Vergleich von motorischen Schwellen und Phosphenschwellen zur Beurteilung kortikaler Erregbarkeit in der transkraniellen Magnetstimulation

Zusammenfassung

Einleitung: Die Reizschwelle zur Auslösung motorischer Antworten (motor threshold; MT) stellt einen Parameter für die kortikale Exzitabilität dar und wird zur Adaptation der individuell genutzten Reizstärke bei der transkraniellen magnetischen Kortexstimulation (TMS) genutzt. Es ist nicht bekannt, ob die MT auch ein Maß für die Erregbarkeit nichtmotorischer Kortexareale wie des visuellen Kortex darstellt und mit den Schwellenwerten zur Auslösung von Phosphenen (phosphene threshold; PT) korreliert. Methodik: Bei 22 gesunden Probanden wurden mit einem Dantec-MagPro-Stimulator und einer achtförmigen Spule biphasische Reize über dem primären, motorischen und visuellen Kortex appliziert und individuell die MT und PT (jeweils in % der maximalen Geräteausgangsleistung) bestimmt. Ergebnisse: Es bestand keine Korrelation der MT mit der PT. Die PT lag mit durchschnittlich 58,5 ± 12,1 % signifikant über der MT von 40,5 ± 6,6 % (p = 0,01). Diskussion: Die MT stellt keinen Parameter zur Beurteilung der transkraniellen Erregbarkeit des visuellen Kortex dar. Bei einer TMS des visuellen Kortex sollte die Reizstärke an die PT adaptiert werden, um interindividuell vergleichbare Reizeffekte zu erzielen.

Abstract

Introduction: Motor threshold (MT) as determined by transcranial magnetic stimulation (TMS) is used as an index for cortex excitability. In TMS stimulus intensity is usually referred to the individual MT. It is unclear whether MT also reflects the excitability of non-motor cortical areas such as the visual cortex or correlates with the phosphene threshold (PT) as demonstrated by TMS over the occipital skull. Methods: In 22 healthy volunteers TMS with single pulses was applied over the motor and visual cortex with a figure-of-eight shaped coil connected to a Dantec MagPro stimulator. Biphasic pulses were used to avoid influence of current direction on excitatory effects. MT and PT were individually measured (% of maximum stimulator output). Results: There was no correlation between MT and PT. Compared to mean MT (40.5 ± 6.6 %) PT was significantly higher (58.5 ± 12,1 %) (p = 0.01). Discussion: MT does not reflect the excitability of the visual cortex. In TMS of the visual cortex PT should be used to define the stimulus intensity to compare stimulus effects interindividually.

Key words

Motor threshold - phosphene threshold - cortical excitability - transcranial magnetic stimulation

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