الفهرس 
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Abstract
Down syndrome is a chromosomal abnormality characterized by the extra copy of genetic material on chromosome 21, occurring in whole or in part. With a prevalence of 1:800 live births Down syndrome is the commonest chromosomal anomaly in humans and the most frequent cause of severe learning disabilities. Maternal age after 35 years is a major contributing factor to the occurrence of trisomy 21. The phenotype of Down syndrome is characterized by more than 80 features. The most common phenotypic manifestations are developmental disabilities, muscular hypotonia, short stature, specific facial features and congenital malformations, particularly cardiac.
An increased prevalence of thyroid disease has been documented in DS, particularly subclinical hypothyroidism, which may be associated with loss of heart function leading to exacerbation of their clinical condition and potentially affect their life expectancy.