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.

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

Literatur

• 1 Hallett M. Transcranial magnetic stimulation and the human brain.  Nature. 2000;  406 147-150
• 2 Pascual-Leone A, Walsh V, Rothwell J. Transcranial magnetic stimulation in cognitive neuroscience - virtual lesion, chronometry, and functional connectivity.  Curr Opin Neurobiol. 2000;  10 232-237
• 3 George M S, Lisanby S H, Sackeim H A. Transcranial magnetic stimulation: applications in neuropsychiatry.  Arch Gen Psychiatry. 1999;  56 300-311
• 4 Loo C K, Taylor J L, Gandevia S C, McDarmont B N, Mitchell P B, Sachdev P S. Transcranial magnetic stimulation (TMS) in controlled treatment studies: are some „sham” forms active?.  Biol Psychiatry. 2000;  47 325-331
• 5 Siebner H R, Tormos J M, Ceballos-Baumann A O, Auer C, Catala M D, Conrad B, Pascual-Leone A. Low-frequency repetitive transcranial magnetic stimulation of the motor cortex in writer's cramp.  Neurology. 1999;  52 529-537
• 6 Hasey G. Transcranial magnetic stimulation in the treatment of mood disorder: review and comparison with electroconvulsive therapy.  Can J Psychiatry. 2001;  46 720-727
• 7 Rossini P M, Barker A T, Berardelli A. Non-invasive electrical and magnetic stimulation of the brain, spinal cord and roots: basic principles and procedures for routine clinical application. Report of an IFCN committee.  Electroencephalogr Clin Neurophysiol. 1994;  91 79-92
• 8 Wassermann E M. Risk and safety of repetitive transcranial magnetic stimulation: Report and suggested guidelines from the International Workshop on the Safety of Repetitive Transcranial Magnetic Stimulation, June 5 - 7, 1996.  Electroencephalogr Clin Neurophysiol . 1998;  108 1-16
• 9 Kammer T, Beck S, Erb M, Grodd W. The influence of current direction on phosphene thresholds evoked by transcranial magnetic stimulation.  Clin Neurophysiol. 2001;  112 2015-2021
• 10 Meyer B U, Diehl R, Steinmetz H, Britton T C, Benecke R. Magnetic stimuli applied over motor and visual cortex: influence of coil position and field polarity on motor responses, phosphenes, and eye movements.  Electroencephalogr Clin Neurophysiol. 1991;  43, Suppl 121-134
• 11 Boroojerdi B, Bushara K O, Corwell B, Immisch I, Battaglia F, Muellbacher W, Cohen L G. Enhanced excitability of the human visual cortex induced by short-term light deprivation.  Cerebral Cortex. 2000;  10 529-534
• 12 Ziemann U. Transkranielle Magnetstimulation: Neue Einsatzmöglichkeiten zur Messung kortikaler und kortikospinaler Erregbarkeit.  Akt Neurol. 2001;  28 249-264
• 13 Mulleners W M, Chronicle E P, Palmer J E, Koehler P J, Vredeveld J W. Visual cortex excitability in migraine with and without aura.  Headache. 2001;  41 565-572
• 14 Amassian V E, Cracco R Q, Maccabee P J, Cracco J B, Rudell A P, Eberle L. Transcranial magnetic stimulation in study of the visual pathway.  J Clin Neurophysiol. 1998;  15 288-304
• 15 Brasil-Neto J P, Cohen L G, Panizza M, Nilsson J, Roth B J, Hallett M. Optimal focal transcranial magnetic activation of the human motor cortex: effects of coil orientation, shape of the induced current pulse, and stimulus intensity.  J Clin Neurophysiol. 1992;  9 132-136
• 16 Niehaus L, Meyer B U, Weyh T. Influence of pulse configuration and direction of coil current on the excitation effects of magnetic motor cortex and nerve stimulation.  Clin Neurophysiol. 2000;  111 75-80
• 17 Niehaus L, Hoffmann K T, Grosse P, Röricht S, Meyer B U. MRI study of human brain exposed to high-dose repetitive magnetic stimulation.  Neurology. 2000;  54 256-258
• 18 Stewart L M, Walsh V, Rothwell J C. Motor and phosphene thresholds: a transcranial magnetic stimulation correlation study.  Neuropsychologia. 2001;  39 415-419
• 19 Rothwell J C. Techniques and mechanisms of action of transcranial magnetic stimulation of the human motor cortex.  J Neurosci Methods. 1997;  74 113-122
• 20 Ziemann U, Lönnecker S, Steinhoff B J, Paulus W. Effects of antiepileptic drugs on motor cortex excitability in humans: a transcranial magnetic stimulation study.  Ann Neurol. 1996;  40 367-378
• 21 Aurora S K, Ahmad B K, Welch K M, Bhardwaj P, Ramadan N M. Transcranial magnetic stimulation confirms hyperexcitability of occipital cortex in migraine.  Neurology. 1998;  50 1111-1114
• 22 Afra J, Mascia A, Gerard P, Maertens de Noordhout A, Schoenen J. Interictal cortical excitability in migraine: a study using transcranial magnetic stimulation of motor and visual cortices.  Ann Neurol. 1998;  44 209-215
• 23 Aurora S K, al-Sayeed F, Welch K M. The cortical silent period is shortened in migraine with aura.  Cephalagia. 1999;  19 708-712
• 24 Werhahn K J, Wiseman K, Herzog J, Förderreuther S, Dichgans M, Straube A. Motor cortex excitability in patients with migraine with aura and hemiplegic migraine.  Cephalalgia. 2000;  20 45-50