Magnetic stimulation: technical aspects.

When the voltage induced in a measuring coil by different magnetic stimulators was assessed, the strongest stimulus was generated by Cadwell followed by Novametrix, Dantec and then Digitimer. The Cadwell produced a polyphasic pulse, while the other stimulators generated monophasic stimuli. This is the reason that reversing coil polarity does not influence the site of nerve excitation with the Cadwell equipment, but does with the other stimulators. The magnetic method has proved to be most useful for transcranial excitation of the motor cortex. Magnetic stimuli at C7 excite cervical roots submaximally. Electrical stimuli excite cervical roots supermaximally. No advantage was found in the use of small coils for cervical transcutaneous root stimulation. Electrical stimulation (60-85% output) excited lumbar motor roots supramaximally in each healthy subject. However, magnetic stimulation at T12/L1 (100% output) evoked responses in only 8 (Novametrix) and 12 (Cadwell) out of 18 trials without any difference between right and left. The most reliable method for transcutaneous non-invasive excitation of motor roots is electrical stimulation.