Sacral neuromodulation (SNM) has been found to improve symptoms for patients who have refractory overactive bladder syndrome and urinary urge incontinence. The mechanism is thought to be due to pulsatile electrical stimulation of sacral nerve fibres to allow for better regulation of pelvic neural networks. Although there is high-level evidence demonstrating its efficacy for subjectively improving urinary urgency and urge incontinence, the objective urodynamic changes are less well studied. 

The aim of our study was to assess urodynamic parameter changes in a real-world setting, with secondary outcome of comparing changes to subjective symptoms in patients who have had an SNM implanted for detrusor overactivity.

We performed a retrospective chart review of patients who had urodynamic studies (UDS) done both before and after SNM implantation for urodynamic confirmed detrusor overactivity (DO) +/- incontinence (DOI). Inclusion criteria were patients >18 years of age of any gender who had an SNM implanted at our single high-volume tertiary care centre during the study time period. Patients who had the device implanted for other diagnoses (such as bladder pain or voiding dysfunction) were excluded. We looked at the urodynamic parameters in these cases to assess for objective changes. We also compared ICIQ-FLUTS scores pre- and post-SNM implantation for patient-reported subjective changes.

Five patients met the inclusion criteria, all of whom were female and had the SNM implanted between 2014 and 2018 at our institution. They presented for repeat UDS within 5 years of SNM implantation and in all cases the device was interrogated to confirm both lead and battery functionality. The majority (80%) of patients presented with stress urinary incontinence (SUI) symptoms and had urodynamics as per local protocol prior to offering SUI surgical options. The remaining patient had ongoing storage LUTS.

In the post-SNM UDS, median cystometric capacity increased from 282mls to 362mls, and median flow rate increased from 19ml/s to 29ml/s. Of the 5 patients, 40% (2/5) reported ongoing symptoms of urgency. The average detrusor pressure at Qmax increased from 16cmH2O (12-18) to 29cmH2O (13-38). The median frequency of DO decreased from 8 (1-20) to 2 (0-13).  The average DO height was also decreased from 14 cmH2O (9-19) to 5 cmH2O (0-13).

Generally, there were improvements in subjective and objective scores following SNM implantation. The cystometric capacity and the voided volume both increased, indicative of an improved bladder capacity. The voiding phase during post-SNM UDS showed the maximum flow rate was almost doubled, and the detrusor pressure at this point was also greater. 

There was a marked decrease in number of episodes of detrusor overactivity, which could explain the greater bladder capacity. The average filling volume during the cystometrogram increased at the first episode of detrusor overactivity as well. The UDS findings show changes in both the filling and voiding phases.
Interestingly patients did not feel any improvement in frequency symptoms with most saying that they passed urine between 7 and 8 times a day although they did report slightly less bother from this level of frequency. They reported less urinary leaking post-SNM but no change in urgency incontinence. However, their symptom score was improved in all areas. The overall bother symptom score dropped by an average 2 points. Most patients still suffered with stress incontinence and some of them have gone on to have surgical management for SUI. 

Further research could investigate using a larger sample size including male patients, repeated urodynamic studies for those who do not report stress incontinence, or investigating individual changes in parameters. For the patients who went on to have SUI surgery, it would be interesting to see if this changed their urgency symptoms. As well, investigating whether patients had as much awareness of the DO with the smaller amplitude contractions will be part of future work.

Our data have shown that in a real-world sample of women who underwent repeat UDS following SNM implantation there is improvement in most parameters of the UDS, but especially in the frequency of DO, the amplitude of DO, bladder capacity, and the flow rate. This is an area that has not been studied extensively, and the repeat UDS have shown the objective effectiveness of SNM implantation.