Niemann-Pick disease refers to a group of diseases passed down through families (inherited) in which fatty substances called lipids collect in the cells of the spleen, liver, and brain.

There are four most commonly recognized forms of the disease: Types A, B, C, and D. Types A and B are also called Type I. Types C and D are also known as Type II.

Each type involves different organs and may or may not involve the central nervous system or respiratory system. Each one can cause different symptoms and may occur at different times throughout life, from infancy to adulthood.

Niemann-Pick disease Type A and B occur when cells in the body lack an enzyme called acid sphingomyelinase (ASM). ASM helps break down (metabolize) a fatty substance called sphingomyelin, which is found in every cell of the body. If ASM is missing or does not work properly, sphingomyelin builds up inside cells. This leads to cell death and makes it hard for organs to work properly. Type A occurs in all races and ethnicities, but higher rates are seen in the Ashkenazi (Eastern European) Jew population.

Niemann-Pick Type C occurs when the body can not properly break down cholesterol and other lipids (fats). This leads to too much cholesterol in the liver and spleen and excessive amounts of other lipids in the brain. There may be reduced ASM activity in some cells. Type C Niemann-Pick disease has been reported in all ethnic groups but it is most common among Puerto Ricans of Spanish descent.

Type D Niemann-Pick involves a defect that interferes with the movement of cholesterol between brain cells. It is now thought to be a variant of type C. This type of Niemann-Pick disease has only been found in the French Canadian population of Yarmouth County, Nova Scotia.

An adult-onset form of Niemann-Pick disease has been suggested. It is sometimes referred to as Type E disease.