Does the Modified Fatigue Impact Scale offer a more comprehensive assessment of fatigue in MS?

Background: As a symptom of multiple sclerosis (MS), fatigue is difficult to manage because of its unknown etiology, the lack of efficacy of the drugs tested to date and the absence of consensus about which would be the ideal measure to assess fatigue. Objective: Our aim was to assess the frequency of fatigue in a sample of MS patients and healthy controls (HC) using two fatigue scales, the Fatigue Severity Scale (FSS) and the Modified Fatigue Impact Scale (MFIS) with physical, cognitive and psychosocial subscales. We also studied the relationship fatigue has with depression, disability and interferon beta. Methods: Three hundred and fifty-four individuals (231 MS patients and 123 HC) were included in this cross-sectional study. Fatigue was assessed using the FSS and MFIS. Depression was measured by the Beck Depression Inventory (BDI), and disability by the Expanded Disability Status Scale (EDSS). A status of fatigue was considered when the FSS >= 5, of non-fatigue when the FSS <= 4, and scores between 4.1 and 4.9 were considered doubtful fatigue cases. Results: Fifty-five percent of MS patients and 13% of HC were fatigued. The global MFIS score positively correlated with the FSS in MS and HC (r=0.68 for MS and r=0.59 for HC, p<0.0001). Nonetheless, the MFIS physical subscale showed the strongest correlation score with the FSS (r=0.75, p<0.0001). In addition, a prediction analysis showed the physical MFIS subscale to be the only independent predictor of FSS score (p<0.0001), suggesting other aspects of fatigue, as cognition and psychosocial functions, may be explored by the FSS to a lesser extent. Depression also correlated with fatigue (r=0.48 for the FSS and r=0.7 for the MFIS, p<0.0001) and, although EDSS correlated with fatigue as well, the scores decreased after correcting for depression. Interferon beta showed no relationship with fatigue. Conclusions: Fatigue is a frequent symptom found in MS patients and clearly related with depression. Each fatigue scale correlates with one another, indicating that they are measuring similar constructs. Nevertheless, spheres of fatigue as cognition and psychosocial functions are probably better measured by the MFIS, although this hypothesis will need to be confirmed with appropriate psychometrical testing.