TMS excitability study in essential tremor: Absence of gabaergic changes assessed by silent period recordings

Abstract

Background: Essential tremor (ET) is thought to emerge from activity in a distributed cerebello-thalamo-cortical network. It has been proposed that the network goes into oscillation because of abnormal GABAergic inhibitory transmission.

Objective: To test this idea by investigating GABAergic circuitry in motor cortex using transcranial magnetic stimulation (TMS).

Methods: Motor cortex excitability was examined using TMS in 21 patients with essential tremor and in 20 control subjects. Resting and active motor threshold (RMT, AMT) and input-output curves examined corticospinal excitability. Contralateral silent period (cSP) at a different range of stimulation intensities, and the ipsilateral silent period (iSP) using a stimulus intensity of 150% RMT were used as measures of GABAergic function.

Results: RMT and AMT were significantly lower in patients than controls and patients had a steeper I/O curve. However, there were no significant differences in either cSP at different intensities or in iSP.

Conclusion: We found no evidence in favour of the GABA hypothesis in ET.

Keywords: Contralateral cortical silent period; Cortical excitability; Essential tremor; Input-output curve; Motor threshold; Transcallosal inhibition; Transcranial magnetic stimulation.