Objective: To evaluate the presence of co-contractions and overflow in cervical dystonia (CD) patients and appraise the applicability of intermuscular coherence as an objective biomarker for CD.

Background: The clinical evaluation and management of CD patients revolve around subjective rating scales, whereas electromyography (EMG) offers a more objective assessment. Coherence analysis (a frequency domain measure that can disclose oscillatory drives common to different motor units) of EMG recordings can identify characteristic features such as overflow and co-contraction, believed but not confirmed to be indicative of dystonic activity. Investigations using this technique have indicated the presence of a “dystonic drive” at 4-12 Hz. In this study, we analyze the bilateral EMG of cervical muscles to investigate 4-12 Hz intermuscular coherences of 15 CD patients and 15 control subjects during co-contraction and overflow-eliciting tasks.

Method: The study includes 15 CD patients and 15 controls, with 9 patients and 7 controls currently enrolled. Clinical assessment included the Burke Fahn Marsden Dystonia Rating Scale (BFMDRS) and Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS). A 16-lead EMG setup measured muscle activity in neck muscles during rest and several motor tasks. Coherence analysis was performed on EMG recordings of antagonistic neck muscles. Differences between groups are assessed using unpaired t-tests, and Pearson correlations explore associations between intermuscular coherence and BFMDRS/TWSTRS scores.

Results: During rest, patients exhibited low 4-12 Hz coherence of the left and right sternocleidomastoid muscles (0.018 ±0.01 SD) compared to controls (0.250 ±0.29 SD). However, coherence increased during head turning (left: 0.089 ±0.01 SD; right: 0.202 ±0.012 SD). In contrast, in controls coherence disappeared during head turning (left: 0.009 ±0.001 SD; right: 0.007 ±0.003SD). Group difference was nonsignificant for rest (p=0.161), but significant during head turning (left: p=0.034; right: p=0.016). Further results of the complete patient cohort, additional tasks and the correlations to clinical scores will be presented at the congress.

Conclusion: These preliminary results show that there is distinctly increased coherence of antagonist muscles in CD patients during a co-contraction-eliciting task. In addition, we aim to investigate this as a biomarker of dystonia, offering an objective alternative to subjective rating scales.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/beyond-the-surface-electromyography-in-cervical-dystonia/