Magnetic stimulation of the spine to produce lower extremity emg responses: Significance of coil position and the presence of bone

Study Design. Magnetic stimulation of the spinal cord in 14 pigs was performed, and compound muscle action potentials (CMAPs) were recorded from lower extremities before and after nerve root and spinal cord lesioning. Objectives. The authors determined whether magnetic stimulation of the spinal cord produced lower extremity CMAPs. Summary of Background Data. Previous studies indicated that the magnetic stimulation of the spinal cord would result in lower extremity CMAP if appropriate amounts of spinal bone were removed to expose the spinal cord. Results. Our findings demonstrated that the presence of intervening bone did not affect the reliability or presence of lower extremity CMAPs. Additionally, magnetic stimulation did not result in spinal cord activation but produced activity in nerve roots lateral to nerve root foramen. Lesioning of the spinal cord and complete rhizotomies did not affect magnetically elicited CMAPs. Conclusions. Magnetic stimulation of the spinal canal does not result in activation of spinal cord motor tracts. Lower extremity CMAPs were elicited by stimulation of nerve roots lateral to nerve root foramen and not of the spinal cord. Magnetic stimulation of the spinal cord is not appropriate for monitoring spinal cord motor tract function.