Reliability of the motor evoked potentials elicited through magnetic stimulation at three sites

BACKGROUND: Motor evoked potentials (MEPs) obtained from magnetic stimulation allow the measurement of the excitability of motor nerve cells. Although this technique is becoming widely used, its reliability has yet to be established. OBJECTIVE: To determine the reliability of MEPs and evaluate their stability. DESIGN: Subjects received magnetic stimulation at 3 different sites: cranial, cervical, and peripheral. Three stimulations were given at each site. Stimulations were given under 4 conditions: eyes open or closed and with or without controlled mental activity. SETTING: The study was performed in a research laboratory setting. PARTICIPANTS: Nine healthy subjects (mean age = 21 years +/- 0.9), asymptomatic of any neural or musculoskeletal dysfunctions. MAIN OUTCOME MEASURES: Normalized peak-to-peak amplitude of MEPs at the opponens pollicis muscle. RESULTS: Intraclass correlations were high for all conditions (R > 0.90). The amplitude of the MEPs elicited from the transcranial site during the controlled mental activity condition was increased by 26.6% (P =.017). No other significant differences (P >.05) were obtained among the other conditions. CONCLUSION: Intraclass correlation results reveal that MEP measurements are highly reliable in a controlled environment. The increase of MEPs during the controlled mental activity condition could be attributed to an elevation of neural activity of different nonmotor areas of the brain increasing corticospinal excitability.