The motor-evoked potentials elicited from the deltoid muscle by transcranial magnetic stimulation with a standardized facilitation - The potential diagnostic utility for C5 radiculopathy

Study Design. We investigated the effects of standardized facilitation on stability in amplitude of the motor-evoked potentials from the deltoid muscle after magnetic stimulation over the scalp in normal study participants. Objectives. To test the potential of this technique in evaluating deltoid weakness and recovery after C5 radiculopathy. Summary of Background Data. Most previous motor-evoked potential studies have emphasized the value of the latency rather than amplitude, probably because the amplitude varies much more than the latency, even in the same individual. Methods. Twenty right-handed healthy men underwent deltoid motor-evoked potential studies on three successive occasions at intervals of >2 weeks. A digital readout of the shoulder abduction force allowed the study participant to adjust voluntary contraction to 20% of the maximal force at the moment of stimulation. The deltoid motor-evoked potentials were compared with the M responses recorded from the relaxed deltoid after electrical stimulation of the brachial plexus. Results. Despite considerable individual variability in the amplitude of deltoid motor evoked potentials a standardized facilitation yielded a relatively small variation of amplitude between sessions in the same individual showing an intersession variability index of 9.1 +/- 7.9%. The side-to-side amplitude ratio, averaging 82 +/- 12%, also provides a more reliable measure in evaluating unilateral deltoid weakness than the absolute value. The measures on the left showed significantly higher amplitude than on the right for the deltoid motor-evoked potentials but not for M responses. Conclusions. The changes of the motor-evoked potential amplitude exceeding 25% between session sand side-to-side amplitude ratios <60% can be outside the normal limits. The asymmetry of deltoid motor evoked potential in favor of the nondominant side is of physiologic interest.