Sacral neuromodulation (SNM) is an approved therapy for patients with OAB, urinary retention without obstruction, and bowel disorders. During surgical implantation, an electrical lead is targeted to the sacral nerve in order to elicit low (<2mA) pelvic floor bellows contractions for each stimulation contact as a predictor of effective therapy. Furthermore, this is often done under fluoroscopic visualization and the images are stored in the electronic medical record (EMR). Quantitative measurements of SNM leads or contact locations, however, are not typically made from the captured images. Therefore, we conducted a retrospective study to capture and quantify contact depths and lead angles in more detail. The utility of making these measurements was then evaluated by comparing them to the motor and sensory thresholds for each therapy contact. Specifically, our retrospective study tested the hypotheses that these thresholds measured during and following surgical implantation will be significantly different across a range of implanted lead angles and contact depths.

Records were reviewed from 16 patients from the EPIC database. To facilitate threshold comparisons across patients, we used only data from Axonics 1201 SNM implantations. Measurements of lead angles and contact depths were made from EMR fluoroscopy images (lateral view). Depths were measured for each contact using the lateral image, as the distance between the posterior margin of the contact and the posterior sacral plane. Lacking magnification level or scale for the images, we calculated contact depths as a percentage of sacral thickness (%ST). Lead angle was measured from the lateral fluoroscopic image in degrees of the inferior angle of the SNM lead relative to the anterior sacral plane. As a test of the functional utility of the fluoroscopy measurements, we compared the lead and contact locations to available bellows motor and sensory thresholds measured during and following surgical implant. Specifically we tested for significant differences across contacts at different depths or across lead angle groupings using Student’s t-test (P<0.05) and using Bonferroni correction for multiple comparisons when needed.

Data were collected for a total of 16 patients with unilateral SNM leads. Bellows motor thresholds were available for all 64 contacts from these 16 patients. Sensory thresholds were only available for 16 contacts (4 patients). Contact depths and lead angles could easily be measured for all patients. Mean contact depths ranged from 81%ST (SD=15) for the shallowest contact (3) to 184 %ST (27) for the deepest (contact 0). Mean contact depths across the contacts (0-3) were all significantly different from one another (P<0.05; t-test, Bonferonni correction, Figure 1). Mean lead angle was 80°(+/- 11°). Contact depth and angle were also related to bellows threshold, measured during implantation. Bellows threshold was significantly smaller for cathodal stimulation at contact 3 (3-,0+) relative to stimulation at contact 0 (0-,3+) (0.9+/-0.1, SEM vs 1.2+/-0.1 mA, P<0.05). We also observed a potential optimal angle for the implanted lead between 75-85°. Within this range, the bellows motor thresholds for contacts were usually significantly smaller (7 of 8 comparisons) across all lead contacts. (Figure 2) The leads within this 75-85° angle range also had smaller variations in thresholds across the lead compared to lead angles > 85° and  < 75°.

SNM contact depths and angles were measurable using saved fluoroscopic images and they appear to provide relevant information for optimal SNM therapy. The shallowest mean location for implanted contacts is visible within the sacral foramen and deeper contacts extend along the SNM lead beyond the foramen with significant differences between the mean implant depths across the contacts. The measured depths of implant contacts may reveal an ideal implantation location for SNM. Bellows motor threshold for cathode stimulation at the shallow contact 3 (3-,0+) was lower than that of the deeper contact 0 (0-,3+) as shown in Figure 1. Implanted SNM lead angle may also be optimal at angles around 80°. Implantation angles of 78-85° are typically associated with the lowest bellows thresholds across the lead contacts (Figure 2). Even with this limited study, we are able to observe potentially useful relationships between measured lead locations and the bellows threshold data.

Functionally relevant SNM contact depths and implant angles can be measured from fluoroscopic images captured during SNM implantations. Future work should confirm these retrospective results in a larger study and extend them to also include their relevance to SNM efficacy and side effects. If confirmed, the lead locations could enable the identification of optimal therapy delivery locations based on lead location alone, potentially eliminating the need for measuring motor thresholds during surgery to determine their usefulness. Future work could also better utilize fluoroscopy during implantation with actual measurements of SNM location, as well as track SNM lead locations and possible therapy efficacy changes over time. Ultimately, an improved focus on identifying, locating, and stimulating the neural targets of SNM can establish evidence-based design inputs for next generation lead designs and implantation techniques.