Evoked magnetic fields as a tool to optimize therapeutic electrical stimulation of the sacral surface

Magnetoencephalography (MEG) was used to measure the somatosensory evoked fields in six healthy males undergoing sacral surface therapeutic electrical stimulation, a recently introduced treatment for chronic urinary dysfunction. This study was performed to investigate the utility of MEG to optimize the stimulation parameters. Strong or weak electrical stimuli were applied to small or large surface electrodes placed over the bilateral sacral surfaces. The peak latency and strength of the equivalent current dipole were evaluated at the first peak originating from the primary somatosensory cortex (M30). There was no difference in M30 latency in response to stimulation intensity or electrode size. However, a larger equivalent current dipole strength was obtained in response to higher stimulus intensities, which indicated a more effective stimulus on the sacral surface. The present MEG study shows that stronger stimuli given through large electrodes evoke larger responses than small stimuli through small electrodes. The study also suggests that MEG can be used as an objective tool to monitor whether therapeutic stimulation parameters are appropriate.