Acute disseminated encephalomyelitis (ADEM) is an often rapidly progressive autoimmune inflammatory demyelinating disease of the central nervous system that may be triggered by infections or immunizations. It is a rare disease and is more common in children than adults. Patients typically present with rapid onset of neurologic symptoms following an incubation period of several days to 2 months, with deterioration requiring hospitalization. Common symptoms include headache, fever, nausea, vomiting, altered mental status ranging from confusion and irritability to coma, motor deficits (paraparesis or tetraparesis), and sensory deficits. Patients often have brain stem involvement with dysarthria, dysphagia, nystagmus, and oculomotor deficits. Increased intracranial pressure and seizures can also occur.

The most common triggers, when identified, are viral or bacterial infections including rubeola, rubella, mumps, varicella, Epstein-Barr virus, human herpesvirus 6, influenza, HIV, and Mycoplasma pneumoniae. There have been reports of ADEM following Zika virus infection. Prognosis is variable and outcomes are more favorable in children. Most patients will improve but over 50% will have residual deficits. It is typically a monophasic illness, although relapses can occur.

Acute hemorrhagic encephalomyelitis is a hemorrhagic variant of ADEM with a more rapid onset and progression of neurologic symptoms. It is associated with hemorrhagic demyelinating lesions and carries a worse prognosis.