Acute disseminated encephalomyelitis (ADEM) is a nonvasculitic inflammatory demyelinating condition that bears a striking clinical and pathological resemblance to multiple sclerosis (MS).
It 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 symptoms of ADEM come on quickly, beginning with encephalitis-like symptoms such as fever, fatigue, headache, nausea and vomiting and in severe cases, seizures and coma. It may also damage white matter (brain tissue that takes its name from the white color of myelin), leading to neurological symptoms such as visual loss (due to inflammation of the optic nerve) in one or both eyes, weakness even to the point of paralysis and difficulty coordinating voluntary muscle movements (such as those used in walking).
ADEM is sometimes misdiagnosed as a severe first attack of multiple sclerosis (MS) because some of the symptoms of the two disorders, particularly those caused by white matter injury, are similar. However, ADEM usually has symptoms of encephalitis (such as fever or coma), as well as symptoms of myelin damage (visual loss, paralysis), 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.
Clinically, acute ADEM is usually readily distinguishable from MS by the presence of certain clinical features, including the following: history of preceding infectious illness or immunization, association with constitutional symptoms and signs such as fever, prominence of cortical signs such as mental status changes and seizures, comparative rarity of posterior column abnormalities, which are common in MS age older than 11-12 years in MS and age younger than 11-12 years in ADEM and .
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.
ADEM is more common in the winter months, with as many as 65-85% of cases occurring between October and March. The mean age at presentation is about 7 years, with a range that extends from the first year of life to adulthood. Typical cases of ADEM arise 1-20 days after a childhood infectious illness, which is febrile in more than 94% of cases.
Various vaccines have been suggested as the exogenous provocation of cases of ADEM and may account for 3-6% of ADEM cases. Measles was associated with ADEM in about 1 out of 800 cases, and in many of these cases, ADEM that was often particularly severe. Measles-associated ADEM had a high rate of both morbidity and mortality.
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. For most, recovery begins within days and half will recover completely. Others may have mild to moderate lifelong impairment. Severe cases of ADEM can be fatal.