What Is Homocystinuria?

Homocystinuria (HCY) is a rare condition that interferes with your body’s ability to break down a protein from the food you eat.

Protein is made of smaller building blocks called amino acids. Normally, your body processes these acids into energy for everything you do.

When you have HCY, your body can’t break down (metabolize) an amino acid called methionine. If left untreated, this acid and another called homocysteine build up in your bloodstream and can cause serious problems.

What Causes It?

A specific enzyme in your body breaks down methionine. If you have HCY, either you don’t make this enzyme or it’s not working right.

A pair of genes tells your body how to make this enzyme correctly. If both don’t work, you’ll get homocystinuria.

This is an inherited illness. The gene must be passed down from both of your parents.

What Are the Symptoms?

Babies born with homocystinuria look perfectly fine. Signs and symptoms start to show up within the first few years of life.

HCY affects all kids differently. If your child has it, she may have many or none of these symptoms:

Other symptoms may include:

How’s It Diagnosed?

Babies in the United States and many other countries are screened for homocystinuria (and other conditions) shortly after birth. It’s part of a blood test known as the newborn screen.

The screening includes a blood test that may show whether your child is at risk of having HCY. If results aren’t normal, she’ll need follow-up blood and urine tests to get a firm diagnosis.

How’s It Treated?

A metabolic doctor trained in genetic diseases will recommend a treatment plan. You need to start it right away to prevent problems.

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There are two types of HCY. The milder version can be treated with prescription-strength vitamin B6 supplements. Your baby’s doctor may perform tests to see if they will be likely to help.

The vitamin helps prevent behavior problems and intellectual problems in some kids. It may lessen the risk of eye and bone problems and blood clots.

If vitamin B6 doesn’t work, your child will start a low-methionine diet. It usually needs to be continued for life. A dietitian with expertise in amino acid disorders can help make an eating plan from infancy and beyond.

Your child probably will have to avoid high protein foods, including:

Some other foods will likely need to be limited or avoided. These include:

  • Nuts and peanut butter
  • Dried beans
  • Regular flour

Fruits and veggies have small amounts of methionine and can be eaten in carefully controlled amounts.

There are foods made especially for people with HCY too. These include flour, pasta, and bread that are low in methionine.

Your child probably will need a special formula to replace milk. It has protein (minus the methionine) and other nutrients needed for normal growth.

Your doctor may decide to add a supplement too:

  • Betaine and folic acid. These supplements can lower dangerously high homocysteine levels. Children who don’t get better with vitamin B6 often respond well to betaine.
  • Vitamin B12 and L-cysteine. HCY may be causing low levels of these needed substances. B12 is given as a shot. L-cysteine is another amino acid that can be taken as a supplement if it’s not a part of the special formula.

Your child’s blood and urine will be tested regularly to make sure treatment is working. Sometimes it needs to be tweaked as your child grows and changes.

What Can I Expect?

If your child begins treatment as a baby and stays on it, she’ll probably grow and develop normally. Treatment also can lower the risk of strokes, heart disease, and blood clots.

Vision problems may still happen even with treatment. These can be fixed with surgery or other methods.

WebMD Medical Reference Reviewed by Amita Shroff, MD on May 17, 2019

Sources

SOURCES:

March of Dimes: “Amino Acid Metabolism Disorders.”

Screening, Technology and Research in Genetics Project: “Disorder name: Homocystinuria.”

Baby’s First Test: “Homocystinuria.”

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