NPC Patients are not able to metabolize cholesterol and other lipids properly within the cell. Consequently, excessive amounts of cholesterol accumulate within the liver and spleen and excessive amounts of other lipids accumulate in the brain. NPC causes a secondary reduction of ASM activity. 

There is considerable variation in when NPC symptoms first appear and in the progression of the disease. Symptoms may appear as early as a few months of age or as late as adulthood.

NPC has about 500 cases diagnosed worldwide.  It is believed, however, that the number of people affected by NPC is higher, but diagnostic difficulties do not allow an accurate assessment of the occurrence rate

Early NPC symptoms include vertical gaze palsy (the inability to move the eyes up and down), enlarged liver, and enlarged spleen.  As the disease progresses, those with NPC lose the ability to walk, talk, or swallow, which requires use of a feeding tube.  They develop seizures, dementia, and cataplexy.

In most cases, neurological symptoms begin appearing between the ages of 4 and 10.  Generally, the later neurological symptoms begin, the slower the progression of the disease.