Hypothesis / aims of study
Diagnosing detrusor underactivity (DU) in women remains challenging. Standardized and accepted nomograms for DU diagnosis have been reported only for men (ref.1), and definitive standardized urodynamic studies (UDS) criteria for defining female DU are lacking. Several urodynamic parameters have been proposed to estimate bladder contractility, including the bladder contractility index (BCI), projected isovolumetric pressure 1 (PIP1), and the maximum value of Watts factor (W) (Wmax). In a previous study, we used the area under the W curve during the voiding phase (WF-AUC) as a new parameter for diagnosing DU before and after transvaginal surgery in women (ref.2). The WF-AUC was used to detect improvement in detrusor contraction during voiding and to assess total detrusor contractility. The main advantage of W is that it can be calculated without considering the infusing bladder volume. A more accurate representation of detrusor contractility during the whole voiding phase might be achieved by comparing the WF-AUC with previously reported parameters. The primary objective of this study was to evaluate the usefulness of the WF-AUC in women.
Study design, materials and methods
Data from consecutive female patients presenting with lower urinary tract symptoms (LUTS) were retrospectively reviewed. Exclusion criteria included history of spinal cord injury, neurologic abnormalities, surgical correction of congenital anomalies, and pelvic surgery.
The W values, which demonstrate the mechanical power per unit area of the bladder surface produced by detrusor contraction during the voiding phase, were computed in accordance with methodology reported by Griffiths et al (ref.3). The Andromeda Ellipse system measured W throughout the voiding phase and presented the results as a continuous line graph. We measured the area under the W curve during the voiding phase. The urodynamic parameters examined in this study included the filling volume, voided volume, maximum flow rate (Qmax), postvoid residual urine volume, detrusor pressure at maximum flow rate (PdetQmax), maximum detrusor pressure, BCI (determined using the following formula: PdetQmax + 5 Qmax), PIP1 (PIP1: PdetQmax + Qmax).
The investigation was conducted in the following order. First, detrusor contractility was classified using previously reported five criteria. Women were stratified based on UDS parameters as follows:
I PdetQmax < 30 cm H2O and Qmax < 10 mL/s
II PdetQmax < 20 cm H2O and Qmax < 15 mL/s
III BCI < 100
IV PIP1 < 30
V Wmax < 7 W/m2
Second, we assessed the potential correlations of PdetQmax, Wmax, BCI, and PIP1, with WF-AUC. Third, receiver operating characteristic (ROC) curve analysis was performed to determine the cut-off value for diagnosing DU based on multiple criteria, along with its accuracy and specificity.
All statistical analyses were performed using GraphPad Prism for Windows Ver. 9.31 (GraphPad Software, San Diego, CA, USA). The Mann-Whitney U test was used to compare nonparametric variables between normal and weak detrusor contractility. Statistical significance was set at P<0.05.
Results
A total of 77 women underwent urodynamic evaluation during the study period. The mean age of the patients was 69 years. During the pressure-flow study, the median filling volume was 414.0 mL, the median Qmax was 16.5 mL/s, and the mean voided volume was 293.4 mL. Detrusor contractility parameters yielded the following results (mean): BCI, 102.9; Wmax, 9.8 W/m2; PIP1, 37.1; and WF-AUC, 247.8. Based on pressure-flow analysis using PdetQmax<30 cm H2O & Qmax<10 mL/s, 58 patients had a "normal," and 19 had a "weak" detrusor. When using PdetQmax<20 cm H2O & Qmax<15 mL/s to grade contractility, 59 patients had a "normal," and 18 had a "weak" detrusor. When using BCI to grade contractility (cut-off value: 100), 38 patients had a "normal," and 39 had a "weak" detrusor. When using PIP1 to grade contractility (cut-off value: 30), 47 patients had a "normal," and 30 had a "weak" detrusor. Additionally, when using Wmax to grade contractility (cut-off value: 7.0 W/m2), 38 patients had a "normal," and 39 had a "weak" detrusor.
The relationships between multiple parameters and the WF-AUC are shown in Figure 1. Spearman's correlation test revealed that Wmax, BCI, and PIP1 were positively correlated with WF-AUC, with the correlation coefficient being 0.63, 0.64, and 0.61, respectively (each P<.001). Table 1 shows the data of ROC curve of the WF-AUC based on previously reported five criteria defining DU.
Interpretation of results
In this study, we compared previously reported five criteria for DU with WF-AUC and analyzed whether the WF-AUC could assess detrusor contraction in women with LUTS. We confirmed a relatively strong correlation between previously reported criteria for DU and the WF-AUC, with no significant deviations observed.
We assessed the combination of parameters (Pdet Qmax and Qmax), Wmax, BCI, and PIP1. This study revealed significant variations in the number of patients diagnosed with DU based on each criterion. The parameters that resulted in the lowest number of patients with DU were PdetQmax<20 cm H2O and Qmax<15 mL/s. In contrast, the highest number of patients diagnosed with DU was observed when using BCI < 100 and Wmax < 7 W/m2 criteria. The most remarkable distinction between the BCI and Wmax classifications lies in the PdetQmax value. In the BCI classification, no difference in PdetQmax was found between the normal and DU groups (21.3 ± 21.3 vs. 20.1± 11.7). This finding may carry more significance in women due to the generally higher Qmax observed in women compared to men. Significant difference in the WF-AUC was confirmed between normal and weak detrusor in each of previously reported five criteria for defining DU.
Broader criteria are expected to exhibit higher sensitivity and lower specificity, whereas stricter criteria are likely to have lower sensitivity and higher specificity. Determining the appropriate cut-off value for the WF-AUC is difficult. However, if PIP1<30 is deemed a useful cut-off value among previous criteria, then WF-AUC<257.3 may serve as a useful cut-off value.