Motor Evoked Potential and Cortical Silent Period in Migraine

Rationale and Background:

Migraine is a neurological disorder that significantly impairs individual quality of life and imposes substantial economic burdens on healthcare systems worldwide. Although its pathophysiology is not fully understood, recent research suggests that migraine is associated not only with vascular changes but also with alterations at neurological and cortical levels (Goadsby et al., 2017). Evidence increasingly indicates enhanced excitability and disrupted inhibitory mechanisms in central nervous system regions, including the motor cortex (Coppola et al., 2007).

Transcranial Magnetic Stimulation (TMS) is a non-invasive and reliable neurophysiological technique that stimulates the motor cortex and records muscle responses. Parameters obtained through TMS, such as Motor Evoked Potential (MEP) and Cortical Silent Period (CSP), are used to assess cortical excitability and inhibitory control, respectively (Chen et al., 1999). CSP is particularly useful for evaluating GABA-B-mediated inhibitory mechanisms. The hypothesis that migraine patients exhibit reduced cortical inhibition can be directly tested using CSP measurements.

Previous studies have reported shortened CSP durations and increased MEP amplitudes in migraine patients, suggesting cortical hyperexcitability (Brighina et al., 2002; Afra et al., 1998). However, these findings are inconsistent, and how they vary across migraine subtypes (with aura vs. without aura) remains unclear. Therefore, this study aims to characterize cortical physiological changes in migraine using objective and quantitative measures, filling gaps in the literature and contributing to clinical evaluations. Additionally, the relationship of these parameters with clinical features such as migraine type, duration, and attack frequency will be investigated to lay the groundwork for individualized neurophysiological profiling.

Primary and Secondary Objectives:

Primary Objective:

The primary objective of this study is to objectively evaluate inhibitory control in the motor cortex of migraine patients using non-invasive methods. Motor Evoked Potentials (MEP) and Cortical Silent Periods (CSP) obtained through TMS will be measured and compared between migraine patients and healthy controls. CSP duration, reflecting GABA-B-mediated inhibitory mechanisms, allows direct assessment of migraine's effect on cortical inhibition (Chen et al., 1999).

Secondary Objectives:

Motor Output Analysis: Electromyographic (EMG) signals will be analyzed using Peristimulus Time Histogram (PSTH) and Peristimulus Frequencygram (PSF) methods to assess the temporal pattern and frequency changes of motor unit responses following TMS. PSTH analyzes the timing of individual motor units after stimulation, while PSF provides a more precise view of post-stimulation frequency changes (Türker \& Powers, 2001). These analyses help reveal the spinal reflection of cortical stimulation and its effect on motor output.

Biomarker Potential: TMS-derived parameters such as CSP and MEP will be evaluated for their potential as biomarkers of migraine pathophysiology, contributing to the development of objective measures for future diagnosis and treatment.

Expected Benefits:

At the end of this study, it is expected to obtain objective data on motor cortical inhibitory capacity in migraine patients. TMS parameters, particularly CSP and MEP, can be used to understand the neurophysiological basis of migraine. The results may provide:

Improved understanding of cortical excitability and inhibition balance in migraine pathophysiology.

Foundational data for the development of objective diagnostic biomarkers for migraine.

Evidence of cortical dysfunction in migraine patients, supporting personalized treatment approaches.

A comprehensive understanding of motor output from cortical to spinal levels via electrophysiological analyses (PSTH/PSF).

All procedures are non-invasive and safe, providing a reproducible and ethical research approach.

Study Methods:

Participants:

The study will include right-handed individuals aged 18-45 years with a prior diagnosis of migraine according to the International Headache Society (IHS) criteria.

Data Collection:

The study will use non-invasive brain stimulation (TMS) along with surface electromyography (sEMG) and needle EMG. Measurements will be performed both at rest and during voluntary muscle contraction. TMS will be applied over the motor cortex, and muscle responses will be recorded simultaneously using sEMG electrodes. For detailed motor neuron analysis, single motor unit (SMU) recordings will also be obtained.

Needle EMG/SMU Recording:

SMU recordings will use sterile, Teflon-coated needles containing copper wires (approximately 70 μm diameter, 25G) inserted into the first dorsal interosseous (FDI) muscle. Needles will be partially retracted to maintain electrode stability, allowing single motor unit activity to be recorded even during movement.

TMS Application:

TMS will be performed using a Magstim 200\^2 Monophasic Stimulator (Magstim Ltd, UK) with a 70 mm figure-of-eight coil placed over the dominant hemisphere's primary motor cortex (M1) corresponding to the FDI muscle. Resting Motor Threshold (RMT) will be determined, followed by stimulation to elicit MEPs.

Surface EMG Recording:

sEMG signals will be obtained using Ag/AgCl electrodes placed over the FDI muscle. Signals will be amplified with a CED 1902 amplifier and digitized via CED 3601 Power 1401 DAC unit. MEP latencies and amplitudes, as well as CSP durations, will be calculated. Participants will maintain \~20% maximal voluntary contraction during measurements.

Data Analysis:

Data will be analyzed using IBM SPSS Statistics 26. Normality will be tested using Kolmogorov-Smirnov and Shapiro-Wilk tests. Parametric data will be analyzed using independent t-tests and ANOVA, while non-parametric data will be analyzed with Mann-Whitney U or Kruskal-Wallis tests. Correlations between clinical parameters (migraine duration, frequency, severity) and neurophysiological measures (MEP, CSP) will be assessed using Pearson or Spearman correlation coefficients, with significance set at p \< 0.05.