 | 2p22 | Son of Sevenless 1 (a RAS-GEF) | 10–13% [1] | Activates RAS by exchanging GDP for GTP |
| RIT1 | 1q22 | RIT1 (a RAS-family GTPase) | ~5% [1] | Higher rate of HCM |
| RAF1 | 3p25 | C-RAF (a serine/threonine kinase) | ~5% [1] | Strongly associated with hypertrophic cardiomyopathy (HCM) |
| KRAS | 12p12 | K-RAS (a GTPase) | *** < 5%*** [1] | Gain-of-function; more severe phenotype |
| BRAF | 7q34 | B-RAF kinase | < 2% | Overlap with CFC syndrome |
| MAP2K1/MEK1 | 15q22 | MEK1 | Rare | Usually CFC syndrome |
| SHOC2 | 10q25 | Leucine-rich repeat scaffold | Rare | "Noonan-like with loose anagen hair" |
| LZTR1 | 22q11 | Leucine-zipper-like regulator | Rare | Can be AR or AD |
| Others | Various | Various pathway components | Rare | ~20% of NS patients have no identified mutation |
High Yield Exam Point
~50% of patients have a pathological variant in PTPN11 which encodes SHP-2 [1][2]. This is the single most testable genetic fact about Noonan syndrome. Remember: PTPNE11 → Pulmonary Stenosis (the most common cardiac lesion). The "11" can remind you of chromosome 12 (close enough as a mnemonic anchor).
- Nearly always autosomal dominant (AD) [1][2]
- Variable expressivity (even within the same family, severity can vary greatly)
- ~2/3 of cases are de novo mutations [2]; ~1/3 are familial
- Rare AR inheritance reported for LZTR1 mutations
- Recurrence risk for an affected parent = 50% (AD); for de novo mutations, recurrence risk for siblings is low (but gonadal mosaicism is a small possibility)
All the implicated genes encode components or regulators of the RAS-MAPK pathway. The mutations are predominantly gain-of-function (i.e. the mutant protein is more active or active for longer):
- PTPN11 mutations: SHP-2 normally acts as a phosphatase that modulates (and in most contexts enhances) RAS-MAPK signalling. Gain-of-function mutations make SHP-2 constitutively active → excessive pathway activation
- SOS1 mutations: SOS1 is a guanine nucleotide exchange factor (GEF) for RAS — it "turns on" RAS by swapping GDP for GTP. Gain-of-function → more RAS activation
- RAF1 mutations: C-RAF is a kinase immediately downstream of RAS. Gain-of-function → more MEK/ERK activation, particularly in cardiac myocytes → explains the high rate of hypertrophic cardiomyopathy with RAF1 mutations
- KRAS mutations: the RAS protein itself is constitutively active
The net effect is enhanced, sustained RAS-MAPK signalling during critical periods of embryonic and postnatal development.
5. Pathophysiology — Mechanism by Organ System
Understanding why each clinical feature occurs requires tracing back to the dysregulated RAS-MAPK pathway:
- RAS-MAPK signalling is essential for semilunar valve development (endocardial cushion → valve leaflet remodelling). Excessive signalling → dysplastic, thickened pulmonary valve leaflets → pulmonary stenosis (the hallmark cardiac lesion, seen in ~50% of NS patients)
- In the myocardium, excess MAPK activation (especially with RAF1 mutations) stimulates cardiomyocyte hypertrophy → hypertrophic cardiomyopathy (HCM), seen in 20–30% of patients
- Septal defects (ASD, VSD) result from abnormal endocardial cushion development
- RAS-MAPK hyperactivation in the growth plate alters chondrocyte proliferation and differentiation
- There may also be relative GH insensitivity (normal or elevated GH but reduced IGF-1 signalling efficiency) or altered GH-IGF-1 axis function
- Short stature is present in ~70% of NS patients [1]; adult height for untreated males averages ~161 cm, females ~150 cm
- Neural crest cells (which give rise to facial bones, cartilage, and connective tissue) rely on RAS-MAPK for migration and differentiation
- Dysregulated signalling → characteristic facial dysmorphism (described below)
- The facial phenotype changes with age (more obvious in childhood, may become subtler in adults)
- Lymphatic vessel development (lymphangiogenesis) depends on VEGF/RAS-MAPK signalling
- Abnormal lymphatic development → lymphatic dysplasia → can manifest as peripheral lymphoedema, cystic hygroma (prenatally), chylothorax, or intestinal lymphangiectasia
- SHP-2 (PTPN11) plays a role in megakaryocyte signalling and clotting factor production
- Dysregulated signalling → abnormal bleeding/bruising [1] due to various coagulopathies (factor XI deficiency, platelet dysfunction, von Willebrand-like defects)
- PTPN11 gain-of-function mutations → predisposition to juvenile myelomonocytic leukaemia (JMML) — a rare but important association
- Testicular descent requires normal signalling cascades; dysregulated RAS-MAPK → cryptorchidism (seen in up to 60–80% of affected males) → can lead to impaired fertility if untreated
- RAS-MAPK is involved in synaptic plasticity, long-term potentiation, and neuronal proliferation
- Intellectual disability (ID) in ~35%, usually mild [1]
- Learning difficulties, speech delay, and motor delay are common even without formal ID
6. Classification
NS can be sub-classified by the causative gene, which has genotype-phenotype correlations:
| Genotype | Key Phenotypic Associations |
|---|---|
| PTPN11 | "Classic" NS facies; pulmonary stenosis predominant; higher risk of JMML; short stature |
| SOS1 | Often normal stature; ectodermal features (keratosis pilaris-like skin); less ID |
| RAF1 | High rate of hypertrophic cardiomyopathy (HCM); may have multiple lentigines |
| RIT1 | High rate of HCM; lymphatic anomalies common |
| KRAS | More severe phenotype; more significant ID |
| SHOC2 | "Noonan-like with loose anagen hair" — sparse, slow-growing hair; darkly pigmented skin; mitral valve prolapse |
Noonan syndrome overlaps with other syndromes involving the RAS-MAPK pathway, now collectively called RASopathies [1][3].
| RASopathy | Key Gene(s) | Distinguishing Features |
|---|---|---|
| Noonan syndrome | PTPN11, SOS1, RIT1, RAF1, KRAS | PS, short stature, characteristic facies |
| Costello syndrome | HRAS | Coarse facies, papillomata, increased cancer risk (rhabdomyosarcoma, neuroblastoma) |
| Cardio-facio-cutaneous (CFC) syndrome | BRAF, MAP2K1/2 | More severe ID, sparse hair, ichthyosis-like skin |
| NS with multiple lentigines (NSML/LEOPARD syndrome) | PTPN11, RAF1 | Lentigines, ECG abnormalities, Ocular hypertelorism, PS, Abnormal genitalia, Retarded growth, Deafness |
| Neurofibromatosis type 1 (NF1) | NF1 (neurofibromin) | Café-au-lait spots, neurofibromas, Lisch nodules |
High Yield — RASopathies
RASopathies refer to any disorders caused by mutations involved in the RAS/MAPK pathway [1]. They share overlapping features (short stature, cardiac defects, facial dysmorphism, neurodevelopmental issues) but differ in degree and specific organ involvement. For the exam, know the major RASopathies and their distinguishing features.
7. Clinical Features
The clinical features of NS evolve with age. The facial phenotype is most distinctive in childhood and may become subtler in adolescence/adulthood.
| Symptom | Pathophysiological Basis |
|---|---|
| Poor feeding / failure to thrive in infancy | Oral motor dysfunction (hypotonia, high-arched palate); cardiac disease causing increased metabolic demand; possible lymphatic dysfunction (protein-losing enteropathy from intestinal lymphangiectasia) |
| "My child is short" — growth concerns | Short stature (~70%) [1]: postnatal growth failure from dysregulated GH-IGF-1 axis and growth plate chondrocyte dysfunction due to excessive RAS-MAPK signalling |
| Developmental delay — late sitting, walking, talking | ID (~35%, usually mild) [1]; motor delay from hypotonia and cerebellar dysfunction; speech delay from combined hearing impairment, oral motor issues, and neurocognitive factors |
| Easy bruising / prolonged bleeding after minor procedures | Abnormal bleeding/bruising [1]: coagulopathy (factor XI deficiency most common, also vWD-like defects, platelet dysfunction) due to SHP-2 dysregulation in megakaryocyte/hepatic signalling |
| Undescended testes (in boys) — absent scrotal contents | Cryptorchidism [1]: failed testicular descent due to abnormal gubernacular signalling; up to 60–80% of affected males |
| Swelling of limbs (lymphoedema) | Lymphatic dysplasia from abnormal lymphangiogenesis → peripheral lymphoedema, can present at birth or later |
| Chest shape abnormality noticed by parents | Pectus excavatum [1]: abnormal costal cartilage growth due to RAS-MAPK effects on chondrocyte development |
| Learning difficulties at school | Neurocognitive effects of dysregulated RAS-MAPK on synaptic plasticity; may include specific learning disabilities, attention difficulties |
| Hearing difficulties | Hearing impairment [1]: sensorineural or conductive hearing loss from inner ear or middle ear structural anomalies |
| Visual problems | Visual impairment [1]: strabismus, refractive errors, amblyopia from ptosis |
7.2 Signs (What You Find on Examination)
| Sign | Description | Pathophysiological Basis |
|---|---|---|
| Low posterior hairline [1] | Hairline extends lower on the back of the neck | Abnormal cervical connective tissue and skeletal development from neural crest cell dysregulation |
| Short, webbed neck (pterygium colli) [1] | Lateral skin folds between mastoid and acromion | Residual cystic hygroma/lymphatic malformation that partially resolved in utero → leaves excess skin/connective tissue |
| Hypertelorism [1] | Widely spaced eyes (increased interpupillary distance) | Abnormal frontonasal development from neural crest cell dysregulation |
| Downslanting palpebral fissures [1] | Outer corners of eyes angled downward | Altered orbital bone development |
| Epicanthic folds [1] | Skin folds covering the inner corner of the eye | Common dysmorphic feature in many syndromes; reflects midface hypoplasia |
| Ptosis [1] | Drooping upper eyelid (unilateral or bilateral) | Levator palpebrae superioris muscle or its innervation (CN III) development affected; be cautious if child looks with head tilting up! [1] — this is a compensatory mechanism for ptosis |
| Strabismus [1] | Misalignment of eyes | Extraocular muscle or CN III/IV/VI development affected |
| Low-set, posteriorly rotated ears [1] | Ears positioned lower than normal and rotated backward; thick helices | Abnormal branchial arch development (ear pinnae derive from 1st and 2nd branchial arches) |
| Micrognathia or retrognathia [1] | Small or posteriorly positioned mandible | Abnormal mandibular (1st branchial arch) development |
| Short nose with broad base [1] | Wide, flattened nasal bridge; short nose | Midface hypoplasia from abnormal frontonasal neural crest cell migration |
| Deeply grooved philtrum | Prominent vertical groove between nose and upper lip | Neural crest cell signalling effect on midface |
| High-arched palate | Narrow, highly arched palate | Can contribute to feeding difficulties |
| Curly or sparse hair | Woolly-textured hair, especially in infancy | Ectodermal effects of RAS-MAPK dysregulation; especially prominent with SHOC2 mutations |
Age-dependent facial changes: In infancy, the face appears round with full cheeks, prominent forehead, and widely spaced eyes. In childhood, the face elongates and becomes more triangular. In adolescence/adulthood, features may become subtler, with a more angular face and prominent nasolabial folds.
| Sign | Frequency | Pathophysiological Basis |
|---|---|---|
| Pulmonary stenosis (PS) | ~50% [1][2] | The most common cardiac lesion; RAS-MAPK dysregulation → dysplastic, thickened pulmonary valve leaflets; characteristically the valve is dysplastic (thick, immobile, myxomatous) rather than simply fused — this is a distinguishing feature from isolated PS |
| Hypertrophic cardiomyopathy (HCM) | 20–30% [1] | Excessive MAPK activation in cardiomyocytes → myocyte hypertrophy without proportionate capillary growth; especially common with RAF1 and RIT1 mutations |
| ASD | 10–15% [1] | Abnormal septation from endocardial cushion dysregulation |
| VSD | 5–10% [1] | Same mechanism as ASD |
| Mitral valve prolapse | 5% | Myxomatous degeneration of mitral leaflets |
| Aortic coarctation | Rare | Less common; more typical of Turner syndrome |
Clinical examination findings:
- Ejection systolic murmur at the upper left sternal edge (pulmonary area) — radiates to back, may have palpable thrill and ejection click → PS
- Systolic murmur at the lower left sternal edge → HCM (dynamic LVOT obstruction) — increases with Valsalva/standing, decreases with squatting
- Fixed, widely split S2 → ASD
- Pansystolic murmur at lower left sternal edge → VSD
High Yield — Cardiac Lesion Contrast
Noonan syndrome = right-sided lesions (pulmonary stenosis). Turner syndrome = left-sided lesions (bicuspid aortic valve, coarctation of the aorta) [2]. This is a commonly tested comparison. Remember: Noonan = pNeumonic = Pulmonary = right; Turner = aorTa = left.
