Approximately three-quarters of these unbalanced translocations are de novo, and the remainder result from familial translocations. In approximately 3% to 4% of persons with the Down syndrome phenotype, the extra chromosomal material is the result of an unbalanced translocation between chromosome 21 and another acrocentric chromosome, usually chromosome 14. In approximately 95% of children with Down syndrome, the condition is sporadic because of nonfamilial trisomy 21, in which there are 47 chromosomes with a free extra chromosome 21 being present. Children with Down syndrome often function more effectively in social situations than would be predicted on the basis of cognitive assessment results. The social quotient may be improved with early-intervention techniques, although the level of function is exceedingly variable. There is a significant risk of hearing loss (75%) obstructive sleep apnea (50%–79%) otitis media (50%–70%) eye disease (60%), including cataracts (15%) and severe refractive errors (50%) congenital heart defects (50%) neurologic dysfunction (1%–13%) gastrointestinal atresias (12%) hip dislocation (6%) thyroid disease (4%–18%) 3, –, 6 and, less commonly, transient myeloproliferative disorder (4%–10%) and later leukemia (1%) and Hirschsprung disease (<1%) ( Table 1).
The degree of cognitive impairment is variable and may be mild (IQ of 50–70), moderate (IQ of 35–50), or occasionally severe (IQ of 20–35). Among the more common physical findings are hypotonia, small brachycephalic head, epicanthal folds, flat nasal bridge, upward-slanting palpebral fissures, Brushfield spots, small mouth, small ears, excessive skin at the nape of the neck, single transverse palmar crease, and short fifth finger with clinodactyly and wide spacing, often with a deep plantar groove between the first and second toes. 1, 2 Although the phenotype is variable, there typically are multiple features that enable the experienced clinician to suspect the diagnosis. Children with Down syndrome have multiple malformations, medical conditions, and cognitive impairment because of the presence of extra genetic material from chromosome 21.
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