The Multiple Sclerosis Severity Score re-examined: Expanded Disability Status Scale rank stability in the MSBase dataset increases five years after onset of multiple sclerosis

Background: The Multiple Sclerosis Severity Score (MSSS) is the Expanded Disability Status Scale (EDSS)-based severity rank for a given disease duration. Roxburgh et al suggested that the MSSS can be used to compare MS severity in large groups of patients for the purposes of, for example, genetic studies of putative disease modifier genes. However, a high rank stability would also suggest utility as an individual outcome predictor. Objective: To assess the stability of MSS scores in the MSBase dataset, a large multi-center cohort study of MS outcomes. Methods: Data was extracted on 4th of April 2008. The dataset comprised 6100 patients with all forms of MS, with 38,683 complete EDSS scores. In individuals, multiple EDSS scores recorded within a specified disease duration interval were averaged. We used the 0-2.5 year interval as 'diagnosis', the 2.5-7.5 year interval as '5-year duration', the 7.5-12.5 year interval as '10-year duration' and the 12.5-15 year interval as '15-year duration'. Groups of patients with EDSS scores spanning these pre-specified disease duration intervals were analysed. Patients were ranked within each interval and 5-year MSSS stability assessed using Spearman rank-sum correlations. Rank stability was also assessed descriptively as the proportion of patients remaining within 15% of their original rank in sequential 5-year intervals. Results: Mean EDSS scores were 2.02 at diagnosis, 2.67 at 5 years, 3.15 at 10 years and 3.49 at 15 years. Spearman rank-sum correlation between MSSS at diagnosis and 5 years was r=0.66 (1692 patients), between 5 and 10 years r=0.80 (1546 patients) and between 10 and 15 years r=0.88 (893 patients). In the diagnosis-5 year interval, 60% of patients remained within 15% of their original MSSS. In the 5-10 year interval, this increased to 75% and, in the 10-15 year interval, to 83%. Conclusions: MSSS rank stability increases with increasing disease duration, probably due to reduced influence of relapses and non-relapse fluctuations of mild neurological signs on EDSS evaluations. MSSS at 5 years from diagnosis is already highly predictive of MSSS at 10 years, and may therefore have predictive prognostic value in individual patients.