Homocystinuria

Homocystinuria is a group of inherited metabolic disorders that cause excess homocysteine or homocystine in blood and urine. Its effects can include eye, skeletal, vascular, and neurological problems.

Cystathionine beta-synthase deficiency: An inherited enzyme deficiency that impairs conversion of homocysteine to cystathionine. This is the most common cause of classic homocystinuria.
Newborn screening: Testing newborns for selected conditions before symptoms appear. Screening can detect some forms before irreversible complications develop.
Ectopia lentis: Displacement of the eye's crystalline lens from its normal position. Lens displacement is a distinctive ocular sign of classic homocystinuria.
Marfan syndrome: An inherited connective-tissue disorder commonly caused by pathogenic variants in FBN1. Both conditions can cause long limbs and lens displacement, but their causes and risks differ.
Methionine metabolism: The network of reactions that processes the essential amino acid methionine. Homocystinuria arises when this pathway fails to process homocysteine normally.
Tandem mass spectrometry: An analytical method that identifies molecules by mass-to-charge ratios in two sequential stages. Newborn programs use it to measure methionine, a marker for some cases.
Marfanoid habitus: A body build resembling Marfan syndrome, often including long limbs and slender fingers. Skeletal features can resemble Marfan syndrome despite a different metabolic cause.
Methylmalonic acidemia: An inherited metabolic disorder in which methylmalonic acid accumulates because of impaired metabolism. Some combined cobalamin disorders elevate both homocysteine and methylmalonic acid.
Remethylation disorders: Inherited disorders that impair conversion of homocysteine back into methionine. These causes of homocystinuria differ from cystathionine beta-synthase deficiency.
Plasma amino acid analysis: Laboratory measurement of amino acid concentrations in blood plasma. Elevated total homocysteine and characteristic amino-acid patterns help classify the disorder.

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