- Niemann-Pick types A and B. In these types, a fatty substance called sphingomyelin cannot be broken down (metabolized) by the body because an enzyme called acid sphingomyelinase (ASM) is missing or not working properly. Sphingomyelin and other fats accumulate in the liver and spleen, causing enlargement of these organs, and accumulate in the brain. Type A occurs mainly in infants who show severe, progressive brain disease. There is no cure, so most children do not live beyond their first few years. Type B usually occurs later in childhood and is not associated with primary brain disease. Most individuals affected with type B survive into adulthood.
- Niemann-Pick type C (NPC). NPC is a rare inherited disease. Mutations in one of the NPC1 or NPC2 genes cause cholesterol and other fats to accumulate in the liver, spleen and brain. All parts of the brain are eventually affected. NPC is a progressive disease and there is no cure. NPC can occur at any age, but about half of those affected have symptoms before age 10.