Comparison of various coils used for magnetic stimulation of peripheral motor nerves: physiological considerations and consequences for diagnostic use.

We compared the ability of 4 magnetic coils to activate peripheral nerves in healthy subjects. No differences in motor threshold intensities were found between the coils, but the intensities needed to elicit maximum compound muscle action potential (CMAP) amplitudes were different. For superficial nerves maximum CMAPs in comparison with electrical stimulation were usually but not always found. CMAPs were at their maximum only when the direction of induced current flowed from proximal to distal and when a certain part of the coil was over the nerve. Distal nerve stimulation was time consuming. Due to artifacts many stimuli were necessary and sometimes no maximum CMAP could be elicited. CMAPs were much less sensitive to position changes of the coil than to changes in an electrical stimulator. Small circular coils were superior to larger coils in terms of the lower intensities necessary to elicit maximum CMAPs, better focusing of the stimulus, and less artifacts. For deep nerves amplitudes were always submaximal. Coactivation of nearby nerves and underlying muscles was another main drawback especially at proximal sites and for coils of large diameter. Despite better focusing, double coils are less useful due to their great diameter. Magnetic stimulation cannot replace electrical neurography at the moment, even if different coils are used at different sites of stimulation.