Objective: To investigate the role of the cerebellum in the pathophysiology of cortical myoclonus (CM).

Background: The putative involvement of the cerebellum in the pathogenesis of CM syndromes has been long hypothesized, as pathological changes in patients with CM have commonly been found in the cerebellum rather than in the suspected culprit, the sensorimotor cortex (1). The hypothesis is that the increased cortical excitability seen in CM is due to loss of the cerebellar inhibitory control via cerebello-thalamo-cortical connections. Here, we explore this hypothesis by modulating cerebellar excitability by means of transcranial Direct Current Stimulation (tDCS), and assessing its effect on sensorimotor cortex excitability in patients with CM.

Method: We recruited 7 patients (1 male, age 47.2 ± 19.8) with CM, wither idiopathic or secondary to different conditions. All of them underwent the following neurophysiological tests: short intracortical inhibition (SICI), somatosensory evoked potentials (SEP) and long-latency reflexes (LLR), tested before and after anodal cerebellar tDCS  applied on the cerebellar hemisphere ipsilateral to the most affected side. The assessment also included eye-blink classical conditioning (EBCC) and the Unified Myoclonus Rating Scale (UMRS).  Data were compared with those obtained in 5 healthy controls (HC).

Results: UMRS scale score was 93.7±33.4. Patients had a smaller number of CR (conditioned responses) in blocks I-VI of EBCC compared to HC. The amplitude of N20-P25 and P25-N33 components of SEP was increased in patients after tDCS, but not in HC. A similar trend was observed in LLR, with a significant increase in amplitude before and after tDCS in patients. Baselince SICI was reduced in patients compared to HC. Whereas tDCS caused a further SICI reduction in patients, it did not change SICI in HC.

Conclusion: According to our data, anodal cerebellar tDCS increases sensorimotor cortex excitability in CM. This is in contrast with previous work showing a decrease in LLR following cerebellar anodal TDCS (2). Overall, the present results suggest a role of the cerebellum in the pathophysiology of CM, and that CM patients might have abnormal homeostatic plasticity within the sensorimotor cortex, possibly responsible for this paradoxical response.

References: 1. The role of the cerebellum in the pathogenesis of cortical myoclonus. Ganos C, Kassavetis P, Erro R, Edwards MJ, Rothwell J, Bhatia KP. Mov Disord. 2014 Apr;29(4):437-43. doi: 10.1002/mds.25867. 2. Anodal transcranial direct current stimulation (tDCS) decreases the amplitudes of long-latency stretch reflexes in cerebellar ataxia. Grimaldi G1, Manto M. Ann Biomed Eng. 2013 Nov;41(11):2437-47. doi: 10.1007/s10439-013-0846-y.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/the-role-of-the-cerebellum-in-the-pathophysiology-of-cortical-myoclonus-a-neurophysiological-study/