Determination of therapeutic threshold in sacral nerve modulation for faecal incontinence

The aim of the study was to determine the therapeutic stimulation threshold in patients with successful sacral nerve modulation for faecal incontinence.

Patients who had undergone successful permanent sacral nerve modulator implantation and had been followed up for a minimum of 3 months were included. The sensitivity threshold and motor threshold were determined and correlated with therapeutic response. Patients went home with the stimulator set at 0.6 V below the sensitivity threshold. Each week the voltage was increased by 0.2 V until the sensitivity threshold was reached. The effects on anorectal physiology and continence were recorded.

Eight patients (seven women) with a median age of 58.5 years were included. The median follow-up was 6.3 months. The median sensibility threshold volume of rectal sensation was 50 ml, the median urge threshold volume was 140 ml and the median maximum tolerated rectal volume 240 ml. The median number of incontinence episodes and days per week affected by incontinence decreased from 5.0 and 3.8 before operation to 0.7 and 0.7 respectively after follow-up for 3 months. At anorectal manometry the median resting and stimulation anal canal pressures were 57 and 85 mmHg respectively, and remained constant over time. The therapeutic response threshold was significantly lower than the sensitivity threshold (median 1.6 versus 1.7 V; P = 0.042). The median motor threshold was 2.1 V, significantly higher than the sensitivity threshold (P = 0.009). The stimulation threshold for suboptimal therapeutic response was 1.4 V. In five of the eight patients the therapeutic response threshold was the same as the sensitivity threshold.

Sacral nerve modulation can produce a therapeutic effect below the sensitivity threshold. A lower stimulation voltage increases the lifespan of the pulse generator.