Longitudinal Changes in Cognitive Test Scores in Patients With Relapsing-Remitting Multiple Sclerosis: An Analysis of the DECIDE Dataset.

BACKGROUND AND OBJECTIVES: Cognitive impairment is a common and impactful symptom of relapsing-remitting multiple sclerosis (RRMS). Cognitive outcome measures are often used in cross-sectional studies, but their performance as longitudinal outcome measures in clinical trials is not widely researched. In this study, we used data from a large clinical trial to describe change on the Symbol Digit Modalities Test (SDMT) and the Paced Auditory Serial Addition Test (PASAT) over up to 144 weeks of follow-up. METHODS: We used the data set from DECIDE (clinicaltrials.gov identifier NCT01064401), a large randomized controlled RRMS trial to describe change on the SDMT and PASAT over 144 weeks of follow-up. We compared change on these cognitive outcomes with change on the timed 25-foot walk (T25FW), a well-established physical outcome measure. We investigated several definitions for clinically meaningful change: any change, 4-point change, 8-point change, and 20% change for the SDMT, any change, 4-point change, and 20% change for the PASAT, and 20% change for the T25FW. RESULTS: DECIDE included 1,814 trial participants. SDMT and PASAT scores steadily improved throughout follow-up: the SDMT from a mean 48.2 (SD, 16.1) points at baseline to 52.6 (SD 15.2) at 144 weeks and the PASAT from 47.0 (SD 11.3) at baseline to 50.0 (SD 10.8) at 144 weeks. This improvement in scores is most likely due to a practice effect. Throughout the trial, participants were more likely to experience improvement than worsening of their SDMT and PASAT performance, whereas the number of worsening events on the T25FW steadily increased. Changing the definition of clinically meaningful change for the SDMT and PASAT or using a 6-month confirmation changed the overall number of worsening or improvement events but did not affect the overall behavior of these measures. DISCUSSION: Our findings suggest that the SDMT and PASAT scores do not accurately reflect the steady cognitive decline that people with RRMS experience. Both outcomes show postbaseline increases in scores, which complicates the interpretation of these outcome measures in clinical trials. More research into the size of these changes is needed before recommending a general threshold for clinically meaningful longitudinal change.