Subsequently, five subtypes have been identified, but only Type A is more frequent in Ashkenazi Jewish populations. Type A disease is a severe neurodegenerative disorder of infancy. By six months of age, affected babies experience feeding difficulty, recurrent vomiting and enlargement of the spleen and liver, which causes the abdomen to appear distended.
Death usually occurs by two to three years of age, due to infections such as pneumonia.
Type B disease is a milder disorder with no neurologic involvement. Affected individuals usually come to medical attention in childhood due to enlarged livers and spleens. With adolescence and adulthood the major symptoms are associated with pulmonary disease, due to involvement of the lungs. Patients with Type B disease may survive into the fourth and fifth decades of life. The specific biochemical defect in both Types A and B Niemann-Pick disease is the deficiency of an enzyme , sphingomyelinase, which normally degrades a fatty substance known as sphingomyelin.