Reduction of bladder outlet resistance by selective stimulation of the ventral sacral root using high frequency blockade: a chronic study in spinal cord transected dogs

The use of electrical neural stimulation as treatment for neurogenic bladder is complicated by simultaneous contraction of the striated sphincter. This result is due to the composition of the ventral sacral roots, which contain somatic fibers innervating the external urethral sphincter and preganglionic parasympathetic fibers innervating the detrusor, among others. The somatic fibers have a larger caliber than the parasympathetic fibers and, as large diameter fibers, need a lower stimulus amplitude for activation than smaller fibers. Activation of the smaller fibers is always accompanied by activation of the larger fibers. We studied the effect of chronic application of selective combined low and high frequency current stimulation of the ventral sacral root on bladder evacuation and urethral resistance in chronically spinalized (spinal cord transected above the spinal micturition center) male dogs for daily bladder evacuation.

A total of 14 male mongrel dogs weighing 20 to 25 kg. underwent transection of the spinal cord at the T10 vertebra. The S2 ventral sacral root was wrapped with a bipolar electrode connected to a subcutaneous microstimulator. Daily bladder evacuation by neural stimulation with determination of the voided and residual urine volumes was done for 8 months. The animals were stimulated only with low frequency current during the 1-month spinal shock phase. Selective parameters of combined low frequency stimulation and high frequency blockade currents were subsequently applied for 6 months. For the last study month the animals were again stimulated by low frequency current alone. Bladder and urethral pressure as well as electromyography of the external urethral sphincter and pelvic floor muscles were evaluated monthly. Histopathological testing of the chronically stimulated nerve and external sphincter was performed.

Of the 14 dogs 12 completed the followup study. During the shock phase with the application of low frequency only stimulation the animals voided 26% of mean functional bladder capacity with the early return of detrusor activity. Mean detrusor pressure plus or minus standard error of mean was 76.4 +/- 21.6 cm. water, while mean urethral pressure was 83.6 +/- 16.8 cm. water. During the application of selective combined low and high frequency stimulation 7 animals (58%) evacuated the bladder completely with post-void residual urine less than 10% of mean individual functional bladder capacity and 5 (42%) had mean post-void residual urine less than 20% of mean individual bladder capacity. Mean detrusor pressure was 73.5 +/- 20 cm. water and mean urethral pressure was 44 +/- 7.3 cm. water. There was a 45.19% reduction in the mean electromyography activity of the external sphincter. Stimulation of the dogs for the last month by low frequency current resulted in the voiding of 33% of mean bladder capacity with an increase in mean urethral pressure and electromyography activity.

These results of reducing external sphincter activity by the simultaneous application of high frequency current blockade of the somatic fibers and activation of the autonomic fibers of the bladder by low frequency stimulation are promising.