Noonan syndrome


Noonan syndrome was initially described by Professor Jackie Noonan who realised that many children with pulmonary stenosis also had similar physical features. Noonan syndrome was incorrectly referred to as ‘male Turner syndrome’ as it also involves short stature and a webbed neck, but the term is wrong as Noonan syndrome affects both males and females and is an inherited disorder with normal sex chromosomes. We now recognise it occurs due to a defect in one of the genes of the RAS-MAPK pathway and as such overlaps with conditions such as LEOPARD syndrome, cardiofaciocutaneous syndrome (CFC), Costello syndrome and neurofibromatosis.


Medical text written October 2010 by Professor Michael Patton, Professsor of Medical Genetics, St George’s Hospital Medical School, London, UK.

What are the symptoms?

Characteristics of Noonan syndrome include:

  • heart defects – the most common being pulmonary valve stenosis, atrial septal defects (see entry Heart Defects) and hypertrophic cardiomyopathy (see entry Cardiomyopathies)
  • characteristic facial features, including ptosis (drooping eyelids), hypertelorism (widely spaced eyes), large downward-slanting eyes, flat nasal bridge and short neck with neck webbing and anterior rotation (slanting forwards) of the ears
  • short stature.

Additional features may include:

  • excess oedema (fluid retention in the body) at birth and slow weight gain
  • feeding difficulties with poor sucking and weaning and frequent and/or forceful vomiting in babies which may be assessed as failure to thrive
  • mild hearing loss
  • dental delay
  • elevated or depressed sternum
  • hypotonia
  • undescended testes (meaning that the testicles stay within the abdomen)
  • mild developmental delay in a small number of children
  • speech and behaviour problems.

In addition, the characteristics of the syndrome, especially facial, appear to change as the individual ages.

Overlapping syndromes
LEOPARD syndrome, CFC syndrome (see entry cardiofaciocutaneous syndrome), Costello syndrome and neurofibromatosis are all due to defects in the same pathway and have different clinical features. However LEOPARD syndrome is the most similar and due to changes in the same gene PTPN11.

The term LEOPARD syndrome stands for:

  • Lentignes (freckles)
  • Electrocardiogram (minor abnormalities in the electrical activity of the heart)
  • Ocular hypertelorism (widely spaced eyes)
  • Pulmonary stenosis
  • Abnormalities of the male genitalia
  • Retardation of growth
  • Deafness.

Inheritance patterns and prenatal diagnosis

Inheritance patterns
Autosomal dominant. A gene PTPN11 on chromosome 12 has been found to account for half of all cases of Noonan syndrome. Other genes that may cause Noonan syndrome include SOS1, KRAS and SHOC2. Molecular testing is now available.

Prenatal diagnosis
Prenatal diagnosis is available by molecular testing if the mutation has been identified. Occasionally it may be diagnosed by the presence of cystic hygroma or fetal hydrops in the womb (where fluid accumulates in certain parts of the unborn baby).

Is there support?

Noonan Syndrome Association


Noonan Syndrome Association (NSA) is a Registered Charity in England and Wales No. 1140671. The NSA supports patients and families affected by Noonan Syndrome through research, nurse services, national and regional events, publications and raising awareness of the condition.

Group details last updated August 2021.

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