Cutaneous extramedullary hemopoiesis in chronic myeloproliferative and myelodysplastic disorders.

Cutaneous extramedullary hemopoiesis (EMH) is a rare complication of chronic myeloproliferative and myelodysplastic disorders. Chronic idiopathic myelofibrosis (CIMF) is the most common underlying condition. To date, fewer than 30 cases have been reported in the literature and there has been significant confusion with regard to the proposed pathogenesis. In this article, we describe two additional cases of cutaneous EMH associated with chronic myeloid diseases and review the literature with the aim of clarifying the underlying pathogenesis of this unusual clinical condition. The diagnosis of cutaneous EMH in both patients with chronic myeloid diseases was made histopathologically, with immunohistochemistry confirming the presence of differentiating hemopoietic cells associated with dermal components. Cutaneous EMH in chronic myeloid diseases occurs as a result of migration of abnormal neoplastic hemopoietic precursor cells into the skin (in effect, metastasis) and subsequent differentiation along divergent myeloid cell lineages. The diagnosis should be considered in any patient with chronic myeloproliferative or myelodysplastic disease who develops a skin rash.