Multiple Sclerosis Phenotypes as a Continuum: The Role of Neurologic Reserve

PURPOSE OF REVIEW: This review presents the hypothesis that loss of neurologic reserve explains onset of progressive multiple sclerosis (PrMS). RECENT FINDINGS: Evidence supporting the separate classification of PrMS and relapsing multiple sclerosis (RMS) is limited and does not explain PrMS or the response of these patients to therapy. SUMMARY: We argue that multiple sclerosis (MS) progresses along a continuum from RMS to PrMS, with differing levels of neurologic reserve accounting for phenotypic differences. In early MS, inflammation causes brain atrophy with symptoms buffered by neurologic reserve. As brain loss from normal aging and MS continues, reserve is depleted and effects of subclinical MS disease activity and aging are unmasked, manifesting as PrMS. Most therapies show limited benefit in PrMS; patients are older, have fewer inflammatory events, and the effects of aging cause continued loss of neurologic function, even if inflammation is terminated. Loss of neurologic reserve means patients with PrMS cannot recover function, unlike patients with RMS.