Responsiveness of the Expanded Disability Status Scale (EDSS) to disease progression and therapeutic intervention in progressive forms of multiple sclerosis

Introduction. The standard approach in relapsing forms of multiple sclerosis (MS) has been to measure therapeutic effects on clinical exacerbations and physical disability as determined by the Expanded Disability Status Scale (EDSS). However, measuring clinical relapses is not a viable option in the progressive forms of MS because of their low frequency. Therefore, the standard approach in clinical trials of progressive forms of MS has been to use the EDSS as primary outcome measure. Patients and methods. We examined the responsiveness of the EDSS to disease progression and treatment effects in the context of clinical trials of secondary progressive (SPMS) and primary progressive (PPMS) MS and compared it to the three functional tasks of the Multiple Sclerosis Functional Composite (MSFC): the Timed 25 Foot Walk (T25FW), the 9 Hole PEG (9HP), and the Paced Auditory Serial Attention Test (PASAT). Results. The effect size of the EDSS after two years on placebo was only 0.2-0.3 in both SPMS and PPMS, similar to the 9HP and the PASAT. In contrast, the effect size of the T25FW was much greater and driven to a large extent by subjects who could not complete the task. Conclusions. The EDSS shows poor responsiveness to both disease progression and treatment effects in SPMS and PPMS. The use of alternative primary outcome measures is recommended for therapeutic trials of progressive MS.