Acute Disseminated Encephalomyelitis (ADEM) is an immune-mediated response to preceding viral infection or vaccination. It is most commonly associated with measles, mumps, rubella, or chickenpox, although it is also seen after other viral infections. It is most common in children ages 5-10 years old. Typical onset is 4 days to 3 weeks after an infection with an abrupt onset and monophasic course. Patients may present with fever, headache, drowsiness and progress to seizures and focal neurological deficits. The typical course lasts several days to weeks with complete recovery. Permanent sequelae may be seen in up to 20% of patients.

ADEM is thought to be caused by multifocal perivascular inflammation resulting in zones of demyelination. It occurs most commonly in the subcortical white matter but gray matter can also be affected. Lesions are bilateral and wide spread. They may also involve the cranial nerves and spinal cord.

On imaging, MRI is superior to CT in detecting lesions. There will be bilateral discrete T2 hyper-intense areas in the subcortical white matter, deep white matter, basal ganglia, thalami, brainstem, or cerebellum. There may be irregular enhancement or optic nerve enhancement.