Selective small nerve fiber activation by anodal block - a chronic study

Following spinal cord injury, the control of bladder and bowel functions is commonly lost. One of the methods to restore bladder and bowel functions is through functional electrical stimulation. The most widely used, commercially available system for assisted emptying of the bladder and the bowel in SCI patients is the Finetech-Brindley bladder system [1]. To obtain voiding, it uses the technique of poststimulus voiding. Poststimulus voiding is an artificial micturition pattern, which causes voiding in spurts at supranormal bladder pressures. To accomplish more natural voiding, it would be necessary to avoid activation of the external urethral sphincter (EUS). That can be achieved by selectively activating only small diameter parasympathetic fibers. Anodal block is a stimulation technique that allows activation of small parasympathetic fibers without coactivation of large somatic fibers innervating the external urethral sphincter (EUS) and the external anal sphincter (EAS). The principle of anodal block is that both small and large fibers are activated and that the propagation of action potentials (APs) in the large fibers is blocked distal to the activation site. To obtain an anodal block, a tripolar cuff electrode is most commonly used. The central contact is the cathode and the lateral contacts are the anodes. The fibers are excited close to the cathode and the APs of large nerve fibers are blocked close to the anodes.