Down syndrome is the most common chromosome abnormality of a generalized syndrome. It occurs in 1 in every 800 to 1000 live births. It is slightly more prevalent in whites than in blacks, but its incidence does not vary with socioeconomic class.
The exact cause of Down syndrome is unknown, but it is believed to occur from multiple causes.
- Approximately 95% of all cases of Down syndrome are linked to an extra chromosome 21 (group G), thus the name non-familial trisomy 21. One of the risk factors in these cases appears to be advanced maternal age (usually greater than 35).
- About 3% to 4% of the cases are linked to translocation of chromosomes 15 and 21 or 22. This type is usually hereditary and is not associated with advanced parental age.
- From 1% to 2% of affected persons demonstrate mosaicism, which refers to cells with both normal and abnormal chromosomes.
People with Down syndrome are at risk for various problems, including but not limited to:
- Congenital heart defects, and particularly septal defects
- Respiratory tract infections - These are very prevalent, and when combined with cardiac defects are the leading cause of death particularly in the first year of life.
- Hypotonicity - of the chest and abdominal muscles - a decrease in muscle tone in these areas
- Dysfunction of the immune system - which predisposes to infections
- Thyroid dysfunction - particularly congenital hypothyroidism
- Leukemia - there is an increased incidence of leukemia with Down syndrome
There is no cure for Down syndrome, but therapies can be used to manage the patient, including surgeries to correct serious anomalies and regular medical care.
Down syndrome is named after John Langdon Down, the British doctor who described the syndrome in 1866. The disorder was identified as a chromosome 21 trisomy by Jérôme Lejeune, a French Catholic pro-life paediatrician and geneticist in 1959.