Sacral neuromodulation (SNM) is an established treatment for underactive bladder (UAB), yet patient selection remains controversial. We aimed to identify predictors of successful Stage II implantation in a real‑world UAB cohort.

Retrospective cohort study of patients with UAB (NOR, neurogenic detrusor underactivity, or neurogenic bladder) who underwent Stage I SNM (2013–2025). Primary outcome was conversion to Stage II (full implant). Univariate and multivariate logistic regression were used to assess predictors including bladder sensation, Bladder Contractility Index (BCI), baseline post‑void residual (PVR), and comorbidities. Among Stage II patients, we correlated reprogramming frequency in the first year with all clinical variables.

Of 45 patients (mean age 38.7 years, 57.8% female), 34 (75.6%) proceeded to Stage II. On multivariate analysis, BCI low (50 100) was the strongest independent predictor of success (OR 17.9, 95% CI 1.71 187, p=0.01). All patients with BCI very low (<50) failed (0/9), all with BCI low succeeded (12/12). Bladder sensation did not significantly predict outcome (p=0.54). All patients with baseline PVR >300 ml was associated with 100% SNM implants, but numbers were small (n=5, p=0.09). Secondary outcomes: 38.2% of Stage II patients achieved complete catheter independence. Overall complication rate was 13.3%, with lead migration in 8.9%. Among Stage II patients, reprogramming frequency in the first year showed no correlation with any clinical variable (all p > 0.05), including age, gender, BMI, primary diagnosis, sensation, BCI, PVR, comorbidities, abdominal surgery, or maximum cystometric capacity.

Low Bladder Contractility Index (BCI 50–100) strongly predicts successful SNM and conversion to full implant (OR 17.9), whereas very low BCI (<50) universally predicts failure. Bladder sensation, PVR, and other clinical variables did not predict success. BCI offers a potential strong predictor for selection of patients with underactive bladder.

The overall conversion rate to Stage II was 75.6%. BCI was the sole independent predictor of this primary outcome, with a low BCI (50 100) strongly associated with success, a very low BCI (<50) universally predicted failure. Bladder sensation and PVR did not independently predict conversion to SNM implantation. Our results provide a potential helping tool for patient selection and counseling, on sacral neuromodulation for underactive bladder.