Focal stimulation of human peripheral nerve with the magnetic coil: a comparison with electrical stimulation.

The hypothenar compound motor action potential (CMAP) response to ulnar nerve stimulation at the elbow was used as a test system in the human to compare excitations by a round magnetic coil (MC), 92 mm in outer diameter, and by electrical bipolar or tripolar stimulation. Optimal focality of excitation was obtained with the MC at 90 degrees to the extended arm and its plane parallel to the arm, i.e., an orthogonal-longitudinal orientation. Tangential orientation of the MC on the arm, i.e., laying it flat on the arm, powerfully excited additional structures. As expected from classical axonology, orthogonal-transverse orientations were the least effective. With orthogonal-longitudinal orientation and submaximal stimulation, the spread of excitation lateral to the median nerve at the wrist was of the order of 10-15 mm, the thickness of the MC being 12 mm. With the same orientation, the site of origin of the distally propagating impulse was estimated by comparing CMAP latencies to bipolar electrical (with cathode distal) and MC stimulation. Tripolar stimulation (with cathode intermediate) had no advantage over bipolar stimulation. The impulse originated 13-22 mm from the midpoint of the contacting edge. Rotating the MC through 180 degrees and thus reversing the field polarity did not significantly change the CMAP latency, indicating that the effective cathode and anode lay within a few mm of each other. Stimulating with a tilted MC resulted in a maximum CMAP when the orthogonal-longitudinal orientation failed to do so. A simple volume conductor model yielded a potential gradient of the right order of magnitude (35 mV/mm) when the MC stimulator output was 25% of maximum, i.e., a little above threshold for exciting ulnar motor axons.