| Sign | Pathophysiological Basis |
|---|---|
| Short stature (~70%) [1] | Postnatal growth failure; mean adult height ~161 cm (males), ~150 cm (females); specific NS growth curves exist |
| Pectus excavatum (or pectus carinatum superiorly / excavatum inferiorly) [1] | Abnormal costal cartilage development → "shield-shaped" chest |
| Widely spaced nipples [1] | Altered thoracic wall development; part of the "shield chest" |
| Cubitus valgus [1] | Increased carrying angle at the elbow (> 15° in males, > 13° in females); abnormal skeletal development of distal humerus/proximal ulna |
| Kyphoscoliosis [1] | Vertebral body development abnormality → spinal curvature |
| Sign | Pathophysiological Basis |
|---|---|
| Peripheral lymphoedema | Lymphatic hypoplasia/dysplasia → impaired lymph drainage → protein-rich interstitial fluid accumulation; most commonly hands/feet; may present at birth |
| Cystic hygroma (prenatal/neonatal) | Abnormal lymphatic development → dilated lymphatic channels, usually in posterior cervical region; may be detected on prenatal ultrasound; can partially resolve, leaving the webbed neck |
| Chylothorax | Chyle leak from malformed thoracic lymphatic vessels into pleural space |
| Blood/lymph vessel malformation [1] | Generalised lymphatic and vascular dysplasia |
| Sign | Pathophysiological Basis |
|---|---|
| Abnormal bleeding/bruising [1] | Multiple mechanisms: factor XI deficiency (most common specific deficiency), platelet dysfunction (defective aggregation), von Willebrand disease-like defects, factor VIII/XII deficiency |
| Hepatosplenomegaly (rare) | JMML or myeloproliferative disorder (especially PTPN11 mutations) |
| Sign | Pathophysiological Basis |
|---|---|
| Cryptorchidism [1] | Bilateral in most; due to abnormal gubernacular/inguinal canal signalling; if untreated → impaired spermatogenesis → reduced fertility |
| Delayed puberty | Hypogonadism from cryptorchidism or primary gonadal dysfunction |
| Sign | Pathophysiological Basis |
|---|---|
| Intellectual disability (~35%, usually mild) [1] | Dysregulated synaptic plasticity from excessive RAS-MAPK signalling; most have IQ 70–85 |
| Hypotonia | Reduced muscle tone, especially in infancy; contributes to feeding difficulties and motor delay |
| Motor delay | Hypotonia + cerebellar involvement; average walking age ~21 months |
| Speech delay | Combined effect of hearing impairment, oral motor dysfunction, neurocognitive factors |
| Arnold-Chiari malformation (rare) | Hindbrain malformation from abnormal posterior fossa development |
| Sign | Pathophysiological Basis |
|---|---|
| Hearing impairment [1] | Sensorineural (cochlear) or conductive (middle ear effusions, ossicular anomalies) |
| Visual impairment [1] | Refractive errors, strabismus, amblyopia from ptosis, nystagmus |
| Dental malocclusion | Micrognathia → crowding |
| Keratosis pilaris-like skin lesions | Follicular keratinisation from ectodermal effects; especially with SOS1 mutations |
| Multiple lentigines | Small brown macules; when prominent → NSML/LEOPARD syndrome |
| Hepatosplenomegaly | Can occur with JMML or extramedullary haematopoiesis |
8. Clinical Approach to the Child with Suspected Noonan Syndrome
Think of NS when you see:
- A child with short stature + characteristic facies + congenital heart disease (especially right-sided)
- A neonate with cystic hygroma or lymphoedema + normal karyotype
- A boy with cryptorchidism + cardiac murmur + facial dysmorphism
- A child with pulmonary valve stenosis where the valve is dysplastic (not just fused commissures)
- A child with HCM + facial dysmorphism (especially consider RAF1/RIT1 mutations)
- A family with AD short stature + cardiac disease + facial features
- Birth history: polyhydramnios, cystic hygroma on prenatal ultrasound, hydrops fetalis, neonatal oedema
- Feeding history: difficulties in infancy, failure to thrive
- Growth: plot on NS-specific growth charts; compare with normal centile charts
- Development: motor milestones (sitting, walking), speech milestones, school performance
- Bleeding history: easy bruising, prolonged bleeding after dental procedures, menorrhagia in adolescent females
- Cardiac: exercise intolerance, cyanosis, murmur detected on newborn examination
- Family history: similar features in parents/siblings (variable expressivity — a parent may have subtle features only recognised in retrospect)
- Genitourinary: undescended testes (in boys), pubertal development
- Growth parameters: height, weight, head circumference — plot on NS-specific growth charts
- Facial features: systematically assess hypertelorism, palpebral fissures, ptosis, ears, nose, chin
- Neck: webbing, posterior hairline
- Chest: pectus deformity, nipple spacing
- Cardiovascular: murmurs, heart sounds (ejection click with PS), blood pressure (all 4 limbs if coarctation suspected)
- Abdomen: hepatosplenomegaly
- Limbs: cubitus valgus, lymphoedema, proportion
- Genitalia: cryptorchidism (bilateral?), Tanner staging
- Spine: scoliosis, kyphosis
- Skin: lentigines, café-au-lait spots, keratosis pilaris
- Neurological: tone, reflexes, coordination
- Developmental assessment: age-appropriate milestones
GH replacement is the main therapeutic approach for short stature in NS [1].
- Recombinant human GH (rhGH) is approved for use in NS-related short stature in many countries
- Mechanism: exogenous GH → increases IGF-1 production → stimulates growth plate chondrocyte proliferation and skeletal growth
- Response is typically modest compared to GH deficiency: average gain of ~3–5 cm in final adult height
- Started when height is < 2 SD below mean for age and growth velocity is declining
- Must exclude and optimise cardiac function before starting (HCM may theoretically worsen with GH, though evidence is reassuring)
- Requires regular monitoring: growth velocity, IGF-1 levels, scoliosis, HCM surveillance
| Age Group | Key Considerations |
|---|---|
| Prenatal | Cystic hygroma, polyhydramnios, increased nuchal translucency on ultrasound; normal karyotype → think NS |
| Neonate | Peripheral oedema (lymphatic), feeding difficulties, cardiac murmur, cryptorchidism; may need chylothorax management |
| Infant | Failure to thrive, growth deceleration, developmental delay, recurrent otitis media |
| Child | Short stature becomes apparent, learning difficulties, cardiac follow-up, consider GH therapy |
| Adolescent | Delayed puberty (especially males with bilateral cryptorchidism), psychosocial support, transition planning; assess fertility potential |
- Counselling: explain AD inheritance, 50% transmission risk if parent affected, but ~2/3 are de novo
- Multidisciplinary team: paediatrician, paediatric cardiologist, clinical geneticist, endocrinologist, developmental paediatrician, audiologist, ophthalmologist, psychologist
- Support groups: connect families with NS support organisations
- Anticipatory guidance: cardiac surveillance schedule, developmental support, educational accommodations
- Consent/assent: obtain parental consent for genetic testing and treatments; involve the adolescent in discussions as appropriate (assent from ~age 7, Gillick competence from ~12–14 depending on maturity)
| Parameter | Normal Range (Age-Dependent) |
|---|---|
| Heart rate (neonate) | 120–160 bpm |
| Heart rate (1–5 years) | 80–130 bpm |
| Heart rate (6–12 years) | 70–110 bpm |
| Blood pressure (1 year) | 80/50 mmHg (approx) |
| Blood pressure (6 years) | 95/60 mmHg (approx) |
| Growth velocity (year 1) | ~25 cm/year |
| Growth velocity (childhood) | ~5–7 cm/year |
| APTT | 25–35 seconds (age-dependent) |
| Factor XI level | 50–150% (deficiency < 50%) |
High Yield Summary
Noonan Syndrome — Key Points for Exams:
- Autosomal dominant; ~50% caused by PTPN11 mutations (encodes SHP-2); ~2/3 de novo [1][2]
- RAS-MAPK pathway disorder — gain-of-function mutations → dysregulated cell growth and differentiation [1][3]
- Clinical triad: short stature + characteristic facies + congenital heart disease (right-sided, especially pulmonary stenosis ~50%, also HCM 20–30%) [1][2]
- Phenotypically similar to Turner syndrome but normal karyotype, AD, affects both sexes [2]
- Contrast with Turner: NS = right-sided cardiac (PS); Turner = left-sided cardiac (coarctation, BAV) [2]
- Key craniofacial features: hypertelorism, downslanting palpebral fissures, ptosis, low-set posteriorly rotated ears, short webbed neck, low posterior hairline [1]
- Other features: cryptorchidism, cubitus valgus, pectus excavatum, widely spaced nipples, bleeding tendency, lymphoedema, mild ID (~35%) [1]
- RASopathies = umbrella term for NS, Costello syndrome, CFC syndrome, LEOPARD syndrome [1][3]
- Management: GH replacement for short stature; cardiac surveillance; multidisciplinary care [1]
- Genotype-phenotype: RAF1/RIT1 → higher risk of HCM; PTPN11 → JMML risk; SOS1 → often normal height
Active Recall - Noonan Syndrome
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 511 — Noonan Syndrome section) [2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p. 852 — Noonan Syndrome section) [3] Lecture slides: Block C - The malformed child: hereditary syndromes and anomalies.pdf (p. 1 — learning objectives and RASopathies content); GC 151. The malformed child hereditary syndromes and anomalies.pdf
Differential Diagnosis of Noonan Syndrome
The differential diagnosis of Noonan syndrome is best understood by asking: "What other conditions can present with this combination of features — short stature, dysmorphic facies, congenital heart disease, and/or webbed neck — in a child?"
The key to navigating the DDx is to anchor on the presenting complaint that brought the child to attention, because NS can present through multiple doors:
- A dysmorphic child with a cardiac murmur — the cardiology clinic door
- A child with short stature — the endocrine/growth clinic door
- Prenatal cystic hygroma or increased nuchal translucency — the fetal medicine door
- A boy with cryptorchidism — the surgical/urological door
- A child with developmental delay — the neurodevelopmental door
- Easy bruising/bleeding tendency — the haematology door
Each door has its own differential. I will organise the DDx systematically by the major presenting feature clusters, then provide a consolidated comparison table.
1. Differential by Presenting Feature Cluster
This is the most classic diagnostic conundrum. Several conditions share overlapping "Turner-like" features:
| Condition | Key Distinguishing Features | Cardiac Lesion | Why It Differs from NS |
|---|---|---|---|
| Turner syndrome (45,X) [4][5] | Short stature, webbing of neck, low hairline, cubitus valgus, broad chest with widely spaced nipples [4]; affects only phenotypic females; caused by monosomy X (45,X) or structural X abnormalities | Left-sided: coarctation of aorta, bicuspid aortic valve, valvular AS, hypoplastic left heart syndrome, MVP [4][5] | Turner = left-sided cardiac, chromosomal (45,X), female only. NS = right-sided cardiac, normal karyotype, AD, both sexes [2][4] |
| Noonan syndrome [1][2] | Both sexes; normal karyotype; AD (PTPN11 ~50%) | Right-sided: valvular PS with thick dysplastic valve cusps, HCM, ASD [4][5] | The index condition |
| Kabuki syndrome (KMT2D or KDM6A) | Long palpebral fissures with eversion of lateral 1/3 of lower eyelid ("Kabuki make-up"), arched eyebrows with sparse lateral third, prominent ears, fingertip pads, short stature, ID | ASD, VSD, coarctation, TOF | Distinguished by the characteristic "Kabuki" facial gestalt and fingertip pads; no dysplastic PS |
High Yield Exam Point — Turner vs Noonan
Noonan syndrome is phenotypically similar to Turner syndrome but with NO aneuploidy of chromosomes, is inherited in an autosomal dominant fashion, and affects both sexes [2]. Turner = left-sided cardiac lesions (coarctation, BAV). Noonan = right-sided cardiac lesions (PS, HCM) [4][5]. The first investigation in a phenotypic female with "Turner-like" features is a karyotype — if 46,XX → think NS (or other RASopathy). If 45,X or mosaic → Turner.
