Voiding dysfunction can be due to bladder outlet obstruction (BOO) or detrusor underactivity (DU). In male above 50 years, BOO owing to Benign Prostatic Hyperplasia is the most frequent cause of voiding dysfunction. Nevertheless, there is not unusual that some of these patients have DU alone or coexisting with BOO. Up to now there is no treatment for DU. A complication of DU is urinary retention, either acute (AUR) or postmicturition voiding residual (PVR). The treatment of this complication is bladder catheterisation. 
However, experimentally it has been demonstrated that BOO removing can improve bladder function (1). Moreover, in human Hamman et al (2). have shown that temporarily suprapubic bladder catheterization decreases PVR and increases urinary flow.
We hypothesize that bladder catheterization is not only a mean to overcome urinary retention, but that it may improve bladder contractility. Therefore, our aim is to investigate urodynamic changes during the voiding phase of male with DU undergoing bladder catheterization for urinary retention

Study design. Longitudinal prospective study
Material and methods.
We carried out a prospective study in a cohort of 64 males submitted to bladder catheterization because of urinary retention. The patients were submitted to a urodynamic study and we withdraw from the study 20 patients without DU. Other 6 patients were excluded for having neurogenic dysfunction and 4 for taking active drugs over lower urinary tract. It was offered to the remained 34 patients to continue with bladder catheterization and to undergo a second urodynamic study 12 months later. Seventeen patients, aged 77 ± 7,6 years (mean± standard deviation), accepted and were submitted to a second urodynamic study.
Sample size was calculated based on the data provided by OU et al (3). Assuming an increase in Pdetmax of 8 cm H2O, a standard deviation of 10 cm H2O, a statistical power of 80%, and alpha level of 5%, the minimum sample size was calculated at 16 patients.
Urodynamic study was performed in accordance with the specifications of the ICS and guidelines for Good Urodynamic Practice. Detrusor contractility was calculated using the Bladder Contractility Index (BCI) (PdetQmax + 5. Qmax), and urethral resistance using the Bladder Outlet Obstruction Index (BOOI) (PdetQmax – 2. Qmax). DU was defined as a BCI < 100

Comparison regarding the first urodynamic study between patients who were submitted to a second urodynamics study and those who do not, demonstrated no significantly statistical differences.
The second urodynamic study showed a significant increase in maximum detrusor pressure (PdetMax), maximum flow rate, BOOI and BCI (table 1) We also observed a significantly increase (p= 0.049) in number of patients who urinated during the second study (table 2).

The increment of BCI confirm our hypothesis that bladder catheterization improves bladder contractility.
	We also found an increment of BOOI. Although we cannot reject that the cause of this BOOI increase might be in relation with the catheter maintenance, the absence of a significantly opening pressure increasing, the increase of the Pdetmax and the significantly number of patients who urinated in the second study, allows us consider that the cause  may be a masked BOO as a consequence that the underactive detrusor is not able to reach a sufficient bladder pressure to demonstrate a BOO.

Bladder catheterization in men with DU significantly improves bladder contractility and revealed BOO that were masked by insufficient detrusor pressure in relation to DU of these patients. These findings could have diagnostic as well as prognostic and therapeutic applications.

Wolffenbuttel KP, de Jong BW, Scheepe JR, Kok DJ. Potential for recovery in bladder function after removing a urethral obstruction. Neurourol Urodyn. 2008;27(8):782-8
Hamann MF, van der Horst C, Naumann CM, Wiederholt C, Seif C, Jünemann KP, Braun PM. [Functional results after temporary continuous drainage of the hypocontractile bladder. The potential rehabilitation of the detrusor]. Funktionelle Ergebnisse nach temporärer Dauerableitung der hypokontraktilen Blase. Urologe A. 2008;47(8):988-93 [Article in German].
Ou R, Pan C, Chen H, Wu S, Wei X, Deng X, Tang P, Xie K. Urodynamically diagnosed detrusor hypocontractility: should transurethral resection of the prostate be contraindicated? Int Urol Nephrol. 2012 ;44(1):35-9.