DISORDERS INVOLVING AN EXTRA AUTOSOME
Down syndrome (trisomy 21).
The most common karyotype is 47, XX, +21. Down syndrome is the most com-mon of the chromosomal disorders. The risk increases with maternal age to anincidence of 1 in 25 live births in women age ≥45. The pathogenesis involves meiotic nondisjunction (95%), Robertsonian translocation (4%), or mosaicism due to mitotic nondisjunction during embryogenesis (1%).
Clinical findings can include intellectual disability; mongoloid facial features (flatface, low-bridged nose, and epicanthal folds); Brushfield spots (speckled appearance of the iris); muscular hypotonia; broad short neck; palmar (simian) crease; and congenital heart defects. Endocardial cushion defect, if present, leads to the forma-tion of an atrioventricular canal (a common connection between all 4 chambers of the heart). Additional clinical problems that can develop include duodenal atresia (“double-bubble” sign); Hirschsprung disease; increased risk (15–20 fold) of acute lymphoblastic leukemia (ALL); and Alzheimer disease (by age 40 virtually all will develop Alzheimer disease).
Prenatal tests include maternal serum tests, ultrasonography, amniocentesis, and chorionic villus sampling.
Median life expectancy is 47 years.
Edwards syndrome (trisomy 18) is caused by nondisjunction. The risk increaseswith maternal age.
Clinical findings can include intellectual disability; low- set ears and micrognathia; congenital heart defects; overlapping flexed fingers; and rocker-bottom feet. There is a very poor prognosis due to severe congenital malformations.
Patau syndrome (trisomy 13) is caused by nondisjunction. The risk increases withmaternal age.
Clinical findings can include intellectual disability; cleft lip and/or palate; cardiac defects; renal abnormalities; microcephaly; holoprosencephaly; and polydactyly. The very poor prognosis is due to severe congenital malformations.