Sensory Evoked Potentials of the Trigeminal Nerve for the Diagnosis of Idiopathic Headshaking in a Horse

A 400-kg 5-year-old Arabian gelding was presented for a 2to 3-year history of progressive uncontrollable violent headshaking that precluded any type of physical activity and compromised quality of life. Although there were no triggering events for the observed behavior, headshaking was exacerbated when a bridle was placed on the horse’s head. One year before presentation, the horse underwent an extensive evaluation at another institution that included hematology and blood chemistry, testing for equine protozoal myelitis, skull radiographs, upper airway endoscopy (including guttural pouches), and magnetic resonance imaging of the head. Subtle asymmetry of the stylohyoid bone at its articulation was noted (left slightly thicker) on endoscopy. The rest of the diagnostic evaluation was within normal limits. Treatment with nonsteroidal anti-inflammatory drugs, gabapentin, and diet modification failed to control the episodes. Additional laboratory testing, as well as oral and ophthalmologic examinations, was performed by the referring veterinarian and found normal. The horse was referred for further evaluation. At presentation, the horse displayed violent continuous episodes of headshaking, which made the horse difficult to handle. Physical and neurologic examinations were normal except for the headshaking. These episodes were seen repeatedly while the horse was hospitalized, independent of time of day, light exposure, exercise, excitement, or confinement. The horse would frequently rub the right side of its face and eye. However, there were no obvious skin or ocular lesions. The horse would eat, but with interruptions attributable to headshaking. Some episodes associated with eating were triggered by hay touching the horse’s muzzle. Therefore, soaked hay and pellets were offered. Laboratory testing showed mild neutrophilia (7,117/ lL; reference range 2,600–6,800/lL) and 245 bands/lL with moderate toxicity, and normal plasma fibrinogen concentration; serum chemistry and blood gases analysis were within normal limits. Skull radiographs and upper airway endoscopy were not performed because computed tomography (CT) of the entire head was part of the diagnostic plan. A CT examination was done under general anesthesia and showed no abnormalities. Complete ophthalmologic and otoscopic examinations also were normal. Collection and evaluation of a spinal fluid sample were offered, but the owner declined. Somatosensory evoked potentials of the trigeminal complex using the infraorbital nerve (branch of the maxillary nerve) were studied in this horse using an evoked potential system (Nicolet Viking IV) as described by Aleman et al. In brief, with the horse in left lateral recumbency, a surface stimulus was applied to the dorsal gingival mucosa at the level of the right maxillary canine tooth. Three pairs of recording electrodes (Disposable EasyGrip monopolar electrode) were placed along the sensory pathway of the trigeminal complex. Recording site 1 (R1: infraorbital nerve) was at the level of the infraorbital foramen; site 2 (R2: maxillary nerve) was at the level of the maxillary foramen; and site 3 (R3: spinal tract of trigeminal complex) was at the level of the spinal cord segment 1. A 4th recording site (R4: cortical somatosensory) utilized subdermal electrodes placed at the level of the frontoparietal cerebral cortex (Fig 1A). A ground electrode was placed between the stimulus site and first recording site. A surface temperature probe was positioned dorsal to the facial crest to monitor regional temperature while recording (local temperature was maintained at 35°C). The stimulus rate was set at 3 Hz with a stimulus duration of 0.2 ms. The sweep speed was set at 5 ms per division. Each recording was the average of 1,000 responses. Acquisition sensitivities were variable with settings at 200, 100, 10, and 10 lV/divsion for recording sites 1, 2, 3, and 4, respectively. Band widths for R1–R3 were 20 Hz to 3 KHz, and 20–250 Hz for R4. Recordings were made at each of the following stimulus intensities (SI 0.1 mA): 2.5, 5, 10, 15, and 20 mA. Two sets of recordings were performed at each SI to test the reproducibility of the data. As a negative control, the study was also done with the SI From the Department of Medicine and Epidemiology, (Aleman, Madigan); the William R. Pritchard Veterinary Medical Teaching Hospital (Rhodes, Williams); and the Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA (Guedes). Corresponding author: M. Aleman, Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95691; e-mail: [email protected]. Submitted August 10, 2013; Revised August 29, 2013; Accepted September 19, 2013. Copyright © 2013 by the American College of Veterinary Internal Medicine 10.1111/jvim.12237 Abbreviations: