Methylmalonic acidemia with homocystinuria

Methylmalonic acidemia with homocystinuria is a disorder in which the body is unable to correctly process certain protein building blocks (amino acids), fat building blocks (fatty acids), and  cholesterol. The body is also unable to convert the amino acid homocysteine to another amino acid, methionine. Individuals with this disorder have a combination of features from two separate conditions, methylmalonic acidemia and homocystinuria. There are several forms of this combined condition, and the different forms have different genetic causes and signs and symptoms. The most common and best understood form, called cblC type (or cobalamin C disease), occurs in about 80 percent of affected individuals. 

The signs and symptoms of methylmalonic acidemia with homocystinuria usually develop in infancy, although they can begin at any age. When the condition begins early in life, affected individuals typically grow more slowly than expected. This sign is sometimes iedentified before the baby is born. These infants can also have difficulty feeding and have an abnormally pale appearance (pallor). Eye abnormalities and neurological problems, including weak muscle tone (hypotonia) and seizures, are also common in people with methylmalonic acidemia with homocystinuria. Many infants and children with this condition have delayed development and intellectual disability, and some have an unusually small head size (microcephaly). 

Some people with methylmalonic acidemia with homocystinuria develop a blood disorder called megaloblastic anemia. Megaloblastic anemia occurs when a person has a low number of red blood cells (anemia), and the remaining red blood cells are larger than normal (megaloblastic). The signs and symptoms of early-onset methylmalonic acidemia with homocystinuria worsen over time, and the condition can be life-threatening if it is not treated.

When methylmalonic acidemia with homocystinuria begins in adolescence or adulthood, it may change an affected person's behavior and personality; the person may become less social and may experience hallucinations, delirium, and psychosis. In addition, these individuals can begin to lose previously acquired mental and physical abilities, resulting in a decline in school or work performance, difficulty controlling movements, memory problems, speech difficulties, a decline in intellectual function (dementia), or an extreme lack of energy (lethargy). Some people with methylmalonic acidemia with homocystinuria whose signs and symptoms begin later in life develop a condition called subacute combined degeneration of the spinal cord, which leads to numbness and weakness in the lower limbs, difficulty walking, and frequent falls.