Magnetic stimulation of the sacral roots

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Abstract
Magnetic stimulation using an external surface coil induces an electrodynamic field that penetrates various tissues and stimulates peripheral nerves in a similar fashion to conventional electrical stimulation. An 83 mm magnetic surface coil was used to stimulate 11 spinal cord injury (SCI) patients, during which time detrusor activity and evoked potentials of the striated urinary sphincter motor pathways were evaluated. AH patients had urodynamic studies and conventional sacral evoked potentials prior to magnetic stimulation. The mean bladder capacity was 337 ml (range 109–590), mean leak point pressure was 50 cm H2O (range 10–80), and mean sacral reflex (afferent‐efferent) latency was 37.9 ms (range 25.1–49.3). Eight patients had detrusor‐sphincter dyssynergia. Magnetic stimulation over the sacral spine at different bladder volumes was performed. Detrusor and striated sphincter responses were recorded during stimulation. In all patients the technique was easy and the results were reproducible. The mean sacral motor pathway (efferent) latency was 27.9 ms (range 18.7–39.6). Using maximal stimulation, no detrusor response was recorded at bladder volumes 10 cm H2O in 2, 200 ml. No complications were seen. Sacral evoked potential measurements assess the function and integrity of the sacral arc but it does not distinguish between afferent and efferent pathways. Magnetic stimulation is a safe and effective method to assess the integrity and function of the detrusor and striated sphincter motor (efferent) pathways. When combined with sacral evoked potential studies, the sensory (afferent) pathways can be evaluated indirectly. Independent assessment of the motor (efferent) and sensory (afferent) pathways of the sacral arc has significant implications in the understanding of the pathophysiology of the different types of neuropathic voiding dysfunction. The magnitude of the detrusor contraction induced by magnetic stimulation was too small to allow any conclusions. However, it appears that the detrusor threshold for stimulation increases with increased bladder volumes.

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