Exploring Neurophysiological Mechanisms and Treatment Efficacies in Laryngeal Dystonia: A Transcranial Magnetic Stimulation Approach

Abstract

Laryngeal dystonia (LD), known or termed as spasmodic dysphonia, is a rare movement disorder with an unknown cause affecting the intrinsic laryngeal muscles. Neurophysiological studies point to perturbed inhibitory processes, while conventional genetic studies reveal fragments of genetic architecture in LD. The study's aims are to (1) describe transcranial magnetic stimulation (TMS) methodology for studying the functional integrity of the corticospinal tract by stimulating the primary motor cortex (M1) for laryngeal muscle representation and recording motor evoked potentials (MEPs) from laryngeal muscles; (2) evaluate the results of TMS studies investigating the cortical silent period (cSP) in LD; and (3) present the standard treatments of LD, as well as the results of new theoretical views and treatment approaches like repetitive TMS and laryngeal vibration over the laryngeal muscles as the recent research attempts in treatment of LD. Neurophysiological findings point to a shortened duration of cSP in adductor LD and altered cSP duration in abductor LD individuals. Future TMS studies could further investigate the role of cSP in relation to standard laryngological measures and treatment options. A better understanding of the neurophysiological mechanisms might give new perspectives for the treatment of LD.

Keywords: ENT; cortical silent period; corticobulbar motor evoked potentials; corticobulbar tract; cricothyroid muscle; deep brain stimulation; laryngeal dystonia; laryngeal vibration; motor evoked potential; thyroarytenoid muscle; transcranial magnetic stimulation; whole-genome sequencing.

Figure 1

TMS over the M1 for laryngeal muscle representation (red-blue arrow) and CoMEPs recorded from TA and CT muscles. Left side: The central sulcus is depicted with a green line. Positive M1 referent spot for upper extremity representation (M1-APB). Legends: APB, abductor pollicis brevis; CoMEP, corticobulbar motor evoked potential; TA, thyroarytenoid; CT, cricothyroid. The magnetic stimulation onset is represented as the zero time. The time on the x-axis is expressed in milliseconds (ms), and the MEP amplitude on the y-axis is expressed in microvolts (µV). Source: single subject; ownership of the authors of the study.

Figure 2

TMS over the single M1 cortical spot for laryngeal muscle representation elicits reproducible CoMEPs followed by cSP in TA and CT muscles. Note: Fifteen trials (different colours) from TA and nine trials (different collors) from CT muscles are superimposed with mean (dashed red line) and median (dashed blue line). The black arrow pointing down represents the offset of the cSP response, while the magnetic stimulation onset is represented as the zero time. The time on the x-axis is expressed in milliseconds (ms), and the MEP amplitude on the y-axis is expressed in microvolts (µV). Source: single subject; ownership of the authors of the study.