Homocystinuria is a rare inherited disease that causes a deficiency of one of several enzymes needed for the breakdown of food (metabolism). This enzyme deficiency may cause a buildup of homocysteine in the blood. Excess homocysteine may be released in the urine.

Babies born with homocystinuria may fail to grow and gain weight (failure to thrive) and may experience developmental delays. People with homocystinuria may develop diseases of the heart and blood vessels at a young age. If homocystinuria is not diagnosed in infancy, other problems may develop, including:

• Partial dislocation of the lens of the eyes (ectopia lentis).
• Severe nearsightedness (myopia).
• Progressive mental retardation.
• Seizures.
• Psychiatric problems.
• Skeletal problems (such as scoliosis, osteoporosis, or protrusion or depression of the breastbone).
• Formation of blood clots in deep veins (deep venous thrombosis, or DVT).
• Stroke.

People with homocystinuria may have a thin appearance, with long, slender arms, legs, fingers, and toes. These features have been described as “marfanoid” because of the similarity to Marfan's syndrome.

Treatment for homocystinuria may include eating foods low in certain amino acids and taking vitamin supplements and a medication (called betaine) to enhance the breakdown of homocysteine.