Multipulse transcranial electrical stimulation (TES): normative data for motor evoked potentials in healthy horses

Abstract

Background: There are indications that transcranial electrical stimulation (TES) assesses the motor function of the spinal cord in horses in a more sensitive and reproducible fashion than transcranial magnetic stimulation (TMS). However, no normative data of TES evoked motor potentials (MEP) is available.

Results: In this prospective study normative data of TES induced MEP wave characteristics (motor latency times (MLT); amplitude and waveform) was obtained from the extensor carpi radialis (ECR) and tibial cranialis (TC) muscles in a group of healthy horses to create a reference frame for functional diagnostic purposes. For the 12 horses involved in the study 95% confidence intervals for MLTs were 16.1-22.6 ms and 31.9-41.1 ms for ECR and TC muscles respectively. Intra-individual coefficients of variation (CV) and mean of MLTs were: ECR: 2.2-8,2% and 4.5% and TC: 1.4-6.3% and 3.5% respectively. Inter-individual CVs for MLTs were higher, though below 10% on all occasions. The mean ± sd of MEP amplitudes was respectively 3.61 ± 2.55 mV (ECR muscle left) and 4.53 ± 3.1 mV (right) and 2.66 ± 2.22 mV (TC muscle left) and 2.55 ± 1.85 mV (right). MLTs showed no significant left versus right differences. All MLTs showed significant (p < 0.05) voltage dependent decreases with slope coefficients of linear regression for ECR: - 0.049; - 0.061 ms/V and TC: - 0.082; - 0.089 ms/V (left; right). There was a positive correlation found between height at withers and MLTs in all 4 muscle groups. Finally, reliable assessment of MEP characteristics was for all muscle groups restricted to a transcranial time window of approximately 15-19 ms.

Conclusions: TES is a novel and sensitive technique to assess spinal motor function in horses. It is easy applicable and highly reproducible. This study provides normative data in healthy horses on TES induced MEPs in the extensor carpi radialis and tibialis cranialis muscles bilaterally. No significant differences between MLTs of the left and right side could be demonstrated. A significant effect of stimulation voltage on MLTs was found. No significant effect of height at the withers could be found based upon the results of the current study. A study in which both TMS and TES are applied on the same group of horses is needed.

Keywords: Horses; MEP; Neurology; Spinal cord function; TES; TMS; Transcranial electric stimulation.

Fig. 1

Schematic drawing of the TES-MEP set-up. A transcranial electrical stimulator is connected to subcutaneously inserted needle electrodes bilaterally from the vertex of the skull. A multipulse stimulation consists of 3 biphasic pulses/train, pulse width 0.1 ms/phase, ipi = 1.3 ms. Elicited action potentials in corticospinal axons cross the midline at the decussatio pyramis and are conveyed to LMNs that relay to peripheral motor axons. After passing the neuromuscular junction muscular motor potentials are generated in muscle fibers and recorded at a pair of needle electrodes that are connected to a physiological amplifier

Fig. 2

Scatter plots showing the correlation between motor MLT and TES intensity. Considered are changes of MLT due to increases in TES-voltage. Motor latency differences ∆MLT_n,i,m are plotted vertically and the TES-intensity is related to the motor threshold MT and plotted along the horizontal axis as V_TES – MT. ∆MLT_n,i,m is obtained after subtraction of the mean MLT, MLT_i,m, from MLT_n,i,m. n denotes case number, i is one of 6 data points per case and m refers to the muscle group. Each plot represents a muscle group: figures (a) and (b) refer to ECR: extensor carpi radialis muscle, respectively left and right. Figures: (c) and (d) refer to TC: tibial cranial muscle, respectively left and right. The parameters of the regression line with correlation and significance are specified in Table 2. All regression lines show decreasing courses with significant correlation

Fig. 3

Scatter plots of MLT_i as a function of the height at withers. Scatter plots of MLT_i as a function of the height at withers where i refers to the case number. MLT_i is the mean MLT of 3 data pairs at stimulation voltages V_stim of 10, 20 and 30 V above motor threshold MT. One point represents the mean value of one case. All 12 cases are included. Each plot represents a muscle group. Figures (a) and (b) refer to ECR: extensor carpi radialis muscle, respectively left and right. Figures (c) and (d) refer to TC: tibial cranial muscle, respectively left and right. The parameters of the regression lines are specified in Table 3. All regression lines show increasing courses of which the left TC muscle group is significant for p ≤ 0.05