A case of acid sphingomyelinase deficiency type B with prominent histiocytes with engulfed nucleated cells and compound heterozygosity.

Acid sphingomyelinase deficiency type B (ASMD-B), also known as Niemann-Pick type B (NPB), is caused by inherited mutations in acid sphingomyelinase that results in accumulation of sphingomyelin and other lipids in monocytes/macrophages leading to splenomegaly, hepatomegaly, and/or cytopenias that typically manifest in mid-childhood. Microscopic examination of bone marrow aspirate and core biopsy specimens frequently reveals the presence of foamy histiocytes. In this case report, we describe a case of a 21-year-old woman who presented with progressive hepatosplenomegaly, gastroparesis, weight loss, and a neutrophilic leukocytosis who was found to have foamy histiocytes with engulfed nucleated cells compatible with emperipolesis or hemophagocytosis. Based on the constellation of clinicopathologic findings, a lysosomal storage disorder was suspected and subsequent genetic testing revealed the presence of two SMPD1 variants, one known pathogenic (c.1829_1831del, p.Arg610del) and one variant of unknown significance (VUS) (c.872G > A, p.Arg291His) (Table 1). Follow-up testing found that acid sphingomyelinase (ASM) activity was low (0.11 nmol/h/mg, reference value > 0.32 nmol/h/mg), consistent with enzyme dysfunction and supportive of the diagnosis of NPB. The patient was started on enzyme replacement therapy with olipudase alfa. To our knowledge, this is the first reported case of NPB in which foamy histiocytes with engulfed nucleated cells were a prominent feature in the bone marrow aspirate. One recent study reported finding emperipolesis in some cases of ASMD. Thus, this may be an uncommon but recurrent finding in some NPB patients.