Sacral neuromodulation (SNM) is a minimally invasive and effective therapy used to manage urinary incontinence, urinary retention, urgency, frequency, and fecal incontinence [1]. While SNM has earned acclaim for its efficacy in alleviating symptoms of overactive bladder (OAB), its application and determinants of success in the context of underactive bladder (UAB) remain areas of ongoing inquiry. In the management of UAB, SNM is a commonly used therapy for patients who do not respond to conservative management [2]. However, not all patients respond to SNM, and identifying predictors of success is important for optimizing patient selection and outcomes. Urodynamic variables, such as detrusor pressure at maximum flow rate (Pdet at Qmax) and bladder capacity, have been suggested as predictors of SNM success in UAB patients, but the evidence supporting their comparative predictive value is limited [3]. This study aims to compare the predictive value of urodynamic variables and patients’ demographics in predicting SNM success in these patients, providing important insights into the use of urodynamic variables and guiding clinical decision-making.

A retrospective chart review was conducted to investigate the outcomes of SNM among patients diagnosed with UAB who underwent treatment at our institution in the last 5 years. The study cohort comprised 32 patients diagnosed with UAB. Data collection included a range of clinical parameters, such as initial visit/preoperative post-void residual (PVR), Valsalva voiding, catheter use, and documentation of comorbidities. Furthermore, comprehensive urodynamic evaluations were performed to capture urodynamic variables. These included maximum detrusor pressure (Pdet max), detrusor pressure at maximum flow rate (Pdet at Qmax), maximum flow rate (Qmax), post-void residual (PVR), and end filling detrusor pressure (End Filling Pdet).  Within this analysis, the success cohort was delineated, comprising patients who underwent implantable pulse generator (IPG) placement with greater than 50% reduction in urinary symptoms. Conversely, patients who did not achieve substantial improvement in urinary symptoms but underwent IPG placement for bowel dysfunction were categorized as non-success cases for study purposes. A primary objective of this study was to assess the proportion of patients who progressed to second stage SNM, characterized by the placement of a permanent IPG subsequent to a successful percutaneous nerve evaluation (PNE) trial. Additionally, the study aimed to identify patients who needed SNM removal due to either lack of efficacy or intolerable side effects. In correlation with the analysis of urodynamic variables, an assessment was undertaken to explore diverse independent factors, including age, comorbidities, race, ethnicity, catheter dependency, and gender, aiming to discern any discrepancies between the success and non-success cohorts. Descriptive statistics were employed, and statistical analysis was performed using SPSS version 26.0.

A total of 32 patients diagnosed with UAB underwent SNM in our study, with female patients constituting 78% (25) of the cohort, while males accounted for 7 (22%) of the participants, exhibiting no disparity in mean age (57 years) between genders (Table 1). The overall success rate of SNM was determined to be 88%, and comparison of success and non-success groups was calculated to be 80%. Post-hoc power analysis was conducted using G*Power3 to test the difference between two independent group means using a two-tailed test, a medium effect size (d= .50), and an alpha of .05. Result showed that a total sample of 32 patients with success (n =28), and non-success (n=4) had a power of .15.

Significant differences were observed between the success and non-success cohorts concerning gender distribution and prior medical history. Notably, a higher proportion of males was found in the non-success group compared to the success group (75% vs. 14%, p = 0.02), along with a notable prevalence of prior bladder outlet obstruction procedures in the non-success cohort relative to the success cohort (50% vs. 4%, p = 0.03).

Furthermore, a marked disparity in catheter dependency was evident between the two groups, with a significantly lower incidence observed in the success cohort compared to the non-success cohort (25% vs. 100%, p < 0.01).

There was a substantial difference in average PVR volume between the success and non-success cohorts at the initial visit (65 mL vs. 675 mL, p < 0.01). Q1 for the Success cohort was 0 mL, median 45 mL, mean 65 mL, and Q3 124 mL. For the Non-Success cohort, Q1 was 512.5 mL, mean and median 675 mL, and Q3 837.5 mL.

Comparison of PVR volumes during urodynamic studies showed significant differences as well (220 mL vs. 535 mL, p < 0.01), while Pdet max and voiding ability during UDS showed no significant discrepancy between the two groups.

Logistic regression analysis identified gender and average PVR as non-significant factors influencing the success of SNM. In the success cohort, a notable distinction was observed in preoperative and postoperative PVR volumes, with a statistically significant difference noted (p < 0.01).

The significantly higher proportion of males within the non-success cohort underscores the need for tailored interventions considering gender-specific factors influencing treatment response. This is also the first study, albeit the small sample size, reporting the association between prior bladder outlet obstruction (BOO) procedures and diminished SNM success rates. This observation accentuates the importance of a comprehensive patient evaluation and assessment, including detailed medical history, to help the provider better delineate potential confounding factors that may impact treatment response. 

Catheter dependency in our study also emerged as a potential factor influencing treatment success, with a stark contrast observed between the success and non-success cohorts. The presence of catheter dependency among all non-success cases emphasizes the profound impact of bladder dysfunction on patients' quality of life. It also highlights the need for effective therapeutic interventions to alleviate symptom burden and restore bladder function. 

Urodynamic parameters, particularly PVR volumes, appeared to serve as valuable prognostic indicators in predicting treatment outcomes following SNM. The substantial disparity in PVR volumes between success and non-success cohorts underscores once again the clinical utility of urodynamic evaluations in guiding treatment decisions. Furthermore, the observed differences in PVR volumes during both initial visits and urodynamic studies underscore the dynamic nature of bladder dysfunction and the potential for SNM to exert meaningful effects on bladder function over time. 

Despite the associations observed between certain patient factors and treatment outcomes, logistic regression analysis revealed gender and average PVR as non-significant predictors of SNM success. This highlights once again the multifactorial nature of treatment response and the need for comprehensive, individualized treatment approaches tailored to patients' unique clinical profiles.

The notable distinction observed in preoperative and postoperative PVR volumes within the success cohort underscores the therapeutic efficacy of SNM in optimizing bladder function and alleviating symptom burden in patients with UAB, and consequently enhancing patients' quality of life.

Our study provides valuable insights into the factors influencing treatment outcomes following SNM in patients with UAB. Despite the multifactorial nature of treatment response, our study identifies male gender and average post-void residual (PVR) volume as potential factors associated with SNM success in this population. Moreover, our findings contribute to bridging the knowledge gap regarding the application and determinants of SNM success specifically in UAB. Importantly, we highlight the need for comprehensive patient evaluation and assessment, including detailed medical history and urodynamic evaluation, to optimize treatment strategies and improve outcomes for patients with this challenging condition.

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