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J Clin Neurophysiol. 2019 Feb 1. doi: 10.1097/WNP.0000000000000568. [Epub ahead of print]

Repeated Transcranial Magnetic Stimulation-Induced Motor Evoked Potentials Correlate With the Subject-Specific Serum Metabolic Profile of Creatine.

Author information

1
Department of Clinical Neurophysiology, Kuopio University Hospital, Kuopio, Finland.
2
Department of Psychiatry, UT Southwestern Medical Center, Dallas, U.S.A.
3
School of Pharmacy, University of Eastern Finland, Kuopio, Finland.
4
Department of Forensic Psychiatry, University of Eastern Finland, Niuvanniemi Hospital, Kuopio, Finland.
5
Institute of Public Health and Clinical Nutrition, University of Eastern Finland, Kuopio, Finland.
6
Department of Clinical Neurophysiology, Institute of Clinical Medicine, Faculty of Health Sciences, University of Eastern Finland, Kuopio, Finland.
7
Department of Clinical Radiology, Kuopio University Hospital, Kuopio, Finland.
8
Doctoral Programme of Clinical Research, School of Medicine, University of Eastern Finland, Kuopio, Finland.
9
Metabolomics Unit, Institute for Molecular Medicine Finland, University of Helsinki, Helsinki, Finland.
10
Department of Adolescent Psychiatry, Kuopio University Hospital, Kuopio, Finland.
11
Institute of Clinical Medicine/Psychiatry, University of Eastern Finland, Kuopio, Finland.
12
Department of Psychology and Logopedics, Faculty of Medicine, University of Helsinki, Helsinki, Finland.
13
Department of Psychiatry, Faculty of Medicine, University of Helsinki, Helsinki, Finland.
14
Department of Psychiatry, Kuopio University Hospital, Kuopio, Finland.

Abstract

PURPOSE:

Transcranial magnetic stimulation-induced motor responses have been considered to mainly reflect the electrophysiological characteristics of the central motor system. However, certain motor phenomena, such as the magnitude of repetition suppression measured with motor evoked potentials (MEPs), could also in part be influenced by metabolic processes in the peripheral muscles and in both the peripheral and central nervous system. Repetition suppression is an inhibitory phenomenon in which the amplitude of MEP decreases in comparison to that of the first MEP in a train of transcranial magnetic stimulation pulses. This study aimed to identify possible metabolic processes influencing repetition suppression.

METHODS:

The metabolic profiles from serum samples and repetition suppression from the right abductor pollicis brevis muscle were measured in 73 subjects (37 female subjects). Repetition suppression was measured using trains of transcranial magnetic stimulation stimuli consisting of 4 identical single pulses at 1-second intervals. The trains were repeated every 20 seconds, and 30 trains were given with a stimulation intensity of 120% of the resting motor threshold of the abductor pollicis brevis. Thus, a total of 120 stimuli were administered.

RESULTS:

The main finding was a significant negative relationship between serum creatine levels and the magnitude of repetition suppression (standardized β coefficient (β) = -0.43; P < 0.001). In other words, higher creatine levels corresponded to a smaller decrement in the MEP amplitude in response to repetition. When MEPs were not repeated, no relationship was observed (β = 0.09; P = 0.454). Creatine is used to form phosphocreatine, which in turn is needed to resynthesize adenosine triphosphate from adenosine diphosphate in situations requiring high amounts of energy in muscles and neural cells.

CONCLUSIONS:

For the first time, this study demonstrated a connection between repeated MEPs and peripheral serum metabolites linked to muscle function. These findings could explain some of the intersubject variability commonly observed in MEPs when the pulses are repeated.

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