Initiation of voiding in humans: the nature and temporal relationship of urethral sphincter responses

The nature and sequence of events during the initiation phase of human micturition are unclear. Disagreement concerning the urethral sphincter response to detrusor contraction may stem from the functional extension of periurethral striated muscle to the bladder neck in many but not all individuals, and the methods used during conventional cystometry, in which individuals are asked to forestall urination for as long as possible (urgency voiding). We reasoned that by instructing individuals to void despite the lack of urgency (volitional voiding), and by stratifying results by whether striated muscle influence extended to the bladder neck, response of the proximal urethra might be more readily determined.

Using a triple microtip transducer catheter and a triple lumen fluid-filled catheter, we investigated 44 consecutive men and women with a variety of urodynamic findings, including 12 whose evaluation was normal.

Despite the diversity of urodynamic diagnoses, once data were stratified as described, results were striking and uniform. The smooth muscle component of the bladder neck region contracted during the initiation of voiding in all subjects. Despite antecedent relaxation of the striated muscle sphincter, voiding did not begin until bladder pressure equaled or exceeded bladder neck pressure.

Proximal urethral pressure increases in the initial phase of human micturition. These findings may have significant physiological, diagnostic and therapeutic implications.