An autosomal recessive lysosomal disease caused by biallelic loss of function variants in the SMPD1 gene. Clinical symptoms in affected individuals occur along a continuum. At the severe end of the spectrum are individuals historically diagnosed with Niemann-Pick disease type A (the neurovisceral form), which is characterized by hepatosplenomegaly with rapid neurological deterioration leading to death in the first few years of life. At the milder end of the spectrum are individuals historically diagnosed with Niemann-Pick disease type B, a later-onset, chronic visceral form, characterized by progressive visceral organ symptoms including hepatosplenomegaly and pulmonary insufficiency, and survival into adulthood. In addition, some affected individuals present with an intermediate phenotype, Niemann-Pick disease type A/B.