Uroflowmetry combined post-void residual (PVR) measurement is well known as a useful tool in diagnosing bladder dysfunction and outlet obstruction. Recently, it has been used in evaluation of different types of urethral stricture[1].The patients were encouraged to drink more water and overfill the bladder, guaranteeing maximum voided volume to keep successful recording the uroflow curve in many hospitals, but questions emerged due to overfill bladder is not natural and might change the bladder function artificially. Our study aims to investigate the effect of bladder overfill on uroflowmetry and PVR of postoperative urethral stricture in children with hypospadias.

A total of 112 children (aged 4～6 years, 5.2±1.5Y) with penile hypospadias performed one stage urethroplasty, involved in this study. Uroflowmetry and PVR measurement follow-up were performed at 3-6 months after urethroplasty. The patients were divided into two groups, urethral stricture (dysuria with a 6F urethral probe unable to go through urethra) and no stricture group (with no voiding difficulty and a 6F urethral probe go through urethra easily), each group consists of two subgroups, bladder overfill and no overfill group. The maximum urine flow rate (Qmax), voided time (VT), voided volume (VV), PVR and urine flow curve shape were compared between different groups. Paired t test was used to analyse the data, P < 0.05 was significant.

In urethral stricture group, a significant decrease in Qmax and increase in VT and PVR compared to those with no bladder overfill were found (Qmax:7.7±3.5 vs.10.8±4.7mL/s，P＜0.05; VT:45.4±10.8 vs.21.9±8.4s，P＜0.001 and PVR: 28.9±7.3 vs.18.1±6.9mL，P＜0.001). In children with no urethral stricture, a significant increase in Qmax, VT and PVR compared to those with no bladder overfill were found (Qmax:15.9±3.4 vs. 13.5±2.6 ml/s，P<0.05；VT:17.9±7.5 vs. 11.7±6.9s, P<0.01 and PVR:16.4±5.1 vs. 7.8±4.3mL, P<0.001). The ratio of normal smooth uroflow curve decreased and Staccato curve increased significantly in all children with bladder overfill compared to those with no bladder overfill (normal smooth curve: 42.9% vs. 48.2%, P<0.05; Staccato curve: 7.1% vs. 1.8%, P<0.05).

The results showed that overfilled bladder produced significant effect on results of uroflowmetry and PVR measurement. It induced significant change in Qmax, VT, PVR and the type of uroflow curve. The mechanism involved in this change is still unclear. However, over fill bladder will induce significant bladder distension might induce detrusor fatigue temporarily; consequently, voiding detrusor pressure decreased resulting Qmax decrease and PVR increase. It has been reported that detrusor sphincter dyssynergia (DSD) might induce voiding staccato curve. Therefore, it is possible that overfilled bladder might induce DSD in some cases. 
Compared to children with no urethral stricture, Qmax decreased, VT prolonged and PVR increased significantly in children with urethral stricture regardless of bladder distension or not. This might indicate that uroflowmetry and PVR measurement is a sensitive tool for evaluating the urethral stricture induced by surgery. However, interpretation of results in these cases must be careful due to uroflowmetry and PVR measurement is easily affected by overfilled bladder. 
The results of present study are in accordance with those of study from Dayanc[2], Yang[3] and Uzun. The number of cases in present study is relatively small, and with no normal control children (with no hypospadias), which should be considered in future study.

Uroflowmetry and PVR is a sensitive tool providing objective evidence for urethral stricture diagnosis in children after hypospadias operation. However, overfilled bladder affected the results of Qmax, VT, PVR and uroflow curve pattern significantly, suggesting that patients should not be encouraged to overfill the bladder at the time of uroflowmetry and PVR measurement.

Lambert E, Denys MA, Poelaert F, Everaert K, Lumen N. Validated uroflowmetry-based predictive model for the primary diagnosis of urethral stricture disease in men. Int J Urol. 2018 Sep;25(9):792-798. doi: 10.1111/iju.13730. Epub 2018 Jul 18. PMID: 30021245.
Dayanc MM, Kibar Y, Irkilata HC, et al. Effect of Voided Volume on Voiding Patterns and Reliability of Uroflowmetry-Electromyography Results in Children with Lower Urinary Tract Dysfunction[J]. Low Urin Tract Symptoms. 2017,9(1):46-51. doi:10.1111/luts.12108.
Yang SS, Chang SJ. The effects of bladder over distention on voiding function in kindergarteners[J]. J Urol. 2008,180(5):2177-2182. doi:10.1016/j.juro.2008.07.063