Sea-blue histiocytosis secondary to Niemann-Pick disease type B: a case report.

Sea-blue histiocytosis is a morphological finding that can be associated both with acquired conditions of increased cellular turnover and inborn errors of lipid metabolism. We report a rare case of sea-blue histiocytosis associated with a mild phenotype of Niemann-Pick disease (NPD) type B in a 44-year-old man who presented with splenomegaly and mild thrombocytopenia. Diagnosis was guided by the morphological finding in bone marrow smears of foamy and sea-blue histiocytes and confirmed by the measurement of acid lysosomal sphingomyelinase activity below normal values. NPD type B is a rare inborn error of metabolism, with a benign course and prognosis, while types A and C are always associated with severe neurological involvement. In our patient diagnosis was confirmed by the specific enzyme assay of leukocytes (deficiency in sphingomyelinase activity). This is a simple and noninvasive method that is useful whenever clinical and morphological finding are relevant, and a primary hematological disorder has been ruled out.