The transitional phase of multiple sclerosis: Characterization and conceptual framework.

The conversion of relapsing-remitting multiple sclerosis (RRMS) to secondary progressive MS (SPMS) cannot be defined by a sharp threshold determined by event-based measures, but rather represents a gradual process. In consequence, there may exist a transitional phase between RRMS and clearly established SPMS. So far, transitional MS has been poorly characterized in terms of patient properties, course of disease and therapeutic interventions that may delay conversion to SPMS. Furthermore, the pathogenesis of transitional MS is incompletely understood, and no definitive imaging or laboratory test informs when exactly a patient has entered the transitional MS phase. Here we review the current knowledge and evidence characterizing the transitional phase of MS and propose potential designs and criteria for a prospective clinical study in patients with transitional MS.