Transcranial magnetic stimulation: normal values of magnetic motor evoked potentials in 84 normal horses and influence of height, weight, age and sex

Abstract

Reasons for performing study: Cervical spinal corddysfunction is a common problem in equine medicine and the currently available tests give no objective information about the functionality of the nervous tracts. Therefore, transcranial magnetic stimulation (TMS) was performed in 84 healthy horses of different height in order to have an objective measure for the integrity of the descending motor tracts in normal horses. Objectives: To obtain reference values for onset latency and peak-to-peak amplitude of magnetic motor evoked potentials (MMEPs) and to evaluate the possible effect of height, age and gender on the neurophysiological measures. Methods: All horses were sedated and stimulated transcranially by using a magnetic coil placed on the forehead. The stimulator triggered the sweep of an electromyogram machine that recorded MMEPs bilaterally from needle electrodes in the extensor carpi radialis and cranial tibial muscles. In that way, it was possible to measure latency between stimulus and onset of response. Results: A significant difference was found between recordings made in the fore-and hindlimbs; MMEPs recorded in the front legs had a shorter onset latency and higher peak-topeak amplitude. Mean ± s.d. normal values for onset latency of 19.32 ± 2.50 and 30.54 ± 5.28 msecs and peak-to-peak amplitude values of 9.54 ± 3.73 and 6.62 ± 3.62 mV were obtained for extensor carpi radialis and cranial tibial muscles, respectively. The left-to-right difference in onset latency and peak-to-peak amplitude was not significant. In the same horse, differences up to 0.82 and 1.53 msecs for the extensor carpi radialis and cranial tibial muscles, respectively, lie within the 95% confidence limit and are considered normal. In contrast to onset latency, peak-topeak amplitude showed a very large intra-and interindividual variability, even in the same muscle. To reduce the variability and predict normal values of new individual cases, influence of height, weight, age and sex on the MMEPs were determined. No significant effects of sex were observed on onset latency and peak-to-peak amplitude. The age of the horse had only a small but significant effect on peak-to-peak amplitude, with larger responses in older horses. Height at the withers and weight of the horse, parameters that strongly correlate with the size of the horse, had an important significant influence on onset latency but not on peak-to-peak amplitude. The age of the horse and height at the withers were used to predict peak-to-peak amplitude and onset latency, respectively, in normal horses. Conclusions and potential relevance: TMS is an excellentaddition to the few tools we have for noninvasive imaging of the equine nervous system. Magnetically evoked potentials are highly reproducible and recent advances suggest that the applications of TMS in horses will continue to grow rapidly.

Transcranial magnetic motor evoked potentials in Great Danes with and without clinical signs of cervical spondylomyelopathy: Association with neurological findings and magnetic resonance imaging

“…A longer neuronal path length means that the impulse will need to travel a larger distance before reaching the muscle and generating a motor evoked potential; thus, yielding longer latencies. The influence of body size on TMMEPs latencies has also been reported in humans and horses (Chu, 1989; Nollet et al, 2004). These results indicate that reference ranges for TMMEPs latency will likely be different across dog breeds with different body sizes.…”

“…However, no study has reported TMMEPs values in clinically normal GDs or investigated its use in GDs with CSM. In humans and horses, TMMEPs latencies are influenced by body size (Chu, 1989; Nollet et al, 2004). GDs are larger than Doberman Pinschers; thus, TMMEPs reference ranges obtained in Doberman Pinschers may not apply to GDs.…”

Central Nervous System Trauma

“…This method has recently been validated and used to distinguish motor tract disorders from other causes of recumbency in clinical cases. [76][77][78] …”

Accuracy of transcranial magnetic stimulation and a Bayesian latent class model for diagnosis of spinal cord dysfunction in horses

“…Transcranial magnetic stimulation (TMS) with recording of magnetic motor evoked potentials (MMEP) is a promising additional test for diagnosis of spinal cord dysfunctions in horses . A magnetic 70 mm coil is placed on the head of the horse, at the level of the brain, to perform a magnetic stimulation.…”

“…This induces descending volleys through the spinal cord, evoking a muscle contraction reflected by the MMEP on the electromyography (EMG) machine. On each MMEP, the latency time, the time between stimulation and onset of muscle contraction, can be measured, which is the most reliable variable . In horses, the mean latency time of 4 MMEP is used, instead of the minimal latency time which is used in humans.…”