trisomy 21

description

trisomy 21 (down syndrome) is the most common chromosome abnormality and the most common cause of mental retardation in newborns. about 65-80% of the trisomy 21 conceptions result in a miscarriage. however, 40% of the pregnancies with trisomy 21 detected at 12 weeks and 30% of those detected at 16 weeks gestation will be lost. the incidence is 1:600-1:800 livebirths and the male/female ration is 1. there are no racial differences and there is a strong association with maternal age. thus, the birth prevalence is only 0.9/1000 when the mother is less than 33 and 38/1000 when the mother is older than 44. most cases are the result of non-disjunction in the first meiotic division of the egg . the phenotype is the result of duplication of region 21q22 and 95% of the cases are the result of an extra chromosome 21, 3% are the result of translocation mainly robertsonian. of the trisomy 21 cases with robertsonian translocation, about 50% of the d/21 translocation and 10% of the g/21 translocation are inherited. in 2% of the cases, mosaicism is detected and most of these cases are the result of postzygotic loss of one of the chromosome # 21 in a trisomy 21 zygote. the most common clinical findings in newborns with down syndrome are hypotonia, poor neonatal reflexes, hyperextensible joints, redundant nuchal skin, upslanting palpebral fissures, short ears, dysplastic pelvis, fifth finger clinodactyly and single palmar creases. other common findings are short stature, mental retardation, brachycephaly, flat occiput, epicanthic folds, brushfield spots, prominent tongue, short broad hands, and ”sandal gap” toes.