Acute disseminated encephalomyelitis (ADEM) is characterized by a brief but intense attack of inflammation in the brain and spinal cord that damages myelin – the protective covering of nerve fibers. It often follows viral infection, or less often, vaccination for measles, mumps, or rubella.
The incidence rate of ADEM is about 8 per 1,000,000 people per year. Children are more likely than adults to have ADEM, although it occurs in all ages. The average age of onset is around 5 to 8 years old.
The symptoms of ADEM come on quickly, beginning with encephalitis-like symptoms such as:
It may also damage white matter (brain tissue that takes its name from the white color of myelin), leading to neurological symptoms such as:
ADEM is sometimes misdiagnosed as a severe first attack of multiple sclerosis (MS), since some of the symptoms of the two disorders, particularly those caused by white matter injury, may be similar. However, ADEM usually has symptoms of encephalitis, as well as symptoms of myelin damage, as opposed to MS, which doesn’t have encephalitis symptoms. In addition, ADEM usually consists of a single episode or attack, while MS features many attacks over the course of time. Doctors will often use imaging techniques, such as MRI (magnetic resonance imaging), to search for old and new lesions (areas of damage) on the brain. Old “inactive” brain lesions on MRI suggest that the condition may be MS rather than ADEM, since MS often causes brain lesions before symptoms become obvious. In rare situations, brain biopsy may show findings that allow differentiation between ADEM and severe, acute forms of MS.
Treatment for ADEM is targeted at suppressing inflammation in the brain using anti-inflammatory drugs. Most individuals respond to intravenous corticosteroids such as methylprednisolone. When corticosteroids fail to work, plasmapheresis or intravenous immunoglobulin therapy has been shown to produce improvement. Additional treatment is symptomatic and supportive.
Corticosteroid therapy can shorten the duration of neurological symptoms and halt further progression of the disease in the short term, but the long term prognosis for individuals with ADEM varies. Full recovery is seen in 50 to 75% of cases, while up to 70 to 90% recover with some minor residual disability. The average time to recover is one to six months. Severe cases of ADEM can be fatal. Some individuals who initially diagnosed as having ADEM will later be reclassified as MS, but there is currently no method to determine whom those individuals will be.