Transcranial magnetic stimulation in myoclonus of different aetiologies

Transcranial magnetic stimulation (TMS) may represent a valuable tool for investigating important neurophysiological and pathophysiological aspects of myoclonus. Moreover, repetitive TMS (rTMS) can influence neural activity. In this review we performed a systematic search of all studies using TMS in order to explore cortical excitability/plasticity and rTMS for the treatment of myoclonus due to different aetiologies. We identified and reviewed 40 articles matching the inclusion criteria; 415 patients were included in these studies. The reviewed TMS studies have detected abnormalities in motor cortex excitability and sensorimotor plasticity. The most consistent finding is a decrease in intracortical inhibition. Short-interval intracortical inhibition (SICI) is reduced in myoclonic epilepsies. Unlike the juvenile and the benign myoclonus epilepsy, long-interval intracortical inhibition, interhemispheric inhibition and sensorimotor integration were altered in patients with progressive myoclonic epilepsies. In patients with myoclonus-dystonia the results are partly conflicting. Cortical membrane excitability was impaired while parameters assessing cortical synaptic activity were normal in DYT11 gene carriers. In other studies normal SICI suggests that the GABAergic cortical circuits are largely intact and that the mechanisms of myoclonus and dystonia are different from those for cortical myoclonus and other dystonic disorders. In conclusion, different TMS study protocols have provided new insights into sensorimotor plasticity and cortical excitability of the different forms of myoclonus, and have shed some light on the pathophysiology of this movement disorder. Well-defined motor cortical excitability patterns can be identified in the different disorders characterized by myoclonus, even if preliminary findings should be confirmed in future studies in larger cohorts of patients. Repetitive TMS might have therapeutic potential at least in some patients with myoclonus, similar to that reported in other neurological and psychiatric disorders.