Neuromodulation on Bladder Capacity in Conscious Sheep

Sacral neuromodulation (SNM) is used for the treatment of patients with overactive bladder when first-line therapies, such as antimuscarinics, do not provide sufficient efficacy [1]. Recently, advances have been made in identifying optimal stimulation parameters [2–5], as well as neural pathways [6] and neurotransmitter systems that contribute to the neuromodulation's inhibitory effects on urinary micturition reflex [7]. However, neuromodulation of bladder function has usually been studied acutely (minutes) in anesthetized rats or cats and cannot address questions regarding continuous therapy in patients.A few studies have used rodent models to evaluate chronic effects of neuromodulation [8–10]. These studies have shown positive effects, such as reducing the frequency of micturition in rat models of cystitis induced by intravesical administration of hydrochloric acid [8]. However, amount of stimulation varied significantly between studies as did the system (exteriorized wires versus an implanted device). There is a distinct need to use and study commercialized devices and clinically relevant parameters to further increase understanding of SNM's effects and to optimize therapy.The goal of this study was to develop and test a large animal model to evaluate effects of SNM in conscious sheep. Bilateral SNM was applied at a frequency of 10 Hz, which has been shown to be optimal for inhibition of bladder contractions [2,3], using clinically relevant current intensity and pulse-width.For device implantation, female sheep (60–80 kg, n = 4) was anesthetized with intravenous propofol (6 mg/kg in combination with 1–3% isoflurane). Two stimulating leads (Model 3889, Medtronic, Inc.) were bilaterally implanted in the sacral foramina (S2 or S3). Leads were connected to two InterStim II devices (Model 3058, Medtronic, Inc.). Motor thresholds (VTh) were tested intra-operatively to confirm lead targeting. After surgery, sheep was allowed to recover for 2 weeks before urodynamics. Urodynamics with 10 voiding trials were performed weekly in conscious, sling-restricted sheep. A double-lumen catheter (12F) was inserted through the urethra into the bladder. The inner lumen of the catheter was connected to a pump for infusing the bladder with saline, and attached to a pressure sensor at the tip to monitor bladder activity (Biopac Systems, Goleta, CA). The outer lumen was attached to a 3–5 ml balloon filled with saline to secure the bladder catheter. In each urodynamic trial, saline was infused into the bladder via the syringe pump (30 ml/min) until voiding occurred. Voiding consisted of a large amplitude bladder contraction (>30 mmHg) with void behaviors (arched back and bent legs). Saline infusion was terminated and the bladder was emptied. The infused volume of saline was recorded as bladder capacity. The stimulation voltage of motor threshold (VTh) was obtained by turning on each InterStim II device independently and increasing voltage until visual confirmation of motor twitching in the appropriate muscle target (perianal area or leg). Maximum tolerable stimulation voltage (VMax) was the highest voltage at which sheep remained calm. For SNM testing, stimulation (10 Hz, 0.21 ms pulse-width) was applied at VTh or VMax intensity during the fourth to fifth bladder void (two voids) or fourth to tenth void (seven voids).All data are expressed as mean ± SEM. The capacity of the first void was excluded from analysis and data were normalized to the mean capacity of the second and third baseline values. Results were analyzed with repeated measure repeated measures analysis of variance (ANOVA) (GraphPad Software, Inc., San Diego, CA). A value of p < 0.05 was considered statistically significant.Figure 1 shows a typical example of urodynamics, summarized in Fig. 2. In this sheep, the bladder capacities were 103 ml and 260 ml prior to and with SNM at VMax, 10 Hz, respectively.There was no significant change in bladder capacity if electrical stimulation was not applied (Fig. 2). Capacity during stimulation at VTh and VMax for two trials changed to 76 ± 1% (p = 0.20) and 204 ± 60% of controls (versus 96 ± 11%, without stimulation, p = 0.18), respectively. There was no functional delay between application of the stimulus and effect on the bladder, i.e., the bladder capacity of the next void was 140 ± 42 ml when the stimulation at VMax was applied while the baseline capacity was 87 ± 23 ml. The capacity returned back close to the baseline value (84 ± 13 ml) after stimulation was terminated. Finally, repeated measure ANOVA analysis demonstrates a significant increase in bladder capacity to VMax stimulation for seven trials (p = 0.02). The mean bladder capacity increased to 245 ± 40% of control (from 42 ± 8 ml to 100 ± 19 ml, versus 94 ± 16%, without stimulation).These data are an early phase demonstration that acute sacral neuromodulation can cause quantitative changes in urodynamic measures in unanesthetized, conscious sheep. The urodynamic changes parallel clinical therapeutic responses to SNM measured in patients using urodynamics and patient diaries [11]. SNM has been tested previously using urodynamics in rats [10,12] and prior data from anesthetized rodents and cats have been used to support neuromodulation therapy for bladder control [2,3]. Here, we provide an initial description of responses to SNM in conscious sheep using urodynamic quantification.This novel model will greatly facilitate testing and development of future neuromodulation therapies for bladder control. Perhaps, most valuable will be the ability to test therapies over long durations (weeks to years) in studies of therapy maintenance that better match clinical use.In this study, electrical stimulation increased bladder capacity in conscious sheep. The increase was induced by relatively high intensity SNM (above VTh). We have reported the bladder inhibitory effects of SNM in the rat are intensity-dependent. Attenuation is stronger with increase in the stimulation current [2,4]. Future work will need to better understand the relationships between stimulation parameters and bladder capacity changes over time as well as the long-term changes that may occur during continuous stimulation in this testing model. Future studies will also focus on the stimulation induced changes in motor and maximal tolerated amplitudes we observed here.Sacral nerve stimulation using permanently implanted SNM increases bladder capacity in conscious, unanesthetized sheep. This model appears to be feasible for testing of implantable neuromodulation devices as well as SNM parameters to test different therapeutic responses to confirm and extend clinical therapeutic concepts.The authors are grateful to Ms. Jennifer Dangers, Ms. Melissa Mattson, Mr. William Schindeldecker, Ms. Stephanie Marsolek, and Dr. Luis Ramon for technical support, Drs. Lance Zirpel and Greg Molnar for helpful comments and Dr. Matt Kelly for study coordination.