EXPERIMENTAL RESULTS ON MECHANISMS OF ACTION OF ELECTRICAL NEUROMODULATION IN CHRONIC URINARY RETENTION

Sacral foramen neuromodulation - initially applied for the treatment o f urinary incontinence - has proved to be effective in patients with c hronic urinary retention. Thus far, the underlying neurophysiological mechanisms have not been elucidated. In an experimental study on the n europhysiological basis of sacral neurostimulation, one objective was to investigate the mechanisms responsible for initiation of micturitio n in chronic urinary retention. In ten female cats anesthetized with o r-chloralose the clinical situation of sacral foramen stimulation was experimentally reproduced by isolated S2 nerve stimulation after L6-S3 laminectomy. Stimulation responses were recorded from the bladder, pe ripheral nerves, and striated muscles of the foot and pelvic floor. Th e effect of sudden cessation of prolonged S2 stimulation, during which the bladder was completely inhibited, was evaluated in 70 stimulation sequences in 5 cats. Sacral nerve stimulation induced excitatory and inhibitory effects on the bladder, depending on the frequency and inte nsity of stimulation. With unilateral S2 stimulation, bladder excitati on was best at frequencies of 2-5 Hz and at intensities ranging betwee n 0.8 and 1.4 times the threshold for the M-response of the foot muscl e. Inhibition was the dominating effect at frequencies of 7-10 Hz and at intensities exceeding 1.4 times the threshold. Prolonged S2 stimula tion above the threshold produced complete bladder inhibition during s timulation but induced strong bladder contractions after sudden interr uption of stimulation, with amplitudes being significantly higher than that of spontaneous contractions preceding the stimulation. These res ults confirm the hypothesis of a ''rebound'' phenomenon as the mechani sm of action for induction of spontaneous voiding in patients with chr onic urinary retention.