Measurement of disability in multiple sclerosis

Despite its limitations, the Kurtzke Expanded Disability Status Scale (EDSS) is the gold standard in assessing physical disability in multiple sclerosis (MS). Sustained progression in EDSS has been used widely as the endpoint in therapeutic studies. However, the patients' disability often fluctuates or improves over time; and up to half of the patients with "sustained progression" of EDSS eventually regress to their baseline. It is, therefore, not a sensitive measure to define irreversible progression of disease. The Multiple Sclerosis Severity Score (MSSS) is a useful measure of MS severity, incorporating the EDSS and disease duration. It has some prognostic significance for individuals with mild or severe disease, but is best suited to cross-sectional comparative studies. Cognitive impairment in MS is often considerable, and principally affects the domains of attention, vigilance, processing speed, working memory and executive function. Tests such as paced auditory serial addition test (PASAT) and the symbol digit substitution test (SDT) are therefore quite sensitive to change over time. The Multiple Sclerosis Functional Composite (MSFC), a score combining a measure of lower limb function (timed walk), upper limb function (nine-hole peg test) and cognitive function (PASAT), is useful to detect disability progression in MS trials, as it is sensitive to change over time, but is too time-consuming to become adopted in clinical practice.