EMG of the External Anal Sphincter: Reference Values of Motor Unit Potentials

Background and Purpose: In proctology electromyography of the external anal sphincter is done using surface electrodes. Concentric needle electrodes are rarely taken, despite of the fact that only those meet the requirements of a quantifying assessment of damage to the external anal sphincter muscle. For quantitative assessment single motor unit potentials have to be measured and analysed. Patients and Methods: Only few values have been published so far, and those mainly drawn from experience. Thus, we examined normal electromyograms of a group of 29 women and 31 men. Results: For the patient population in general duration and amplitude of a motor unit potential averaged 4.53 ms and 0.71 mV, respectively. 95% of the potentials were in the range of 3.11 ms to 5.95 ms and 0.25 mV to 1.15 mV, respectively. Few potentials only measured more than 8 ms, none was longer than 10 ms. Less than 1% of the potentials exceeded 1.5 mV, none of them reached 3 mV. Polyphasia was about 2%. Conclusion: According to our results, potentials of the external anal sphincter are different to those of other striated muscles. We hope with these findings to participate in increasing clinical accessibility of electromyography of the external anal sphincter.