Objective: To compare somatosensory temporal discrimination threshold (STDT) previously and 4 months after unilateral (left) high intensity focused ultrasound (HIFU) thalamotomy. Secondly, to observe the effect of tremor on the STDT and to compare it with the record of somatosensory evoked potentials (SEP).

Background: STDT is the ability to identify two successive stimuli as separate in time. In healthy individuals it is about 30-90ms, and in ET it yields similar values. In a previous pilot study we observed that tremor raised STDT and that STDT did not modify with HIFU VIM in a small sample. Here we enlarge our cohort to further analyze these results.

Method: 15 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 7 patients. STDT was retested 4 months after HIFU on both sides. Underlying polyneuropathy was discarded by means of electroneurography of median nerves.

Results: 9 patients (60%) were men. Mean age was 70.33 (SD 5.34) y.o. Mean disease duration was 15.46 (SD 7.22) years. Mean basal FTM scale was 78.86 (SD 21.09). Basal STDT on the right arm with tremor was 126.81ms and without tremor 102.1ms (t paired test; p<0.001). Four months after HIFU STDT on the right arm was 105.29 ms (W test; p=0.421). An attenuation of N20 amplitude of SSP >30% was observed in 3 out of 7 patients during tremor.

Conclusion: In our ET cohort, VIM HIFU thalamotomy did not change STDT which could indicate a spare of the somatosensory pathway under this procedure. STDT significantly increases in the tremor comparing to the resting state, but a correlation with an attenuation of the cortical potential in PESS was observed only in 3 out of 7 patients, results that need to be confirmed in a larger cohort.

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