136. Magnetic stimulation of the cervical spine: Effect of coil current configuration on the MEPs
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文摘
Magnetic stimulation over the spine is san available method to stimulate proximal nerve axon: the probable site of excitation is considered very close to exit of cervical roots. It is argued that the spatial derivative of induced electric field parallel to the nerve axon is crucial factor to elicit nerve action potential. We study the effect of coil current configuration in magnetic stimulation of cervical roots on MEPs, assuming that the direction of coil current occurring at the intersection of figure eight coil (transverse vs perpendicular to the nerve) might be related to oriented axons. Five healthy subjects were stimulated by eight shaped coil placed over the cervical spine between C5 and T2 spine processes; the coil direction was orthogonally changed (vertical vs horizontal) along the midline (cervical spine) and along parallel lines (1, 2, 3 cm outer on right and left side). The amplitude of MEPs are significantly higher by using vertical vs horizontal coil position at any site of stimulation and results are referred to the former experimental trial. MEP amplitude at 60 % output: the mean maximum amplitude was along C7 line at position +2 cm, both on the right (4,4 mV) and left (4,6 mV) side. MEP amplitude at 70 % output: the mean maximum amplitude was along C7 line at position +2 cm, both on the right (8,82 mV) and left (7,96 mV) side. MEP amplitude along T1 line was slightly lower than at C7 level, without significant difference. Magnetic stimulation using commercially available device is easy to perform, but the MEP characteristics must be rigorously used to couple anatomy with physical properties of the stimulus enough to improve the clinical relevance of this method and the understanding of mechanism of stimulation as well.