Introduction: Essential Tremor is the most common movement disorder; however its pathophysiology is still unclear. Severe cases have been treated successfully with deep brain stimulation of the Ventral Intermediate nucleus of the Thalamus (Vim).
Methods: Microelctrode recordings were made from 9 patients as part of the intra-surgical exploration during the implantation of deep brain stimulation electrodes. Stereotactic surgery was performed with a Cosman-Roberts-Wells CRW stereotactic head frame. Image based targeting of the Vim was performed on MRI and stereotactic-CT fused images on a Medtronic Stealthstation machine. Recordings were taken with a single microelectrode, with a Medtronic Leadpoint system, on a linear trajectory at one millimeter steps. Only recordings taken up to 5 mm above the lower border of the Vim were considered for the analysis. Signals were acquired with Spike2 software. Unit spike sorting was based on a supevised hierachical clustering with Wavelet and principal components decomposition and performed with custom Matlab code. Accelerometric signal of the contralateral hand was also acquired, during intention tremor and rest state. All patients were awake during recording.
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 did not find differences in firing rate, action potential duration, burst index, mean spikes per burst, burst rate and proportion of spikes in bursts between rest and tremor (Wilcoxon rank-sum test p>0.05). Four out of 23 units tested during tremor were phase locked at tremor frequency (4-5Hz). Two out of 28 units were phase locked at 15-20 Hz during baseline (Rayleigh test, corrected p<0.01).
Local Field Potential (LFP) was coherent with tremor at tremor frequency (4-5 Hz), with a tendency of tremor to lead the LFP by a variable delay time (~250-650 ms). Tremor frequency appeared to fluctuate at ~0.5Hz, within tremor epoch.
Conclusions: We found a periodical fluctuation of the tremor frequency at ~0.5 Hz both in LFP as well as the accelerometer signal (FIG5), which to our knowledge is a new pathological feature of intention tremor.
Patient Care: This was an experimental analysis, and will help patient care
Learning Objectives: Understanding the activity in the VIM during essential tremor and its relationship to the tremor