Since all RASopathies share dysregulated RAS-MAPK signalling, they have overlapping features. The key is distinguishing which RASopathy based on the unique additional features:
| RASopathy | Gene(s) | Shared Features with NS | Distinguishing Features |
|---|---|---|---|
| Costello syndrome [1][6] | HRAS | Short stature, coarse facies, cardiac defects, developmental delay | Deep palmar/plantar creases, papillomata (perioral, perianal), loose redundant skin in infancy, increased malignancy risk (rhabdomyosarcoma, neuroblastoma, bladder carcinoma); more severe ID |
| Cardio-facio-cutaneous (CFC) syndrome [1][2][6] | BRAF, MAP2K1, MAP2K2 | Short stature, cardiac defects (PS, HCM), facial dysmorphism | More severe ID than NS, sparse/absent eyebrows and eyelashes, ichthyosis-like/hyperkeratotic skin, curly friable hair; skin findings are more prominent |
| NS with multiple lentigines / LEOPARD syndrome [1][6] | PTPN11, RAF1 | PS, short stature, hypertelorism | Lentigines (multiple small dark macules — the hallmark), ECG abnormalities, Ocular hypertelorism, PS, Abnormal genitalia, Retardation of growth, Deafness (sensorineural) |
| Neurofibromatosis type 1 (NF1) [7] | NF1 (neurofibromin) | Café-au-lait spots, short stature, learning difficulties; both are RASopathies | ≥ 6 café-au-lait macules, axillary/inguinal freckling, ≥ 2 neurofibromas, Lisch nodules (iris hamartomas), optic pathway glioma; NS may have café-au-lait macules > 5 mm [7] but lacks neurofibromas and Lisch nodules |
| Legius syndrome | SPRED1 | Café-au-lait macules, axillary freckling, macrocephaly | NO neurofibromas, NO CNS tumours [7]; milder than NF1; can overlap phenotypically with NS if café-au-lait macules present |
Distinguishing RASopathies — Practical Tips
Think of a spectrum from mildest to most severe:
- Legius → mild (mainly skin)
- Noonan → moderate (facies + cardiac + growth)
- LEOPARD → Noonan + lentigines + deafness
- CFC → Noonan + severe skin + severe ID
- Costello → Noonan + papillomata + cancer risk
All share the RAS-MAPK pathway; the specific gene and mutation determine which end of the spectrum.
When you see a dysmorphic child with a cardiac murmur, the DDx includes the major syndrome-CHD associations:
| Syndrome | Dysmorphic Features | Cardiac Lesions | How to Distinguish from NS |
|---|---|---|---|
| Down syndrome (Trisomy 21) [4][5] | Hypotonia, prominent medial epicanthic fold, upslanting palpebral fissure, flat nasal bridge, low-set ears, protruding tongue, brachycephaly, single transverse palmar crease, wide 1st toe web space [4] | AVSD (most characteristic), VSD, secundum ASD, PDA, TOF [4] | Upslanting (not downslanting) palpebral fissures; brachycephaly; single palmar crease; karyotype shows trisomy 21; AVSD is the hallmark (not PS) |
| Williams syndrome (7q11.23 deletion) [4][5][6] | Elfin facies with full cheeks, flat nasal bridge, anteverted nostrils, long philtrum, prominent lips with open mouth; hypercalcaemia; intellectual disability; "cocktail party" personality [4][6] | Supravalvular AS (most characteristic), peripheral branch pulmonary arterial stenosis, systemic arterial stenosis, renal artery stenosis, coronary artery ostial stenosis [4][5] | Supravalvular AS (not valvular PS); hypercalcaemia (not seen in NS); "cocktail party" personality; elastin gene deletion on FISH |
| DiGeorge syndrome / 22q11.2 deletion [4][5] | Abnormal facies (low-set posteriorly rotated ears, hypertelorism), thymic hypo/aplasia, cleft palate, hypocalcaemia [4] | Conotruncal abnormalities: interrupted aortic arch, truncus arteriosus, TOF, ASD/VSD, vascular rings [4][5] | CATCH-22 mnemonic: Cardiac, Abnormal facies, Thymic hypoplasia, Cleft palate, Hypocalcaemia, 22q11.2 deletion; immunodeficiency (T-cell); conotruncal lesions (not PS) |
| Marfan syndrome (FBN1) [6] | Tall stature (opposite to NS!), arm span > height, arachnodactyly, high-arched palate, upward lens dislocation, joint hypermobility, pectus deformity, scoliosis [6] | MVP, aortic regurgitation, aortic root dilatation/dissection [6] | Tall (not short); lens dislocation; arachnodactyly; aortic root disease (not PS) |
| Alagille syndrome (JAG1/NOTCH2) | Triangular face, broad forehead, deep-set eyes, prominent chin; cholestasis (bile duct paucity) | Peripheral pulmonary stenosis, TOF, PS | Cholestatic liver disease is key differentiator; butterfly vertebrae on spinal X-ray; both can have PS but Alagille has prominent liver involvement |
| CHARGE syndrome (CHD7) | Coloboma, choanal atresia, cranial nerve dysfunction | TOF, ASD, VSD, DORV, aortic arch anomalies | Coloboma, Heart defects, Atresia choanae, Retardation of growth/development, Genital anomalies, Ear anomalies; coloboma is a strong differentiator |
| Fetal alcohol spectrum disorder | Smooth philtrum, thin upper lip, short palpebral fissures, microcephaly | VSD, ASD, TOF | Maternal alcohol exposure history; smooth philtrum (not deeply grooved as in NS); microcephaly |
| Condition | Key Features | How to Distinguish from NS |
|---|---|---|
| Turner syndrome | See above | Karyotype 45,X; female only; left-sided cardiac |
| GH deficiency (isolated) | Proportionate short stature, central adiposity, immature facies, delayed bone age | No dysmorphism; low IGF-1 and GH on stimulation testing; responds well to GH |
| Constitutional delay of growth and puberty | Family history of late puberty; short for age but normal growth velocity; delayed bone age; spontaneous catch-up | No dysmorphism; no cardiac lesion; normal final adult height |
| Skeletal dysplasias (e.g. achondroplasia) | Disproportionate short stature (rhizomelic limb shortening in achondroplasia); macrocephaly | Disproportionate (NS is proportionate); specific skeletal radiographic features |
| Silver-Russell syndrome | Intrauterine growth restriction, relative macrocephaly, limb asymmetry, triangular face, feeding difficulties | IUGR is prominent (NS usually normal birth weight); limb asymmetry; no cardiac lesion |
| Chronic disease (coeliac, IBD, CKD) | Growth failure with systemic symptoms | Appropriate investigations (coeliac serology, inflammatory markers, renal function) |
| Condition | How to Distinguish from NS |
|---|---|
| Isolated cryptorchidism | No dysmorphism, no cardiac lesion; very common (3% of term males) |
| Prader-Willi syndrome | Severe neonatal hypotonia, poor feeding → later hyperphagia/obesity, hypogonadism, small hands/feet, ID; chromosome 15q11.2 deletion (paternal) |
| Klinefelter syndrome (47,XXY) | Tall stature (opposite to NS), small testes, gynaecomastia, infertility; karyotype diagnostic |
| Disorders of sex development | Ambiguous genitalia; hormone panel and karyotype needed |
| Condition | How to Distinguish from NS |
|---|---|
| Von Willebrand disease | Most common inherited bleeding disorder; no dysmorphism; vWF antigen and activity assays diagnostic |
| Haemophilia A/B | X-linked recessive; no dysmorphism; specific factor VIII/IX deficiency |
| Immune thrombocytopenia (ITP) | Acquired; isolated thrombocytopenia; no dysmorphism |
| Fanconi anaemia | Pancytopenia, short stature, café-au-lait spots, polydactyly, radial ray anomalies [8]; chromosomal breakage study diagnostic |
When a child with NS (especially PTPN11 mutation) develops hepatosplenomegaly, monocytosis, and failure to thrive:
| Condition | Distinguishing Feature |
|---|---|
| JMML (NS-associated) | Can be a transient myeloproliferative disorder in NS neonates that resolves spontaneously; true JMML requires sustained features |
| Chronic myelomonocytic leukaemia | Adults; not paediatric |
| Viral infection-related monocytosis | Self-limiting; no hepatosplenomegaly progression |
| Langerhans cell histiocytosis | Skin rash, bone lesions, diabetes insipidus |
This is a must-know table for paediatric exams:
| Feature | Down (T21) [4] | Turner (45,X) [4] | Noonan [4] | Williams (7q11.23 del) [4] | DiGeorge (22q11.2 del) [4] |
|---|---|---|---|---|---|
| Inheritance / Genetics | Trisomy 21 | Monosomy X | AD (PTPN11 ~50%) | 7q11.23 microdeletion (elastin) | 22q11.2 microdeletion |
| Sex | Both | Female only | Both sexes | Both | Both |
| Stature | Short | Short | Short | Short | Variable |
| Palpebral fissure | Upslanting | Normal | Downslanting | Normal | Normal |
| Hallmark cardiac | AVSD | Coarctation, BAV | Valvular PS (dysplastic), HCM | Supravalvular AS | Interrupted aortic arch, truncus arteriosus, TOF |
| Cardiac side | Septal | Left | Right | Arterial | Conotruncal |
| Key extra-cardiac | ID, hypothyroid, atlantoaxial instability | Gonadal dysgenesis, renal | Cryptorchidism, lymphoedema, bleeding | Hypercalcaemia, "cocktail party" personality | Thymic aplasia, hypocalcaemia, cleft palate |
| Karyotype | 47,XX/XY,+21 | 45,X | Normal | Normal (FISH for deletion) | Normal (FISH for deletion) |
Step 1: Recognise the pattern — short stature + dysmorphic facies + congenital heart disease
Step 2: Determine the cardiac lesion side
- Right-sided (PS, HCM) → Noonan / RASopathy [4][5]
- Left-sided (CoA, BAV) → Turner [4][5]
- Conotruncal (IAA, truncus, TOF) → DiGeorge
- Supravalvular AS → Williams
- AVSD → Down syndrome
Step 3: Check the karyotype — this is the single most important first-line investigation to distinguish Turner from Noonan in a phenotypic female
Step 4: Assess for additional features that point to a specific RASopathy (lentigines → LEOPARD; papillomata → Costello; ichthyosis → CFC)
Step 5: Confirm with molecular genetics — RAS-MAPK gene panel (PTPN11 tested first, then SOS1, RIT1, RAF1, KRAS, BRAF, etc.)
Clinical Pearl — The 'Noonan Phenotype' in a Female
When you see a girl with Turner-like features but a normal karyotype (46,XX), the most important next step is to consider Noonan syndrome and request molecular testing for RAS-MAPK pathway genes [2]. Do NOT dismiss the diagnosis because "she looks like Turner" — NS affects both sexes and has a normal karyotype.
Common Exam Mistake
A common mistake is to confuse the cardiac lesion associations. Remember the mnemonic: Noonan = Pulmonic (right); Turner = aorTic (left); Down = Defect of septum (AVSD); Williams = Supravalvular (above valve); DiGeorge = Conotruncal. Getting these mixed up is a frequent source of lost marks.
Active Recall - Differential Diagnosis of Noonan Syndrome
References
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 511 — Noonan Syndrome section) [2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p. 852–854 — Noonan Syndrome section) [4] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 184 — Syndromes associated with congenital heart diseases table) [5] Senior notes: Ryan Ho Cardiology.pdf (p. 185 — Syndromes associated with congenital heart diseases table) [6] Senior notes: Maksim Paediatric Notes.pdf (p. 206–207 — Williams syndrome, Marfan syndrome, Noonan syndrome, Fragile X) [7] Senior notes: Ryan Ho Rheumatology.pdf (p. 172 — NF1 differential diagnosis including Noonan syndrome) [8] Senior notes: Block A - Pallor: diagnosis of anaemia; nutritional anaemia; anaemia of systemic diseases.pdf (p. 42 — Fanconi anaemia)
Diagnostic Criteria for Noonan Syndrome
Noonan syndrome is a clinical diagnosis supported by molecular confirmation. Because the phenotype is variable (even within the same family due to variable expressivity), overlaps with other RASopathies, and changes with age, a standardised clinical scoring system was developed to guide clinicians on when the clinical picture is strong enough to warrant the diagnosis — even before or without genetic confirmation.
The most widely used clinical scoring system is the van der Burgt scoring system (originally published 1994, updated 2007). Understanding this system from first principles: each feature is weighted by how specific it is for NS (i.e. how much it discriminates NS from the general population and from other genetic syndromes).
The system evaluates six feature categories, each scored as either Major (A) or Minor (B):
| Feature Category | Major (A) | Minor (B) |
|---|---|---|
| 1. Facial features | Typical face dysmorphology — the classic NS "gestalt" with hypertelorism, downslanting palpebral fissures, ptosis, low-set posteriorly rotated ears [1][2] | Suggestive face dysmorphology — some features present but not the full classic pattern |
| 2. Cardiac | Pulmonary valve stenosis (dysplastic valve) and/or hypertrophic cardiomyopathy (HCM) [1][2][5] | Other congenital heart defect (ASD, VSD, peripheral PS, etc.) |
| 3. Height | Height < 3rd percentile (using standard population growth charts) | Height < 10th percentile |
| 4. Chest wall | Pectus carinatum/excavatum [1][2] | Broad thorax |
| 5. Family history | First-degree relative with definite NS | First-degree relative with suggestive features |
| 6. Other features (at least one of: intellectual disability, cryptorchidism, lymphatic dysplasia) | All three present: mild ID + cryptorchidism + lymphatic dysplasia [1][2] | One of the three present |
Diagnostic Rule:
| Facial Features | Required Additional Features for Diagnosis |
|---|---|
| Typical face (A) | One other Major (A) feature OR two other Minor (B) features |
| Suggestive face (B) | Two other Major (A) features OR three other Minor (B) features |
High Yield — Diagnostic Criteria Logic
The facial gestalt is the gateway criterion — you must have either typical or suggestive facial features to make the clinical diagnosis. Without facial features, the diagnosis of NS should not be made on clinical grounds alone (although a pathogenic RAS-MAPK gene variant in the right clinical context can still confirm the diagnosis molecularly). The more typical the face, the fewer additional features you need.
