Chromosomal Diseases
Abnormal total number of chromosomes (aneuploidy) or structural rearrangement (deletion, translocation, duplication).
Clinical manifestations: mental retardation (most common), variable congenital malformations.
Trisomy 21 (Down Syndrome)
Extra copy of chromosome 21. Incidence 1:800 live births; M:F ratio 3:2. Risk increases with increased maternal age: 1:350 at age 35; 1:110 at age 40.
Clinical Features
Round face, short nose, flat nasal bridge. Up-slanting palpebral fissures; epicanthal folds. Small mouth; large protruding tongue. Conductive hearing loss. Short stature; short, broad hands; increased space between first and second toes. Small pelvis.
Atlanto-axial or atlanto-occipital instability in 15% to 20%. Increased incidence of congenital heart defects and intestinal malformations (e.g., duodenal atresia). Infertility in males. Hypothyroidism. Hematologic abnormalities, including increased risk of leukemia.
Neurologic signs: marked hypotonia in infancy; later, mental retardation; after age 35, dementia. Increased incidence of seizures.
Neuropathology
Low neuronal density in diverse cortical areas. Pathology similar to Alzheimer disease (AD), including senile plaques, neurofibrillary tangles.
Cytogenetics
Complete extra chromosome 21 in 95%. Mosaic trisomy or translocation in remaining 5%.
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