Objective: Essential Tremor (ET) is the most common movement disorder; however, its pathophysiology is still unclear. We recorded neuronal activity in ventral intermediate nucleus of the thalamus (Vim) and tremor from the contra-lateral limb to further elucidate the involvement of Vim in the pathophysiology of ET.

Background: Severe cases of ET have been successfully treated with Deep Brain Stimulation of the Vim. The nucleus receives primarily cerebellar afferents with projections to the motor cortex and adjoining cortical areas involved in movement initiation and control. Kinesthetic, tremor, and voluntary cells are found within the Vim.

Methods: Microelectrode recordings were made intra-operatively from 9 awake patients during the implantation of Deep Brain Stimulation electrodes. Movements from the contralateral hand were also recorded during intention tremor and rest state using accelerometers.

Results: Spike sorting yielded 66 units (27 single cell activity and 39 multi-cell activity) recorded during rest (n= 54) and tremor (n= 32). We characterised the firing rate, action potential duration, burst index, mean spikes per burst, burst rate and proportion of spikes in bursts between tremor and rest. No significant difference was found. Single and multi-units showed phase locking with local field potential (LFP) at a range of frequencies (2.6-25 Hz) during tremor (22% of cells) and rest (11% of cells). During tremor, LFP power was selectively increased at tremor frequencies and showed a strong correlation with tremor intensity at a sub-second time scale. A phase coherence analysis revealed that LFP lagged behind the accelerometric signal with a delay time of 260(35) ms (mean(SEM)). Further analysis showed strong frequency coupling between LFP and tremor at a sub-second time scale, which included a ~0.5 Hz oscillation of the tremor frequency.

Conclusions: This study revealed differences in phase locking between tremor and baseline activity in Vim neurons, despite no differences being found with classical firing indexes. LFP and accelerometer measures of hand tremor were coupled both in amplitude and frequency, with a tendency for the tremor to lead the LFP. We also found a periodical fluctuation of the tremor frequency at ~0.5 Hz both in LFP as well as the accelerometer signal, which appears to be a previously unreported pathological feature of intention tremor.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/multiple-single-unit-activity-in-ventral-intermediate-thalamus-of-essential-tremor-patients-during-intention-tremor/