Niemann-Pick disease types A and B (NORD)

Niemann-Pick disease type A and type B, or NPD-A and NPD-B, which are subtypes of acid sphingomyelinase or ASM deficiency, are rare, genetically inherited conditions characterized by the inability to break down a fat called sphingomyelin due to a deficiency of the enzyme, acid sphingomyelinase.

There’s also Niemann-Pick disease type C, which is known to be caused by mutations in the genes NPC1 and NPC2, and is therefore considered to be distinct from types A and B.

Sphingomyelin is a fat that's included in the membrane of many different cells.

When cells become old or damaged, they are often phagocytized, or eaten, by macrophages, which are cells of the immune system.

They contain organelles called lysosomes that are said to function as recycling centers because they break down large, potentially harmful substances to be reused by the body.

They break down sphingomyelin by using an enzyme called acid sphingomyelinase, which is a product of the sphingomyelin phosphodiesterase 1, or SMPD1 gene.

In Niemann-Pick disease types A and B, there’s a mutation in the SMPD1 gene that causes a defect in the production of sphingomyelinase, leading to an inability to break down sphingomyelin.

In NPD-A there’s almost a complete absence of sphingomyelinase activity, while NPD-B has some residual sphingomyelinase activity remaining.

While the mechanism isn’t completely understood, sphingomyelin primarily accumulates in the lysosomes of macrophages, which travel throughout the body and cause damage in multiple organs and tissues.

The macrophages develop a characteristic lipid-laden appearance under microscopes and are called “foam cells.”

Sphingomyelin can also build up in other cell types in the body, reflecting impaired intracellular recycling of membranes and damaged organelles in lysosomes due to sphingomyelinase deficiency.

Signs and symptoms of NPD-A present early in life, and are usually life-threatening.

These include enlargement of the liver and/or spleen, jaundice, feeding difficulties, and progressive loss of reflexes and muscle tone.

Infants also often develop a “cherry red spot” in the eye that affects the macula, which corresponds with a decrease in central vision.