Internal Anal Sphincter Electromyography: Correlation with Resting Anal Pressure

Abstract

Little is known about the electromyographic (EMG) activity of the internal anal sphincter (IAS) in vivo, in man. IAS activity is responsible for approximately 80% of resting anal canal pressure (RP), but EMG activity is said to correlate poorly with RP. To determine the relationship of IAS activity and RP, we have recorded RP (air filled manometry system) synchronously with IAS EMG (frequency range 0.1–15H3) both at rest and during rectoanal inhibitory reflex testing (RAIR) in 26 patients (10 men, 15 women). RP (median, 62.5cm H2O; range, 15–150) failed to correlate with EMG amplitude (median, 0.22mV; range, 0.10–0.45; r = 0.156; p > 0.1), but correlated with EMG frequency (median, 0.38 H2; range, 0.21–0.62; r = 0.829; p 0.05), but correlated with decrease in EMG frequency (median, 0.24 H2; range, 0.04–0.50; r = 0.850; p < 0.001). We conclude that the IAS component of RP is related to EMG activity, but this is frequency and not amplitude related.