It is simply the clinical course. ADEM is monophasic, and replaces by large the old terms post- or parainfectious encephalitis. Marburg variant is a variant of MS with foudrouyant clinical presentation, and relapses, be it relatively benign or another big relapse. Neuropathology makes no real difference except for the Hurst variant of ADEM.

I hope that following paper be useful.

Regards

Article Fulminant Demyelinating Diseases

My little knowledge is that Marburg is an acute MS variant with numerous large multifocal demyelinating lesions occurring in the deep white matter. The clinical course seems to be different; being more fulminant & death is inevitable without aggressive immunotherapy. ADEM in the contrary is more of transient and  self limiting disease.

You may would like to scan this review.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3726117/