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

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.

Serum Amyloid A (SAA) Concentration after Vaccination in Horses and Mules.

Serum amyloid A (SAA) is a sensitive acute-phase response (APR) marker in equids. Prominent APRs with elevations of SAA concentrations ([SAA]) have been reported after vaccination. The authors hypothesized that vaccination with an inactivated EHV-1/-4 vaccine would cause increase in [SAA] and antibody responses and that higher [SAA] would be positively correlated with the antibody titer in both equids. Twelve Haflinger horses and 12 mules were included in this longitudinal prospective study. All horses and mules were vaccinated with a commercially available EHV-1/-4 vaccine. Blood was sampled before and after vaccination to measure [SAA] and virus-neutralizing response (VN-T). In horses and mules, significantly higher [SAA] were measured on days 1, 3, and 5 after EHV-1/-4 vaccination; [SAA] on day 1 after vaccination were only measured in animals that developed fever, where mean [SAA] were significantly higher in horses than in mules (horses: 1,365.75 ± 87.64 mg/L, mules: 615.5 ± 153.444 mg/L) (P > .05). Four horses and 2 mules developed fever after vaccination, lasting for ≤24 hours. Increased antibody responses (VN-T) on days 7 and 14 after vaccination were observed in all animals, whereas mules showed higher overall antibody responses. Nevertheless, [SAA] did not correlate with the intensity of the antibody responses (VN-T) stimulated by the vaccine (P < .05). EHV-1/-4 vaccination caused a prominent APR, higher in horses than in mules, but [SAA] did not correlate with antibody responses. Measuring [SAA] after vaccination could help identify severe APRs that may require longer resting intervals before training or competition.