Fatal encephalitis in a case of hypereosinophilic syndrome: MRI and autopsy findings.

A 34-year-old man developed fever and headache, followed by finger tremor and gait disturbance, and was admitted to our hospital about two months after onset. Blood tests showed a white blood cell count of 32,600 /µL with an eosinophil count of 22,300 /µL. There was no evidence of allergic drug reaction or parasitic infection. Cerebrospinal fluid examination demonstrated mononuclear pleocytosis without eosinophils or atypical cells. Brain MRI showed symmetric lesions bilaterally in the medial temporal lobe, frontobasal and insular regions and medulla oblongata. Herpes simplex virus-DNA was negative in the cerebrospinal fluid. The patient died about four months after onset. Histopathologically, there was infiltration of T cells, B cells and macrophages throughout the whole brain, but eosinophils or atypical cells were absent. Immunohistochemistry for herpes simplex virus type 1 and human herpesvirus 6 was negative. This case suggests that fatal encephalitis may develop in association with hypereosinophilic syndrome.