Double-pulse magnetic brain stem stimulation: mimicking successive descending volleys

Magnetic stimulation with a double-cone-coil over the back of the head activates the motor tracts at the level of pyramidal decussation (brain stem stimulation [BST]). However, single-pulse BST (single BST) sometimes cannot elicit motor-evoked potentials (MEPs) in patients with corticospinal tract involvement. We developed a technique using double-pulse BST (double BST) to elicit MEPs even in patients whose threshold is abnormally elevated. Subjects were 11 healthy volunteers and 12 patients with corticospinal tract involvement in whom single BST evoked no discernible MEP. Double BST was performed at the intensities of resting and active motor threshold for single BST; MEPs were recorded from the first dorsal interosseous muscle. Interstimulus intervals (ISIs) between two pulses were 1.5, 2, 3, 5, and 10 ms in healthy subjects. Double BST enlarged MEPs at ISIs of 1.5-5 ms with a peak at 2 ms in the relaxed condition, but not in the active condition. At an ISI of 2 ms in the relaxed condition, the MEP amplitude was 15 times as large as that to single BST in relaxed muscles. The onset latency of the enlarged MEP from the second pulse in relaxed muscles was the same as that by single BST in active muscles. Double BST at a 2-ms interval elicited MEPs in eight patients. Double BST can enhance MEPs probably by temporal summations of excitatory postsynaptic potentials at the spinal motoneurons. Using this new technique, we can obtain more information about the central motor conduction even when single BST fails to elicit any MEP.