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

Abstract

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. A total of 50 such patients with (25) and without (25) detrusor instability was investigated urodynamically by cystometry and pressure-flow study. 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. 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.