A study of the continence mechanism of the external urethral sphincter with identification of the voluntary urinary inhibition reflex

PURPOSE: The role of the external urethral sphincter in the opposition and interruption of the act of voiding was investigated. MATERIALS AND METHODS: The study included 7 men and 5 women with a mean age plus or minus standard deviation of 38.6 +/- 11.2 years. The bladder was filled with saline up to the urge sensation. Detrusor and posterior urethral pressures were recorded before and upon resisting the reflex detrusor contraction, and upon interrupting voiding by voluntary external urethral sphincter contraction. The test was repeated by interrupting the urinary stream with external urethral sphincter electrostimulation. The electromyography response of the internal urethral sphincter to the suppression and interruption of voiding was documented before and after internal urethral sphincter anesthetization. RESULTS: Suppression of the reflex detrusor contraction as well as of urinary stream interruption by external urethral sphincter contraction voluntarily or by electrostimulation resulted in a significant detrusor pressure decrease (p <0.01) and urethral pressure increase (p <0.001). Internal urethral sphincter electromyography activity, which normally disappears during voiding, was still present. After internalurethral sphincter anesthetization subjects suppressed the reflex detrusor contraction by voluntary external urethral sphincter contraction for a mean of 62.6 +/- 9.6 seconds, after which involuntary voiding occurred. The internal urethral sphincter showed no electromyography activity. CONCLUSIONS: The external urethral sphincter induces continence by preventing internal urethral sphincter relaxation at the detrusor contraction, which is suggested to be reflex in nature and is called the voluntary urinary inhibition reflex, and by mechanically compressing the urethra. Contraction of the external urethral sphincter, which is a striated muscle, mechanically occludes the urethra for a few seconds, by which time the detrusor has relaxed as an effect of the voluntary inhibition reflex.