Effect of acute changes in sacral nerve stimulation amplitude on anorectal function in faecal incontinence

Sacral nerve stimulation improves faecal continence. Preliminary evidence suggests that the effect may be mediated via altered rectal and anal smooth muscle activity, as well as facilitation of external sphincter contraction. Clinical benefit however, appears unrelated to whether stimulation amplitude is above or below that required to produce threshold perineal sensation. Formal evaluation of the effect of different levels of stimulation has not been undertaken. This study aimed to assess the effect of varying stimulation amplitude on pelvic floor motor and sensory function in patients permanently implanted for faecal incontinence.

Three female patients with passive faecal incontinence secondary to internal sphincter weakness, rendered continent with a permanently implanted sacral nerve stimulating electrode and pulse generator a mean of 19 months previously, were assessed. Patients underwent a series of anorectal physiological tests whilst stimulating amplitude was altered but all other stimulation parameters were kept constant. Baseline studies were initially performed at the stimulation amplitude providing continence over recent months for individual patients. Amplitude was then initially reduced, and then increased, in increments of 0.2 V, initially down to zero, and then to the maximal tolerated stimulation amplitude, at which pain was felt. Anorectal physiological testing was repeated 10 min after adjustment to each new amplitude of stimulation.

Procedures were well tolerated by all patients. There were no significant differences in any of the physiological variables measured dependent upon intensity of stimulation between zero and the maximum tolerated stimulation. The recto-anal inhibitory reflex was unaltered in the one patient in whom it was demonstrable prior to stimulation.

Acute changes in stimulation amplitude alone do not produce acute measurable differences in standard anorectal physiological variables. The clinical benefits of sacral nerve stimulation for faecal incontinence are not mediated primarily by improvement in anal canal pressures or alteration of anorectal sensation. Trials of different stimulation parameters over weeks or days may be necessary to demonstrate other functional differences.