Objective: To identify the existence of ipsilateral responses by magnetic stimulation, and to elucidate the influences of ipsilateral tonic contraction on bilateral MEP responses, and to attain the proper facilitation technique for bilateral MEP responses.
Method: Ten normal healthy volunteers who were right handed, were recruited. They performed the voluntary contraction of thenar muscle via lateral prehension with three stage, contralateral, bilateral, ipsilateral contraction by 70 mm bipolar coil stimulator (figure-of-eight shaped). The excitability threshold (ET) at rest was determined, and then, three facilitation techniques with combination of both voluntary contraction and stimulus intensity were also performed: A technique, minimal facilitation (10% of MVC) with ET intensity; B technique, moderate facilitation (30% of MVC) with 110% of ET intensity; C technique, minimal facilitation (10% of MVC) with 140% of ET intensity. Contralateral, bilateral and ipsilateral voluntary contractions were performed in each technique. In 90 mm circular coil stimulator, same procedure as above was followed.
Results: There were no differences of ET between the two coil stimulators. Ipsilateral MEP responses were not detected after bipolar coil stimulation except one case in C technique, but they were developed over 70% in B and C technique with ipsilateral muscle contraction. However, only 2 cases of ipsilateal responses could be detected in C technique, and not detected in A and B technique. The latency of ipsilateral responses were similar to that of contralateral responses, but the amplitude was much lower than that of contralateral responses. Ipsilateral muscle contraction reduced the amplitude of contralateral MEP in moderate facilitation (B technique). Especially, ipsilateral response was never evoked in B technique with contralateral contraction.
Conclusion: In normal subjects, ipsilateral MEPs can't be obtained in focal magnetic stimulation. Ipsilateral tonic contractions are regarded as reducing facilitative effects of contralateral MEP responses. Bilateral responses should be attained when contralateral target muscle is contracted with moderate facilitation (30% of MVC and 110% of threshold intensity). In this facilitation, the stimulation by circular coil is no less effective than focal stimulation by 70 mm bipolar.
