Introduction Conventional anal manometry (ARM) suggested that the internal anal sphincter (IAS) contributes 60–85% of anal resting pressure, the remaining 15–40% being attributable to the tonic contraction of the external anal sphincter (EAS) and anal cushions. Anal acoustic reflectometry (AAR) is a more specific and sensitive indicator of sphincter function than ARM. Our aim was to determine the relative contributions of the IAS and EAS to anal resting pressure by isolating the IAS for interrogation using a bilateral pudendal nerve block (bPNB).
Method Prospective cohort study of patients with pudendal neuralgia undergoing bPNB. Blocks were trans-vaginal (Women) and trans-gluteal (Men). EAS electromyography (EMG) was performed before and 5 mins after bPNB to establish its success. Patients received AAR and manometry (maximum resting pressure MRP, maximum squeeze pressure MSP and incremental squeeze pressure ISP) before and 30 mins after bPNB. Variables were compared using a paired t-test for parametric data and Wilcoxon signed rank test or Mann-Whitney U test* for non-parametric data.
Results 15 patients, 10 (67%) women, median age 59 (25–89) and median reduction in EMG motor unit potential activity of 45% (Range -25–48%). There was no difference in reduction of EMG activity between trans-vaginal and trans-gluteal bPNB approach (30.3 v 38.5% p = 0.762*). There was no significant change in any AAR parameter at rest. AAR squeeze opening pressure showed a significant reduction post bPNB (121 v 104 cmH2O p = 0.039). MSP and ISP showed a significant reduction post bPNB; MRP 75 v 71 cmH2O p = 0.5, MSP 154 v 118 cmH2O p = 0.02, ISP 79 v 47 cmH2O p = 0.02.
Conclusion We found a significant reduction in all measurements of EAS function after bPNB. No change was found after a 45% median reduction in EAS activity in measurements of IAS function, suggesting that AAR is primarily an investigation of the IAS.
Disclosure of interest None Declared.