ARTERIAL AS WELL AS VENOUS OCCLUSIONS OCCUR WITH HOMOCYSTINURIA.

GENETICS/BIOCHEMISTRY: Homocystinuria, an autosomal recessive inborn error of metabolism, is most commonly due to reduced activity of the enzyme cystathionine-beta-synthetase. It results in elevation of serum homocysteine levels and excretion of homocysteine in the urine. Boers and coworkers (cited in 1) have demonstrated high blood levels of homocysteine after methionine loading in patients who are heterozygous for homocystinuria. The frequency of heterozygosity for cystathionine-beta-synthetase deficiency causing homocystinuria in the normal population is estimated to be 1.4%. Wenzler and others (cited in 1) reported the incidence of hyperhomocystinemia to be 21% in patients less than 50 years of age with either retinal vein or retinal artery occlusion.

CLINICAL MANIFESTATIONS: Because of the varied expression of the genetic defect, there is considerable variation in the severity of clinical complications of the disease. Homozygotes are at a high risk for premature arteriosclerotic vascular disease and venous thrombosis. Many patients heterozygous for homocystinuria have also been reported to have premature arteriosclerotic disease. Mudd and coworkers (cited in 1) reported that patients with homocystinuria have a 40% probability of having the first thromboembolic episode in the first 2 decades of life. Homocystinuria predisposes to venous thromboembolic phenomenon at a young age, leading to cerebrovascular occlusion, deep venous thrombosis, retinal vessel occlusion, and renovascular abnormalities.