Cognitive progression independent of relapse in multiple sclerosis

Background: Substantial physical-disability worsening in relapsing-remitting multiple sclerosis (RRMS) occurs outside of clinically recorded relapse. This phenomenon, termed progression independent of relapse activity (PIRA), is yet to be established for cognitive decline. Methods: Retrospective analysis of RRMS patients. Cognitive decline was defined using reliable-change-index cut-offs for each test (Symbol Digit Modalities Test, Brief Visuospatial Memory Test-Revised, California Verbal Learning Test-II). Decline was classified as PIRA if the following conditions were met: no relapse observed between assessments nor within 9 months of cognitive decline. Results: The study sample (n = 336) was 80.7% female with a mean (standard deviation (SD)) age, disease duration, and observation period of 43.1 (9.5), 10.8 (8.4), and 8.1 (3.1) years, respectively. A total of 169 (50.3%) subjects were cognitively impaired at baseline relative to age-, sex-, and education-matched HCs. Within subjects who experienced cognitive decline (n = 167), 89% experienced cognitive PIRA. A total of 141 (68.1%) cognitive decline events were observed independent of EDSS worsening. Cognitive PIRA was more likely to be observed with increased assessments (p < 0.001) and lower assessment density (p < 0.001), accounting for baseline clinical factors. Conclusion: These results establish the concept of cognitive PIRA and further our understanding of progressive cognitive decline in RRMS.