Malignant multiple sclerosis
The term malignant multiple sclerosis is used to describe MS patients who reach significant level of disability in a short period of time. 
The National MS Society Advisory Committee on Clinical Trials of New Agents consensus defined it as: disease with a rapid progressive course, leading to significant disability in multiple neurologic systems or death in a relatively short time after disease onset. Reaching Expanded Disability Status Scale of 6.0 or higher, which is equivalent of needing unilateral support to ambulate  (or worse) is generally considered this significant disability level.
Patients with severe forms of more common relapsing remitting or progressive MS subtypes, as well as rare Marburg variant and Balo concentric sclerosis, could be considered to have malignant MS. Patients should be carefully worked up to rule out Neuromyelitis optica (Devic's disease) due to the distinctive pathophysiology and management strategies of this disease.
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