Extramedullary hematopoiesis within a frontoethmoidal encephalocele in a newborn with holoprosencephaly

We present the first report of extramedullary hematopoiesis (EMH) in an enc ephalocele. The patient was a new-born with semilobar holoprosencephaly, a frontoethmoidal encephalocele, and a large subdural hematoma. The encephalo cele appeared as a hemorrhagic mass, protruding from the forehead to cover the right eye, without involvement of the sinuses or nasopharynx. Computeri zed tomography and magnetic resonance imaging studies ruled out other forms of holoprosencephaly and confirmed the continuity of the brain with the ex truded mass. Immunohistochemistry confirmed the presence of an atrophic epi thelium covering the mass. Histologic examination of the encephalocele reve aled EMH both within and adjacent to malformed cerebral cortex, with a tend ency for the hematopoietic cells to line up in columns within malformed cer ebral cortex. We propose that a single event during the fourth week of gest ation could both interrupt closure of the neural tube, giving rise to the e ncephalocele, and impair migration of the neural crest, leading to holopros encephaly secondary to failure of neural crest derivatives to induce basome dial telencephalic differentiation. EMH may have been induced from hematopo ietic stem cells in the richly vascular meningeal component of the encephal ocele, in response to anemia and hypoxia.