Abstract
Fatigue is a common and highly disabling symptom of multiple sclerosis. Patients experience an effort-independent general subjective feeling of fatigue as well as excessive fatigability when engaging in physical or mental activity. Previous research using functional magnetic resonance imaging (fMRI) has revealed heterogeneous findings, but some evidence implicates the motor system. To identify brain correlates of fatigue, 44 mildly impaired patients with relapsing-remitting multiple sclerosis and 25 age- and gender-matched healthy controls underwent functional magnetic resonance imaging at 3 Tesla, while they performed alternating blocks of rest and a non-fatiguing precision grip task. We investigated neural correlates of fatigue using the motor subscore of Fatigue Scale for Motor and Cognitive Functions (FSMCMOTOR) using the bilateral motor cerebellum, putamen, and dorsal premotor cortex as regions of interest. Patients and healthy controls performed the grip force task equally well without being fatigued. In patients, task-related activity in lobule VI of right motor cerebellum changed in proportion with individual FSMCMOTOR scores. In right dorsal premotor cortex, linear increases in activity across consecutive task blocks scaled with individual FSMCMOTOR scores in healthy controls, but not in patients. In premotor and dorsomedial prefrontal areas, patients were impaired at upscaling task-related activity the more they were affected by motor fatigue. The results support the notion that increased sensorimotor processing in the cerebellum contributes to the experience of motor fatigue and fatigability in multiple sclerosis. Additionally, downscaling of motivational input or sensorimotor processing in prefrontal and premotor areas may constitute an additional pathophysiological factor.
Highlights
Fatigue is one of the most common and disabling symptoms in multiple sclerosis (MS) [1,2,3]
The total Beck Depression Inventory II (BDI-II) score indicated the presence of severe symptoms of depression, these patients had no clinical diagnosis of depression
We show that the functional activation of the motor cerebellum during a non-fatiguing tonic grip force task reflects the severity of motor fatigue and fatigability experienced during daily life
Summary
Fatigue is one of the most common and disabling symptoms in multiple sclerosis (MS) [1,2,3]. Effort-independent fatigue is often referred to as the “trait” fatigue, whereas fatigability is referred to as a “state” feature of fatigue. The precise mechanisms that determine the emergence and magnitude of effort-independent “state” fatigue and effort-evoked “trait” fatigability in a given patient are still largely unknown and treatment remains a challenge [7]. Whatever the causal mechanisms may be, the experience of excessive fatigue and fatigability are underpinned by pathophysiological changes in functional brain networks [8]. Identifying brain activity that scales with the experience of fatigue and fatigability during everyday life may reveal important insights into the pathophysiology of fatigue, but may be a step towards establishing neuroimaging-based biomarkers of fatigue that can supplement the subjective clinical scores
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