Urethral resistance (UR) refers to the pressure-flow relationship in a urodynamic study (UDS) during the voiding phase. The evolution of flow and pressure from the maximum flow (Qmax) at its corresponding pressure (pdetQmax) to the end of voiding, represented by the minimal urethral opening pressure (pmuo), is defined by the Passive Urethral Resistance Relation (PURR).[1] Several measures, such as bladder outflow obstruction index (BOOI), Urethral Resistance Factor A (URA) and the linearized PURR (linPURR) were established to grade UR in a unidimensional value based on this PURR.

The anatomy of the lower urinary tract (LUT) in pre-pubertal boys is different from that of grown men. The prostate is underdeveloped, while the urethra itself has a smaller radius and a shorter length. This result in different voiding dynamics, when comparing to adult men with or without prostatic enlargement. This study aims to describe the UR after Qmax in pre-pubertal boys, in order to work towards a specific grading system of UR in pre-pubertal boys.

The study is a single-center retrospective study. All UDSs performed in boys aged ≤12 years between 2004 and 2019 were reviewed. UDSs of insufficient quality, showing pelvic floor activity (assessed by EMG) during voiding or with a sustained or after contraction following Qmax were excluded. The resulting UDSs were corrected for the time-delay between the pressure and flow. Qmax, pdetQmax and pmuo were recorded. In addition, the base model of UR pdetQmax=pmuo+1/C*Qmax^2, was used to describe the data, more specifically, to calculate the parameter C.[2]

Qmax, pdetQmax and pmuo were compared to the expected relation according to linPURR using slope A: A=(pdetQmax-pmuo)/Qmax. In addition, the Spearman correlation between the terms in the base model of UR was investigated.

104 UDSs were included after applying the exclusion criteria. The actual pmuo was significantly lower than expected from the linPURR, with a median difference of 22 cmH2O (Wilcoxon p < 0.001), see table 1. No correlation between pmuo and Qmax (-0.15, p=0.141) and a moderate correlation between pmuo and pdetQmax (0.48, p<0.001) was found. 

Regarding the slope A, a moderate correlation was found between Qmax (-0.47, p < 0.001) and pdetQmax (0.64, p < 0.001), while there was no correlation between Qmax and pmuo (-0.08, p=0.448). Similar results were observed for the correlation of those parameters with C, see figure 2.

The model of the PURR describes the UR in two terms (pmuo and C), which are correlated in men with LUTS with a Spearman coefficient of 0.58, enabling a one-dimensional BOO grading system.[2] Our results show no relation between pmuo and C (Spearman 0.02) in boys, indicating that this simplification cannot be made. Additionally, the actual slope was found to be significantly steeper than expected by linPURR, while this slope is quite accurate in predicting pmuo for adult men with LUTS.[3] 
These results suggest different voiding dynamics in pre-pubertal boys compared to men. UR is relatively less ‘elastic’ (thus more “constrictive”) in boys, creating an independence between pressure needed to open the urethra (expressed by pmuo) and the resistance of the urethra during voiding (expressed by C). This demonstrates that currently used UR measures to assess BOO cannot be used in pre-pubertal boys. Additionally, the independence of Qmax from pmuo indicates that the flow-rate is independent of the pressure needed to open the urethra. In adult men this is different: a larger prostate generally leads to a lower Qmax and requires a higher opening pressure.[1]

The currently used standard urethral model used in adult men does not represent the voiding dynamics in pre-pubertal boys. Therefore, additional UR metrics and reference values need to be established in this population.

Schäfer W. The contribution of the bladder outlet to the relation between pressure and flow rate during micturition. In: F Hinman, S Boyarsky (eds). Benign Prostatic Hypertrophy. Springer; 1983. doi:10.1007/978-1-4612-5476-8_44
Griffiths D, van Mastrigt R, Bosch R. Quantification of urethral resistance and bladder function during voiding, with special reference to the effects of prostate size reduction on urethral obstruction due to benign prostatic hyperplasia. Neurourol Urodyn. 1989; 8: 17-27. doi:10.1002/nau.1930080104
van Dort W, Rosier PFWM, Geurts BJ, van Steenbergen TRF, de Kort LMO. Quantifying bladder outflow obstruction in men: A comparison of four approximation methods exploiting large data samples. Neurourol Urodyn. 2023 Nov;42(8):1628-1638. doi: 10.1002/nau.25270.