Conditions Homocystinuria

Homocystinuria (HCY) is a condition in which the body is unable to break down certain proteins. HCY is considered an amino acid condition because individuals with homocystinuria cannot process certain amino acids, small molecules that make up proteins. These amino acids build up in the body and can cause serious health problems. However, if homocystinuria is detected early and treatment is begun, children with homocystinuria can often lead healthy lives.

Condition Type

Amino Acid Disorders


Homocystinuria (HCY) affects about 1 out of every 200,000 to 300,000 babies born in the United States. The condition is more common in certain ethnic groups and populations.

In the United States, HCY is more common among white people from New England and among people of Irish ancestry. In these groups, the condition affects 1 out of every 50,000 babies. The condition is also more common in Ireland, Germany, Norway, and Qatar.

Also known as

  • HCY
  • Homocystinemia
  • Cystathionine beta-synthase deficiency
  • CBS deficiency

Follow-Up Testing

Your baby’s doctor may ask you if your baby is showing any of the signs of HCY (see Early Signs below). If your baby has certain signs, your baby’s doctor may suggest starting immediate treatment.

If your baby’s newborn screening result for homocystinuria (HCY) was out of the normal range, your baby’s doctor or the state screening program will contact you to arrange for your baby to have additional testing. It is important to remember that an out-of-range screening result does not necessarily mean that your child has the condition. An out-of-range result may occur because the initial blood sample was too small or the test was performed too early. However, as a few babies do have the condition, it is very important that you go to your follow-up appointment for a confirmatory test. Because the harmful effects of untreated HCY can occur soon after birth, follow-up testing must be completed as soon as possible to determine whether or not your baby has the condition.

Follow-up testing will involve checking your baby’s urine and blood samples for harmful levels of acids and toxins. Certain acids and toxins build up in the body when a child has an amino acid condition, so measuring the levels of these substances in your baby’s body can help doctors determine if your baby has a condition. A high amount of certain amino acids (homocysteine and methionine) in the blood and a high amount of homocysteine in the urine might indicate that your baby has HCY.

About Homocystinuria

Early Signs

Babies with homocystinuria (HCY) are usually healthy at birth. Early screening and treatment can often prevent the signs and symptoms of HCY. If children are not treated, signs of HCY usually begin between ages one and three. The signs and symptoms are highly variable among individuals with HCY. Your baby may have many of these signs or none at all.

Signs of HCY may include:

  • Poor growth
  • Trouble gaining weight
  • Developmental delays
  • Behavioral difficulties
  • Weak muscle tone (also known as hypotonia)
  • Pale skin and hair
  • Seizures (Epilepsy)

Many of these signs may occur when your baby eats foods that their body cannot break down. They can be triggered by long periods of time without eating, illnesses, and infections.

If your baby shows any of these signs, be sure to contact your baby’s healthcare provider immediately.


Dietary Treatments

Your baby may need to be on a restricted diet to avoid foods containing methionine, a substance in proteins, which individuals with homocystinuria (HCY) cannot break down. A dietician or nutritionist can help you plan an appropriate diet for your child.

Your baby’s doctor might also recommend special formulas and foods for children with HCY. These formulas will likely need to continue through adulthood.

Supplements and Medications

Certain types of homocystinuria (HCY) can be treated with prescription vitamin B6 supplements. These natural supplements may reduce or prevent some of the health complications commonly associated with HCY, such as blood clots, eye problems and learning and intellectual disabilities. Your baby’s doctor may be able to perform tests to determine if your baby will benefit from vitamin B6 supplements and write an appropriate prescription.

Betaine and folic acid are other natural vitamin supplements that can help lower the dangerously high homocystine levels in individuals with HCY. Your baby’s doctor can also write a prescription for these supplements.

Vitamin B12 injections or L-cysteine supplements may help your child if they have low levels of these substances due to HCY. Vitamin B12 is a natural vitamin, and L-cysteine is a substance found in proteins. Talk to your baby’s healthcare provider before starting this type of treatment.

Expected Outcomes

With early and on-going treatment, children with homocystinuria (HCY) can have healthy growth and development. Treatment can reduce the chances for developing blood clots, vision trouble, heart disease, and strokes. This is why it is important to have newborn screening for HCY.

Some children who receive treatment still experience vision trouble, but this can be corrected.

Children who do not receive treatment for HCY may develop behavioral difficulties, learning disabilities, or intellectual disabilities.


When we eat food, enzymes help break it down. Some enzymes help break down proteins into their building blocks, called amino acids. Other enzymes break down these amino acids. In homocystinuria (HCY), the enzymes that break down the amino acids methionine and homocysteine are not working correctly.

In most cases of HCY, the enzyme cystathionine beta-synthase (CBS) is not working. After methionine is broken down into homocysteine, CBS helps to break down homocysteine.

If your baby’s body has non-working copies of CBS, then homocysteine cannot be broken down. This causes high levels of methionine and homocysteine to build up in the body. Everyone has these substances in their blood, but high levels can be toxic.

HCY is an autosomal recessive genetic condition. This means that a child must inherit two copies of the non-working gene for HCY, one from each parent, in order to have the condition. The parents of a child with an autosomal recessive condition each carry one copy of the non-working gene, but they typically do not show signs and symptoms of the condition. While having a child with HCY is rare, when both parents are carriers, they can have more than one child with the condition. Learn more about autosomal recessive inheritance.

Support for Homocystinuria

Support Services

Support groups can help connect families who have a child or other family member affected with homocystinuria with a supportive community of people who have experience and expertise in living with the condition. These organizations offer resources for families, affected individuals, health care providers, and advocates.

Accessing Care

Work with your baby’s doctor to determine the next steps for your baby’s care. Your baby’s doctor may help you coordinate care with a doctor specializing in metabolism, a dietician who can help plan your child’s specialized diet, and other medical resources in your community. Depending on the signs and symptoms your baby experiences, your baby’s doctor may work with other doctors who specialize in treating the eyes, bones, or heart and blood vessels. Some children with homocystinuria (HCY) have developmental delays. If you think that your baby is not meeting their developmental milestones, ask your baby’s doctor about the next steps in accessing a developmental evaluation and care.

Because HCY is a genetic condition, you may want to talk with a genetics specialist. A genetic counselor or geneticist can help you understand the causes of the condition, discuss genetic testing for HCY, and understand what this diagnosis means for other family members and future pregnancies. Speak with your baby’s doctor about getting a referral. The Clinic Services Search Engine offered by the American College of Medical Genetics and Genomics (ACMG) and the Find a Genetic Counselor tool on the National Society of Genetic Counselors (NSGC) website are two good resources for you or your baby's health care provider to use to identify local specialists.

Families' Experiences

Benjamin was diagnosed with homocystinuria (HCY) through newborn screening and he received immediate treatment. Now in school, Benjamin continues to stay healthy with the help of a strict diet. Watch his story.

References & Sources

Visit MedlinePlus Genetics for more condition information

Visit the (STAR-G) Project for more information on amino acid disorders

Visit Wikipedia to view the metabolic pathway for homocystinuria

ACT Sheets

Healthcare professionals can learn more about confirmatory testing by reading the American College of Medical Genetics and Genomics' Algorithm for Diagnosis and ACT Sheet, a guide for follow-up after newborn screening. You can visit this page of the ACMG website here.

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