The effect of vaginal distension on the female urinary bladder and urethral sphincters

A mention of effect of vaginal distension, as induced by penile thrusting at coitus, on urinary bladder (UB) and urethral sphincters could not be traced in literature. We investigated the hypothesis that, upon vaginal distension, UB undergoes inhibited activity, while external and internal urethral sphincters (IUS) exhibit increased activity in order to guard against urine leakage during coitus.

Response of UB and external and IUS to vaginal balloon distension was recorded in 28 healthy women (age 35.6+/-3.3 years). A vaginal condom was inflated with air in increments of 50 up to 200 ml, and vesical pressure as well as electromyographic (EMG) activity of external and IUS were registered. The test was repeated after separate anaesthetisation of vagina, UB and external and IUS.

On vaginal distension, vesical pressure was reduced in the ratio of expansion of vaginal volume up to a certain capacity, beyond which vesical pressure ceased to decline when distending volume was augmented. Similarly, IUS EMG activity increased progressively on incrementally added vaginal distension up to 150-ml distension, beyond which any further vaginal distension did not produce an additional increase of EMG activity; the external urethral sphincter (EUS) EMG activity showed no response. Vaginal distension, while the vagina, UB and external and IUS had been separately anaesthetised, produced no significant change.

Vaginal balloon distension seems to effect vesical wall relaxation and increase IUS tone. This appears to provide a mechanism that prevents urine leak during coitus. Vesical and IUS response to vaginal distension are suggested to be mediated through a reflex we term 'vagino-vesicosphincteric reflex', which seems to be evoked by vaginal distension during penile thrusting. The reflex may prove of diagnostic significance in sexual disorders.