Commentary on Selective Detrusor Activation by Electrical Sacral Nerve Root Stimulation in Spinal Cord Injury

Abstract

The feasible restoration of bladder control in patients with complete or near-complete suprasacral spinal cord injuries has been demonstrated through sacral deafferentation and sacral ventral root stimulation either intradurally' or extradurallyz by different investigators. Sacral dorsal rhizotomy diminishes detrusor spasticity, improving bladder's reservoir function and often restoring continence. Stimulation of ventral sacral roots after deafferentation, in most cases, induces bladder contraction and bladder evacuation. Unfortunately simultaneous activation of both detrusor and urethral external sphincter by sacral nerve stimulation has been identified as main problem in achieving complete bladder emptying. The ventral sacral root carries both parasympathic fibers to urinary bladder as well as somatic fibers to external urethral sphincter. Sacral ventral root stimulation results occasionally in detrusor/sphincter dyssynergia. This may be avoided by intermittent stimulation of ventral root, taking advantage from characteristic responses of striated and smooth muscle fibers which can result in intermittent lowering of urethral resistance while inducing sustained high detrusor pressure (principle of post stimulus voiding). Various approaches have been performed to obtain selective detrusor contraction. An attempt was made to localize spinal cord sacral motoneuron of parasympathetic nucleus as well as those of pudendal nucleus and to stimulate each group separately. Direct stereotactic microstimulation of sacral spinal cord succeeded in inducing detrusor contraction without sphincteric contraction in cats.3 Similar experiments by our own lab confirm these results. However, although this approach would give purely selective detrusor contraction, it wodd be extremely difficult to apply clinically. An alternative is intradural dissection of sacral ventral root and selective rootlet stimulation which is technically feasible. In acute canine experiments Hohenfellner M. et al. demonstrated the possibility to distinguish those rootlets predominantly carrying fibers to bladder from those carrying fibers to urethral ~phincter.~ Neuroanatomical investigations in dogs as well as in humans showed sacral ventral roots to be composed of several bundles that correspond to number of rootlets that originate from spinal cord and that they maintain their integrity and number, although with a different configuration and orientation, throughout entire intradural course. Stimulation of sacral rootlet bundles isolated from rest of sacral root gave essentially same responses when stimulated close to exit from dura as well as in mid segment and close to origin in spinal cord. It was evident that none of them was 100% pure, but there was a definite predominance of autonomic fibers in some and somatic fibers in others as well as a group was evenly mixed.5 Step by step procedure including intradural deafferentation followed by dissection of ventral root under magnification into its bundles, interruption of those with purely or extensively sphincteric response is feasible. This will permit easy extradural sacral root stimulation that will provide pure detrusor response. This surgical approach is technically feasible and also more selective. At present time we are trying to confirm these exciting results in a chronic animal model.