Figure 2.11 Vestibular system.

      On rare occasions, central unilateral or asymmetric CNS lesions, particularly if in the region of the caudal cerebellar peduncle, may result in paradoxical vestibular signs. This is seen as any head tilt and circling being away from, and the fast phase of nystagmus being toward, the side of the lesion. Defining the side of such a lesion will depend on identifying other signs of cerebellar and/or medullary disease.

      A rather forgotten component to the vestibular system is proprioceptive input from the cranial cervical vertebrae, ligaments, and muscles.^23,24 From this region, afferent fibers pass via at least the C_1–3 dorsal spinal nerve roots to ascend the spinal cord to the caudal part of the medial vestibular nuclei that receive no other afferent inputs (Figure 2.11).²⁵ Like damage to the vestibular nerve, lesions involving these cervical nerves or the cervical spinal input to the vestibular apparatus can result in signs of vestibular disease. This certainly can be seen with asymmetric lesions of the dorsal nerve roots of C_1–3 when loss of balance, eye deviation, and a head tilt can be seen.

       Glossopharyngeal nerve—CN IX; vagus nerve—CN X; accessory nerve—CN XI

      A major function of these cranial nerves is achieved through innervation of the pharynx and larynx with both sensory and motor fibers. This is tested by listening for normal laryngeal sounds, observing normal swallowing of food and water, assessing the swallowing reflex by passage of an esophageal tube, and finally by inspection of the larynx and pharynx manually and preferably with the aid of an endoscope. The major centers for control of the pharynx and larynx via these three cranial nerves are the nucleus ambiguus and nucleus solitarius in the caudal medulla oblongata. The most important clinical signs of dysfunction are related to paralysis of the pharynx and larynx, and the severity of signs depends on whether there is unilateral or bilateral involvement. In pharyngeal paralysis, usually food, water, and saliva can be seen at the nostrils in horses, and at the nostrils and mouth in ruminants, and spilling onto the floor. During exercise, unilateral paralysis of the larynx results in an abnormal respiratory noise—so‐called roaring in physically active horses, but is usually subclinical in sedentary animals. Stertorous breathing may be detected with diseases that cause bilateral pharyngeal or laryngeal paralysis.

In the horse at least, the thoraco‐laryngeal adductor response, or slap test (Figure 2.12), should be performed.²⁶ The skin just caudal to the dorsal aspect of the scapula is slapped firmly with a hand while the horse is exhaling, and while the larynx is observed with an endoscope, the normal response being brief adduction of the contralateral arytenoid cartilage. Contraction of the contralateral dorsal and lateral laryngeal musculature over the muscular process of the arytenoid cartilage can also be palpated with the fingers while the withers are slapped. This has been detected in cooperative foals as early as 2 weeks of age. Usually, this response can be felt externally as a slight tap or muscular contractions under the fingers. The afferent pathway is through the segmental dorsal thoracic nerves to the spinal cord, cranially in the contralateral cervical spinal cord white matter, and thus to the contralateral vagal motor nucleus in the medulla oblongata. From there, the efferent pathway is through the contralateral vagus nerve to the cranial thorax and then back up the neck in the recurrent laryngeal nerve to the larynx. The reflex can be interrupted at any of these sites.