Objective: To investigate left-right motor cortex asymmetry in the pathophysiology of essential tremor (ET).

Background: ET is typically bilateral but usually manifests with mild asymmetry between the left and right upper limbs. Early studies with accelerometric measures in clinic-based samples reported a predominance of right-hand tremor, whereas later investigations with clinical measures in community-based samples suggested an opposite predominance of left-hand tremor (Louis et al., Arch Neurol 1998). Since cortical excitability studies in ET have focused only on the left cortex, the possible role of left-right motor cortex asymmetry in the pathophysiology of ET remains unknown.

Method: We studied a total of 43 patients with ET. First, we assessed hand postural tremor with accelerometry and kinetic tremor with part B of the clinical rating scale for tremor (CRST) in order to quantify possible left-right clinical asymmetry. Second, we adopted a non-invasive brain stimulation approach, by performing a randomized double-blind study to test whether 30-min of transcranial static magnetic field stimulation (tSMS) of the left (n=13) or right (n=14) primary motor cortex (M1) symmetrically reduces contralateral tremor in ET patients (n=27; NCT03780426). Third, we adopted a mechanistic neurophysiological approach, by applying paired-pulse transcranial magnetic stimulation (TMS) protocols to measure possible left-right asymmetry of M1 excitability in ET patients (n=23) compared to healthy controls (n=20).

Results: The amplitude and frequency of postural tremor did not show any left/right-hand predominance, whereas kinetic tremor displayed a significant left-hand predominance. Unilateral tSMS of M1 significantly reduced contralateral postural and bilateral kinetic tremor, but only when tSMS was applied to the right hemisphere. ET patients compared to controls displayed no differences in M1 excitability in the left hemisphere, but showed a significant reduction of short-interval intra-cortical inhibition (SICI) in the right hemisphere.

Conclusion: These results suggest that the right motor cortex is specifically involved in the pathophysiology of essential tremor.

References: Louis, E. D., Wendt, K. J., Pullman, S. L., & Ford, B. (1998). Is Essential Tremor Symmetric? Archives of Neurology, 55(12), 1553. https://doi.org/10.1001/archneur.55.12.1553

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MDS Abstracts - https://www.mdsabstracts.org/abstract/left-right-motor-cortex-asymmetry-in-the-pathophysiology-of-essential-tremor/