Effect of micturition on the external anal sphincter: identification of the urethro-anal reflex.

BACKGROUND/

OBJECTIVE

A study on the response of the external anal sphincter (EAS) to the passage of urine through the urethra during micturition could not be found in the literature. We investigated the hypothesis that urine passage through the urethra effects EAS contraction to guard against possible flatus or stool leakage during micturition.

METHODS

The study was performed in 23 healthy volunteers (age, 38.6 +/- 10.8 [SD] years; 14 men and 9 women). The EAS electromyogram (EMG) was performed during micturition by surface electrodes applied to the EAS. Also, the EAS EMG response to urethral stimulation by a catheter-mounted electrode was registered. The test was repeated after individual anesthetization of the EAS and urethra.

RESULTS

The EAS EMG recorded a significant increase (P < 0.01) during micturition and on urethral stimulation at the bladder neck. Stimulation of the prostatic, membranous, or penile urethra produced no significant change in the EAS EMG. Urethral stimulation after individual EAS and urethral anesthetization did not cause any changes in the EAS EMG.

CONCLUSIONS

Urine passing through the urethra or urethral stimulation at the vesical neck produced an increase in the EAS EMG, which presumably denotes EAS contraction, which seems to guard against flatus or fecal leakage during micturition. EAS contraction on urethral stimulation is suggested to be mediated through a urethro-anal reflex. Further studies on this issue may potentially prove the diagnostic significance of this reflex in micturition and defecation disorders.