Is the EDSS an Adequate Measure in Secondary Progressive MS?

Jennie Smith

January 29, 2021

Clinical trials enrolling patients with progressive multiple sclerosis (MS) commonly use the Expanded Disability Status Scale (EDSS), an instrument that looks at impairment across several different functional domains, as a primary outcome measure. But results from a new analysis of trial data suggest that the EDSS, in use since 1983, may be a noisier or more variation-prone measure of MS disease progression than two other widely used measures: the timed 25-foot walk and the 9-hole peg test.

For their research, published in the Jan. 5 issue of NeurologyMarcus W. Koch, MD, PhD, of the department of neurosciences at Hotchkiss Brain Institute at the University of Calgary (Alta.) and colleagues looked at data from the placebo arms of two randomized trials that collectively enrolled nearly 700 patients with secondary progressive MS (SPMS). The trials were similar in terms of baseline patient characteristics and level of disability.

Comparing Three Outcome Measures

The investigators compared disability progression and improvement across each of the three instruments and their combinations. Because improvement is understood to occur only rarely in untreated secondary progressive MS, most improvement picked up in the placebo arm of a trial is assumed to be noise from random variation or measurement error.

Koch and colleagues found that the EDSS showed higher rates of improvement than the other tests. The EDSS also showed the smallest differences between progression and improvement among the three instruments, with improvement rate over time increasing in parallel with disability progression rates. With the other two tests, improvement rates remained low – at 10% or less – while disability was seen steadily increasing over time.

The results, the investigators wrote in their analysis, suggest that the timed 25-foot walk and 9-hole peg test are the more reliable outcome measures. The reason "may simply lie in the fact that both the timed 25-foot walk and 9-hole peg test are objective and quantitative interval-scaled measures while the EDSS is a graded categorical measure." As primary outcome measures in clinical trials, "the lower noise of the timed 25-foot walk and 9-hole peg test may make them preferable over the EDSS," Koch and colleagues concluded. The investigators noted that a 2019 analysis of different MS disability scales across more than 13,000 patients in 14 trials did not find such stark differences – but that the patients in the pooled trials had less disability at baseline (median EDSS score of 2.5, compared with 6.0 for the two trials in Koch and colleagues' study). This suggests, the investigators wrote, "that the timed 25-foot walk and 9-hole peg test may be more useful outcomes in patients with a progressive disease course and with greater baseline disability."

'Considerable Implications' for the Design of Future Clinical Trials

In an accompanying editorialTomas Kalincik, MD, PhD, of the University of Melbourne, along with colleagues in Italy and Britain, praised Koch and colleagues' study as having "considerable implications for the design of future clinical trials because detecting a treatment effect on an outcome that is subject to large measurement error is difficult." Most trials in progressive MS use change in EDSS score as their primary or key secondary outcomes. "However, as the authors elegantly show, other, more reliable clinical outcomes are needed. As we are revisiting our biological hypotheses for treatment of progressive MS, perhaps the time has come that we should also revisit the instruments that we use to examine their efficacy."

The editorialists allowed for the possibility that something besides noise or measurement error could be responsible for the disparities seen across the instruments. "An alternative interpretation of the presented results could be that recovery of neurologic function is more common in SPMS than what we had previously thought and that EDSS is more sensitive to its detection than the other two measures," they wrote.

Koch and colleagues' study received no outside funding. Koch disclosed consulting fees and other financial support from several drug manufacturers, and three coauthors also disclosed financial relationships with pharmaceutical companies. All three editorial writers disclosed similar relationships.

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