Intermittent cortical involvement in the preservation of tremor in essential tremor

Abstract

Cortical involvement in essential tremor, an involuntary action tremor supposedly of subcortical origin, is uncertain. Conflicting results of corticomuscular coherence studies in essential tremor suggest an intermittent corticomuscular coupling. On the basis of the literature, we hypothesized that corticomuscular coupling is influenced by bilateral motor synchronization and "cognitive states" such as awareness of tremor. In the present study, we investigated ) the existence of intermittent corticomuscular coherence (CMC) in essential tremor and) factors that influence CMC strength. In 18 essential tremor patients and 18 healthy controls, who mimicked tremor, we simultaneously recorded 64-channel EEG and 6-channel bipolar surface EMG from right and left wrist extensors and flexors. Right-sided (mimicked) hand tremor was recorded with and without a cognitive arithmetic task and with left-sided (mimicked) hand tremor. CMC values per task were compared within and between groups. Changes in CMC strength during tasks were calculated. Our main findings are ) significant CMC around the (mimicked) tremor frequency across all tasks in both groups;) significant differences in CMC between unilateral tasks, with the highest values during the cognitive task only in the essential tremor group; and ) significant fluctuations of CMC strength over time, independent of the tremor intensity, only in the essential tremor group. Our results suggest a limited role, and certainly not a continuous steering role, of sensorimotor cortical neurons in the generation of tremor. In clinical practice, these findings might help to standardize tremor registration and the interpretation of the analysis. The part of the motor cortex involved in essential tremor is uncertain. The current electrophysiological study is the first to assess corticomuscular coherence systematically. The study shows a dynamic nature of corticomuscular coherence and a possible influence of cognitive states. The results elucidate the involvement of the motor cortex in tremor and help interpret the varying results in the literature. In clinical practice, the findings may guide in standardizing tremor registration and its interpretation.

Corticomuscular Coherence and Its Applications: A Review

“…In addition, it has been proved that CMC strength is modified in healthy subjects after immobilization ( Lundbye-Jensen and Nielsen, 2008 ) or in neurological conditions such as essential ( Muthuraman et al, 2010 ), neuropathic tremor ( Weiss et al, 2010 ) and Parkinson disease ( Weiss et al, 2012 ). Different choice of patients, analysis techniques and recording methods, types of sport duties, and possibly cognitive state (e.g., awareness of tremor) might be explained the reasons for inconsistent results ( Sharifi et al, 2017 ). Through the comparative study of CMC between healthy controls and patients, we can find the potential clinical application of CMC, and the most direct application is motor rehabilitation.…”

“…Tremor is one of the most common disorders. To confirm the motor cortex involved in essential tremor and factors that affect CMC strength, Sharifi et al (2017) collected 18 essential tremor patients and the result showed that essential tremor CMC is desultory and subject to different functional duties. This result may serve to standardize tremor classification and the explanation of the analysis in clinical research.…”

Unilateral Thalamic Deep Brain Stimulation for Voice Tremor

“…Based on the proposed pathophysiology of ET, VT may be related to a deficient cortical control and sensory feedback, along with an abnormal cerebellar control of the vocal fold tension [33-36]. Recent evidence suggests that the speech circuit is lateralized to the language-dominant side and this may explain why the benefit of thalamic DBS was lateralized in most of our patients [37, 38].…”

Tremor habituation to deep brain stimulation: Underlying mechanisms and solutions

“…It is unclear whether oscillatory activity in this circuit is caused by a single pacemaker, multiple pacemakers, or altered network dynamics (such as unstable feedback loops). An intriguing finding is that corticomuscular coherence is intermittent in ET despite ongoing tremor . This suggests that there are multiple oscillators involved.…”

“…An intriguing finding is that corticomuscular coherence is intermittent in ET despite ongoing tremor. 78,88 This suggests that there are multiple oscillators involved.…”