Merck Manual

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Matt Demczko

, MD, Sidney Kimmel Medical College of Thomas Jefferson University

Last full review/revision Apr 2020| Content last modified Apr 2020
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Homocystinuria is a disorder of amino acid metabolism that is caused by a lack of the enzyme cystathionine beta-synthase, which is needed to metabolize homocysteine. This disorder can cause a number of symptoms, including decreased vision, intellectual disability, and skeletal abnormalities. Homocystinuria occurs when parents pass the defective genes that cause this disorder on to their children.

  • 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.)


Infants with this disorder are normal at birth. The first symptoms, including dislocation of the lens of the eye causing severely decreased vision, usually begin after 3 years of age. Most children have skeletal abnormalities, including osteoporosis. Children are usually thin with a curved spine, chest deformities, elongated limbs, and long, spiderlike fingers. Without early diagnosis and treatment, mental (psychiatric) and behavioral disorders and intellectual disability are common. Homocystinuria makes the blood more likely to clot spontaneously, resulting in strokes, high blood pressure, and many other serious problems.


  • Newborn screening test

Since 2008, nearly every state in the United States has required that all newborns be screened for homocystinuria with a blood test.

A test measuring enzyme function in the liver and DNA tests are done to confirm the diagnosis of homocystinuria. Tests of skin cells may also be done.


  • Vitamin B6 and a special diet

Some children with homocystinuria improve when given vitamin B6 (pyridoxine). Some children need to follow a special diet that restricts certain substances in protein.

Doctors may give supplements of betaine, which can help lower the levels of homocystine in the blood, and folic acid.

Children who have other forms of homocystinuria are given vitamin B12 (cobalamin) and folic acid.

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