Down syndrome, also called Down’s syndrome (DS), is the most common chromosomal condition associated with intellectual disability and is characterized by a variety of additional clinical findings. It occurs in approximately one in 8,000 births worldwide. In the U.S., DS accounts for about 500 live births annually, and more than 200,000 persons are living with the disorder. The original description of the syndrome, in 1866, has been attributed to John Langton Down, a physician from Cornwall, England. It wasn’t until more than 90 years later that the chromosomal cause was found, and the condition was called Down syndrome. There are wide differences in intellectual disability and social function in those with DS, and often the degree of social dysfunction is linked to the degree of intellectual disability.
A third copy of chromosome 21 (normally chromosomes occur in pairs), called trisomy 21, has long been recognized as the cause of DS. Chromosome 21 has 200 to 300 genes on it, and factors affecting these genes account for the clinical features of DS. The actual cause of the genetic mutation remains largely unknown. Until recently, to diagnose DS in the fetus required invasive and potentially dangerous procedures like amniocentesis (using a syringe to puncture the abdominal wall and withdraw amniotic fluid). Fetal testing would be recommended based upon the appearance of the fetus on sonogram. Today, non-invasive screening and testing of the mother’s blood for a type of DNA from the fetus can detect DS, and this test is 99.7% accurate for the diagnosis of fetal DS. Remember, a mother and her unborn baby share a common blood circulation.
The physical appearance of a person, or even a fetus, with DS is singular. There are upslanted palpebral fissures (the crease above the eyelid), flattened bridge of the nose, nuchal folds (mounds and creases in the skin behind the neck), a single flexion crease on the palm, clinodactyly of the fifth finger (the finger curves toward the fourth finger), and hypotonia in an infant (abnormally low muscle tone). DS is associated with multiple medical conditions. Among these conditions are congenital heart disease, and improvements in managing this have led to an increase in life expectancy, from 30 years in 1973 to 60 years in 2002. Autoimmune conditions are common and include Hashimoto’s disease (thyroid disorder), type 1 diabetes, alopecia (hair loss), celiac disease, juvenile arthritis, and vitiligo (whitening of the skin in darker-skinned individuals). In addition, impairment of the immune system can occur and is why many people with DS die from infections such as pneumonia.
Persons with DS and their families generally have a positive attitude and express a desire for a high quality of life that builds on the strengths and skills of the affected child or adult. Early intervention with tutoring, therapy, and strong family interaction has led to many with DS leading near-normal lives and becoming contributing members of human society.
By Peter Galvin, MD
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