Homocystinuria

Early diagnosis and intervention reduces risk of visual impairment and thromboembolism

Although homocystinuria was described in 19621 and the ocular features were well characterised in 1973,2 reports from different parts of the world suggest that the diagnosis is sometimes missed or delayed.3 4 In this week's BMJ, Cruysberg et al (p 1037) present data on 34 patients with homocystinuria in whom the mean delay in diagnosis was 11 years.5 Myopia was an early clinical sign in many patients, while some patients were diagnosed later because of subluxation of the lens (ectopia lentis), which may present as decreased vision, monocular diplopia or pain secondary to pupillary glaucoma, and vascular signs.

Ectopia lentis is present in 85% of patients with homocystinuria,6 while as many as 5% of people with non-traumatic dislocation of the optic lens may be found to have homocystinuria.7 Why then is the diagnosis of homocystinuria problematic? Firstly, homocystinuria is rare, and other commoner diagnoses have to be considered. Secondly, there are technical problems in measuring the increased concentration of sulphur amino acids in body fluids.

Homocystinuria is due to deficiency of the enzyme cystathionine-ß-synthase, which converts methionine to cystathionine. The deficiency leads to the build up in the plasma of the intermediate chemicals homocysteine and homocystine (jointly refered to as homocyst(e)ine) and …