Detrusor instability as an energy-saving device in prostatic obstruction.

ABSTRACT

PURPOSE: We assessed whether voiding dynamics differ in patients with infravesical obstruction from benign prostatic hyperplasia between those with detrusor instability and those with stable bladders. MATERIALS AND METHODS: A total of 50 such patients with (25) and without (25) detrusor instability was investigated urodynamically by cystometry and pressure-flow study. RESULTS: In the unstable group there was greater obstruction (as assessed by the linear passive urethral resistance relation pressure-flow nomogram) but the detrusor expended less energy (p = 0.001) for voiding the unit volume (54.4 +/- 22.8 mJ./ml versus 74.4 +/- 31.8 mJ./ml. in the stable series). Spared energy was converted into more powerful micturition contractions and more efficient voiding, with a mean maximum contraction power and mean fraction of bladder volume voided of 17.9 +/- 4.9 microW./mm.2 and 93 +/- 11%, respectively, in the unstable population versus 12.7 +/- 2.1 microW./mm.2 (p < 0.001) and 79 +/- 13% (p < 0.001) in the stable subjects. CONCLUSIONS: Detrusor instability possibly works within physiological limits as an energy-saving device, preventing voiding efficiency in patients with prostate disease from decreasing too much with increasing obstruction.