Noonan Syndrome

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Noonan's Syndrome is phenotypically very similar to Turner's Syndrome, though it has no chromosomal abnormality and occurs in both sexes.

  • Described in 1963 by Noonan and Ehmke.


Contents

Incidence and Aetiology

Males and females are affected equally. Aetiology is not fully understood, but can either be sporadic or autosomal dominant. Importantly, karyotyping is normal, distinguishing the syndrome from Turner's.

  • Incidence thought be as much as 1:1000 live births (worldwide / all ethnic groups).

Four genes have been identified at present: PTPN11, SOS1, RAF1, and KRAS. All are part of a pathway which regulates cell growth (RAS/RAF/MEK/ERK).


Diagnosis

Diagnosis remains clinical. Absence of known genetic mutations does not exclude the diagnosis. Antenatal history is usually normal; polyhydramnios, oedema or cystic hygroma are associated.

  • Facies become more obvious in early childhood, then less so again once older.

Some adults may be diagnosed when the syndrome is recognised in their offspring.

  • Karyotyping may be necessary
  • Mutation analysis


Phenotype

  • Short stature
    • Normal birth weight. Slightly reduced birth length. Failure to thrive in first year ~40%
  • Unusual facies
    • Triangular facies (broad forehead, micrognathia)
    • hypertelorism
    • Down-slanting eyes
    • Low posterior hairline with excessive nuchal skin
    • Low set ears
    • Prominent grooved philtrum
    • Small upturned nose
    • high arched palate
  • Webbed neck
  • Oedema (often of hands and feet)
  • Pectus carinatum / excavatum
  • Congenital heart disease ~50%
    • Pulmonary valve stenosis
    • VSD (less common)
  • Coagulopathy (mild) ~ 50%
  • Intellectual impairment ~ 25%
  • Blunt square finger tips
  • Cryptorchidism
  • Eyes: strabismus, amblyopia, refractive errors.
  • Other abnormalities may be present.


Differential Diagnosis

  • Turner syndrome
  • Costello syndrome
  • Craniofaciocutaneous syndrome
  • Fetal hydantoin syndrome
  • Lentigines, electrocardiographic (conduction abnormalities), ocular (hypertelorism), pulmonary (stenosis), abnormal (genitalia), retardation (of growth), and deafness (LEOPARD) syndrome
  • XO/XY mosaicism


Investigations

  • Coagulation studies (particularly prior to any surgical proceedure)
  • Detailed cardiac assessment
  • Developmental assessment
  • Audiology (high frequency sensorineural loss ~50%)


Prognosis

  • Females: normal puberty and fertility
  • Males: cryptorchidism may reduce fertility.


References

  • Emedicine article
  • Noonan JA. Hypertelorism with Turner phenotype. A new syndrome with associated congenital heart disease. Am J Dis Child. Oct 1968;116(4):373-80.
  • Noonan JA. Noonan syndrome. An update and review for the primary pediatrician. Clin Pediatr (Phila). Sep 1994;33(9):548-55.
  • Noonan Syndrome Support Group
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