Objective: To compare somatosensory temporal discrimination threshold (STDT) with and without tremor in a cohort of essential tremor (ET) patients. Additionally, to compare STDT previously and 4 months after unilateral (left) high intensity focused ultrasound (HIFU) thalamotomy in a subgroup of this cohort.

Background: STDT is the ability to identify two successive stimuli as separate in time. In healthy individuals it is about 30-80ms, and in ET it yields similar values. STDT is known to raise with movements of the analyzed body part (the so-called sensory gating mechanism), but this has not been tested during involuntary movements. On the other hand, STDT has not been assessed in subjects treated with ventral intermediate nucleus (VIM) HIFU thalamotomy to date.

Method: 21 ET patients were recruited. Basal characteristics were collected. STDT was determined by a 10ms ascending step paradigm, with paired stimuli delivered over the index finger, starting with a simultaneous pair. The first of three consecutive interstimulus intervals at which patients recognized the pair as separate was considered the STDT. STDT was tested with and without tremor on the right arm and without tremor on the left arm. Somatosensory evoked potentials (SSP) were recorded with and without tremor in 2 patients. STDT was retested in 5 patients 4 months after HIFU on both sides. Underlying polyneuropathy was discarded by means of electroneurography of median nerves.

Results: 14 patients (66.6%) were men. Mean age was 69.73 (SD 10.43) y.o. Mean disease duration was 22.19 (SD 10.49) years. Mean basal FTM scale was 59.8 (SD 19.34). Basal STDT on the right arm with tremor was 126.13ms and without tremor 91.96ms (t paired test; p<0.001). An attenuation of N20 amplitude of SSP was observed in both patients during tremor. In the group of 5 patients who were treated by HIFU, STDT on the right arm at baseline without tremor was 96.6 and four months afterwards 93.3 ms (W test; p=0.715).

Conclusion: In our ET cohort, STDT significantly increases in the tremor comparing to the resting state, which could be related to a sensory gating mechanism during the involuntary movement. VIM HIFU thalamotomy did not change STDT in the analyzed subgroup, which could point to a respect of the somatosensory pathway under this procedure. Larger sample sizes are needed to verify the reproducibility of these findings.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/somatosensory-temporal-discrimination-in-essential-tremor-treated-with-hifu-vim-thalamotomy-tremoric-and-lesional-effects-on-the-stdt/