Considerations for safety in the use of extracranial stimulation for motor evoked potentials

The possibility of neural damage during extracranial brain stimulation for motor evoked potentials (MEPs) is discussed from the perspective of animal studies in which the stimulating electrodes were in direct contact with the brain. These data indicate that the charge per phase used in most of the extracranial MEP protocols is sufficient to induce neural damage if the stimulation is applied continuously for several hours. However, in most cases dispersion of the stimulus current in the extracranial tissue and skull is probably adequate to attenuate the stimulus charge density at the brain surface to a safe level (less than approximately 40 microC/cm2 X ph). However, the possibility exists that low resistance paths between the stimulating electrode and the brain may give rise to foci of high charge density. The possibility of such focusing may be less with magnetic field than with direct electrical field stimulation. We stress the need for additional animal studies designed to delineate a range of safe stimulus parameters for this particular technique.