Acute disseminated encephalomyelitis (ADEM) is a white matter disease that is indistinguishable from multiple sclerosis by all except
Monophasic
Rapidly lethal
Associated with brainstem and spinal cord disease
Associated with MRI  lesions which may resolve
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.