Histologic findings in skin biopsy in a JMML rash: a case report and review of literature.

Juvenile myelomonocytic leukemia (JMML), belonging to the category of myeloproliferative/myelodysplastic syndromes, is a rare pediatric hematologic malignancy with frequent skin manifestations commonly in the form of rashes. However, these rashes are not always biopsied and their immunophenotype studied in details. We report one such case in a 2-year-old boy who presented with a 1-month history of nonresolving fever, fatigue, and pallor along with a generalized maculopapular skin rash. The child also had mild hepatomegaly. A complete blood count with differential revealed a hemoglobin value of 8.6 g/L, leukocytosis (white blood cell count of 55.3 × 109/L), absolute monocytosis (27 × 109/L), immature granulocytes, and a platelet count of 126 × 109/L. The bone marrow aspirate showed a hypercellular marrow with trilineage hematopoiesis, 10% blasts (including promonocytes), increased monocytes (46%), and dysplastic changes in the erythroid and myeloid cell lines. These findings along with absence of a BCR-ABL1 fusion gene and a hemoglobin F level of 3.4% were consistent with the diagnosis of JMML, which was confirmed by subsequent positive granulocyte macrophage-colony stimulating factor hypersensitivity and NRAS mutation studies. A skin biopsy of the rash revealed a dermal infiltrate composed predominantly of atypical monocytic cells that were positive for CD68, myeloperoxidase, and lysozyme and negative for CD117, CD1a, and S100, consistent with JMML.