Fatigue and processing speed are related in multiple sclerosis

Background: Fatigue is common in multiple sclerosis (MS) and could be related to impaired processing speed caused by MS specific brain alterations. The objective of this study was to examine the relationship between processing speed and fatigue in patients with relapsing remitting MS. Methods: Patients with EDSS score <= 3.5 were grouped as fatigued [Fatigue Severity Scale (FSS) score >= 5.0] or non-fatigued (FSS score <= 4.0). Patients with FSS scores >= 5 were categorized as primary or secondary fatigued according to various indices. A cognitive test battery obtained from Wechsler's Adult Intelligence Scale-III/Wechsler's Memory Scale-III was applied. Results: Processing speed (Digit Symbol Coding) was lower amongst all MS patients being 9.4(2.9) in primary fatigued, 8.3(2.8) in secondary fatigued and 10.3(2.7) in non-fatigued versus 12.3(3.0) in healthy controls. In the combined group of primary and secondary fatigued MS patients, processing speed was slower than that in non-fatigued MS patients and inversely related to fatigue (r = -0.35; P < 0.05). No such relationship could be established in non-fatigued MS patients or in healthy controls. Conclusion: The degree of fatigue in MS is related to processing speed impairment and longitudinal studies should clarify their mutual dependency.