Magnetic stimulation of peripheral nerve; site of stimulation estimated from H reflex

In order to find out the difference in the site of stimulation between electrical and magnetic stimulation of a peripheral nerve, H reflexes were recorded in 20 healthy persons. Electrical stimulation was performed with the stimulator placed along the tibial nerve (A) and at right angles to the tibial nerve (B). Magnetic stimulation (Magstim Model 200) was performed with a large (14 cm in diameter) magnetic coil (C) and a small (7 cm in diameter) magnetic coil (D). The magnetic coil was held parallel to the skin and its center was placed on the point where the cathode had been placed in the electrical stimulation. The mean value of the sum of M wave latency and H reflex latency in C was 1.4 msec, 1.7 msec and 0.8 msec shorter than those in A, B and D, respectively. The mean value of the sum of M wave latency and H reflex latency in D was 0.6 msec shorter than that in A and 0.9 msec shorter than that in B. When the center of the magnetic coil is placed over the peripheral nerve, two loci of the nerve, 7-8 cm apart from each other with the large magnetic coil and 3-4 cm apart from each other with the small magnetic coil, are considered to be stimulated simultaneously.