Down syndrome features: Children with Down syndrome have multiple malformations and cognitive impairment due to the presence of extra material from chromosome 21.
Among the more common physical features are hypotonia (floppiness), small head with brachycephaly, epicanthic folds across the inside corners of the eyes, upward outward slanting palpebral fissures (eye slits), Brushfield spots in the iris, small mouth, small ears, excessive skin at the nape of the neck, a single transverse palmar crease, and short fifth finger with clinodactyly (incurving). A wide space, often with a deep fissure between the first and second toes, is also common.
There is an increased risk of congenital heart defects (50%); leukemia (<1%); hearing loss (75%); otitis media (ear infections) (50%-70%); Hirschsprung disease with absence of nerves from the bowel (<1%); gastrointestinal atresia (12%); eye disease (60%), including cataracts (15%) and severe refractive errors (50%); acquired hip dislocation (6%); obstructive sleep apnea (50%-75%); and thyroid disease (15%). There is no increased risk of solid tumors. There is mental impairment. The degree is variable, ranging from mild (IQ: 50-70) to moderate (IQ: 35-50), and only occasionally to severe (IQ: 20-35). The social quotient may be improved with early intervention techniques, although the level of function is exceedingly variable. Children with Down syndrome often function better in social situations than might be expected from their IQ. An excellent set of guidelines for the health care of children with Down syndrome is available from the American Academy of Pediatrics.
Downregulation: An decrease in the number of receptors on the surface of target cells, making the cells less sensitive to a hormone or another agent. For example, insulin receptors may be downregulated in type 2 diabetes.
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