Effect of vaginal distension on anorectal function: identification of the vagino-anorectal reflex

Sexual stimulation produces not only reflex changes in the female external and internal reproductive organs but also extragenital reactions. A mention of the response of the anal sphincters and the rectum to penile thrusting could not be traced in the literature. We investigated the hypothesis that the anal sphincters and the rectum respond to penile thrusting in a way that prevents gas and fecal leakage during sexual intercourse.

The response of the external anal sphincter (EAS) and the internal anal sphincter (IAS) and the rectum to vaginal balloon (condom) distension was recorded in 23 healthy women (age: 33.7 +/- 7.3 years). The vaginal condom was inflated with air in increments of 50-300 ml, and the electromyographic (EMG) activity of the EAS and the IAS, as well as rectal pressure, was recorded. The test was repeated after separate anesthetization of the vagina, the rectum, the EAS and the IAS and after the use of normal saline instead of lidocaine.

Vaginal distension reduced the rectal pressure in the ratio of expansion of the vaginal volume up to a certain volume, beyond which the rectal pressure ceased to decline when more distending volume was added. Similarly, the internal sphincter EMG activity increased progressively on incremental vaginal distension increase until the 150-ml distension was reached after which more vaginal distension caused no further increase of the EMG activity; external sphincter EMG activity showed no response. Vaginal distension, while the vagina, the rectum, the EAS, and the IAS had been separately anesthetized, produced no significant change, but saline did.

Vaginal balloon distension appears to effect rectal wall relaxation and increase of the internal sphincter tone. This seems to provide a mechanism to avoid rectal contents leakage during coitus. Rectal and internal sphincter response to vaginal distension is suggested to be mediated through a reflex we term 'vagino-anorectal reflex', which seems to be evoked by vaginal distension during penile thrusting. The reflex may prove of diagnostic significance in sexual disorders; further studies are needed in order to investigate this point.