Motor Evoked Potentials Following Highly Frequent Transcranial Magnetoelectrical Motor Cortex Stimulation: Normal Data and Potential Modulation by Stimulation-Dependent Inhibitory and Activating Mechanisms

Abstract

Objective: To determine the normal values (latency, amplitude) of motor evoked potentials (MEP) of the abductor pollicis brevis (APB) and tibialis anterior (TA) muscle after high-frequency repetitive transcranial magnetic stimulation of the motor cortex (rTCMS), and to evaluate stimulation-dependent MEP modulations. Patients and Methods: 29 healthy volunteers underwent rTCMS with 2 and 4 stimuli. The interstimulus interval (ISI) was 2, 3, and 4 ms respectively, which corresponded to frequencies between 250 and 500 stimuli/s. The evoked potentials of the relaxed and voluntarily contracted APB and TA were registered. Results: Depending on the frequency and number of stimuli, the mean corticomuscular latency to the relaxed APB varied between 22.2 and 22.9 ms, and to the relaxed TA between 30.4 and 32.0 ms. The intra- and interindividual variability of the amplitudes was substantial. Voluntary contraction of the target muscle always led to a decrease in latency and increase in amplitude (p < 0.05). Conclusion: The high variability of the amplitudes does not allow the computation of meaningful normal values. The latencies after rTCMS are close to those of normal data after single TCMS, which indicates that in awake humans identical cortical and spinal structures are similarly activated. The discrete variations of latency and amplitude after changing the frequency and stimulus number suggest that inhibitory and excitatory mechanisms on the cortical and/or spinal level modulate the muscle response.

Zusammenfassung

Zielsetzung: Erstellung von Normwerten (Latenz, Amplitude) motorisch evozierter Potentiale (MEP) des M. abductor pollicis brevis (APB) und M. tibialis anterior (TA) nach hochfrequenter transkranieller Magnetstimulation des Motorkortex (rTKMS), und Erfassung stimulationsabhängiger Potenzialmodulationen. Patienten und Methoden: Bei 29 wachen Probanden wurde der Motorkortex transkraniell mit 2 und 4 magnetischen Stimuli bei einem Interstimulusintervall (ISI) von 2, 3 und 4 ms (entsprechend einer Frequenz von 250, 333, 500 Stimuli/s) gereizt. Latenz und Amplitude der MEP des relaxierten und vorinnervierten APB und TA wurden registriert. Ergebnisse: Die mittlere kortikomuskuläre Latenz zum relaxierten APB betrug, je nach Stimuluszahl und Frequenz, 22,2 bis 22,9 ms, zum relaxierten TA 30,4 bis 32,0 ms. Die Amplituden zeigten eine große intra- und interindividuelle Variabilität. Vorinnervation führte immer zu einer Latenzreduktion und Amplitudenzunahme (p < 0,05). Schlussfolgerungen: Wegen der hohen Variabilität ist die Festlegung normwertiger Amplituden nicht indiziert. Die Latenzen nach rTKMS gleichen den bekannten Normwerten nach magnetischem Einzelreiz. Wahrscheinlich werden trotz Reizrepetition beim wachen Probanden identische kortikale und spinale Strukturen in vergleichbarer Art aktiviert. Die Änderungen von Latenz und Amplitude in Abhängigkeit von Stimuluszahl und -frequenz lassen kortikale und/oder spinale Inhibitions- und Fazilitierungsmechanismen vermuten.

magnetic stimulation - motor cortex - transcranial motor cortex stimulation - motor evoked potentials - MEP

Magnetstimulation - Motorkortex - transkranielle Motorkortexstimulation - motorisch evozierte Potentiale - MEP

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