Types A and B Niemann-Pick are both caused by the deficiency of a specific enzyme activity, acid sphingomyelinase (ASM). If ASM is absent or not functioning properly, sphingomyelin cannot be metabolized properly and is accumulated within the cell, eventually causing cell death and the malfunction of major organ systems.

Type C Niemann-Pick is very different from types A and B. Patients with Type C are not able to metabolize cholesterol and other lipids properly. Consequently, excessive amounts of cholesterol accumulate within the liver and spleen and excessive amounts of other lipids accumulate in the brain. The defect in metabolism occasionally leads to a secondary reduction in ASM activity in some cells.

Type C Niemann-Pick disease has been reported in all ethnic groups.  There is a higher than average prevalence of the disease in Puerto Ricans of Spanish descent. 

Type D Niemann-Pick has only been found in the French Canadian population of Yarmouth County, Nova Scotia and is now thought to be a variant of Type C. Geneological research indicates that Joseph Muise (c. 1679 - 1729) and Marie Amirault (1684 - c. 1735) are common ancestors to all Type D cases. This couple is the most likely origin for the Type D variant.

Based on tissue and chemical changes similar to Type C, but with very late adult onset, a Type E Niemann-Pick has also been suggested.

Pick's Disease is sometimes confused with Niemann-Pick but it is a different disease.

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