Fumarase Deficiency: Causes, Symptoms, and Treatment
Medically reviewed by Harsha Moole, MD, Internal Medicine. Last reviewed: October 23, 2026.
What kind of doctor treats Fumarase Deficiency?
A medical geneticist (a doctor for inherited conditions) and a child neurologist (a children's brain and nerve doctor) usually care for a child with fumarase deficiency. It is a very rare condition. A direct primary care or concierge doctor can be a first stop for an evaluation and can refer you to a specialist when needed.
What is fumarase deficiency?
Fumarase deficiency is a condition that mainly affects the nervous system, especially the brain. It is also called fumaric aciduria or fumarate hydratase deficiency.
It is very rare. About 100 affected people have been reported worldwide. Signs usually begin in the newborn period or in infancy.
Affected people usually survive only a few months, but a few have lived into early adulthood.
Symptoms
Affected infants may have a very small head size, called microcephaly. They may also have abnormal brain structure. Other signs can include:
- Severe developmental delay, which means slow learning of skills such as sitting and talking.
- Weak muscle tone, called hypotonia.
- Trouble gaining weight and growing as expected.
- Seizures.
- Feeding difficulties in infancy.
Some people have unusual facial features. These can include a prominent forehead, low-set ears, a small jaw, widely spaced eyes, and a flat nasal bridge.
An enlarged liver and spleen may also occur. In infancy, some have too many red blood cells or too few white blood cells.
Causes and who gets it
Fumarase deficiency is caused by changes, called mutations, in the FH gene. This gene gives the body instructions for making an enzyme called fumarase, also known as fumarate hydratase.
Fumarase works in a series of steps called the citric acid cycle, or Krebs cycle. This cycle helps cells make energy. Cells in the developing brain are harmed most when this energy process is impaired.
The condition is inherited in an autosomal recessive pattern. This means both copies of the gene in each cell have mutations. Each parent carries one changed copy of the gene, but parents typically show no signs of the condition.
How doctors diagnose it
Doctors look at the child's signs and symptoms. Low fumarase enzyme activity is one finding. Some people also have a high level of fumaric acid in the urine.
The NIH Genetic Testing Registry lists genetic tests for it.
Treatment
The medical sources reviewed here do not describe a specific treatment for fumarase deficiency. A child's care team can explain what care is available for that child.
When to see a doctor
Talk to your child's doctor if your baby has a very small head or weak muscle tone. Also talk to the doctor about trouble feeding or gaining weight.
Call 911 or go to the emergency room if your child has a seizure, trouble breathing, or is hard to wake up.
How a direct primary care or concierge doctor fits in
A direct primary care doctor charges a flat monthly fee instead of billing insurance, while a concierge doctor usually charges more and often works alongside insurance. A direct primary care doctor can review your child's symptoms and help with follow-up. They can also refer you to a children's specialist or a genetics clinic.
Common questions
Is fumarase deficiency inherited?
Yes. It is inherited in an autosomal recessive pattern, which means both copies of the FH gene in each cell have mutations. Each parent carries one changed copy of the gene, but parents typically show no signs of the condition.
Is fumarase deficiency the same as fumaric aciduria?
Yes. Fumaric aciduria is another name for fumarase deficiency.
How rare is fumarase deficiency?
It is very rare. About 100 affected people have been reported worldwide.
Can the parents of an affected child have health problems?
Yes, they can. People with one changed copy of the FH gene, such as these parents, tend to develop benign tumors. These are growths that are not cancer. They are made of smooth muscle tissue and appear in the skin and, in females, the uterus. They also have an increased risk of kidney cancer. This is called hereditary leiomyomatosis and renal cell cancer (HLRCC).
Sources
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.