Important Limitation
The van der Burgt criteria were designed before comprehensive molecular testing was available. In current practice (2025–2026), molecular confirmation with a RAS-MAPK gene panel is the gold standard. A child with a pathogenic variant in a known NS gene and compatible clinical features (even if they don't meet the full van der Burgt score) is considered to have NS. Conversely, ~20% of clinically diagnosed NS patients have no identifiable mutation — in these cases, the clinical score becomes the definitive diagnostic tool [2].
In the era of next-generation sequencing, the molecular diagnosis is increasingly the confirmatory step:
- Definite NS: Classic clinical phenotype (meets van der Burgt criteria) AND pathogenic/likely pathogenic variant in a known NS gene
- Probable NS: Classic clinical phenotype WITHOUT identifiable mutation (~20% of cases) — diagnosed on clinical grounds
- Possible NS: Suggestive but incomplete clinical features WITH a variant of uncertain significance (VUS) in a RAS-MAPK gene — requires further workup, segregation analysis, functional studies
NS can be suspected prenatally when the following findings are present on ultrasound [2]:
- Increased nuchal translucency (NT) — a thickened NT (> 3.5 mm or > 99th percentile at 11–14 weeks) with a normal karyotype should prompt consideration of NS
- Cystic hygroma — especially posterior cervical; large cystic hygromas with normal karyotype are a strong pointer
- Polyhydramnios — from fetal swallowing dysfunction
- Hydrops fetalis — generalised oedema from lymphatic dysplasia
- Hydrothorax / pleural effusions — chylothorax from lymphatic malformation
- Cardiac anomalies detected on fetal echocardiography (PS, HCM)
- Renal anomalies — pyelectasis, structural malformations
NS mutation testing is indicated in the presence of increased nuchal translucency, cystic hygroma, polyhydramnios, generalised hydrops, hydrothorax, cardiac anomalies, or renal anomalies with a normal karyotype [2].
5. Step-by-Step Clinical Approach
The diagnostic approach to a child with suspected Noonan syndrome follows a logical sequence:
Step 1: Clinical recognition → Identify the characteristic phenotype (facial gestalt + cardiac lesion + short stature ± other features)
Step 2: Exclude chromosomal disorders → Karyotype (especially in phenotypic females to exclude Turner 45,X)
Step 3: Apply van der Burgt clinical scoring → Determine if clinical criteria are met
Step 4: Molecular confirmation → RAS-MAPK gene panel (PTPN11 tested first as it accounts for ~50%)
Step 5: Comprehensive phenotyping → Full multi-system assessment to characterise the extent of disease
Step 6: Family screening → Cascade genetic testing and clinical evaluation of first-degree relatives
Investigation Modalities, Key Findings, and Interpretations
6. Genetic Investigations
| Parameter | Details |
|---|---|
| Why | To exclude Turner syndrome (45,X) which is the most important phenotypic mimic, especially in phenotypic females |
| Method | G-banded karyotype from peripheral blood lymphocytes |
| Expected in NS | Normal karyotype (46,XX or 46,XY) [2] |
| If abnormal | 45,X or mosaic 45,X/46,XX → Turner syndrome; other aneuploidies may indicate different chromosomal syndromes |
| Turnaround | 1–2 weeks |
This is the FIRST genetic test to order in a phenotypic female with Turner-like features. A normal karyotype in a child with the right phenotype redirects you towards NS and molecular testing.
| Parameter | Details |
|---|---|
| Why | Definitive confirmation of NS; identifies the specific causative gene for genotype-phenotype correlation and family counselling |
| Method | Next-generation sequencing (NGS) panel targeting RAS-MAPK pathway genes; or whole-exome sequencing (WES) with focused analysis |
| Genes tested | PTPN11 (test first, ~50%), SOS1 (10–13%), RIT1 (~5%), RAF1 (~5%), KRAS ( < 5%), BRAF, MAP2K1, MAP2K2, SHOC2, LZTR1, CBL, NRAS [1][2] |
| Expected findings | Heterozygous pathogenic missense variant (most NS mutations are missense, not truncating — because the mutations are gain-of-function) |
| Interpretation pitfalls | Variants of uncertain significance (VUS) are common; requires careful clinical correlation; ~20% of clinically diagnosed NS patients have no identifiable mutation [2] |
| Turnaround | 4–8 weeks for NGS panel |
High Yield — Why PTPN11 Is Tested First
50% of NS patients have a pathological variant in PTPN11 which encodes SHP-2, a protein tyrosine phosphatase with diverse roles in signal transduction [2]. Because it is the most common causative gene, many labs offer single-gene Sanger sequencing for PTPN11 first as a rapid screen; if negative, the full RAS-MAPK NGS panel is sent. In current practice, most centres now go straight to the full panel.
| Parameter | Details |
|---|---|
| Why | To exclude copy number variants (CNVs) — particularly 22q11.2 deletion (DiGeorge) and 7q11.23 deletion (Williams) which can overlap phenotypically |
| Expected in NS | Normal (no pathogenic CNVs) |
| When to order | If the phenotype is atypical, if molecular testing for RAS-MAPK genes is negative, or as part of a broader dysmorphology workup |
| Setting | Test | Details |
|---|---|---|
| Prenatal ultrasound findings suggestive | Amniocentesis or CVS | Karyotype + RAS-MAPK gene panel on fetal DNA |
| Known familial mutation | Targeted mutation analysis | CVS at 11–12 weeks or amniocentesis at 15–16 weeks |
| Non-invasive prenatal testing (NIPT) | Cell-free fetal DNA | Can screen for aneuploidies but cannot detect single-gene mutations like NS; a normal NIPT does not exclude NS |
7. Cardiac Investigations
| Parameter | Details |
|---|---|
| Why | Cardiac defects are present in 50–80% of NS patients [2][5]; echo is essential for diagnosis and ongoing surveillance |
| Key findings | |
| — Pulmonary stenosis | Valvular PS with thick, dysplastic valve cusps [5] — this is characteristic of NS. Unlike typical congenital PS where the valve cusps are thin and fused at commissures (amenable to balloon valvuloplasty), NS-associated PS shows thickened, immobile, myxomatous cusps that respond poorly to balloon dilatation |
| — HCM | Asymmetric septal hypertrophy or concentric LV hypertrophy; LVOT obstruction may be present; especially in RAF1 and RIT1 mutations [1] |
| — ASD | Secundum ASD with left-to-right shunt on colour Doppler |
| — Peripheral PS | Branch pulmonary artery narrowing; may cause RV pressure overload |
| When | At diagnosis; repeat echo every 1–2 years for ongoing cardiac surveillance; more frequently if HCM present |
Clinical Pearl — Dysplastic vs Non-dysplastic PS
The pulmonary valve in Noonan syndrome is characteristically dysplastic — meaning the cusps are thickened, myxomatous, and relatively immobile. This is clinically important because balloon pulmonary valvuloplasty (the standard treatment for typical PS) is less effective for dysplastic valves. These patients may require surgical valvotomy or valve replacement instead. When a cardiologist sees a dysplastic pulmonary valve on echo in a child, NS should always be considered [5].
| Parameter | Details |
|---|---|
| Why | To assess for conduction abnormalities, rhythm disturbances, and hypertrophy patterns |
| Key findings in NS | |
| — Left axis deviation | Unusual for a right-sided lesion; but characteristic of NS (possibly related to conduction system anatomy) |
| — RV hypertrophy | Right axis deviation, tall R waves in V1, deep S in V5-V6 — from PS causing RV pressure overload |
| — LV hypertrophy | Tall R waves in V5-V6, deep S in V1 — if HCM present |
| — Giant negative T waves | May be seen with HCM |
| — Superior QRS axis | Can be seen in NS; distinguishes from typical PS |
| — Wide QRS | May indicate conduction delay |
Paediatric ECG interpretation note: Always use age-appropriate normal values. RV dominance is normal in neonates; what matters is whether the degree of RV hypertrophy is disproportionate to age.
| Finding | Interpretation |
|---|---|
| Cardiomegaly | RV enlargement from PS; LV enlargement from HCM |
| Prominent pulmonary artery segment | Post-stenotic dilatation of the main pulmonary artery (upstream to the stenotic valve) |
| Pulmonary oligaemia | Reduced pulmonary vascular markings if PS is severe |
| Pectus deformity | Visible on lateral CXR |
8. Growth and Endocrine Investigations
| Parameter | Details |
|---|---|
| What to measure | Height, weight, head circumference, sitting height, arm span |
| How to interpret | Plot on NS-specific growth charts (Ranke/Noonan charts) as well as standard population charts; NS children track along lower centiles on standard charts but may be appropriate on NS charts |
| Key findings | Short stature in ~70% [1]; mean adult height ~161 cm (males), ~150 cm (females) untreated; growth deceleration typically begins in infancy |
| Bone age | Left wrist and hand X-ray; typically delayed by 1–2 years in NS → implies remaining growth potential |
| Test | Interpretation |
|---|---|
| Serum IGF-1 and IGFBP-3 | May be low-normal or low; used to screen for GH deficiency/insensitivity |
| GH stimulation test (glucagon, insulin, clonidine, or arginine) | Variable results: some NS children have true GH deficiency (low peak GH < 7–10 µg/L), others have normal GH secretion but relative GH insensitivity |
| Why it matters | Recombinant human GH is recommended to optimise adult height [2]; GH stimulation testing helps characterise the mechanism of short stature but GH treatment is approved for NS regardless of GH stimulation test results (i.e. it is a licensed indication based on the diagnosis, not the GH level) |
| Parameter | Details |
|---|---|
| Why | Hypothyroidism has been reported in some NS patients and contributes to growth failure |
| Tests | TSH, free T4 |
| Expected | Usually normal; subclinical hypothyroidism may be found in some patients |
| Parameter | Details |
|---|---|
| Why | Delayed puberty is common, especially in males with bilateral cryptorchidism |
| Assessment | Tanner staging; testicular volume (orchidometer); serum LH, FSH, testosterone (males) / oestradiol (females) |
| Key findings | Males with bilateral cryptorchidism may have elevated gonadotropins (hypergonadotrophic hypogonadism if testes are damaged) or normal gonadotropins but delayed pubertal onset |
| Test | Key Findings in NS | Interpretation |
|---|---|---|
| Complete blood count (CBC) | Usually normal; monocytosis with hepatosplenomegaly → suspect JMML [2] | JMML is particularly associated with PTPN11 mutations; a transient neonatal myeloproliferative disorder can mimic JMML |
| Coagulation screen (PT, APTT) | Prolonged APTT may be seen (factor XI or XII deficiency) | Normal paediatric APTT: 25–35 seconds (varies by lab and age) |
| Factor XI level | Most common specific factor deficiency in NS; levels < 50% considered deficient | Factor XI deficiency causes variable bleeding tendency — not as severe as haemophilia A/B but can cause post-surgical bleeding |
| Factor VIII, XII, vWF antigen and activity | May be low; vWD-like pattern reported | |
| Platelet function tests (PFA-100 or aggregation studies) | Platelet dysfunction may be present with normal platelet count | Explains bleeding tendency despite normal platelet count |
| Peripheral blood film | Look for blasts or monocytosis; rule out JMML | In JMML: monocytosis > 1 × 10⁹/L, < 20% blasts in marrow |
Clinical Pearl — Pre-Operative Coagulation Screening
| Test | Key Findings | Interpretation |
|---|---|---|
| Renal ultrasound (USG) [2] | Renal pelviectasis (dilated renal pelvis), duplex kidney, renal ectopia, structural anomalies | Renal anomalies are reported in 10–15% of NS; USS is a non-invasive baseline screen |
| Urinalysis | Usually normal | Screen for proteinuria in case of renal structural abnormality |
| Renal function tests (RFTs) | Usually normal | Baseline for patients who may undergo contrast studies or nephrotoxic medication |
| Assessment | Details |
|---|---|
| Formal developmental assessment | Age-appropriate tools (e.g. Bayley Scales of Infant Development in infants; Griffiths in young children; WISC/WAIS for IQ in older children) |
| Expected findings | Mild intellectual disability in ~35% [1]; most have IQ 70–85; specific learning difficulties in language and mathematics; attention difficulties common |
| Speech and language assessment | Often delayed; articulation difficulties from oral motor dysfunction and hearing loss |
| Motor assessment | Gross and fine motor delay; hypotonia in infancy; average walking age ~21 months |
| Test | Key Findings |
|---|---|
