Acute disseminated encephalomyelitis (ADEM) is a monophasic demyelinating disease of the central nervous system typically affecting the gray and the white matter of the brain and spinal cord in multiple locations. In the acute stages, ADEM is characterized histologically by perivenous edema, demyelination, and infiltration with macrophages and lymphocytes, with relative axonal sparing, whereas the late course of the disease is characterized by perivascular gliosis.