Radiographic Position of the Electrode as a Predictor of the Outcome of InterStim Therapy

Abstract

Purpose: Sacral neuromodulation (SNM) therapy is indicated for some refractory urological conditions. The electrode lead position in sacral x-rays during routine follow-up may predict the outcome of SNM therapy. To determine whether the radiographic position of the electrode in the sacral foramen predicted the long-term outcome of SNM therapy.

Methods: This was a retrospective study of patients who underwent InterStim SNM at Toronto Western Hospital by 2 surgeons from July 2013 to March 2014. The position of electrodes in relation to the sacral bone was assessed on follow-up sacral x-rays. In the lateral view, we determined the location of the radio-opaque marker of the electrode relative to the inner surface of the sacrum (P3, D3, P2, D2, P1, D1, P0, and D0). In the anteroposterior view, the angle between a line through the spinous process shadow and the electrode was measured (0°-30°, 30°-60°, 60°-90°, >90°, or medial). Dissatisfied patients were defined as those who did not improve based on a voiding diary or those who needed salvage treatment after SNM. The primary endpoint was to determine whether the electrode lead position on sacral x-rays predicted the outcome of SNM therapy.

Results: A total of 69 patients (61 female and 8 male patients) were included, with a median age of 55 years. Forty-two of the patients (60.9%) had refractory overactive bladder, 21 (30.4%) suffered from chronic urinary retention, and 6 (8.7%) had lower urinary tract symptoms and chronic pelvic pain syndrome. The univariate analysis did not show any correlation between SNM response and the electrode position or angle. Dummy regression analysis using response to implantation as the dependent outcome variable did not show any significance for any of the predictors.

Conclusions: Our study did not show a correlation between the long-term response to SNM and the electrode position on follow-up sacral x-rays. In this study, electrode lead position in sacral x-ray at follow-up was not correlated with the outcome of SNM therapy.

Keywords: Sacral Neuromodulation; Urinary Bladder, Overactive; Urinary Incontinence; Urinary Retention.

Fig. 1.

Change in electrode position. (A) At the time of insertion. (B) Six months postoperatively.

Fig. 2.

Lateral view of the sacral bone with an electrode that was marked as being at P3.

Fig. 3.

Anteroposterior view. The angle is 23° in this example.

Fig. 4.

The number of satisfied and dissatisfied patients according to the electrode position on lateral sacral x-rays. Despite the trend favoring P3, D3, and P2, the differences were not statistically significant.

Fig. 5.

The number of satisfied and dissatisfied patients according to the electrode position on anteroposterior sacral x-rays. Despite the trend favoring 0°–30° and 30°–60°, the differences were not statistically significant.