| Full ophthalmological examination | Strabismus, amblyopia (from ptosis), hypermetropia, astigmatism [2]; anterior segment anomalies (prominent corneal nerves) rarely |
| Refraction | Refractive errors common; early correction prevents amblyopia |
| Fundoscopy | Usually normal |
| Test | Key Findings |
|---|---|
| Spinal X-ray | Kyphoscoliosis [1]; vertebral anomalies occasionally |
| Left wrist X-ray (bone age) | Delayed bone age; important for GH treatment planning |
| Chest X-ray | Pectus deformity; cardiac silhouette changes as above |
| System | Investigation | Primary Purpose |
|---|---|---|
| Genetic | Karyotype | Exclude Turner syndrome |
| RAS-MAPK gene panel | Confirm NS; identify specific gene | |
| CMA | Exclude CNVs (DiGeorge, Williams) if atypical | |
| Cardiac | ECG + Echocardiography [2] | Identify and monitor PS, HCM, ASD |
| CXR | Assess cardiac size, lung fields | |
| Growth | Auxology on NS-specific charts | Track growth trajectory |
| Bone age X-ray | Assess remaining growth potential | |
| GH stimulation test, IGF-1 | Evaluate GH axis before GH therapy | |
| Haematology | CBC, coagulation screen, factor XI | Detect coagulopathy before surgery |
| Peripheral blood film | Screen for JMML | |
| Renal | Renal USS [2] | Detect structural renal anomalies |
| Endocrine | TFTs | Exclude hypothyroidism |
| LH, FSH, testosterone/oestradiol | Assess pubertal status | |
| Development | Formal developmental assessment | Quantify developmental status |
| Ophthalmology | Full eye exam + refraction | Detect refractive errors, strabismus |
| Audiology | Audiometry / ABR | Detect hearing loss |
| Skeletal | Spinal X-ray | Detect scoliosis/kyphosis |
High Yield Summary — Diagnosis of Noonan Syndrome
- NS is a clinical diagnosis supported by molecular confirmation using the van der Burgt scoring system
- Facial gestalt is the gateway criterion — must have typical or suggestive facies
- First genetic test in a phenotypic female: karyotype → if 46,XX with Turner-like features, think NS
- Gold standard molecular test: RAS-MAPK gene panel; PTPN11 first (~50% of cases) [1][2]
- ~20% have no identifiable mutation — diagnosis remains clinical
- Cardiac: Echo + ECG mandatory at diagnosis [2]; characteristic finding is dysplastic pulmonary valve [5]
- Coagulation screen (including factor XI) before any surgery — unrecognised bleeding tendency is a preventable complication
- Prenatal indicators: increased NT, cystic hygroma, hydrops with normal karyotype → prompt NS mutation testing [2]
- Multi-system baseline assessment required: cardiac, renal, growth, development, hearing, vision, coagulation
Active Recall - Diagnosis of Noonan Syndrome
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 511 — Noonan Syndrome section) [2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p. 852–854 — Noonan Syndrome: Etiology, Clinical Manifestation, Diagnosis, Treatment) [5] Senior notes: Ryan Ho Cardiology.pdf (p. 185 — Syndromes associated with congenital heart diseases table) [7] Senior notes: Ryan Ho Rheumatology.pdf (p. 172 — NF1 diagnostic criteria and differential diagnosis including Noonan syndrome)
Management of Noonan Syndrome
Noonan syndrome is a multi-system genetic condition with no cure. Management is therefore:
- Anticipatory — proactive surveillance to detect complications early
- Organ-specific — treating each affected system according to best paediatric practice
- Multidisciplinary — coordinated care involving multiple specialties
- Longitudinal — lifelong follow-up with transition planning from paediatric to adult services
- Family-centred — supporting parents/caregivers, genetic counselling, psychosocial support
Management of complications of NS with ongoing health surveillance [2] — this is the foundational principle. There is no single treatment for NS; rather, each complication is managed individually while the whole child is monitored.
3. Cardiac Management
The cardiac system is the most important determinant of morbidity and mortality in NS. Cardiac defects are present in 50–80% of NS patients [5].
| Severity | Definition | Management | Rationale |
|---|---|---|---|
| Mild | Peak gradient < 40 mmHg on echo | Observation; echocardiography every 1–2 years | Low haemodynamic burden; most mild PS does not progress significantly |
| Moderate | Peak gradient 40–64 mmHg | Consider intervention if symptomatic or progressive | RV pressure overload developing; exercise tolerance may be affected |
| Severe | Peak gradient ≥ 64 mmHg | Intervention required | RV hypertrophy, risk of RV failure, syncope, arrhythmia |
Intervention options for PS:
| Procedure | Indication | Considerations in NS |
|---|---|---|
| Balloon pulmonary valvuloplasty | First-line for non-dysplastic valves (fused commissures) | NS-associated PS characteristically has dysplastic valve cusps [5] — thickened, myxomatous, immobile; balloon dilatation is less effective because the problem is thick leaflets, not fused commissures. Response rate in NS is lower (~50% vs > 90% in typical PS) |
| Surgical valvotomy | For dysplastic valves that fail or are unsuitable for balloon | Involves open excision/thinning of dysplastic valve tissue; may require cardiopulmonary bypass |
| Pulmonary valve replacement | Severely dysplastic/destroyed valve; significant pulmonary regurgitation post-valvotomy | Options: bioprosthetic valve (Contegra conduit, homograft), transcatheter pulmonary valve (e.g. Melody valve) in older children/adolescents |
High Yield — Dysplastic PS in NS
The characteristic cardiac defect of NS is valvular PS with thick, dysplastic valve cusps [5]. This is the single most important cardiac fact for exams. The dysplastic nature means balloon valvuloplasty is often suboptimal, distinguishing NS-associated PS from isolated congenital PS. Always assess valve morphology on echo before deciding on intervention.
HCM occurs in ~20–30% of NS patients [1][2], particularly those with RAF1 and RIT1 mutations. NS-associated HCM can present in infancy (often more severe) or later in childhood.
| Severity | Management | Rationale |
|---|---|---|
| Asymptomatic, mild hypertrophy | Observation; echo every 6–12 months; avoid competitive sports and isometric exercise | Risk of sudden cardiac death during exertion from dynamic LVOT obstruction or arrhythmia; serial monitoring for progression |
| Symptomatic or LVOT obstruction | Beta-blockers (e.g. propranolol 1–4 mg/kg/day in 3 divided doses; or atenolol 0.5–2 mg/kg/day) | Beta-blockers reduce heart rate → prolong diastolic filling time → reduce LVOT gradient; also reduce myocardial oxygen demand |
| Calcium channel blockers (e.g. verapamil — only in children > 1 year; contraindicated in infants due to risk of cardiovascular collapse) | Alternative if beta-blockers not tolerated; negative inotrope and chronotrope | |
| Refractory symptoms | Surgical septal myectomy (Morrow procedure) | Excision of obstructing septal muscle; reserved for severe symptomatic LVOT obstruction refractory to medical therapy |
| Severe, progressive, infantile HCM | Heart transplantation may be required | NS-associated infantile HCM can be rapidly progressive and fatal; transplant is the definitive option for end-stage disease |
Drugs to AVOID in HCM:
- Digoxin — positive inotrope worsens LVOT obstruction
- Vasodilators (ACE inhibitors, nitrates) — reduce afterload → worsen dynamic obstruction
- Diuretics — reduce preload → worsen obstruction (use only cautiously if genuine fluid overload)
- Inotropes (dobutamine, milrinone) — worsen obstruction
Avoid These Drugs in HCM
In HCM with LVOT obstruction, the obstruction is dynamic — it worsens with anything that reduces preload (dehydration, diuretics), reduces afterload (vasodilators), or increases contractility (inotropes, digoxin). The management principles are the opposite of dilated cardiomyopathy management. This is a common exam trap.
| Defect | Management |
|---|---|
| Small ASD/VSD | Observation; many close spontaneously (especially small muscular VSDs) |
| Haemodynamically significant ASD | Transcatheter device closure (e.g. Amplatzer) or surgical closure |
| Haemodynamically significant VSD | Surgical closure if Qp:Qs > 2:1 or signs of heart failure |
| Age | Frequency | Modalities |
|---|---|---|
| At diagnosis | Baseline | ECG + Echo |
| Infancy to 5 years | Every 6–12 months | Echo (more frequent if HCM) |
| 5–18 years | Every 1–2 years | Echo + ECG |
| If new symptoms | Immediate | Echo + ECG + Holter if arrhythmia suspected |
4. Growth and Endocrine Management
Recombinant human growth hormone (GH) is recommended to optimise adult height [2].
| Parameter | Details |
|---|---|
| Indication | Short stature in confirmed NS — height < -2 SD (or < 3rd percentile) on standard population growth charts, with growth velocity declining or height significantly below mid-parental height target [1][2] |
| Drug | Recombinant human GH (somatropin) — various formulations available (Norditropin, Genotropin, etc.) |
| Dose | 0.033–0.066 mg/kg/day subcutaneously (typical starting dose ~0.05 mg/kg/day); administered as a daily SC injection at bedtime (GH is normally secreted in pulses during deep sleep; bedtime dosing mimics physiology) |
| Route | Subcutaneous injection — teach parents injection technique; pen devices available for ease of use in children |
| Response | Modest compared to GH deficiency: average gain in final adult height ~3–5 cm over untreated NS. First-year response is best (~3–5 cm/year increase in growth velocity); response diminishes over time |
| Duration | Continue until near-final height achieved (growth velocity < 2 cm/year) or epiphyseal fusion |
| Monitoring | Height velocity every 3–6 months; IGF-1 levels (target age-appropriate range, avoid supra-physiological levels); bone age annually; scoliosis assessment; cardiac surveillance for HCM progression |
Contraindications to GH therapy:
| Contraindication | Rationale |
|---|---|
| Active malignancy | GH promotes cell proliferation via IGF-1; theoretical risk of tumour growth (important given JMML risk in PTPN11 patients) |
| Severe HCM with LVOT obstruction | Theoretical concern that GH/IGF-1 could worsen cardiac hypertrophy; however, current evidence is reassuring — GH therapy in NS does not appear to worsen HCM in most cases. Requires close cardiac monitoring |
| Closed epiphyses | No further linear growth possible |
| Active proliferative diabetic retinopathy | Not typical in NS children but a general GH contraindication |
| Prader-Willi syndrome with severe obesity | Not relevant to NS but important to know for general GH prescribing |
High Yield — GH in Noonan Syndrome
GH replacement is the main therapeutic approach for short stature in NS [1]. Key points: (1) It is an approved indication for rhGH regardless of GH stimulation test results; (2) The response is modest (~3–5 cm gain in final height); (3) Must monitor cardiac status (echo) throughout therapy; (4) Administer as daily SC bedtime injection.
| Scenario | Management |
|---|---|
| Euthyroid | No treatment; monitor TFTs annually |
| Subclinical hypothyroidism (elevated TSH, normal fT4) | Repeat in 3–6 months; treat with levothyroxine if TSH persistently > 10 mIU/L or if fT4 low-normal with growth failure |
| Overt hypothyroidism | Levothyroxine 2–4 µg/kg/day in young children (adjust by weight and TSH) |
| Scenario | Management |
|---|---|
| Delayed puberty (no signs by age 14 in boys, 13 in girls) | Endocrine assessment (LH, FSH, testosterone/oestradiol); consider short course of low-dose sex steroids to initiate puberty |
| Males with bilateral cryptorchidism (post-orchidopexy) | Monitor testicular volume and pubertal development; FSH/LH may be elevated (hypergonadotrophic hypogonadism from gonadal damage) |
5. Surgical Management
| Parameter | Details |
|---|---|
| Indication | Cryptorchidism — present in up to 60–80% of affected males [1] |
| Timing | By age 6–12 months (ideally before 12–18 months) — early orchidopexy reduces risk of infertility and testicular malignancy |
| Pre-operative essential | Full coagulation screen (PT, APTT, factor XI, platelet function) before any surgery — NS-associated bleeding tendency must be identified and managed perioperatively |
| Procedure | Standard inguinal orchidopexy; bilateral if both testes undescended |
| Post-operative | Monitor for wound haemorrhage (given coagulopathy risk); assess testicular growth on follow-up |
Already discussed above — surgical valvotomy for dysplastic PS, septal myectomy for severe HCM, and cardiac transplantation for end-stage disease. Key perioperative consideration: always check coagulation status pre-operatively.
