Sacral root stimulation for bladder evacuation in dogs: Selective rootlet neurotomy and local hyperthermia

Purpose: To investigate and compare anatomical with functional findings after selective transection of rootlets with a predominantly sphincteric response to stimulation, and to assess the effects of local hyperthermia on bladder and sphincteric responses to sacral nerve stimulation. Methods: 15 male mongrel dogs were used to evaluate the effects of selective ventral rootlet stimulation in acute (n = 5) and chronic (n=5) studies by means of neurostimulation and electron microscopy. In addition, the effects of local hyperthermia on bladder and sphincteric pressure responses to electrical stimulation (n = 5) were investigated. Results: Functional studies consistently demonstrated that, when the roots were subdivided intradurally into rootlets (range: 3-7; mean: n=4), electrical stimulation distinguished 3 qualities of individual rootlets: predominantly autonomic fibers, predominantly somatic fibers and a group that was mixed. In the chronic experiments, extradural stimulation after selective ventral sacral rootlet neurotomy caused a significant decrease of the sphinteric response at 3 months. Electron microscopic studies revealed surviving small myelinated and unmyelinated axons within the distal Ventral root after selective neurotomy of rootlets with a predominantly sphincteric response. Local heat applied to sacral roots diminished both detrusor and sphincteric pressure responses to stimulation, with a greater reduction in the latter. Initial changes in pressure responses were detected at temperatures of 44.5+/-1.4 degreesC. If local hyperthermia was immediately terminated when pressure responses were abolished, both sphincteric (33.8+/-11.8%) and bladder (25+/-15.4%) showed partial recovery at this time. Conclusions: The intradural identification and transection of predominantly somatic rootlets is feasible. When this is combined with extradural electrode placement, sacral root stimulation will induce bladder contraction for low-pressure voiding. Furthermore, there is evidence that small myelinated and unmyelinated axons may represent the sacral efferent pathway. In addition, the preliminary results of our hyperthermia studies indicate that, with further investigation of temperature threshold and treatment duration, local hyperthermia on the sacral root may result in more selective bladder stimulation to reduce detrusor-sphincter dyssynergia.