The role of the motor cortex in essential tremor

Objectives: Essential tremor (ET), the most prevalent hyperkinetic movement disorder, is characterized by an action tremor (4-7 Hz). The role of the sensorimotor cortex in tremor generation is uncertain. Previous corticomuscular coupling studies in ET showed either a cortical drive, an afferent input, or both. In contrast, familial cortical myoclonic tremor with epilepsy (FCMTE) with tremulous movements, clinically resembling ET, is characterized by a cortical drive around 15-20Hz.

Methods: Corticomuscular directionality was investigated with renormalized partial directed coherence (rPDC) in ET (n=16), FCMTE (n=7), and healthy controls (HC; n=16). EEG-EMG was recorded during 2 minutes of postural tremor (ET and FCMTE right arm outstretched; HC mimicking tremor ±5Hz), and 1 minute of self-paced slow movements <3Hz (ET and HC).

Results: In ET and HC, predominantly, muscular to cortical directionality was evident around (mimicked) tremor frequency. FCMTE patients revealed a cortical to muscular directionality. During voluntary slow movements, below 3 Hz, corticomuscular coupling was bidirectional in ET and HC.

Conclusions: Our results indicate cortical activity at (mimicked) tremor frequencies resulting from afferent input in ET and HC, instead of a cortical drive. In FCMTE, rPDC was able to confirm a cortical drive tremulous movement frequencies. Furthermore, our findings indicate direct sensorimotor cortex output during slow voluntary movements. Concluding, tremor in ET seems not to result from a direct cortical drive. Subcortical circuits, possibly normally involved in high paced oscillatory movements, seem to be involved in ET.