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Type B Niemann–Pick disease

A 39-year-old man was referred to our centre with hepatosplenomegaly that had been present since childhood. He was asymptomatic and his full blood count showed only mild thrombocytopenia (platelet count 123 9 10/l). A blood film was normal. A bone marrow aspirate revealed hyperplasia of foamy and, especially, sea-blue histiocytes (images), with haemophagocytosis. The activity of acid sphingomyelinase from dried blood was below the normal range (54 pmol/ spot*20 h, normal range 200–3500) in leucocytes, leading to the diagnosis of type B Niemann–Pick disease (NPD). Acid lipase activity was within the normal range (0 56 nmol/ spot*3 h, normal range 0 10–2). Molecular analysis showed compound heterozygosity for two mutations in SMPD1 (p.Tyr469Ser/p.Arg610del). No mutation was detected in NPC1 or NPC2. NPD is a sphingolipidosis that includes at least three different disease types (A, B, C), all inherited by autosomal recessive transmission. Complete and partial deficiency of the lysosomal enzyme acid sphingomyelinase (ASM) is responsible for NPD types A and B, respectively. Type A is the most common form in which symptoms begin shortly after birth, with hepatosplenomegaly, failure to thrive, neurological impairment and cherry red retinal spots. Type B, the nonneuropathic form, is a relatively benign disorder with splenomegaly, thrombocytopenia, sea-blue histiocytes in the bone marrow and, rarely, hepatomegaly and pulmonary infiltrates; survival to adulthood is frequent and its incidence is believed to be underestimated. Type C is due to mutations in NPC1, which cause a massive lysosomal accumulation of cholesterol leading to progressive neurological impairment in early childhood. As NPD type C produces a secondary reduction of ASM activity, all three types are considered forms of the same disease. Morphological features of NPD are undistinguishable from those of acid lipase deficiency. Although demonstration of decreased specific enzymatic activity is mandatory, the diagnosis of lysosomal storage disorders can be guided by the morphological findings in bone marrow films.