Persistently increased voiding frequency despite relief of bladder outlet obstruction

Bladder outlet obstruction (BOO) can increase urinary frequency. Even after surgical relief of obstruction, up to 30% of patient are still bothered by irritative voiding symptoms. We tested the hypothesis that deligation of a partial bladder outlet obstruction model mimics this clinical observation.

Female Wistar rats were obstructed for 3 weeks by partial urethral ligation and then were relieved of obstruction by urethral deligation. Measurements of voiding frequency and voided volumes were measured preoperatively, after ligation, and after deligation. Relief of obstruction was confirmed by measuring flow rates through ex vivo perfusion of deligated urethras. Urine osmolality and bladder weights were determined. Awake cystometrograms (CMGs) were performed 3 weeks after deligation to measure bladder function.

Neither sham ligation nor sham deligation altered voiding frequency. Ligation doubled mean voiding frequency (in cc) from 2.01 +/- 0.32 to 3.96 +/- 0.22 per 4 hours (p = 0.0002). Three weeks after deligation, voiding behavior of the animals segregated into 2 groups: 20% had persistent hyperactive voiding frequency (6.67 +/- 1.23 per 4 hours) while 80% normalized voiding frequency (1.53 +/- 0.20 per 4 hours). The difference in voiding frequency in these 2 groups could not be attributed to alterations in urine osmolality, persistence of urethral obstruction, difference in bladder weights or severity of initial obstruction created. Awake CMGs revealed a higher peak micturition pressure and lower voided volume in the hyperactive voiders.

20% of the animals after urethral deligation had persistent hyperactive voiding which parallels clinical observations. Because the CMG data suggested persistent obstruction, yet urethral perfusion and bladder weights indicated no obstruction, we propose that these 20% of animals have a "functional" bladder outlet obstruction and can be used to study mechanisms underlying hyperactive voiding.