Noonan syndrome

Alternative names

Turner-like syndrome of males

Definition

Noonan syndrome is a genetic disorder that causes abnormal development of multiple parts of the body. In particular there is webbing of the neck and different shapes to the chest that are reminiscent of Turner syndrome, hence the former name Turner-like syndrome.

Causes, incidence, and risk factors

Noonan syndrome can be inherited in an autosomal dominant manner. It affects at least 1 in 2,500 children. The fact that some children do not have a parent with Noonan syndrome likely reflects sporadic inheritance, that is, presumably the occurrence of a new mutation. The first specific gene that causes Noonan syndrome, called PTPN11, was discovered in 2001. It is expected that other genes will be discovered that cause Noonan syndrome.

Frequently seen abnormalities include webbing of the neck, changes in the sternum (usually a sunken chest called pectus excavatum), facial abnormalities, and congenital heart disease (especially pulmonic stenosis). Because these abnormalities resemble those of Turner syndrome (which only affects females), Noonan syndrome used to be called "male Turner syndrome," this term is no longer used because Noonan syndrome can affect females also.

Facial abnormalities may include low-set or abnormally shaped ears, sagging eyelids (ptosis), wide-set eyes (hypertelorism), epicanthal folds, and a small jaw (micrognathia). Mild mental retardation is present in about 25% of cases. Hearing loss varies. Puberty is usually delayed, and males may have undescended testicles and a small penis. Adult height is usually decreased.

Symptoms

• webbed and short appearing neck
  
• sternum abnormalities (pectus excavatum, occasionally pectus carinatum)
  
• sagging eyelids (ptosis)
  
• wide-set (hypertelorism) or down-slanting eyes
  
• low-set or abnormally shaped ears
  
• undescended testicles
  
• delayed puberty
  
• mental retardation in only a quarter of patients
  
• short stature
  
• small penis
  

Signs and tests

Examination may show an extra fold of skin above the eyes (epicanthal folds). The eyes may also appear down-slanted (antimongoloid palpebral slant). The arms may be held at an unusual angle (cubitus valgus). There may be signs of congenital heart disease (often pulmonic stenosis, occasionally ASD). There may be a bleeding tendency revealed by low platelet count or coagulation tests and measuring the levels of specific coagulation factors in the blood (factors XI-XIII).