Fumarase deficiency: a rare genetic condition that has only been reported in about 100 people worldwide. The condition results from a mutation in a gene known as FH that codes for an enzyme called fumarase (also known as fumarate hydratase). Fumarase is involved in chemical reactions in the body that help cells use oxygen and generate energy. Fumarase deficiency often is fatal in infancy, but some people have lived to adulthood.
Fumarase deficiency mainly affects the brain and nervous system. Symptoms and signs can include an abnormally small head size (microcephaly), other brain abnormalities, severe developmental delay, weak muscle tone (hypotonia), seizures, and failure to grow as expected (failure to thrive). Other possible birth defects associated with fumarase deficiency include unusual facial features, a prominent forehead (frontal bossing), a small jaw (micrognathia), widely spaced eyes (ocular hypertelorism), low-set ears, and a depressed nasal bridge. Enlargement of the liver and spleen (hepatosplenomegaly) may be present, as well as abnormalities of blood cells.