Title: The effect of transcutaneous tibial nerve stimulation on the corticomotor excitability of the abductor hallucis and pelvic floor muscles
Thesis Supervisor: Dr. Tania Lam
Committee Members: Dr. John Kramer, Dr. Jean-Sébastien Blouin
Chair: Dr. Mark Beauchamp
Abstract: Background: Urinary incontinence negatively impacts quality of life for over 420 million people worldwide. The pelvic floor muscles (PFM) are crucial for maintaining urinary continence in humans. Transcutaneous tibial nerve stimulation (TcTNS) is a promising therapeutic technique used to manage urinary incontinence. There is compelling evidence from upper and lower limb studies that peripheral nerve stimulation alone can trigger neuroplastic changes in primary motor cortex in the absence of motor training. Since the tibial nerve (L4-S3) shares segmental innervation with the pudendal nerve (S2-S4), which supplies the PFM, TcTNS may lead to neuromodulation of the corticospinal projections to the PFM as well as abductor hallucis (AH) muscle, which is innervated by the tibial nerve. Purpose: To evaluate the effects of two distinct patterns of TcTNS (intermittent vs. continuous) on corticomotor excitability of the PFM and AH. We hypothesized that intermittent TcTNS would increase, while continuous TcTNS would suppress, the excitability of both muscles. Methods: Twelve able-bodied adults (20-33 years of age) enrolled in this study. TcTNS was delivered either intermittently (1-ms pulses delivered at 30Hz with an on:off duty cycle of 600:400 ms, for 60 min), or continuously (1-ms pulses delivered at 30Hz for 36 min). The order of the TcTNS patterns was randomized and tested on separate days. Surface electromyography (EMG) was used to record motor evoked potentials (MEPs) from PFM and AH by transcranial magnetic stimulation (TMS) over the motor cortical areas controlling these muscles before and after TcTNS. Results: Our results suggest that intermittent, but not continuous stimulation of the tibial nerve might have increased the excitability of corticospinal projections to AH without altering the excitability of those controlling the PFM. Conclusion: Based on our observations, to induce corticomotor excitability of the lower limb muscles, intermittent stimulation should be used. Moreover, it is possible that there are differences in the mechanisms underlying TcTNS-induced modulation of the corticospinal projections to AH and PFM, which require further investigation.