Methylmalonic Acidemia with Homocystinuria: Causes, Symptoms, and Treatment
Medically reviewed by Harsha Moole, MD, Internal Medicine. Last reviewed: October 2, 2026.
What kind of doctor treats Methylmalonic Acidemia with Homocystinuria?
A primary care doctor often coordinates care, and most people also see a metabolic geneticist who guides treatment. A direct primary care or concierge doctor, who charges a membership fee, may have more time per visit and would refer to a metabolic specialist when needed.
What it is
Methylmalonic acidemia with homocystinuria is a rare inherited condition that affects how the body breaks down protein. People with this condition cannot properly process two substances, methylmalonic acid and homocysteine, which then build up in the blood and urine and can harm the body over time. It is caused by changes in a gene called MMACHC, which the body needs to process vitamin B12.
Symptoms
Symptoms often start in the first months or years of life. Common signs include poor feeding, vomiting, tiredness, slow weight gain, and floppiness. Later problems can include developmental delay, vision and balance trouble, and an enlarged liver. Without treatment, the buildup can damage the brain, eyes, kidneys, and blood vessels.
Causes and who gets it
A child must receive one changed copy of the gene from each parent to have the condition. Parents who carry only one changed copy usually have no symptoms. It can appear in families with no history of the disease.
How doctors diagnose it
Many babies are diagnosed through newborn screening, a blood test done shortly after birth. Doctors follow up with blood and urine tests measuring methylmalonic acid and homocysteine, and may run genetic testing. Early diagnosis matters because treatment works best when started soon.
Treatment
Treatment aims to lower harmful substances. Many people respond to vitamin B12. Doctors may also prescribe medication that helps remove homocysteine. Diet changes, guided by a dietitian, often limit protein, and some people take supplements such as carnitine. Illness or infection can trigger a metabolic crisis, which needs urgent care.
When to see a doctor
Contact a doctor if a baby or child has repeated vomiting, extreme sleepiness, poor feeding, or poor weight gain. Anyone diagnosed with this condition who becomes very sick, stops eating, or seems confused needs emergency care right away.
Common questions
Can this condition be cured?
There is no cure, but treatment can lower the harmful substances in the body and help prevent complications. How well a person does depends on the severity of the condition and how early treatment starts. Many people manage the condition well with lifelong care.
Will my other children have it?
If both parents carry one changed copy of the gene, each pregnancy has a one in four chance of producing a child with the condition. Genetic testing can tell whether family members carry the gene change. A genetic counselor can help families understand these risks.
Can adults have this condition?
Yes, some people have milder forms that are not found until childhood or adulthood. These cases may show up as tiredness, nerve problems, or thinking difficulties. Doctors can test for the condition at any age.
Does diet really matter?
Yes, for many people diet is a key part of treatment. Protein contains the substances that build up in the body. A dietitian helps create a plan that still supports growth and health. The exact diet depends on how the person responds to treatment.
Sources
- MedlinePlus Genetics - Methylmalonic acidemia with homocystinuria
- MedlinePlus - Methylmalonic acidemia: MedlinePlus Genetics
Medical Disclaimer
This content is for general educational and informational purposes only and is not medical advice. It is not a substitute for professional medical advice, diagnosis, or treatment, and reading it does not create a doctor-patient relationship. Always seek the advice of your physician or another qualified health provider with any questions about a medical condition, and never disregard or delay seeking professional advice because of something you read here. If you think you may have a medical emergency, call 911 (or your local emergency number) immediately.
