Physical and psychological aspects of multiple sclerosis: Revisiting the Multiple Sclerosis Impact Scale (MSIS-29).

Background: The MSIS-29 measures the physical and psychological impact of MS.

Objective: The associations between MSIS-29 domains and demographic/clinical aspects were examined and trajectories analysed over time.

Methods: Data were collected in the Trajectories of Outcome in Neurological Conditions study for a diverse population of people with MS, with follow-up for up to 5 years. Following Rasch analysis, minimal important change (MIC) was computed for ensuing total, physical and psychological domains.

Results: Fit to the Rasch model using data from 5921 participants validated physical, psychological and total domains, and the conversion table transforms raw scores to interval-level metric equivalents. These domains showed significant differences across demographic (age, gender, employment, education, and marital status) and clinical (subtype, treatment, and duration) factors with large effect sizes. The MIC scores were physical: 9.1, total: 14.1, which were both above measurement error, and psychological: 5.5 which was not, so 1.6% of participants reported psychological change which was clinically important but not statistically significant. Trajectory analysis showed three groups, one stable and two with significant slopes, improving and deteriorating.

Conclusion: The MSIS-29 has shown adequate fit to the Rasch model after accommodating problems with local item dependency, through a bi-factor solution. The domains showed good discrimination across key factors.