Microgranular acute promyelocytic leukemia: a proposed role for a greater deformability of the leukemic cell.

A case of microgranular acute promyelocytic leukemia (APL), M-3 variant, is reported in a boy aged 5 years. The disease, which was rapidly fatal, presented with acute disseminated intravascular coagulation (DIC) and leukocytosis. Different cytomorphologic subtypes of promyelocytes were identified on the basis of cytoplasmic granular patterns: the microgranular type with barely visible cytoplasmic granulations and deeply basophilic cytoplasm and the more characteristic type with large promyelocytes containing azurophil granules. We observed a ratio of large promyelocytes to microgranular promyelocytes of 1:1.2 in the marrow and 1:4 in the peripheral blood. To explain this discrepancy, we hypothesize that the microgranular promyelocytes may be more deformable than the typical promyelocyte and that this intrinsic cellular characteristic may promote marrow egress and increase the likelihood of hyperleukocytosis in the M-3 variant.