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.

Exam domain	One-glance essentials
Definition / diagnosis	RASopathy with characteristic facies, short stature and congenital heart disease ^[1] Usually autosomal dominant with variable expression ^[1]
Epidemiology / risks	Many cases are de novo; an affected parent has 50% transmission risk ^[1] Paternal-age effect occurs with some RAS/MAPK variants ^[1]
Core mechanism	Gain-of-function RAS/MAPK signalling alters development and growth ^[1] Genotype influences cardiac and haematological risks ^[1]
Clinical picture	Hypertelorism, webbed neck, pectus deformity and short stature ^[1] Pulmonary-valve stenosis, hypertrophic cardiomyopathy and cryptorchidism ^[1]
Investigations	Clinical assessment plus a RASopathy gene panel ^[2] Echocardiography, ECG, hearing, vision, coagulation and development review ^[2]
Management	Treat cardiac lesions and support growth/development ^[3] Consider growth hormone only after specialist risk assessment ^[3]
Complications	Heart failure, arrhythmia and bleeding tendency ^[4] Feeding difficulty, hearing loss and myeloproliferative disease can occur ^[4]
Prevention / follow-up	Genetic counselling and scheduled cardiac surveillance ^[3] Check coagulation before surgery and monitor growth carefully ^[3]
Exam traps	*Noonan occurs in all sexes and usually has a normal karyotype* ^[2] Do not assume bleeding screens are normal before procedures ^[3]

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 511 — Noonan Syndrome section)

^[2] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 511 — Noonan Syndrome section)

^[3] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 511 — Noonan Syndrome section)

^[4] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 511 — Noonan Syndrome section)

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

Any child with Noonan syndrome should have a full coagulation workup (PT, APTT, factor XI, platelet function tests) BEFORE any surgical procedure — including orchidopexy, cardiac surgery, or even dental procedures. Unrecognised coagulopathy can lead to unexpected bleeding ^[1]^[2].

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
Pure tone audiometry (age-appropriate)	Sensorineural or conductive hearing loss ^[1]^[2]
Otoacoustic emissions / ABR (neonates/infants)	Screen for congenital hearing loss
Tympanometry	Middle ear effusions common (conductive component); Eustachian tube dysfunction

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

Noonan Syndrome

1. Definition

2. Epidemiology

3. Anatomy and Function (Relevant Normal Developmental Biology)

3.1 The RAS-MAPK Signalling Pathway — Why It Matters

3.2 Normal Paediatric Growth and Development Context

4. Aetiology

4.1 Genetic Basis

4.2 Inheritance Pattern

4.3 Why These Mutations Cause Disease — Pathophysiological Link

5. Pathophysiology — Mechanism by Organ System

5.1 Cardiac

5.2 Growth/Short Stature

5.3 Craniofacial

5.4 Lymphatic

5.5 Haematological

5.6 Genitourinary

5.7 Neurodevelopmental

6. Classification

6.1 Within Noonan Syndrome — By Genotype

6.2 RASopathies — The Broader Family

7. Clinical Features

7.1 Symptoms (What the Family Reports)

7.2 Signs (What You Find on Examination)

A. Craniofacial Dysmorphology

B. Cardiac Signs

C. Musculoskeletal Signs

D. Lymphatic Signs

E. Haematological/Coagulation Signs

F. Genitourinary Signs

G. Neurological/Developmental Signs

H. Other Signs

8. Clinical Approach to the Child with Suspected Noonan Syndrome

8.1 Recognition — When to Suspect NS

8.2 History Taking — Key Points

8.3 Physical Examination — Systematic Approach

9. Growth Hormone Replacement in Noonan Syndrome

10. Important Age-Specific Considerations in Paediatrics

11. Family-Centred Care and Communication

12. Normal Paediatric Values — Quick Reference for NS Assessment

Active Recall - Noonan Syndrome

References

1. Differential by Presenting Feature Cluster

1A. "Turner-like" Phenotype (Short Stature + Webbed Neck + Shield Chest + Cardiac Defect)

1B. Other RASopathies (Overlapping Pathway, Overlapping Phenotype)

1C. Syndromes with Congenital Heart Disease + Dysmorphism (The "Syndromic CHD" DDx)

1D. Short Stature DDx (When Growth Failure Is the Presenting Complaint)

1E. Cryptorchidism DDx (in Males)

1F. Bleeding Tendency DDx

1G. Haematological Malignancy (JMML) DDx

2. Consolidated Comparison Table — The "Big Five" Syndromic CHD

3. Diagnostic Flow — Mermaid Diagram

4. How to Approach the DDx Systematically in an Exam or Clinical Setting

Active Recall - Differential Diagnosis of Noonan Syndrome

Diagnostic Criteria for Noonan Syndrome

1. Overview — Why Formal Diagnostic Criteria Exist

2. Van der Burgt Clinical Scoring System

3. Molecular Diagnostic Criteria

4. Prenatal Diagnostic Indicators

Diagnostic Algorithm

Investigation Modalities, Key Findings, and Interpretations

6. Genetic Investigations

6A. Karyotype

6B. Molecular Genetic Testing — RAS-MAPK Gene Panel

6C. Chromosomal Microarray (CMA)

6D. Prenatal Genetic Testing

7. Cardiac Investigations

7A. Transthoracic Echocardiography (TTE)

7B. Electrocardiogram (ECG)

7C. Chest X-Ray (CXR)

8. Growth and Endocrine Investigations

8A. Auxology (Growth Assessment)

8B. GH-IGF-1 Axis Assessment

8C. Thyroid Function Tests (TFTs)

8D. Pubertal Assessment

9. Haematological and Coagulation Investigations

10. Renal Investigations

11. Developmental and Neurocognitive Assessment

12. Ophthalmological Assessment

13. Audiological Assessment