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Homocystinuria Primary Defect Deficiency or absence of an enzyme necessary for the breakdown of the amino acid methionine results in build up of methionine in the blood and elevated excretion of homocystine in the urine. Screening Test Historically screening has been based on measurement of methionine in the dried blood spot using a bacterial inhibition assay similar to the original Guthrie assay for PKU. Screening is now possible using tandem mass spectrometry (MS/MS). Predictive value should be high. Because of delayed accumulation of methionine if residual enzyme activity is present, screening may be more effective at 2 to 4 weeks of age for these infants. Etiology & Prevalence Genetic, autosomal recessive. A number of specific genetic defects have been identified. With some, residual enzyme activity may occur, resulting in moderation of symptoms and delay in accumulation of elevated levels of methionine. About one in every 200,000 babies in the United States is born with homocystinuria. If Untreated There is wide variation in clinical course for affected infants. Clinical features include: circulatory blood clotting (thromboembolism), physical and mental developmental disabilities. Approximately half die by age 25 due to thromboembolism. Developmental delay and physical defects affect most. The most common defect (cystathionine b-synthase deficiency) can be classified as either responsive or non-responsive to treatment with vitamin B6. This may be related to residual enzyme activity needed for response. Those who are not responsive have more severe clinical course. Therapy Vitamin B6 supplementation for those who are responsive. Dietary restriction of methionine with supplementation of cystine for those who are not. Other treatment focused on clinical features such as aspirin to combat thromboembolism. With Treatment Mortality and mental retardation are prevented or reduced. Clinical variability of other features remains. |
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Washington
State Department of Health Last
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05/21/2009 06:37 PM
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