Homocystinuria is a genetic disorder in which the body cannot break down a particular amino acid.

The disorder passes from parents to children in an autosomal recessive manner, which means that two copies of an irregular gene must be present for it to develop.

In this article, we define homocystinuria and discuss its causes and symptoms. We also explain how doctors diagnose and manage the disorder.

a man wearing glasses because he experiences nearsightedness from homocystinuriaShare on Pinterest
A person with homocystinuria may experience nearsightedness.

In people with homocystinuria, the body is unable to break down certain proteins. Specifically, the condition affects the metabolism of an amino acid called methionine.

Amino acids are essential human building blocks. Methionine is a naturally occurring protein that babies need for growth and adults require to regulate nitrogen in the body. All amino acids contain nitrogen, making nitrogen an important part of human DNA.

There are several different forms of homocystinuria, which differ in their symptoms.

In the United States, doctors screen for homocystinuria at birth. However, screening does not detect all forms of homocystinuria. A person with the condition may, therefore, not receive a diagnosis for up to 5 years following birth.

Homocystinuria develops in about 1 out of every 200,000 to 300,000 babies in the U.S.

The condition is more prevalent among people of certain nationalities. In the U.S., it is more common in white people from New England and those of Irish ancestry. In these groups, the condition affects 1 in 50,000 newborns.

The most common form of homocystinuria is called cystathionine beta-synthase (CBS). Genetics Home Reference advise that 1 in every 200,000–335,000 people worldwide have this type.

As homocystinuria is genetically autosomal recessive, a child must inherit a copy of the mutated gene for homocystinuria from each parent to develop the condition.

Parents who carry just one mutated gene may not have any symptoms of homocystinuria.

According to researchers, the child of parents who both are carriers of the gene has:

  • a 25% chance of having homocystinuria
  • a 50% of being a carrier of the gene but not experiencing symptoms
  • a 25% chance of being unaffected and not carrying the mutated gene

In most cases of homocystinuria, the enzyme cystathionine beta-synthase (CBS) is not effective. As a result, the body cannot break down the amino acids methionine and homocysteine.

High levels of both amino acids build up throughout the body, which can be toxic and damage the nervous system. Damage might occur in the brain, as well as in the vessels carrying blood and lymph around the body.

The signs and symptoms of homocystinuria become clear in the first year of life. While some infants only experience mild effects, the symptoms may get worse as the individual progresses into childhood and even adulthood.

Symptoms may include:

  • nearsightedness
  • dislocation of the lenses of the eyes
  • pale skin and hair
  • weak bones
  • sideways curvature of the spine
  • long, thin arms and legs
  • chest deformities
  • developmental delays and learning disabilities
  • behavioral problems

Symptoms that are less common include:

  • megaloblastic anemia, which is a disorder in which the number of red blood cells is lower than normal, and the blood cells are unusually large
  • infants not growing or gaining weight at the expected rate
  • movement and gait problems
  • seizures

Many of these symptoms may get worse when a baby consumes food that their body cannot break down. Periods of illness and infection can also trigger these symptoms.


The most serious complication of homocystinuria is blood clots, which can be life threatening and increase the risk of stroke.

Cognitive disability is also possible, but early diagnosis and successful treatment can reduce the risk.

Dislocated lenses of the eyes can damage vision. Lens replacement surgery may be necessary to help some people.

Here, learn how to recognize the symptoms of a blood clot.

Clear signs that may lead a doctor to test for homocystinuria include a child being extremely thin and too tall for their age or not growing as expected.

Additionally, the doctor will look for signs of chest deformity, spinal curvature, and dislocated eye lenses.

Eye exams may reveal vision problems, while X-rays may show signs of weak bones. Genetic testing, amino acid screening, liver biopsies, and liver enzyme tests can also help a doctor confirm a diagnosis of homocystinuria.

Genetic testing

Researchers recommend genetic testing for anyone who has a family history of homocystinuria and wants to have children.

However, because this condition is a result of a genetic mutation, it is not possible to prevent it. The only reason for genetic testing is to give people an idea of their risk of passing on a gene mutation that can lead to homocystinuria.

There is no cure for homocystinuria. However, it is possible to manage the condition with dietary restrictions, supplements, and medication.

High dosages of vitamin B-6 supplements have been helpful for some people. Most individuals with the disorder will need to take these supplements for the rest of their lives.

Another treatment option is betaine, which is a nutrient that helps remove homocysteine from the bloodstream. Taking folic acid with betaine might also be beneficial.

Read more about the effects of vitamin B-6 on the body.

Foods to avoid

Individuals who do not respond to vitamin B-6 treatments should follow a low methionine diet. They will need to avoid foods that contain high levels of the amino acid methionine, such as:

  • some meats and fish
  • eggs
  • soy
  • dairy
  • nuts and beans

People with homocystinuria can lead active lives if they receive a diagnosis early enough, and the management of the disorder is successful.

However, blood clots can sometimes cause health problems and life threatening complications.

Although there is no cure for homocystinuria, treatments are available to help people manage the condition.