SNM is a highly effective treatment for voiding dysfunction in women, with reported efficacy close to 80%. Better immediate and long-term SNM outcomes in patients with voiding dysfunction have been reported when there is a finding of an abnormal external urethral sphincter electromyography (EUS-EMG) [1,2]. Other studies document a correlation between abnormal EUS-EMG and higher values of maximum urethral closure pressure (MUCP) [3]. Therefore, a similar correlation between MUCP and SNM outcome could be expected. We aimed to review the outcomes of SNM treatment for female voiding dysfunction in a high-volume U.K. SNM unit, to explore potential clinical, urodynamic and/or neurophysiological parameters that could help to predict the efficacy and better select patients for this therapy.

Retrospective review of all female patients with voiding dysfunction referred to a single tertiary referral centre between 2012 and 2022. All patients with neurological conditions or anatomical bladder outlet obstruction were excluded. Only patients with an invasive urodynamic study, urethral pressure profile (UPP), EUS-EMG and a trial of SNM were included in analysis. Age, presenting lower urinary tract symptoms (LUTS), comorbidities, urodynamic (filling cystometry and pressure-flow study), UPP and EUS-EMG data was collected. Follow-up data was recorded, including outcome of SNM trial, return of symptoms (ROS), need for battery exchange or removal of SNM device. All SNM trials were 2-stage procedures using a quadripolar permanent tined lead for the first stage trial. For the purpose of this study, success was retrospectively defined as conversion to second stage implantation of pulse generator, which in our centre is made based on a careful clinical assessment by our expert team when there has been >50% clinical improvement during the trial. Tined lead was removed in all cases of failed trials. Statistical analyses were performed using Stata 18.0 (StataCorp, Texas, USA). Parametric and non-parametric tests were used to compare means and medians depending on the distribution of variables. Fisher’s exact test was used to compare frequencies and association between variables. Logistic regression models were used were indicated. Two-sided p values <0.05 were considered for statistical significance.

Clinical and demographic data is summarised in Table 1. Fifty-two patients were included in the study. Forty-four (86.4%) had successful SNM trial. There was poor documentation of detailed objective and patient-reported outcome measures in each case as to provide a more detailed description on the parameters that were improved. Median age of patients with a successful trial was similar to those who failed to improve. Prevalence of LUTS was similar between success and failure groups, with a higher prevalence of urge- and stress- urinary incontinence symptoms amongst those who failed the trial, but this did not reach statistical significance. Most patients were unable to identify a triggering event for their symptoms and when identifiable, the triggering events were found to be similar between both groups. Distribution of comorbidities were also similar between groups, with anxiety/depression being the most common. There was an apparent higher proportion of patients with an indwelling (urethral/suprapubic) catheter in the failure group but did not reach statistical significance.

Neurophysiology and urodynamic findings are summarised in Table 2. EUS-EMG findings were similar between groups, and although the MUCP tended to be higher in the successful trial patients, this was not statistically significant. Bladder capacity and bladder sensations were the only parameters from filling cystometry that were associated with the SNM trial outcome. Higher capacity and reduced/delayed sensations were associated with success, whereas early sensations and pain due to bladder filling were associated with failure. Detrusor overactivity was an uncommon finding despite the high prevalence of reported frequency/urgency symptoms, suggesting a significant component of sensory-urgency in these patients. Only 24 (54.5%) and 5 patients (62.5%) were able to perform a void during the pressure-flow study in the successful and failure groups, respectively. The finding of detrusor acontractility, hypocontractility or functional bladder outlet obstruction (BOO) did not correlate with the outcome of SNM trial. 

Patients with a successful trial were followed up for a median of 32 months (range 4-142). Fifteen (34.1%) of them reported return of symptoms (ROS) at a median time of 9 months (range 0-94). Battery exchange was necessary in 9 patients (20.5%) at a median time of 53 months (range 32-73). Finally, removal of SNM device was necessary in 10 (22.7%) patients at a median time of 26 months (range 1-58). EUS-EMG results and MUCP failed to demonstrate any associations with ROS, need for battery exchange or SNM removal in all logistic regression models (p>0.05).

The success rate of SNM trial is comparable with outcomes reported in the literature. Our data suggests that patients with reduced bladder sensations and larger capacities are more likely to have a successful outcome from SNM trial. Those with early sensations or pain with bladder filling are less likely to succeed. No other clinical, urodynamic or neurophysiological parameters were able to predict the outcome of the SNM trial. Similarly, EUS-EMG and MUCP did not correlate with long term outcomes in regards to ROS, need for battery exchange or removal of device. This study has inherent limitations due to its retrospective nature and size of the sample.

SNM is a highly effective therapy for female patients with voiding dysfunction, delivering sustained benefit in the long term. 

The observed high conversion rate after first-stage SNM trial offers an opportunity to challenge the current paradigm of staged SNM procedures and suggests that we should advocate for one-stage implantation of SNM in selected cases, such as women with voiding dysfunction and large bladder capacity or reduced sensations. This recommendation hinges upon satisfactory intra-operative positioning of the lead and motor responses.

Moreover, the inability of invasive tests such preoperative UPP and EUS-EMG to predict SNM outcome, coupled with the association between SNM success and non-invasive urodynamic parameters or bladder diaries, prompts a reevaluation of the necessity for invasive tests prior to SNM trials.

Streamlining preoperative assessments and transitioning to one-stage procedures would expedite treatment, reduce lengthy waiting periods for invasive investigations, alleviate strain on healthcare resources, optimise theatre utilisation, and streamline the patient journey. However, to validate these assertions and endorse this proposition, larger prospective studies are warranted.

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De Ridder, D., Ost, D., Bruyninckx, F. The presence of Fowler’s syndrome predicts successful long-term outcome of sacral nerve stimulation in women with urinary retention. European Urology (2007); 51(1): 229-234.
Wiseman, O. J., Swinn, M. J., Brady, C. M., Fowler, C. J. Maximum urethral closure pressure and sphincter volume in women with urinary retention. The Journal of urology (2002); 167(3): 1348-1352.

Continence 12S (2024) 101577
DOI: 10.1016/j.cont.2024.101577