SPARC: Current Steering To Selectively Recruit Nerves Of The Lower Urinary Tract
Authors: Jantz Maria K. , Gopinath Chaitanya , Kumar Ritesh , Greenlee Natalie M. , Wong Liane , Ogren John I. , McLaughlin Bryan L. , Gaunt Robert A. 
Published: April 2020

Lack of bladder control has a substantial impact on quality of life, with consequences ranging from sleep deprivation and embarrassment to bladder and kidney infection. Current methods to address lower urinary tract (LUT) dysfunction, including pharmacological interventions and catheterization, are insufficient to restore bladder control and may lead to rehospitalization, especially after spinal cord injury. Previously, we have used epidural spinal cord stimulation (SCS) to selectively recruit nerves innervating the LUT, a promising result that suggests this method of stimulation could be used to address LUT dysfunction. Epidural SCS has been in use clinically for pain relief for over 50 years, and is a minimally‐invasive way to access these neural pathways. However, current clinical leads have large electrodes that make it difficult to spatially limit the region targeted by SCS. Here, we use a high‐density epidural spinal cord electrode and multipolar stimulation patterns to reduce off‐target neural recruitment and refine the selective activation of LUT nerves.

We placed a high‐density epidural spinal cord electrode (MicroLeads Inc) at three locations of the sacral spinal cord and cauda equina in cats under isoflurane anesthesia. To record antidromic compound action potentials evoked by SCS, we recorded from nerve cuffs (MicroLeads Inc) on the sciatic nerve, pelvic nerve, as well as the pudendal nerve and its branches. To measure muscle recruitment during stimulation, we recorded electromyogram (EMG) activity from the external urethral sphincter, external anal sphincter, pelvic floor and gluteus. Using catheters in the bladder and urethra, we measured pressure changes during stimulation. We stimulated on all electrodes of the spinal cord arrays individually, as well as in bipolar and tripolar sets of electrodes to determine the effect of current steering on LUT nerve recruitment.

We were able to recruit LUT nerves with all configurations of stimulation electrodes. However, monopolar stimulation resulted in substantial selective recruitment of the off‐target sciatic nerve, representing 59% of selective responses at the most rostral sacral location, 72% at the more caudal sacral location, and 27% at the cauda equina. By using bipolar and tripolar stimulation across the same electrodes, we were able to steer current so that electrodes selective for the sciatic nerve with monopolar stimulation were selective for LUT responses. Additionally, recruitment thresholds of monopolar stimulation were typically lower, between 50–1000 μA, than those of multipolar stimulation, which ranged from 100–2800 μA. We were also able to change the EMG recruitment with changes in electrode configurations, allowing targeting of functionally‐relevant muscles.

We were able to map the recruitment of LUT nerves and muscles across several levels of the spinal cord using both monopolar and multipolar stimulation. By using these stimuli to access LUT nerves and muscles, we hope to systematically modify function in the LUT, providing a potential route to treat LUT dysfunction.

Support or Funding Information

NIH SPARC award 1OT2OD024908 and Craig H. Neilsen Foundation, award 476681