Homocystinuria is caused by a lack of the enzyme needed to metabolize homocysteine.
Symptoms include intellectual disability, eye problems, and abnormalities of the skeleton.
The diagnosis is based on a blood test.
A special diet and supplements of vitamin B6, betaine , and folic acid may help some children.

Amino acids are the building blocks of proteins and have many functions in the body. Children with homocystinuria are unable to break down (metabolize) the amino acid homocysteine, which, along with certain toxic by-products, builds up to cause a variety of symptoms. Symptoms of homocystinuria range from mild to severe.
There are different types of inherited disorders. In homocystinuria, both parents of the affected child carry a copy of the abnormal gene. Because usually two copies of the abnormal gene are necessary for the disorder to occur, usually neither parent has the disorder. (See also Overview of Hereditary Metabolic Disorders.)