Niemann-Pick disease (NPD) is an autosomal recessive lysosomal storage disorder caused by deficient acid sphingomyelinase (ASM) activity, resulting in accumulation of sphingomyelin and cholesterol in many organs. ASM plays an important role in normal membrane turnover and is one of the key enzymes responsible for the production of ceramide. Type A NPD is the infantile form characterized by a rapidly progressive neurodegenerative course that leads to early death. Type B NPD is the later-onset form in which patients exhibit little or no neurological symptoms, but may have severe and progressive visceral organ abnormalities, including hepatosplenomegaly and cardiovascular disease. The different clinical presentations of Types A and B NPD are likely due to small differences in the amount of residual, functional ASM activity.
