INITIATION OF VOIDING IN HUMANS - THE NATURE AND TEMPORAL RELATIONSHIP OF URETHRAL SPHINCTER RESPONSES

Purpose: The nature and sequence of events during the initiation phase of human micturition are unclear. Disagreement concerning the urethra l sphincter response to detrusor contraction may stem from the functio nal extension of periurethral striated muscle to the bladder neck in m any 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 exten ded to the bladder neck, response of the proximal urethra might be mor e readily determined. Materials and Methods: Using a triple microtip t ransducer catheter and a triple lumen fluid-filled catheter, we invest igated 44 consecutive men and women with a variety of urodynamic findi ngs, including 12 whose evaluation was normal. Results: Despite the di versity of urodynamic diagnoses, once data were stratified as describe d, results were striking and uniform. The smooth muscle component of t he bladder neck region contracted during the initiation of voiding in all subjects. Despite antecedent relaxation of the striated muscle sph incter, voiding did not begin until bladder pressure equaled or exceed ed bladder neck pressure. Conclusions: Proximal urethral pressure incr eases in the initial phase of human micturition. These findings may ha ve significant physiological, diagnostic and therapeutic implications.