AKT · Child health/Genetic syndromes

Noonan syndrome

Autosomal dominant RASopathy (e.g. PTPN11) — normal karyotype

Overview

An autosomal-dominant condition (RAS-MAPK pathway, often PTPN11), sometimes called "male Turner" for the overlapping phenotype — but it affects BOTH sexes, has a NORMAL karyotype, and the classic cardiac lesion is pulmonary stenosis (not coarctation).

Recognise

Short stature, webbed neck, widely-spaced nipples, low-set ears, ptosis, down-slanting eyes
Pulmonary valve STENOSIS (± hypertrophic cardiomyopathy)
Bleeding/clotting abnormalities; pectus deformity; variable learning difficulty
Affects boys and girls; cryptorchidism in boys

Red flags

Significant cardiac disease (pulmonary stenosis/HCM); bleeding tendency before surgery

Differentials & how to tell them apart

Turner syndrome45,X — GIRLS only, abnormal karyotype, COARCTATION/bicuspid valve, streak ovaries

Williams syndromesupravalvular aortic stenosis, sociable personality, hypercalcaemia

Fetal alcohol syndromematernal alcohol history, different facies

Investigations

Clinical + molecular genetics (PTPN11 etc.); echocardiogram; karyotype is NORMAL (distinguishing it from Turner).

Management

Genetic confirmation + echocardiogram; MDT surveillance

1Confirm genetically; echocardiogram for pulmonary stenosis/HCM; assess bleeding; growth and developmental support.Gate: A normal karyotype with a Turner-like phenotype in a BOY (or pulmonary stenosis rather than coarctation) → think Noonan, not Turner
2Cardiology, growth (± growth hormone), genetic counselling (autosomal dominant).

No specific drug — manage cardiac lesion, growth and bleeding; MDT

Key points

The discriminator from Turner: Noonan affects both sexes, the karyotype is normal, and the lesion is pulmonary stenosis (Turner = coarctation/bicuspid valve, girls, 45,X).

Monitor & prognosis

Cardiac, growth, development; family screening.

Depends on cardiac disease; many do well.

Source: Clinical genetics; StatPearls