Proper function of the pelvic floor muscles (PFMs) depends of the integrity and function of central and peripheral nerve pathways. There is a wide spectrum of dysfunctions related to the female PFMs, and, among those, certain conditions such as superficial and deep dyspareunia, urinary incontinence (UI) and dysfunctional voiding may be related to alterations within the cortical motor areas representing these muscles and/or their associated corticomotor pathways [1]. Transcranial magnetic stimulation (TMS) is a pain-free, non-invasive, approach to evaluate the excitability of corticospinal projections to the PFMs. A magnetic field generated by a stimulating coil results in current reaching the cortical neurons beneath the skull. The resulting neuronal depolarization ultimately depolarizes the motor cortical areas beneath the stimulating coil, generating evoked potentials (MEPs) in the muscles innervated by the depolarized region. TMS has been used successfully to demonstrate alterations in corticomotor excitability; studies mainly focus on the upper extremity, while some have focused on the lower limbs and trunk. However, the application of TMS to study corticomotor pathways to the PFMs remains challenging, as PFM MEP amplitudes and latencies have demonstrated poor reproducibility [2]. The aim of this study is to present a novel approach to assessing the corticomotor excitability of the PFM pathways, and to evaluate the reliability of MEP parameters using this approach.

This is a feasibility and reliability study. The protocol was approved by local Research Ethics Board. Healthy females over 18 years of age were recruited from the local community. Exclusion criteria were pregnancy, menopause, diagnosed gynecologic conditions (e.g., pelvic organ prolapse, UI, vaginal infection) other than dyspareunia, tendency to faint, metal implants in head or neck and pacemaker. Participants attended two laboratory-based assessments, the first of which was within one week following the start of their menstrual cycle. In the first session, after providing informed consent, demographic data were collected and a standardized PFMs assessment was performed by a pelvic floor physiotherapist to evaluate muscle function and to instruct each participant on how to perform a correct PFM contraction. At each of the two assessments, participants were instrumented with adhesive electromyography (EMG) electrodes (Delsys D.E.2.1) on the skin overlying the right tibialis anterior muscle, the abdominal muscles, and the hip adductor muscles; the latter two being beyond the scope of this report. Custom monopolar suction electrodes, similar to differential suction electrode [3], were placed intravaginally, with the active pole over the pubovisceralis (PV) muscle on the lateral sidewall and the reference pole placed anteriorly over the pubis, just within the introitus to avoid crosstalk from the urethral sphincters, Adhesive electrode pairs were placed unilaterally over the bulbocavernosus (BC) muscle and unilateralally over the external anal sphincter (EAS). A common reference electrode was placed on the skin overlying the right anterior superior iliac spine. The protocol consisted of applying single TMS pulses over the vertex using the Magstim® 200 system coupled with a double cone coil (96 mm loops, P/N 9902). Resting motor threshold (rMT) was determined using tibialis anterior as the target muscle. For the assessment, TMS pulses were delivered at 1.3 rMT while MEPs were recorded from all instrumented muscles. To assess MEP amplitude and latency characteristics, 12 TMS pulses were delivered while participants were instructed to remain relaxed. To assess the cortical silent period (sCP), 5 TMS pulses were delivered while participants were instructed to gently contract their PFMs. The outcome variables of interest were the peak to peak amplitude, latency, and cSP from MEPs recorded over the PV, BC and EAS.

Thirty-one women participated, with demographic data presented in Table 1. Twenty-four participants reported no history of vulvar pain, while 7 reported symptoms of vulvar pain including PVD and deep dyspareunia. The MEPs recorded from the PV, BC and EAS had distinct timing and shape characteristics (Figure 1) and were also distinct from the signals recorded from the hip adductors and the abdomen. Intra-class correlation coefficients (ICCs) showed that the TMS intensity used as well as cSP from all the muscles evaluated had excellent between-day reproducibility. The peak-to-peak amplitude of MEPs recorded from PV at rest and with PFM contraction, and those recorded from BC showed good reproducibility, whereas the other outcomes showed fair to poor reproducibility (Table 1).

These results suggest that the approach presented has adequate between-day reliability in terms of cSP to evaluate corticomotor excitability to the different female PFMs (PV, BC, EAS) and to measure changes in cSP that may indicate improvements or declines in corticomotor excitability as a result of time or intervention. Because intracortical and corticospinal excitability can modify motor output, the capacity to reliably assess the excitability of these projections may be important to our understanding of several conditions where altered corticomotor excitability is suspected to play an important role as well as a role in monitoring the effectiveness of interventions.

This novel approach for generating MEPs from the PFMs appears to be effective, generating distinct signals for the PV, BC and EAS all of which have adequate repeatability in cSP to assess changes in corticomotor excitability over time. This approach may help us to improve our understanding the pathophysiology of conditions such as dyspareunia, UI, anismus, and dysfunctional voiding, and to determine the effectiveness of interventions targeting central pathways.

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