A common complication of myelofibrosis presenting as a rare finding in cerebrospinal fluid cytology.

Herein, we present a rare case of intracranial extramedullary hematopoiesis (EMH) diagnosed by cerebrospinal fluid (CSF) cytology and describe the clinical presentation, radiologic, and pathologic findings. A 65 year-old man with a history of progressing primary myelofibrosis was admitted for headaches and right facial numbness. A brain MRI revealed focal abnormalities that were suspicious for leptomeningeal involvement of acute leukemia. Cytologic examination of CSF demonstrated a hypercellular specimen composed of hematopoietic cells including few blasts, as well as maturing red blood cells and granulocytic cells. The integration of morphologic findings, peripheral blood and bone marrow counts, as well as flow cytometric analysis of CSF and bone marrow, excluded leptomeningeal involvement by leukemic blasts and helped establish the diagnosis of intracranial EMH. Inclusion of EMH in the differential diagnosis of intracranial pathology in patients with known conditions predisposing them to EMH is important because recognizing this rare event has implications for treatment and prognosis.