Although similar in name, Type C Niemann-Pick is very different from Types A and B at the biochemical and genetic level. 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.
This defect in metabolism occasionally leads to a secondary reduction in acid sphigomyelinase activity in some cells, which is why these diseases were all called Niemann-Pick Disease in the early days, before the biochemical differences were understood.
Niemann-Pick Type C notably affects the Spanish-American population of southern New Mexico and Colorado.