Trigeminal sensory input elicited by electric or magnetic stimulation interferes with the central motor drive to the intrinsic hand muscles

In 6 normal subjects, unilateral supraorbital magnetic or electric stimulat ion resulted in a consistent symmetrical inhibition of the motor evoked pot entials (MEPs) of the relaxed and preactivated first dorsal interosseus (FD I) muscle. A supraorbital stimulus caused a significant reduction in amplit ude when the trigeminal CS was given 30 to 65 ms before transcranial magnet ic stimulation (TR IS). In addition, supraorbital magnetic stimulation indu ced a bilateral EMG suppression of the isometrically contracting FDI muscle s, starting about 40 to 50 ms after the magnetic stimulus. In 4 subjects, M EPs evoked by transcranial electric stimulation or by TMS during slight mus cle contraction showed a comparable trigeminomotor inhibition. These findin gs demonstrate that electromagnetic stimulation of trigeminal afferents int erferes with the motor output to the intrinsic hand muscles inducing a bila teral inhibition which is probably mediated by a multisynaptic subcortical network. In all 6 subjects, TMS over the motor hand area or the cerebellum elicited a reproducible blink reflex. Since the blink reflex is a sensitive indicator of trigeminal excitation, one has to assume that TMS is associat ed with a significant excitation of trigeminal afferents. Therefore, trigem inomotor inhibition has to be considered in all TMS studies that use a cond itioning-test design. (C) 1999 Elsevier Science Ireland Ltd. All rights res erved.