Features of the Noonan phenotype are webbed neck with low posterior
hairline, pterygium colli, flattened midface, cystic hygroma, pectus
carinatum or excavatum, short stature (50%), high-arched palate, ptosis,
hypertelorism, down-slanting palpebral fissures, dental malocclusion,
micrognathia, strabismus, cryptorchidism, mental retardation (25%),
kyphoscoliosis, and several skeletal abnormalities (spina bifida occulta,
hemivertebra, narrow spinal canal, cubitus valgus). Approximately 50% of
patients have a cardiac problem. The most common cardiac anomalies are
pulmonary stenosis, hypertrophic cardiomyopathy, atrial septal defect,
tetralogy of Fallot, atrioventricular canal anomaly, and coarctation of the
aorta. Stature is short. Lymphedema may occur. Coagulopathy is frequent and
most commonly caused by factor XI deficiency, although thrombocytopenia and
deficiencies of factors XII and VIII are well described.