Acute disseminated encephalomyelitis (ADEM) is a brief (several weeks to months) but intense attack of inflammation in the brain and spinal cord (and occasionally the optic nerve) that causes damage to the myelin — the protective covering around nerve fibers in the central nervous system.
The symptoms of ADEM come on quickly, typically beginning with symptoms of encephalopathy, including behavioral changes (e.g., confusion, excessive irritability) and an alteration in consciousness (e.g., lethargy, coma). The damage to myelin may also cause neurological symptoms similar to those seen in MS — such as visual loss (due to inflammation of the optic nerve) in one or both eyes, weakness, and loss of coordination. ADEM may occur in association with a viral or bacterial infection, as a complication of inoculation or vaccination, or without a preceding cause.
ADEM is sometimes misdiagnosed as a severe first attack of MS since some of the symptoms of the two disorders may be similar. However, ADEM usually (but not always) consists of a single episode or attack, while MS features many attacks over the course of time. Doctors will often use magnetic resonance imaging (MRI) to search for old and new lesions (areas of damage) on the brain and spinal cord because old areas of damage seen on MRI suggest that the condition is ongoing and may be MS rather than ADEM.
In the absence of old lesions, it is difficult to discriminate severe first attacks of what will eventually be diagnosed as MS from ADEM. For this reason, the physician carefully monitors the person with ADEM to see if a recurrence or progression suggests that it might, in fact, be MS. Children are more likely than adults to have ADEM.
Differentiating ADEM from MS is important because the treatment recommendations differ significantly; people with MS will usually require some form of continuing immune treatments while those with ADEM will not.