Factors Affecting Anal Sphincter Recruitment During Intraoperative Pudendal Nerve Stimulation: An Observational Study.

Introduction and hypothesis: The relationship between pudendal neuromodulation and patient factors is not well understood. This observational study was aimed at identifying and quantifying physiological, demographic, and stimulation factors that significantly affect external anal sphincter (EAS) recruitment and outcomes in participants receiving pudendal neuromodulation for treatment of lower urinary tract symptoms and pelvic pain.

Methods: Participants (N = 16) provided demographic and diagnostic information upon entry to this observational study. EAS activation at different stimulation amplitudes and pulse widths was recorded during lead implantation. Magnetic resonance imaging and computed tomography were used to determine the distance of the electrodes on the implanted lead from the nerve. Linear mixed modeling was used to quantify the impact of each variable on EAS recruitment.

Results: Participant sex, age, and body mass index did not significantly affect EAS recruitment. Participant diagnoses had significant relationships with EAS recruitment, likely because of unbalanced group sizes. A pulse width of 210 µs required less current than 60 µs (p = 0.005) and less charge than 450 µs (p = 0.02) to activate the EAS. Increased electrode-to-nerve distance decreased the magnitude of the EAS response (p = 0.0011), increased the EAS activation threshold (p < 0.001), and was related to reduced bladder symptom improvements.

Conclusions: Of the three tested pulse widths, 210 µs best balances current and charge for EAS recruitment. Minimizing the distance between the electrode and pudendal nerve should be a priority during lead implantation. External sphincter activation threshold and response magnitude could be useful clinical indicators of electrode-to-nerve distance.