Reinforcement of motor evoked potentials by remote muscle contraction

We investigated the effects of remote muscle contraction on the amplitude and latency of motor-evoked potentials (MEPs) recorded from tibialis anterior and lateral gastrocnemius after transcranial magnetic stimulation of the motor cortex. In particular, consideration was given to the effects of unilateral handgrip of different strengths, jaw clench, and combined bilateral handgrip (Jendrassik maneuver) and jaw clench. Eight healthy adult volunteer subjects (six women, two men) participated in the main study. The clinical usefulness of this form of facilitation was subsequently demonstrated in three patients with myelopathy. Transcranial magnetic stimulation of the motor cortex was delivered from a Cadwell MES-10 (100% intensity) with a 9.5-cm circular coil positioned with the rim over the vertex. Stimuli were delivered ∼ 1 s after initiation of reinforcement. In the main study of normal subjects, surface recordings of the evoked compound muscle action potentials were made bilaterally from anterior tibialis and lateral gastrocnemius. In the individual studies of patients, MEPs were also recorded from abductor pollicis brevis and other muscles innervated from roots below the level of lesion. The principal results showed that unilateral handgrip (50 and 100% maximum voluntary contraction, MVC) of either hand enhanced the amplitude of MEPs bilaterally in both tibialis anterior and lateral gastrocnemius in all normal subiects (p < 0.05). The mean increase in amplitude associated with 100% MVC unilateral handgrip was >300% of the control value when the subject was relaxed (p < 0.01). The bilateral Jendrassik maneuver and jaw clench were also effective in increasing (p < 0.05) MEP amplitude in both muscle groups. These two procedures did not yield the same degree of reinforcement as 100% unilateral handgrip. Remote muscle contractions induced a reduction in MEP latency of 1-2 ms (p < 0.05). The clinical importance of these observations was demonstrated by the fact that remote muscle reinforcement enabled identification of low-amplitude MEPs that might otherwise have been missed in the three patients with spinal cord trauma.