| Procedure | Indication | Special Considerations |
|---|---|---|
| Ptosis repair | Significant ptosis causing amblyopia or functional visual impairment | Ophthalmological assessment first to determine if amblyopia is present; correct before critical period ends (age ~7 years) |
| Strabismus surgery | Significant strabismus not correctable with glasses | Requires post-operative occlusion therapy compliance |
| Scoliosis surgery | Progressive kyphoscoliosis (Cobb angle > 40°) | Bracing first; surgery for severe or progressive curves |
| Dental/orthodontic | Malocclusion from micrognathia | Orthodontic referral in late childhood; coag screen before dental extractions |
| Lymphatic procedures | Severe chylothorax, chylous ascites | Thoracic duct ligation or sclerotherapy; lymphovenous anastomosis in specialised centres |
6. Haematological Management
| Scenario | Management |
|---|---|
| Identified factor XI deficiency | Fresh frozen plasma (FFP) infusion pre-operatively (raises factor XI levels); factor XI concentrate where available (not widely available); tranexamic acid (antifibrinolytic) as adjunct |
| vWD-like defect | DDAVP (desmopressin) 0.3 µg/kg IV or 300 µg intranasal — stimulates endothelial release of vWF and factor VIII. Perform a DDAVP trial before planned surgery to confirm response |
| Platelet dysfunction | Platelet transfusion if severe bleeding; avoid aspirin and NSAIDs (further impair platelet function) |
| Perioperative general measures | Avoid IM injections; ensure IV access early; crossmatch blood; have FFP/cryoprecipitate/platelets available |
Pre-Operative Checklist for NS
Before any surgical procedure in a child with NS:
- ✅ Full coagulation screen (PT, APTT, TT, fibrinogen, factor XI, vWF panel, platelet function)
- ✅ DDAVP trial if vWD-like defect identified
- ✅ Echocardiography to assess cardiac status and anaesthetic risk
- ✅ Anaesthetic review (difficult airway assessment — micrognathia, short neck, cervical spine issues)
- ✅ Crossmatch blood and have blood products available
| Scenario | Management |
|---|---|
| Transient neonatal myeloproliferative disorder (common in PTPN11-NS) | Often resolves spontaneously within months; close haematological monitoring with serial CBC and clinical examination |
| True JMML (persistent monocytosis > 1 × 10⁹/L, hepatosplenomegaly, HbF elevation, blasts in blood/marrow) | Haemato-oncology referral; treatment is haematopoietic stem cell transplantation (HSCT) — the only curative option for JMML |
| Monitoring | CBC every 3–6 months in first 2 years of life (especially PTPN11 patients); palpate for hepatosplenomegaly at each visit |
| Domain | Intervention | Rationale |
|---|---|---|
| Speech and language therapy | Early referral (from first year of life if delay suspected); targeted articulation, language comprehension, and expressive language therapy | Speech delay is multifactorial: hearing loss + oral motor dysfunction + neurocognitive factors; early intervention improves outcomes |
| Occupational therapy | Fine motor skills, self-care skills, sensory integration | Hypotonia and motor delay affect fine motor coordination |
| Physiotherapy | Gross motor development; postural support; scoliosis exercises | Hypotonia, delayed walking (average ~21 months); kyphoscoliosis management |
| Special educational support | Individualised educational plan (IEP); classroom accommodations (extra time, preferential seating, visual aids) | Mild ID in ~35% [1]; specific learning difficulties in language and mathematics are common even in those with normal IQ |
| Psychological support | Cognitive behavioural therapy for anxiety; social skills training; family counselling | Children with NS may experience bullying, low self-esteem, and social difficulties related to short stature, facial differences, and learning challenges |
| Condition | Treatment |
|---|---|
| Refractive errors | Corrective spectacles; early correction to prevent amblyopia |
| Amblyopia (from ptosis or strabismus) | Patching of the stronger eye + corrective lenses; surgical ptosis repair if severe |
| Strabismus | Spectacles/prisms first; surgical correction if persistent |
| Condition | Treatment |
|---|---|
| Conductive hearing loss (middle ear effusions) | Grommets (ventilation tubes) if persistent otitis media with effusion > 3 months and affecting hearing/development |
| Sensorineural hearing loss | Hearing aids; cochlear implant referral if severe bilateral SNHL |
| Monitoring | Annual audiometry throughout childhood |
| Severity | Management |
|---|---|
| Mild peripheral lymphoedema | Compression garments (graduated compression stockings/sleeves); skin care (moisturise, avoid cuts/infections); manual lymphatic drainage physiotherapy |
| Moderate lymphoedema | Complex decongestive therapy (CDT): multi-layered bandaging + manual drainage + exercise + skin care |
| Chylothorax | Conservative: medium-chain triglyceride (MCT) diet (MCTs are absorbed directly into portal circulation without requiring lymphatic transport, reducing chyle flow); nil by mouth + TPN if severe; thoracentesis for symptomatic relief |
| Refractory chylothorax | Octreotide (reduces splanchnic blood flow and lymphatic flow); thoracic duct ligation; pleurodesis |
| Chylous ascites / intestinal lymphangiectasia | MCT-based formula/diet; high-protein diet to compensate for protein loss; albumin infusions if hypoalbuminaemic |
| Age | Consideration |
|---|---|
| Neonates/infants | Feeding difficulties common (hypotonia, poor suck-swallow, high-arched palate); may need NG tube feeding; high-calorie formula; OT/SLT for oral motor support |
| Children | High-caloric diet due to increased metabolic demand [9] from cardiac disease; dietitian involvement; monitor weight gain alongside linear growth |
| Adolescents | Encourage balanced diet; monitor for obesity if mobility is limited |
| Topic | Details |
|---|---|
| Inheritance | AD; 50% recurrence risk if a parent is affected [1][2]; ~2/3 of cases are de novo |
| Cascade testing | Offer molecular testing (for the identified familial variant) to all first-degree relatives; subtle phenotype in parents may be missed — examine parents carefully |
| Prenatal options | For families with known mutation: preimplantation genetic testing (PGT-M), chorionic villus sampling (CVS), or amniocentesis for targeted mutation analysis |
| Psychosocial support | Connect families with NS support groups (e.g. Noonan Syndrome Association); peer support is invaluable |
| Transition planning | Begin transition from paediatric to adult services at age 14–16; coordinate with adult cardiologist, endocrinologist, and genetic counsellor |
| System | Assessment | Frequency | Who |
|---|---|---|---|
| Growth | Height, weight, HC on NS charts | Every 3–6 months (more frequent on GH) | Paediatrician / Endocrinologist |
| Cardiac | Echo + ECG | Every 6–12 months (more frequent if HCM) | Paediatric cardiologist |
| Development | Formal assessment + school report | Annually | Developmental paediatrician |
| Haematology | CBC + coag screen | At diagnosis; pre-operatively; annually if PTPN11 (JMML risk) | Paediatric haematologist |
| Ophthalmology | Full eye exam + refraction | Annually until stable | Paediatric ophthalmologist |
| Audiology | Pure tone audiometry | Annually | Audiologist |
| Renal | USS | At diagnosis; repeat if abnormal | Paediatric nephrologist |
| Endocrine | TFTs, bone age, pubertal assessment | Annually from age 8–10 | Paediatric endocrinologist |
| Musculoskeletal | Spine exam, scoliosis screening | Annually | Paediatrician / Orthopaedics |
| Dental | Orthodontic assessment | From age 6–7 | Paediatric dentist |
| Therapy | Status | Mechanism | Notes |
|---|---|---|---|
| MEK inhibitors (e.g. trametinib, selumetinib) | Clinical trials and compassionate use (2024–2026) | Directly inhibit MEK1/2, the kinase downstream of RAF in the RAS-MAPK cascade; reduces excessive pathway activation | Showing promise in severe NS-associated HCM (especially infantile onset); case reports of significant regression of cardiac hypertrophy; also used in other RASopathies and NF1 |
| Mavacamten | Early evaluation for NS-HCM | Cardiac myosin inhibitor; reduces sarcomere contractility and LVOT obstruction | Approved for adult obstructive HCM; paediatric trials ongoing |
| RAS pathway-targeted agents | Preclinical/early clinical | Various targets along the pathway | Personalised medicine approach based on specific genotype |
Emerging Therapy — MEK Inhibitors in NS-HCM
MEK inhibitors (e.g. trametinib) represent a paradigm shift from treating the downstream complications of NS to treating the upstream cause — excessive RAS-MAPK signalling itself. Early results in severe infantile HCM are encouraging, with documented regression of ventricular hypertrophy. However, these are not yet standard of care and are used primarily in life-threatening cases under specialist guidance. Watch this space for exam updates.
| Drug | Indication in NS | Dose (Paediatric) | Key Points |
|---|---|---|---|
| Somatropin (rhGH) | Short stature | 0.033–0.066 mg/kg/day SC at bedtime | Recommended to optimise adult height [2]; modest response; monitor cardiac/IGF-1 |
| Propranolol | HCM with LVOT obstruction | 1–4 mg/kg/day PO in 3 divided doses | First-line for symptomatic HCM; avoid if asthma |
| Atenolol | HCM | 0.5–2 mg/kg/day PO once daily | Alternative beta-blocker; better compliance (OD dosing) |
| Levothyroxine | Hypothyroidism | 2–4 µg/kg/day PO (young children) | Standard thyroid replacement |
| DDAVP (desmopressin) | vWD-like bleeding defect | 0.3 µg/kg IV or 300 µg intranasal | Pre-operative; trial dose first to confirm response |
| Tranexamic acid | Adjunct for bleeding | 15–25 mg/kg PO TDS (max 1.5 g/dose) | Antifibrinolytic; useful for dental procedures, menorrhagia |
| FFP | Factor XI deficiency (perioperative) | 10–15 mL/kg IV | Raises factor XI; short half-life so may need repeat dosing |
| Trametinib (MEK inhibitor) | Severe/refractory HCM (experimental) | Weight-based dosing under specialist protocol | Emerging therapy; not standard care |
High Yield Summary — Management of Noonan Syndrome
- Management is complication-directed with ongoing health surveillance [2]
- Cardiac: PS → observe if mild; balloon valvuloplasty often ineffective for dysplastic valves [5] → surgical valvotomy may be needed. HCM → beta-blockers; avoid digoxin, vasodilators, inotropes
- Recombinant human GH is recommended to optimise adult height [2] — modest gain (~3–5 cm); daily SC bedtime injection; monitor cardiac
- Coagulopathy: full coagulation screen before any surgery; factor XI deficiency most common; DDAVP for vWD-like defect; tranexamic acid as adjunct
- Cryptorchidism: orchidopexy by age 6–12 months; coag screen pre-op
- Development: early intervention (speech, OT, physiotherapy); IEP for school
- Multidisciplinary team: cardiologist, endocrinologist, geneticist, developmental paediatrician, haematologist, ophthalmologist, audiologist, surgeon, psychologist
- Genetic counselling: AD inheritance, 50% risk if parent affected, cascade screening, prenatal options
- Emerging: MEK inhibitors (trametinib) for severe HCM — promising but experimental
- Surveillance: lifelong, system-based, with transition to adult services at 14–16 years
Active Recall - Management of Noonan Syndrome
References
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 511 — Noonan Syndrome section) [2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p. 852–854 — Noonan Syndrome: Etiology, Clinical Manifestation, Diagnosis, Treatment) [5] Senior notes: Ryan Ho Cardiology.pdf (p. 185 — Syndromes associated with congenital heart diseases table) [9] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 200 — Cardiac failure management: high caloric diet due to increased metabolic demand)
Complications of Noonan Syndrome
Noonan syndrome is a multi-system disorder caused by constitutive activation of the RAS-MAPK signalling pathway. Because this pathway governs cell proliferation, differentiation, survival, and migration across virtually every organ system during embryogenesis and postnatal life, complications are diverse and can affect nearly every organ. Understanding the complications from first principles means asking: "Where does the RAS-MAPK pathway play a critical role, and what happens when it is dysregulated there?"
