Acute disseminated encephalomyelitis (ADEM) is a white matter disease that is indistinguishable from multiple sclerosis by all except
ADEM is a demyelinating disease of the brain ,brainstem and spinal cord that is indistinguishable from MS on MRI. However,it is monophasic,  that is  it occurs acutely on a single occasion and not in a recurrent fashion like MS.
Monophasic
Rapidly lethal
Associated with brainstem and spinal cord disease
Associated with MRI  lesions which may resolve