Self-reported sleep problems, but not fatigue, lead to decline in sustained attention in MS patients

Objective: According to the compensation theory, fatigue in MS results from efforts to compensate for a reduction in capacity due to demyelination or neurodegeneration. Recently, it has been argued that fatigue may result from poor sleep. Both explanations predict a worsening of fatigue and a cognitive decline during sustained attention tasks ( higher fatigability). Method: We compared MS patients with and without self-reported cognitive fatigue, in three sessions with a two-back working memory task, registering hits and response latencies as well as changes in fatigue. In the two breaks between the sessions, either a video instruction to relax or a stimulating video was presented. Subsequently, patients were divided into those with and those without self-reported sleep problems and the analyses were repeated. Results: Patients with fatigue performed worse than healthy controls, irrespective of task duration and type of video during the break. The task-related increase of fatigue also did not differ between the groups and no differential effect of the videos was observed in the MS patients with fatigue. In contrast, patients with sleep problems did show a performance decline as predicted by the compensation theory. Conclusion: MS patients with fatigue were impaired in working memory, but did not show greater fatigability, whereas MS participants with self-reported sleep problems showed fatigability, which could be improved with a restorative rest period. Our data therefore do not support the compensation theory of fatigue, and we argue that sleep problems and fatigue in MS patients differ with respect to their functional consequences.