Complications can be divided into:
- Complications intrinsic to the syndrome — direct consequences of the genetic mutation (e.g. congenital heart disease, short stature)
- Complications of the complications — secondary problems arising from the primary features (e.g. infertility from cryptorchidism, amblyopia from ptosis)
- Treatment-related complications — adverse effects of interventions (e.g. GH therapy effects, post-surgical bleeding)
2. Cardiac Complications
The cardiac system is the leading determinant of morbidity and mortality in NS. Cardiac defects are present in 50–80% of patients [5].
| Aspect | Details |
|---|---|
| Mechanism | Valvular PS with thick, dysplastic valve cusps [5] may be progressive — the dysplastic, myxomatous tissue can further thicken and stiffen over time, gradually increasing the transvalvular gradient |
| Consequence | Chronic RV pressure overload → RV hypertrophy → eventual RV failure (hepatomegaly, peripheral oedema, raised JVP — though JVP assessment is difficult in young children) |
| Why it matters | RV failure in childhood is a serious complication; exercise intolerance, fatigue, and in severe cases syncope and arrhythmias. Unlike typical congenital PS (which often remains stable), dysplastic PS in NS may progress |
| Monitoring | Serial echocardiography every 6–12 months; intervene if gradient increases to ≥ 40–64 mmHg or if symptoms develop |
HCM occurs in ~20–30% of NS patients [1][2], and is the most serious cardiac complication because of its potential for:
| Complication of HCM | Mechanism | Clinical Impact |
|---|---|---|
| Sudden cardiac death (SCD) | Disorganised myocyte architecture → re-entrant ventricular arrhythmias (VT/VF) during exertion or stress | The most feared complication; risk factors include severe septal hypertrophy ( > 30 mm or Z-score > 6), syncope, family history of SCD, non-sustained VT on Holter |
| Heart failure | Progressive diastolic dysfunction (stiff, hypertrophied ventricle cannot relax properly → impaired filling → reduced cardiac output); or systolic dysfunction in end-stage "burnt-out" HCM | Dyspnoea, exercise intolerance, failure to thrive in infants |
| LVOT obstruction | Asymmetric septal hypertrophy + systolic anterior motion (SAM) of mitral valve → dynamic obstruction of LVOT | Exertional syncope, presyncope, chest pain; worsens with dehydration, exercise, Valsalva |
| Atrial fibrillation/flutter | Left atrial dilatation secondary to diastolic dysfunction → atrial arrhythmia | Palpitations, haemodynamic deterioration; thromboembolic risk (stroke) |
| Infective endocarditis | Turbulent flow across thickened structures → endothelial damage → nidus for bacterial colonisation | Fever, new murmur, embolic phenomena; requires antibiotic prophylaxis for high-risk dental procedures in certain scenarios |
High Yield — Infantile HCM in NS
NS-associated HCM that presents in infancy (particularly with RAF1 or RIT1 mutations) tends to be more severe and more rapidly progressive than childhood-onset HCM. These infants may develop refractory heart failure requiring transplantation or MEK inhibitor therapy. In contrast, HCM presenting after infancy tends to be milder with better prognosis. Genotype matters for prognostication.
| Arrhythmia | Mechanism |
|---|---|
| Supraventricular tachycardia (SVT) | Altered conduction system development from RAS-MAPK dysregulation; accessory pathways |
| Ventricular tachycardia (VT) | Myocyte disarray in HCM creates re-entrant circuits |
| Conduction defects | Left axis deviation, bundle branch blocks — from conduction tissue developmental anomalies |
Children with NS-associated valvular disease (especially dysplastic PS, prosthetic valves post-surgery, or residual VSD) are at risk of infective endocarditis. Current guidelines (AHA/ESC) recommend antibiotic prophylaxis for high-risk cardiac conditions (prosthetic valves, previous endocarditis, unrepaired cyanotic CHD) undergoing dental procedures involving gingival manipulation or perforation of oral mucosa. Not all NS cardiac lesions meet high-risk criteria — clinical judgement required.
3. Growth and Endocrine Complications
| Aspect | Details |
|---|---|
| Mechanism | Short stature in ~70% [1]; mean untreated adult height ~161 cm (males), ~150 cm (females); caused by dysregulated growth plate signalling + possible GH-IGF1 axis dysfunction |
| Psychosocial impact | Short stature in childhood and adolescence → bullying, low self-esteem, social isolation, reduced quality of life; particularly impactful during adolescence when peer comparison is intense |
| Complication of GH therapy | GH itself can theoretically: (1) worsen HCM (increased IGF-1 → cardiomyocyte hypertrophy, though current evidence is reassuring); (2) promote tumour growth (JMML concern in PTPN11 patients); (3) cause injection-site reactions, fluid retention, benign intracranial hypertension (rare). Requires careful monitoring |
| Aspect | Details |
|---|---|
| Mechanism | Multifactorial: bilateral cryptorchidism → gonadal damage → hypergonadotrophic hypogonadism in males; possible hypothalamic-pituitary dysregulation; chronic illness effect |
| Consequence | Delayed secondary sexual characteristics; reduced bone mineral density (from sex steroid deficiency); psychosocial distress; potential infertility |
- Reported in a subset of NS patients
- Mechanism not fully understood — may relate to RAS-MAPK effects on thyroid follicular cell function or autoimmune thyroiditis
- If untreated → contributes to growth failure, fatigue, constipation, cold intolerance, cognitive slowing
4. Haematological Complications
Blood/lymph vessel malformation and abnormal bleeding/bruising [1] is a significant complication that must be proactively identified:
| Bleeding Complication | Mechanism | Clinical Scenario |
|---|---|---|
| Post-surgical haemorrhage | Factor XI deficiency (most common specific deficiency), platelet dysfunction, vWD-like defects | Unexpected bleeding after orchidopexy, cardiac surgery, dental extraction — preventable if coagulopathy identified pre-operatively |
| Easy bruising | Platelet dysfunction, factor deficiencies | May be the presenting complaint; parents report "he bruises from nothing" |
| Menorrhagia (adolescent females) | Same coagulopathy | Heavy menstrual periods; may require tranexamic acid or hormonal management |
| Intracranial haemorrhage (rare) | Severe coagulopathy + trauma | Extremely rare but devastating; emphasises need for coagulation workup |
Preventable Complication
Post-surgical bleeding in NS is a PREVENTABLE complication. The key is to always perform a full coagulation screen (including factor XI and platelet function tests) before ANY surgical procedure — even minor ones like dental extraction or grommet insertion. Failure to identify and manage the coagulopathy pre-operatively is a medical error.
JMML [2] is a rare but life-threatening haematological complication particularly associated with PTPN11 mutations:
| Aspect | Details |
|---|---|
| What is JMML? | A clonal myeloproliferative / myelodysplastic disorder of early childhood characterised by excessive proliferation of monocytic and granulocytic lineage cells; it is a RAS-pathway-driven leukaemia |
| Why NS patients are at risk | PTPN11 gain-of-function mutations cause constitutive SHP-2 activation → excessive RAS-MAPK signalling in haematopoietic stem cells → uncontrolled monocyte/granulocyte proliferation. Somatic PTPN11 mutations are found in ~35% of sporadic JMML (i.e. JMML even in non-NS children); germline PTPN11 mutations in NS children further predispose |
| Clinical features | Hepatosplenomegaly, failure to thrive, pallor, petechiae/bruising, lymphadenopathy, skin rash, recurrent infections |
| Laboratory features | Monocytosis > 1 × 10⁹/L; WBC usually 10–100 × 10⁹/L; thrombocytopenia; elevated HbF for age; hypersensitivity of myeloid progenitors to GM-CSF in vitro; < 20% blasts in bone marrow (distinguishes from AML) |
| Prognosis | Without treatment: median survival ~1 year. HSCT is the only curative treatment for true JMML |
Important distinction — Transient myeloproliferative disorder vs true JMML:
| Feature | Transient MPD of NS | True JMML |
|---|---|---|
| Age | Neonates/young infants | Usually < 4 years but can be later |
| Course | Self-resolving within weeks to months without treatment | Progressive, does not resolve |
| Splenomegaly | Mild, transient | Progressive, marked |
| Blasts | Minimal | May increase over time |
| Management | Observation with close monitoring | HSCT |
Approximately 10% of NS infants with PTPN11 mutations develop a transient MPD; only a small proportion progress to true JMML. Serial monitoring (CBC every 1–3 months in the first 2 years of life) is essential to distinguish these.
Beyond JMML, there is a modestly increased lifetime risk of certain cancers due to constitutive RAS-MAPK activation (RAS pathway mutations are common in many adult cancers). Reported associations include:
- Rhabdomyosarcoma (more common in Costello syndrome but reported in NS)
- Neuroblastoma (rare)
- Acute lymphoblastic leukaemia (ALL) (rare case reports)
- Giant cell granular tumour of the jaw — a non-malignant but locally destructive lesion [2]
The overall malignancy risk in NS is lower than in Costello syndrome but higher than the general population.
| Complication | Mechanism | Clinical Impact |
|---|---|---|
| Peripheral lymphoedema | Lymphatic hypoplasia/dysplasia from abnormal lymphangiogenesis (RAS-MAPK required for VEGF-C/VEGFR-3 signalling in lymphatic endothelial cells) | Chronic limb swelling; risk of cellulitis (stagnant lymph is a good culture medium for bacteria, especially Streptococcus); skin thickening over time |
| Chylothorax | Malformed thoracic lymphatic vessels → chyle leak into pleural space | Respiratory distress; protein/immunoglobulin/lymphocyte loss in chyle → nutritional depletion and secondary immunodeficiency |
| Chylous ascites | Abdominal lymphatic malformation → chyle leak into peritoneum | Abdominal distension; hypoalbuminaemia; failure to thrive |
| Intestinal lymphangiectasia | Dilated intestinal lymphatics → protein-losing enteropathy | Chronic diarrhoea, hypoalbuminaemia, oedema, immunoglobulin loss → increased infection risk |
| Prenatal hydrops fetalis | Severe generalised lymphatic dysplasia in utero → diffuse fluid accumulation | Can be fatal in utero or neonatally; presents as generalised oedema, pleural effusions, ascites, skin oedema on prenatal USS |
Lymphatic Complications — Why They Are Under-Recognised
Lymphatic complications are frequently under-recognised in NS because they may present subtly (mild peripheral oedema, recurrent cellulitis of the legs, chronic loose stools) and are often attributed to other causes. A high index of suspicion is needed. Any NS child with unexplained oedema, recurrent skin infections, chronic diarrhoea with hypoalbuminaemia, or pleural effusions should be evaluated for lymphatic complications.
| Complication | Frequency | Mechanism |
|---|---|---|
| Renal structural anomalies | 10–15% | Abnormal nephrogenesis from RAS-MAPK dysregulation; includes renal ectopia (pelvic kidney), duplex kidney, hydronephrosis, horseshoe kidney |
| Renal pelviectasis | Common | Dilated renal pelvis; may predispose to urinary tract infections (UTI) |
| Vesicoureteric reflux (VUR) | Occasional | Abnormal ureteric bud development; risk of reflux nephropathy if UTIs occur |
| Complication | Mechanism | Prevalence/Details |
|---|---|---|
| Intellectual disability | ~35%, usually mild (IQ 70–85) [1]; dysregulated RAS-MAPK in neurons → abnormal synaptic plasticity, long-term potentiation, and neuronal proliferation | Most can attend mainstream school with support |
| Specific learning difficulties | Difficulties in language, reading, mathematics; attention problems (ADHD-like) | Even those with normal IQ may struggle academically |
| Motor delay | Hypotonia → delayed sitting, walking (average walking age ~21 months); fine motor difficulties persisting into school age | Affects handwriting, self-care activities |
| Speech and language delay | Multifactorial: hearing loss + oral motor dysfunction + neurocognitive factors | Articulation difficulties, limited vocabulary in early childhood |
| Behavioural problems | Stubbornness, social immaturity, attention difficulties, anxiety | May be misdiagnosed as ADHD or ASD |
| Psychosocial difficulties | Short stature, facial differences, learning difficulties → bullying, low self-esteem, social isolation, depression (especially in adolescence) | Requires proactive psychological support; peer support groups invaluable |
| Arnold-Chiari malformation (rare) | Abnormal posterior fossa development → cerebellar tonsillar herniation | Headaches, syringomyelia, motor difficulties; MRI diagnosis |
| Complication | Mechanism | Management |
|---|---|---|
| Kyphoscoliosis [1] | Abnormal vertebral body/disc development from RAS-MAPK effects on chondrocytes/osteoblasts | Bracing for curves 25–40°; surgical correction (spinal fusion) for curves > 40° or progressive |
| Pectus deformity | Abnormal costal cartilage growth | Observation if mild/asymptomatic; Nuss procedure or Ravitch procedure for severe pectus excavatum causing cardiopulmonary compromise |
| Joint hypermobility | Connective tissue laxity | Physiotherapy; joint protection strategies |
| Osteoporosis (adolescent/adult) | Delayed puberty → sex steroid deficiency → reduced bone mineral density; possible direct skeletal effect of RAS-MAPK dysregulation | DEXA scan in adolescence if delayed puberty; calcium/vitamin D supplementation; sex steroid replacement if hypogonadal |
| Talipes equinovarus (clubfoot) [2] | Abnormal musculoskeletal development | Ponseti method (serial casting + Achilles tenotomy) if present |
| Complication | Mechanism | Consequence |
|---|---|---|
| Amblyopia | Ptosis → occlusion of visual axis → failure of visual cortex development for that eye | Permanent visual loss in the affected eye if not treated before the critical period (~age 7); ptosis repair + patching |
| Strabismus | Extraocular muscle developmental anomaly | Diplopia; abnormal binocular vision; cosmetic concern |
| Refractive errors | Hypermetropia, astigmatism common [2] | Blurred vision; correctable with spectacles |
| Anterior segment abnormalities | Prominent corneal nerves; rare anterior segment dysgenesis | Usually asymptomatic |
| Complication | Mechanism | Impact |
|---|---|---|
| Conductive hearing loss | Eustachian tube dysfunction → chronic otitis media with effusion → middle ear fluid dampens sound transmission | If untreated → speech/language delay, academic difficulties; manage with grommets |
| Sensorineural hearing loss | Inner ear (cochlear) developmental anomaly from RAS-MAPK dysregulation | Usually mild-moderate; hearing aids; cochlear implant if severe bilateral |
| Complication | Mechanism | Details |
|---|---|---|
| Cryptorchidism [1][5] | Failed testicular descent; up to 60–80% of affected males | If not corrected early → progressive germ cell loss → infertility |
| Male infertility | Even after successful orchidopexy, some men with NS have impaired spermatogenesis (primary gonadal dysfunction + post-orchidopexy testicular damage) | Semen analysis in adulthood; consider sperm banking in adolescence if bilateral cryptorchidism |
| Hypergonadotrophic hypogonadism | Damaged/underdeveloped testes → low testosterone, elevated LH/FSH | Testosterone replacement may be needed in adolescence/adulthood |
| Female fertility | Generally preserved; females with NS usually have normal fertility | Important reassurance for families |
| Pregnancy in women with NS | AD inheritance → 50% chance of transmitting to offspring; cardiac status must be assessed before pregnancy (HCM may worsen) | Requires pre-pregnancy counselling by clinical geneticist and cardiologist |
| Complication | Mechanism |
|---|---|
| Malocclusion | Micrognathia/retrognathia → crowded teeth, abnormal bite |
| High-arched palate | Midface hypoplasia → feeding difficulties in infancy, orthodontic issues later |
| Delayed dental eruption | General developmental delay may extend to dental development |
| Dental caries risk | Poor oral hygiene related to motor difficulties; malocclusion making cleaning difficult |
| Bleeding risk with dental procedures | Coagulopathy → always check coagulation status before extractions |
Children with NS pose specific anaesthetic challenges:
| Risk | Mechanism | Mitigation |
|---|---|---|
| Difficult airway | Micrognathia, short neck, limited cervical extension, cervical spine instability (rare) | Pre-operative airway assessment; video laryngoscopy availability; involve experienced paediatric anaesthetist |
| Cardiac risk | PS (RV outflow obstruction), HCM (LVOT obstruction — worsened by hypovolaemia, anaesthetic-induced vasodilation) | Avoid drugs that reduce preload/afterload; maintain euvolaemia; invasive BP monitoring for major procedures |
| Bleeding risk | Unrecognised coagulopathy | Pre-operative coagulation screen; have blood products available |
| Malignant hyperthermia | Rare case reports of association; debated | Use non-triggering agents (avoid succinylcholine and volatile agents if concerned); have dantrolene available |
| Lymphatic complications | Post-operative chylothorax (especially after thoracic/cardiac surgery) | Awareness; low-fat (MCT-based) diet post-operatively; monitor for pleural effusion |
Pre-Anaesthetic Checklist for NS
Every anaesthesia team should be aware of NS-specific risks: (1) Difficult airway — micrognathia, short neck; (2) Cardiac — type and severity of lesion (PS? HCM?); (3) Bleeding — coagulation screen results; (4) Malignant hyperthermia — rare but reported. These risks should be communicated clearly in the pre-operative briefing.
