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

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 the bladder's reservoir function and often restoring continence. Stimulation of the ventral sacral roots after deafferentation, in most cases, induces bladder contraction and bladder evacuation. Unfortunately simultaneous activation of both the detrusor and the urethral external sphincter by sacral nerve stimulation has been identified as the main problem in achieving complete bladder emptying. The ventral sacral root carries both parasympathic fibers to the urinary bladder as well as somatic fibers to the external urethral sphincter. Sacral ventral root stimulation results occasionally in detrusor/sphincter dyssynergia. This may be avoided by intermittent stimulation of the ventral root, taking advantage from the characteristic responses of the striated and smooth muscle fibers which can result in intermittent lowering of the 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 the spinal cord sacral motoneuron of the parasympathetic nucleus as well as those of the pudendal nucleus and to stimulate each group separately. Direct stereotactic microstimulation of the 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 the 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 the bladder from those carrying fibers to the urethral ~phincter."~ Neuroanatomical investigations in dogs as well as in humans showed the sacral ventral roots to be composed of several bundles that correspond to the number of rootlets that originate from the spinal cord and that they maintain their integrity and number, although with a different configuration and orientation, throughout the entire intradural course. Stimulation of sacral rootlet bundles isolated from the rest of the sacral root gave essentially the same responses when stimulated close to the exit from the dura as well as in the mid segment and close to the origin in the 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 the ventral root under magnification into its bundles, interruption of those with purely or extensively sphincteric response is feasible. This will permit the easy extradural sacral root stimulation that will provide pure detrusor response. This surgical approach is technically feasible and also more selective. At the present time we are trying to confirm these exciting results in a chronic animal model.