Methylmalonic acidemia represents a group of disorders that affect the way a person breaks down proteins and fats. In general, symptoms of methylmalonic acidemia can occur at any time between the neonatal period and adulthood. The cblB type is usually associated with onset of symptoms shortly after birth. During metabolic decompensation (episodic crisis due to infection or other external stressors) symptoms include vomiting, dehydration, lethargy, seizures, metabolic acidosis (too much acid in the blood), and hyperammonemia (elevated ammonia levels). Long-term complications can include increased risk for infections, low muscle tone (hypotonia), encephalopathy (brain damage), epilepsy, developmental delay and intellectual disability, anorexia and poor growth (failure to thrive), pancreatitis, enlarged liver (hepatomegaly), and chronic kidney disease that progresses to kidney failure.
The estimated incidence of methylmalonic acidemia is 1 in 50,000 to 1 in 100,000 people. The portion of methylmalonic acidemia attributed to cblB type is ~13%.
There is no cure for methylmalonic acidemia. Treatment is divided between managing an acute crisis (metabolic decompensation) and management of symptoms between crises. Metabolic crisis is managed in a hospital and involves increasing the amount of fluid in the body, ensuring proper nutrition (reduced or no protein and increased glucose-intake), and monitoring laboratory work for signs of further complications (such as those related to elevated ammonia levels), which is best done by a metabolic expert. Long-term management includes a high-calorie diet that is low in protein, carnitine and other supplements, antibiotics, and hydroxocobalamin (vitamin B12) intramuscular injections in rare cases. Transplant may become necessary if organ failure occurs.
Methylmalonic acidemia is associated with a number of chronic issues and higher than average mortality rates. Even with treatment, the cblB type can be associated with symptoms, like intellectual disability or kidney failure. Metabolic crisis can lead to coma, especially if left untreated. The median age of death for patients with the cblB type is ~3 years.
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