Selective Transcutaneous Electrical Nerve Stimulation (TENS) of Lower Leg Afferents

Sharan, Eshani 1 ; Hunter, Kelly 2 ; Yoo, Paul 1, 2  

1. Institute of Biomaterials and Biomedical Institute, University of Toronto; 2. Department of Electrical and Computer Engineering, University of Toronto

Background: Overactive bladder (OAB) is characterized by sudden and repeated urges to urinate regardless of bladder capacity. OAB can affect the individual’s ability to perform everyday tasks, social interactions, and even sleeping habits Electrical neuromodulation is used as a treatment approach that utilizes electrical pulses to control the central nervous system circuits that, in turn, modulate urinary function. Percutaneous tibial nerve stimulation (PTNS) is a type of electrical neuromodulation used as a form of OAB therapy. Clinically, PTNS therapy is administered by a percutaneous approach, where a needle is inserted 3 finger-widths above the medial malleolus.

Preclinical work: Preclinical animal work in our lab is uncovering the physiological mechanisms that contribute to the therapeutic effects of PTNS therapy. As recently published by Kovacevic and Yoo, we showed that electrical activation of individual branches of the posterior tibial nerve can reflexively control bladder function. More recently, in an anesthetized rat model we have shown that electrical stimulation of the saphenous nerve can also inhibit bladder function. In PTNS therapy, the active tip of the needle electrode is typically located between the posterior tibial nerve and the saphenous nerve. Ongoing preclinical research in our lab suggests there are multiple neural targets that could be used to treat OAB.

Goal: The goal of this study is to determine the feasibility of selectively activating 3 nerve targets in participants (saphenous nerve, SAFN, medial plantar nerve, MPN, and lateral plantar nerve, LPN) by using a non-invasive method of electrical stimulation (i.e., transcutaneous electrical nerve stimulation, TENS).

Method: Using a handheld TENS stimulator, 15 participants (healthy adult volunteers), were tested at four target nerve sites (knee, ankle, medial and lateral sole) at 20 Hz with a pulse width of 200 µs. The relative current amplitudes were recorded while an image was shaded indicating where sensation was felt. 3 thresholds were measured for each nerve: cutaneous activation (Tskin), nerve recruitment (Tnerve) and tolerance (Tlimit).

Results: There was no significant difference between the skin thresholds of the SAFN nerve and the PTNS location (p=0.11). At low current amplitudes, at the conventional PTNS location we experience coactivation of the two nerves. Whereas, there was a noticeable difference between the cutaneous thresholds of the SAFN nerve compared to the MPN and LPN nerves indicating selective activation. The SAFN nerve requires the most current to activate the entire nerve trunk compared to the other nerve targets. LPN and SAFN nerves have the highest limit thresholds making them the most feasible targets for OAB therapy.  Our results reveal that selective activation of each neural target is feasible with TENS.

Impact: TENS may provide patients with a convenient at-home treatment for OAB allowing them to manage their symptoms better.