An introduction to the basic principles of magnetic nerve stimulation.

Magnetic nerve stimulation is a new method for the noninvasive stimulation of neuromuscular tissue. The technique, developed at the University of Sheffield, United Kingdom, is being increasingly used for both clinical studies and basic research, with some 500 stimulators presently in use worldwide. This paper looks at the development of magnetic stimulation as a clinical tool. The basic physics principles of the technique are outlined, and the different magnetic field waveforms, coil geometrics, and orientations that can be used are discussed. The depth of penetration of magnetic stimulation is compared to that of conventional electrical stimulation using surface electrodes. It is shown that the former generates lower electric fields at the surface of the body, resulting in greater penetration and the ability to stimulate deep nerves without pain. Magnetic stimulation has several other advantages over electrical stimulation, including being able to stimulate the human brain without discomfort due to the magnetic fields passing through the skull without attenuation. These advantages, along with the limitations of the technique, are discussed. Finally, data relating to the safety of brain stimulation are summarised in terms of the electromagnetic parameters used. It is concluded that the present generation of magnetic stimulators do not cause acute hazards, provided their electrical and mechanical design meets the relevant electromedical safety standards.