Selective sacral root stimulation for bladder control: acute experiments in an animal model.

High bladder pressure is a potential side effect of poststimulus voiding, used to date for stimulation-induced bladder emptying in spinal cord injured patients. To prevent this side effect, selective activation of the bladder without activation of the urethral sphincter by selective stimulation of sacral roots was studied in a canine animal model. On-line registration of bladder, urethral and rectal pressure was performed, and EMG of tail muscles and urethral sphincter was recorded. After laminectomy, intradural left and right sacral root S2 were stimulated with a tripolar cuff electrode. A self-made stimulator generating adjustable pulse shapes was used. Using 200 microsec. rectangular pulses, contraction of bladder and urethral sphincter could be elicited. Selective activation of the bladder occurred with pulses of 600 to 800 microsec. due to anodal blocking of the large nerve fibers in the sacral roots. During acute animal experiments we were able to achieve selective activation of the detrusor without simultaneous activation of the external urethral sphincter, and complete, low-pressure voiding occurred.