| System | Key Complications | Underlying Mechanism | Surveillance |
|---|---|---|---|
| Cardiac | Progressive PS; HCM → SCD, HF, arrhythmia; endocarditis | Dysplastic valve progression; myocyte hypertrophy; re-entrant circuits | Echo + ECG every 6–12 months |
| Haematology | Bleeding; JMML [2]; other malignancies | Factor XI def; platelet dysfunction; RAS-driven myeloproliferation | Coag screen; CBC 1–3 monthly in infancy if PTPN11 |
| Growth | Short stature; psychosocial impact | GH-IGF1 axis dysfunction; growth plate dysregulation | Auxology every 3–6 months |
| Endocrine | Delayed puberty; hypothyroidism | Gonadal damage from cryptorchidism; thyroid dysfunction | Pubertal staging; TFTs annually |
| Lymphatic | Lymphoedema; chylothorax; protein-losing enteropathy | Lymphatic hypoplasia/dysplasia | Clinical examination; albumin if suspected |
| Neurological | ID; learning difficulties; motor/speech delay; behavioural problems | Synaptic plasticity disruption | Developmental assessment annually |
| Renal | Structural anomalies; VUR; UTIs | Abnormal nephrogenesis | Renal USS at baseline |
| Ophthalmology | Amblyopia; strabismus; refractive errors | Ptosis; EOM anomaly; globe development | Annual eye exam |
| Audiology | Conductive/SNHL | Eustachian tube dysfunction; cochlear anomaly | Annual audiometry |
| Orthopaedic | Scoliosis; pectus; osteoporosis | Chondrocyte/osteoblast dysregulation; hypogonadism | Annual spine exam; DEXA in adolescents |
| Fertility | Male infertility; hypergonadotrophic hypogonadism | Cryptorchidism → germ cell loss | Orchidopexy by 6–12 months; semen analysis in adulthood |
| Dental | Malocclusion; bleeding post-extraction | Micrognathia; coagulopathy | Orthodontic referral; coag screen pre-extraction |
| Anaesthetic | Difficult airway; cardiac instability; bleeding | Micrognathia; HCM/PS; coagulopathy | Pre-op assessment by paediatric anaesthetist |
| Factor | Detail |
|---|---|
| Overall prognosis | Good if no severe cardiac abnormalities [6]; most individuals with NS have a normal or near-normal life expectancy |
| Main determinants of mortality | Severity of cardiac disease (especially infantile HCM, severe PS with RV failure); JMML |
| Life expectancy | Most patients reach adulthood; those with mild cardiac lesions or no cardiac involvement have near-normal lifespan |
| Quality of life | Significantly influenced by neurodevelopmental status, psychosocial support, height, and cardiac limitations; proactive multidisciplinary care improves outcomes |
| Adult transition | Ongoing cardiac surveillance, endocrine management, genetic counselling for reproductive decisions; adult congenital heart disease services |
High Yield Summary — Complications of Noonan Syndrome
- Cardiac is the most important system — PS (dysplastic valve) and HCM (20–30%) [1][2][5] are the main lesions; HCM can cause sudden cardiac death, especially in infancy with RAF1/RIT1 mutations
- Bleeding tendency — factor XI deficiency, platelet dysfunction, vWD-like defects [1]; always screen before surgery — post-surgical haemorrhage is preventable
- JMML [2] — rare but life-threatening; particularly with PTPN11 mutations; distinguish from transient neonatal MPD (which self-resolves)
- Lymphatic — lymphoedema, chylothorax, protein-losing enteropathy; under-recognised
- Short stature [1] — psychosocial impact significant; GH therapy provides modest benefit
- Cryptorchidism [1][5] — if untreated → male infertility; orchidopexy by 6–12 months
- Mild ID in ~35% [1] — early intervention (speech, OT, educational support) is key
- Amblyopia from ptosis — treat before critical period (age ~7 years)
- Anaesthetic risks — difficult airway + cardiac instability + bleeding tendency; requires specialist assessment
- Prognosis: good if no severe cardiac abnormalities [6]; near-normal life expectancy in mild cases
Active Recall - Complications of Noonan Syndrome
References
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 511 — Noonan Syndrome section) [2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p. 852–854 — Noonan Syndrome: Clinical Manifestation, JMML, giant cell tumour) [5] Senior notes: Ryan Ho Cardiology.pdf (p. 185 — Syndromes associated with congenital heart diseases table — Noonan syndrome cardiac defects) [6] Senior notes: Maksim Paediatric Notes.pdf (p. 207 — Noonan syndrome prognosis: "Good if no severe cardiac abnormalities")
High Yield Summary
Noonan Syndrome — Key Points for Exams:
- Autosomal dominant; ~50% caused by PTPN11 mutations (encodes SHP-2); ~2/3 de novo [1][2]
- RAS-MAPK pathway disorder — gain-of-function mutations → dysregulated cell growth and differentiation [1][3]
- Clinical triad: short stature + characteristic facies + congenital heart disease (right-sided, especially pulmonary stenosis ~50%, also HCM 20–30%) [1][2]
- Phenotypically similar to Turner syndrome but normal karyotype, AD, affects both sexes [2]
- Contrast with Turner: NS = right-sided cardiac (PS); Turner = left-sided cardiac (coarctation, BAV) [2]
- Key craniofacial features: hypertelorism, downslanting palpebral fissures, ptosis, low-set posteriorly rotated ears, short webbed neck, low posterior hairline [1]
- Other features: cryptorchidism, cubitus valgus, pectus excavatum, widely spaced nipples, bleeding tendency, lymphoedema, mild ID (~35%) [1]
- RASopathies = umbrella term for NS, Costello syndrome, CFC syndrome, LEOPARD syndrome [1][3]
- Management: GH replacement for short stature; cardiac surveillance; multidisciplinary care [1]
- Genotype-phenotype: RAF1/RIT1 → higher risk of HCM; PTPN11 → JMML risk; SOS1 → often normal height
High Yield Summary — Diagnosis of Noonan Syndrome
- NS is a clinical diagnosis supported by molecular confirmation using the van der Burgt scoring system
- Facial gestalt is the gateway criterion — must have typical or suggestive facies
- First genetic test in a phenotypic female: karyotype → if 46,XX with Turner-like features, think NS
- Gold standard molecular test: RAS-MAPK gene panel; PTPN11 first (~50% of cases) [1][2]
- ~20% have no identifiable mutation — diagnosis remains clinical
- Cardiac: Echo + ECG mandatory at diagnosis [2]; characteristic finding is dysplastic pulmonary valve [5]
- Coagulation screen (including factor XI) before any surgery — unrecognised bleeding tendency is a preventable complication
- Prenatal indicators: increased NT, cystic hygroma, hydrops with normal karyotype → prompt NS mutation testing [2]
- Multi-system baseline assessment required: cardiac, renal, growth, development, hearing, vision, coagulation
High Yield Summary — Management of Noonan Syndrome
- Management is complication-directed with ongoing health surveillance [2]
- Cardiac: PS → observe if mild; balloon valvuloplasty often ineffective for dysplastic valves [5] → surgical valvotomy may be needed. HCM → beta-blockers; avoid digoxin, vasodilators, inotropes
- Recombinant human GH is recommended to optimise adult height [2] — modest gain (~3–5 cm); daily SC bedtime injection; monitor cardiac
- Coagulopathy: full coagulation screen before any surgery; factor XI deficiency most common; DDAVP for vWD-like defect; tranexamic acid as adjunct
- Cryptorchidism: orchidopexy by age 6–12 months; coag screen pre-op
- Development: early intervention (speech, OT, physiotherapy); IEP for school
- Multidisciplinary team: cardiologist, endocrinologist, geneticist, developmental paediatrician, haematologist, ophthalmologist, audiologist, surgeon, psychologist
- Genetic counselling: AD inheritance, 50% risk if parent affected, cascade screening, prenatal options
- Emerging: MEK inhibitors (trametinib) for severe HCM — promising but experimental
- Surveillance: lifelong, system-based, with transition to adult services at 14–16 years
High Yield Summary — Complications of Noonan Syndrome
- Cardiac is the most important system — PS (dysplastic valve) and HCM (20–30%) [1][2][5] are the main lesions; HCM can cause sudden cardiac death, especially in infancy with RAF1/RIT1 mutations
- Bleeding tendency — factor XI deficiency, platelet dysfunction, vWD-like defects [1]; always screen before surgery — post-surgical haemorrhage is preventable
- JMML [2] — rare but life-threatening; particularly with PTPN11 mutations; distinguish from transient neonatal MPD (which self-resolves)
- Lymphatic — lymphoedema, chylothorax, protein-losing enteropathy; under-recognised
- Short stature [1] — psychosocial impact significant; GH therapy provides modest benefit
- Cryptorchidism [1][5] — if untreated → male infertility; orchidopexy by 6–12 months
- Mild ID in ~35% [1] — early intervention (speech, OT, educational support) is key
- Amblyopia from ptosis — treat before critical period (age ~7 years)
- Anaesthetic risks — difficult airway + cardiac instability + bleeding tendency; requires specialist assessment
- Prognosis: good if no severe cardiac abnormalities [6]; near-normal life expectancy in mild cases
Achondroplasia
Achondroplasia is the most common form of skeletal dysplasia in children, caused by a gain-of-function mutation in the FGFR3 gene that impairs endochondral ossification, resulting in rhizomelic short-limbed dwarfism typically evident at birth.
Marfan Syndrome
Marfan syndrome is an autosomal dominant connective tissue disorder caused by mutations in the fibrillin-1 gene, presenting in childhood and adolescence with tall stature, long limbs, arachnodactyly, lens subluxation, and potentially life-threatening aortic root dilation.