A. Familial Hypercholesterolemia
This is a primary genetic abnormality. In very rare cases, marked elevation of cholesterol (800–1500 mg/dl) is caused by a genetic defect. A receptor on the surface of cells (LDL receptors) removes LDL cholesterol from the blood. In this disorder there is decreased production or function of the LDL receptor. This autosomal disorder may involve abnormalities in the synthesis, transport, or clustering of the LDL receptor.
Homozygous familial hypercholesterolemia fortunately is rare and occurs in approximately one per million individuals in the United States. These patients have no functioning LDL receptors and have markedly elevated LDL cholesterol as high as 1200 mg/dl (31 mmol/L) and extensive coronary and peripheral atherosclerosis. Acute MI may occur within the first one to two years of childhood.
Heterozygotes have a reduction of 50% of the circu¬lating LDL receptors and may have serum cholesterol levels in the range of 300–800 mg/dl and manifest coro¬nary artery atherothrombosis, peripheral vascular disease, or atheromatous obstruction to the abdominal aorta in the third or fourth decade. Racial differences may deter¬mine the number of LDL receptors, and thus the ability to remove LDL cholesterol gradually from the bloodstream is affected.

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