A developmental disability due to abnormal chromosome number or structure. It is characterized by physical and behavioral features and has been considered the most common form of genetic aberration. Incidence among the newborn is estimated at 3 in 1000, in the general population approximately 1 in 1000. The difference reflects the early mortality.
The most common type (trisomy 21) is due to a nondisjunction of chromosome 21 during the original cell division, resulting in an extra chromosome 21. These children have a total of 47 chromosomes instead of the usual 46. However, the extra material from chromosome 21 can also be attached to another chromosome through translocation; such children have Down syndrome but only 46 chromosomes. More rarely, the trisomy 21 breaks up, giving some cells with 47 chromosomes and some with 46 (mosaicism).
The characteristic physical features include almond-shaped eyes; a rounded, brachycephalic skull with flattened occipital region; a broad, flattened bridge of the nose; an enlarged fissured tongue; broad hands with stubby fingers; often a single “simian” palmar crease; hypotonic muscle development; thick, everted, and cracked lips; dry, rough skin; subnormal height; and infantile genitalia. Not all of these physical signs are present in every case, and some may be observed in individuals without Down syndrome. However, Down syndrome is diagnosed when most of the anomalies are present.
The degree of mental defect is not directly related to the number or gravity of the physical signs, but rather to a combination of those anomalies and the specific chromosomal defect. Few children with Down syndrome are classified today as severely retarded. Most are moderately to mildly retarded and are often educable and highly trainable. They tend to be curious, observant, skillful at mimicry, and usually, very affectionate. Aggression and hostility are rare; however, they are often stubborn and compulsive and are not easily frustrated. They are excellent candidates for vocational training.
Pathological research suggests nonspecific, generalized defective brain development. There is a tendency toward thyroid dysfunction and congenital heart defects. There may also be vision problems, but below-average dental caries. Medication has little effect on the physical condition or on the mental retardation. See Alzheimer's disease, Congenital anomalies
Although there are some reports of more than one child with Down syndrome in a single family, it is not a classical hereditary disease. Incidence is increased if the mother is under 16 or over 35 years old or the father is of advanced age. Furthermore, the Down syndrome child may result from a late or problem pregnancy or the last of numerous pregnancies. Thyroid deficiency, hypopituitarism, and pathology of the ovary have been observed in the mothers, and the probability of upset in their endocrine balance may increase with age. However, the basic etiology is still very much in doubt.
Prenatal identification of Down syndrome in the fetus is possible through amniocentesis. See Human genetics, Mental retardation