Reduced Threshold Potential of the Trigeminal Nerve in Equine Headshaking

Abstract

AimAlthough the aetiopathogenesis of headshaking remains elusive, trigeminal neuralgia is regarded as the likely explanation of observed clinical signs. The aim of this study was to compare sensory nerve conduction threshold and velocity of the trigeminal nerve using the infraorbital nerve in control and headshaking horses.MethodsControl group (n = 6 horses) and headshaking group (n = 6) were subject to general anaesthesia. A Nicolet Viking IV evoked potential system was used for sensory nerve conduction study. Stimulating, recording and reference electrodes were used. A pair of stimulating electrodes was placed at the gingival mucosa, one each at the rostral and caudal aspects of the maxillary canine tooth. Four pairs of recording electrodes were placed at 4 different points along the tract of the infraorbital nerve (point 1 at infraorbital foramen), maxillary nerve (point 2 at exit of trigeminal canal), spinal somatosensory (point 3 at level of C1 spinal cord segment), and cortical somatosensory evoked potentials (point 4 at level of frontal cerebral cortex). A reference electrode was placed at a distance half way between the stimulating and recording electrodes at point 1. Stimuli were applied at 2.5, 5, 10, 15, 20 mA. The duration of each stimulus was 0.1 ms. Thresholds were recorded and conduction velocities calculated for each group.ResultsThe threshold of sensory nerve action potential occurred at low stimuli (2.5 and 5 mA) in horses with headshaking; and at higher stimuli (10 mA in 3 horses, 15 mA in 1 horse, and 25 mA in 2 horses) in control horses. Although there were differences in conduction velocity between groups, this was not significant.Conclusions and practical significanceHeadshaking horses have a low threshold for inducing sensory action potentials upon minimal stimulation compared with control horses supporting involvement of the trigeminal nerve in the pathogenesis of affected horses.Ethical animal researchStudy approved by the UC Davis IACUC. Sources of funding: Private donation. Competing interests: None.