Objective: We present our pilot data on the Intermuscular coherence (IMC) and cortico-muscular coherence (CMC) pattern in writer’s cramp (WC) patients, healthy participants and patients with Essential tremor (ET).

Background: The diagnosis of WC is essentially clinical and rarely diagnostic tests are required to exclude essential or psychogenic tremor.  IMC is an analytical tool to interpret the EMG signals and it has potential for clinical application. In addition, it might also indicate the origin of neural drive if supported by CMC.  

Methods: Surface EMG was recorded from three forearm and hand muscles while the participants performed a repetitive pinch grip task on a force transducer; the target force was set to 10% to 80% of maximum voluntary contraction. IMC between first dorsal interosseous and both flexor digitorum superficialis and extensor digitorum communis were calculated from the period of steady contraction. Average IMC across all participants and across both muscle pairs were compared among healthy volunteers, WC and ET patients. We also performed CMC analysis between EEG signals from scalp electrodes placed over sensory-motor cortex with EMG electrodes placed on forearm and hand muscles.

Results: The average IMC plot for 10 healthy volunteers and 20 WC patients demonstrated a characteristically different pattern. A peak IMC was noted at 4- 7 Hz in WC patients, in contrast to healthy volunteers who had negligible IMC in that band. No difference in IMC was noted between WC patients with or without writing tremor. Beta band coherence of WC patients was like healthy volunteers. The IMC pattern of ET patients was also significantly different from WC patients at 4-5 Hz frequency band. Pilot data of CMC was devoid of any significant peak at 4-7 Hz unlike characteristic IMC of WC patients. 

Conclusions: We report, for the first time, a clear IMC at 4- 7 Hz in WC patients. The similar coherence patterns in patients with and without writing tremor possibly indicate that they belong to the same spectrum of dystonia with a diverging clinical presentation. As expected the beta band coherence (biomarker for corticospinal tract) was intact in WC patients. This simple and low-cost electrophysiological method has the potential for high diagnostic utility. In addition, we also hypothesise that the low frequency peak did not have a cortical origin.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/intermuscular-and-cortico-muscular-coherence-of-patients-with-writers-cramp/