Cortical Silent Period in Laryngeal Dystonia

The goal of this observational study is to evaluate the cortical silent period (cSP) in cricothyroid muscle (CT) in laryngeal dystonia and control healthy subjects. The study will provide norms related to latency and amplitude of motor evoked potentials (MEPs) and duration of cSP in CT muscle in laryngeal dystonia and control healthy subjects. Findings may give a baseline in comparison to findings in laryngeal diseases and insight into maladaptive cortical control function during phonation in laryngeal diseases like laryngeal dystonia.

Transcranial magnetic stimulation (TMS)-induced cSP provide a noninvasive "in vivo" insight into how the human motor cortical inhibitory pathway function by generating electric current at a specific brain area through electromagnetic induction. The stimulator generates a changing electric current within the coil, which induces a magnetic field that then causes a second inductance of inverted electric charge within the brain itself. A single supra-threshold pulse of TMS over the human motor cortex elicits multiple descending volleys (I-waves) that generate a motor evoked potential (MEP) followed by a period of electromyographic silence in the tonically contracted target muscle (cSP). To date, methodologies for mapping the primary motor cortex (M1) with TMS and intraoperatively by electrical stimulation techniques have been previously developed to record corticobulbar motor evoked potentials (MEP) from laryngeal muscles. Except for estimating the amplitude and latency of MEPs recorded from laryngeal muscles, the cSP was investigated from thyroarytenoid muscle (TA) as a measure of M1 excitability in the TMS study. Previous work using TMS has indicated reduced inhibition to be characteristic of focal laryngeal dystonia. According to investigators' knowledge, cSP from other laryngeal muscles other than TA has not been investigated so far. The proposed study analyses neurophysiological CNS data, including motor evaluation of corticospinal and corticobulbar pathways by recording MEPs and cSP using TMS. Firstly, a questionnaire will be given to check for any previous neurological conditions, and subjects will provide signatures on participation in the study. The MRI of the subject's brain will be performed, and then 3D reconstructed for the use of TMS. The evaluation of the MRI will be done by radiologists at University Hospital Split. With the subject comfortably seated, the MRI is co-registered to the subject's head using the tracking system with TMS's unique forehead tracker. After TMS measurement of the MEP and cSP of CT muscle of individual subjects will be recorded. Data collected will be analyzed using MATLAB 2022a (MathWorks, USA). Measurements and data evaluation will be performed at the School of Medicine University in Split. An otorhinolaryngology specialist will be consulted for the correct placement of the recording electrodes (Hookwire IOM electrodes, disposable subdermal needle electrode, 0.4 x 13 mm, SGM d.o.o, G. Novaka 22 a, 21000 Split, Croatia) into the CT muscle.