Mechanism of Action

The precise mechanism of action by which electrical stimulation works remains to be proven. It is thought that the efficacy is dependent on stimulation of the afferent nerves that modulate sensory processing of the voiding reflex and its pathways to the central nervous system. Several voiding reflexes are affected by neuromodulation, and it is useful to review these. The guarding reflex serves to maintain continence with increased bladder volume. The detrusor muscle relaxes, the levator ani muscles contract, and urinary leakage is prevented. The micturition reflex facilitates voiding. Normally, a voluntary signal to empty the bladder originates from the cerebral cortex. The pontine micturition center acts as a switch between the guarding reflex and the micturition reflex. After sensory information is processed in the pontine micturition center, micturition is initiated under voluntary control. This is mediated by afferent delta nerve fibers and is composed of two reflexes. The bladder-bladder reflex is a positive reflex that activates the full bladder allowing a sustained contraction until the bladder empties. The bladder-urethra reflex allows smooth muscle of the proximal urethra to remain open while the bladder contracts. One theory is that these reflexes become pathological with voiding dysfunction. For bladder over-activity, SNS may work by inhibition of several neurons: spinal tract neurons involved in the micturition reflex, interneurons involved in spinal segmental reflexes, and postganglionic neurons, thus inhibiting the primary afferent pathway.2 For nonobstructed urinary retention, SNS

may suppress the guarding reflex by turning off bladder afferent input to the internal sphincter sympathetic or external urethral sphincter interneurons, and activation of postganglionic neurons and including bladder activity.2 Stimulation is not curative for the underlying condition, and symptoms will recur when stimulation has been turned off. However, by modulating these reflexes that have become aberrant, symptoms improve.

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