Factors affecting anal sphincter recruitment during intraoperative pudendal nerve stimulation

Introduction: The factors that impact external anal sphincter (EAS) recruitment in clinical pudendal neuromodulation are not well understood. Therefore, the goal of this study was to identify and quantify physiological, demographic, and stimulation factors that significantly affect EAS recruitment in participants receiving pudendal neuromodulation for treatment of lower urinary tract symptoms. Methods: Demographic and diagnostic information were collected from participants upon entry to the study. MRI and CT were used to capture patient pudendal nerve anatomy and register the location of the implanted lead. EAS activation by pudendal nerve stimulation at different amplitudes and pulse widths was recorded during neurostimulator lead implantation in the operating room. Linear mixed modeling was used to quantify the impact each variable had on EAS recruitment in the sixteen enrolled participants. Results: Participant sex, age, and BMI did not significantly affect EAS recruitment with pudendal nerve stimulation. Participant diagnoses had significant relationships to EAS recruitment, likely due to unbalanced group sizes. A pulse width of 210 μs required significantly less current than 60 μs (p = 0.005) to activate the EAS at threshold and significantly less charge than 450 μs (p = 0.02) to activate the EAS at threshold. Increased distance between the stimulating electrode and the pudendal nerve decreased the magnitude of the EAS response (p = 0.0011) and increased EAS activation threshold (p < 0.001). Conclusions: Of the three tested pulse widths, 210 μs requires the least charge and is a good choice for stimulation. The distance between the electrode and pudendal nerve plays an important role in EAS and pudendal nerve recruitment and minimizing this distance should be a priority during lead implantation.