Subjective fatigue is not associated with cognitive impairment in multiple sclerosis: cross-sectional and longitudinal analysis

Background Studies in multiple sclerosis (MS) report conflicting conclusions regarding fatigue and cognition, which may partly be due to the use of small sample sizes and frequent reliance on a cross-sectional approach. Objective The ability to distinguish between these two disabling symptoms is necessary in order to properly assess and treat MS patients. Methods In a retrospective analysis, we assessed the correlation between fatigue and neuropsychological (NP) testing using a cross-sectional (n = 465) and longitudinal approach (n = 69). Cognition was measured using a comprehensive battery called the Minimal Assessment of Cognitive Function in MS (MACFIMS), and fatigue was measured with the Fatigue Severity Scale (FSS). FSS scores were categorized as normal (<= 4.0), borderline fatigue (4 < FSS < 5.0), and fatigued (>= 5.0). Repeat assessments (n = 69) were categorized as improved or worsened by a change in FSS of either 0.5 or 1.0. Results MS patients had significantly higher FSS scores than normal controls (P < 0.001). No correlation was found between FSS and NP scores in either cross-sectional or longitudinal analyses. Fatigue was moderately correlated with depression, assessed using the Beck Depression Inventory Fast Screen (BDIFS) (r = 0.44, P < 0.001). Longitudinally, there was a medium correlation between change in FSS and BDIFS (r = 0.34, P = 0.001), but no significant differences on NP scores using either definition of change. Conclusion We conclude that self-reported fatigue, while correlated with self-reported depression, is not significantly related to cognitive capacity in MS. Multiple Sclerosis 2009; 15: 998-1005. http://msj.sagepub.com