Transcranial magnetic stimulation (TMS) has been successfully used in horses to evaluate function and integrity of descending motor pathways in patients affected by neurological gait abnormalities. In preceding studies, lengthening latency times (LT) of cranially evoked limb muscle potentials have been considered a reliable diagnostic parameter. Standardized settings use device output signal intensities of 100%. The aim of this study was to determine the effect of submaximal stimulation intensities (SI) and to determine the minimum coil output necessary to evoke motor unit potentials. As an additional effect, lower stimulation intensities are supposed to decrease sensory irritation of the equine patient. Altogether, 36 neurologically healthy horses underwent TMS under sedation with a dome coil at stimulation intensities varying from 40% to 100% of device output intensity. Motor potentials were recorded by surface electrodes from all four limbs and LT was calculated in milliseconds. To further refine the stimulation settings, cortical motor thresholds (CMT) were assessed in triplets, using IFCN recommendations. The electromyographic recordings were evaluated in 30 horses. Increasing stimulation intensities resulted in significant (P < .05) LT shortening until application of 80% of maximal output intensity. Further increase to maximal SI of 100%, brought up no significant differences (P > .05). Gating effects were excluded as there was no difference of LT upon ascending and descending SI changes (P > .05). CMT revealed a large inter-individual variability amongst horses independent of their body size. There was a strong linearity in between CMT and LT even within submaximal SI ranges (P < .001). The inverse impact of SI on LT may be explained by deeper penetration of the magnetic field, circumvention of interposed neurons and subsequent activation of fast acting motor pathways. However, in warmblood horses a stimulation intensity of 80% coil output already appeared sufficient for reproducible activation of lower motor neurons in all limbs. Furthermore, due to the strong linear correlation of CMT and LT, the tested CMT algorithms may be used to estimate the normal LT on submaximal stimulation for equine myelopathy patients in future.
|Journal||Journal of Equine Veterinary Science|
|State||Published - May 2022|
Bibliographical noteFunding Information:
The authors thank Dr. H-P. Remler, Main and State stud farm Schwaiganger, Bavaria, Katleen Vanschandevijl, Equine Hospital ‘De Bosdreef’, Moerbeke, Belgium and Gunther van Loon, Ghent University, Belgium, for technical advice. Special thanks also to Dipl.-Math. Ulrike von Hehn for initial statistical consultation. This study was performed at the Equine Hospital, Center for Clinical Veterinary Medicine, Faculty of Veterinary Medicine, Ludwig-Maximilians-Universitaet Muenchen, Munich, Germany, and at Bavaria's Main Stud Farm ‘Haupt- und Landgestuet Schwaiganger’, Ohlstadt, Germany. The study was supported through internal funds of the Equine Hospital, Center for Clinical Veterinary Medicine, Ludwig-Maximilians-Universitaet Muenchen, Munich, Germany. Conflict of Interest Statement: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
© 2022 Elsevier Inc.
- Cortical motor threshold
- Motor unit potential
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