Evaluation and Utility of Submaximal Stimulation Intensity in Transcranial Magnetic Stimulation in the Standing Horse☆.

The research focuses on how using less than maximum stimulation intensities in transcranial magnetic stimulation (TMS) can affect the function and integrity of descending motor pathways in horses. The aim of the study was to identify the minimum coil intensity needed to evoke motor unit potentials in horses and to see if using lower stimulation intensities can decrease sensory irritation in horses.

Methodology

• The study involved 36 neurologically healthy horses who went through TMS procedure while under sedation.
• The horses were treated with a dome coil at varying stimulation intensities, ranging from 40% to 100% of device output intensity.
• The researchers recorded motor potentials from the limbs of the horses and calculated latency times in milliseconds.
• To fine-tune the stimulation settings, Cortical Motor Thresholds (CMT) were also assessed in triplicates, adhering to the International Federation of Clinical Neurophysiology (IFCN) recommendations.

Findings

• As the stimulation intensities were increased, the latency times reduced significantly until 80% of the maximum output intensity was applied.
• No significant difference was observed when the intensity was further increased to the maximum of 100%.
• On testing the gating effects, it was found that there were no differences in latency times with increasing or decreasing stimulation intensity changes.
• Researchers noted a large interindividual variability among horses regardless of their body size.
• There was a strong linearity observed between Cortical Motor Thresholds and latency times, even within submaximal stimulation intensity ranges.
• Researchers surmised the inverse impact of the stimulation intensity on latency times could be attributed to deeper penetration of the magnetic field, avoidance of intermediated neurons, and then activation of quick acting motor pathways.

Outcome and Recommendations

• Researchers found that in warmblood horses, a stimulation intensity of 80% coil intensity was already adequate for consistently activating lower motor neurons in all limbs.
• They also suggested that, due to the strong linear correlation between CMT and LT, the tested CMT algorithms can potentially be used to estimate the normal latency times on submaximal stimulation for equine patients suffering from myelopathy in the future.