Attention-related changes in short-term cortical plasticity help to explain fatigue in multiple sclerosis

Background: In multiple sclerosis (MS), pathophysiology of fatigue is only partially known. Objective: The aim of this study was to investigate whether the attention-induced modulation on shortand long-term cortical plasticity mechanisms in primary motor area (M1) is abnormal in patients with MS-related fatigue. Methods: All participants underwent 5-Hz repetitive transcranial magnetic stimulation (rTMS), reflecting short-term plasticity, and paired associative stimulation (PAS), reflecting long-term plasticity, and were asked to focus their attention on the hand contralateral to the M1 stimulated. A group of agematched healthy subjects acted as control. Results: In patients with MS, 5-Hz rTMS and PAS failed to induce the normal increase in motor-evoked potential (MEP). During the attention-demanding condition, 5-Hz rTMS- and PAS-induced responses differed in patients with MS with and without fatigue. Whereas in patients with fatigue neither technique induced the attention-induced MEP increase, in patients without fatigue they both increased the MEP response, although they did so less efficiently than in healthy subjects. Attention-induced changes in short-term cortical plasticity inversely correlated with fatigue severity. Conclusion: Short-term and long-term plasticity mechanisms are abnormal in MS possibly owing to widespread changes in ion-channel expression. Fatigue in MS reflects disrupted cortical attentional networks related to movement control.