Acute disseminated encephalomyelitis (ADEM) is a severe inflammatory, demyelinating disease of the central nervous system that typically occurs following vaccination or infection and may sometimes occur spontaneously.1,2 The pathogenesis of this disease is likely related to a transient autoimmune response to myelin. ADEM has been reported to follow a number of different immunizations and diseases, with the most commonly reported associated illness being a nonspecific upper respiratory infection.2

Disease symptoms begin approximately six weeks following antigenic challenge. The patient often experiences a prodrome of headache, low-grade fever, myalgias, and malaise several days prior to the onset of ADEM.2 Neurologic symptoms then develop rapidly, and may include meningeal signs, seizures, focal neurologic deficits, stupor, and coma.1 Hemiparesis, tetraparesis, cranial nerve palsies, and movement disorders may be apparent during the acute phase. CSF analysis may demonstrate abnormalities in the presence of ADEM, with moderately increase protein and leukocyte levels. Protein electrophoresis of the CSF often reveals oligoclonal bands of IgG, indicating intrathecal antibody production.3 The course of the disease is usually monophasic and affects children more often than adults. Peak severity of the disease occurs within several days, though it may occasionally evolve over several months. The mortality rate is 10-25%, and about 20-30% of survivors are left with permanent neurologic sequelae. Preferred treatment is with IV corticosteroids, which seem to halt disease progression and allow recovery sooner.2,3

In MR imaging, ADEM causes multiple sclerosis-like, but more asymmetrical, white matter lesions. Lesions have been reported to be evident by MR shortly after the onset of symptoms. Disappearance of the lesions is suggested to be associated with clinical recovery, although some reports indicate that the lesions may appear to worsen initially on follow-up imaging, while the patient recovers clinically.3 The lesions are usually multiple and tend to occur in the brainstem, cerebellum, basal ganglia, and deep white matter. T2-weighted MRI is significantly more sensitive than CT in identifying lesions of increased signal intensity in the white matter. The lesions are hypointense on T1-weighted images and may demonstrate solid or ring enhancement following administration of gadolinium.1 Lesions may later become hemorrhagic in a variant of ADEM known as acute hemorrhagic encephalomyelitis (AHEM).3