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Noonan syndrome is a condition usually involving a heart problem found at birth, short stature, a broad or webbed neck, pectus excavatum and pectus carinatum (chest deformities), as well as a range of developmental delays. Occasionally, café-au-lait spots (a skin finding) and other features of neurofibromatosis may be present.
First described by the pediatrician and heart specialist Jacqueline Noonan in 1963, Noonan syndrome includes numerous specific features. However, no two affected individuals typically have the exact same combination of these characteristics. As of 2001, there still is no defined list of criteria to diagnose the condition, and no molecular genetic testing exists to confirm a diagnosis. Therefore, attributing an individual's features to Noonan syndrome is based upon a careful review of medical and family history, a detailed physical examination, and study of other possible diagnoses.
There are three major groups of Noonan syndrome. The classical type is Noonan syndrome, Type 1 (NS1). This is also known as Noonan syndrome, Male Turner syndrome, Female pseudo-Turner syndrome, Turner phenotype with normal karyotype, and Pterygium colli syndrome.
Individuals with NS1 may often have a heart defect, pulmonic stenosis, found at birth. A chest wall abnormality is common, typically with pectus carinatum at the upper portion (near the neck) and pectus excavatum below it, creating a "shield-like" appearance. Developmental delays are sometimes a part of the condition.
Facial features such as a tall forehead, wide-set eyes, low-set ears, and a short neck are common. Young children with NS1 often have very obvious facial features, and may have a "dull" facial expression, similar to conditions caused by muscle weakness. However, facial features may change over time, and adults with Noonan syndrome often have more subtle facial characteristics. This makes the face a less obvious clue of the condition in older individuals. Other associated features in NS1 are smaller genitalia in males, as well as cryptorchidism. Some individuals with the condition develop thrombocytopenia, or a low number of blood platelets, as well as other problems with normal blood coagulation (clotting).
Another type of the condition is Noonan syndrome, Type 2 (NS2). This involves the same characteristic features as Type 1, but the inheritance pattern is proposed as recessive, rather than the more commonly seen dominant pattern.
The final type of the syndrome is neurofibromatosis-Noonan syndrome, also known as Noonan-neurofibromatosis syndrome, and neurofibromatosis with Noonan Phenotype. In this, individuals often have some features of both neurofibromatosis and NS1. It has been proposed that this may simply be a chance occurrence of two conditions. This is because these conditions have two distinct gene locations, with no apparent overlap.
Author Info: Deepti Babu MS, The Gale Group Inc., Gale, Detroit, Gale Encyclopedia of Genetic Disorders Part I, 2002
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