Fatigue in multiple sclerosis: relationship to depression, disability, and disease pattern.

In order to investigate the associations between fatigue and depression, disability, and disease subtype, 207 individuals with clinically definite Multiple Sclerosis (MS) were administered the Fatigue Severity Scale and the Zung Self-rating Depression Scale during a regular clinic appointment. Their current level of disability was established using the Expanded Disability Status Scale. Fatigue and depression were highly correlated (r=0.58), even when the depression measure was corrected for items overlapping with fatigue and other symptoms or consequences of MS (r=0.44). Fatigue and disability were also correlated (r=0.33). Multiple regression revealed that both depressed mood and disability were significant predictors of fatigue, together accounting for approximately 23% of the variance in patients' self-reported fatigue. The combined groups of primary progressive (n=45) and secondary progressive patients (n=25) appeared to have higher fatigue scores than relapsing-remitting patients (n=137). However, an analysis of covariance revealed that this apparent difference was in fact attributable almost exclusively to differences in disability among the three subtypes of MS. Other reports of differences in fatigue between subtypes of MS should be re-examined in light of this finding.