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PEER REVIEWED 38 JULY/AUGUST 2018 ■ Coordination of movement of the eyes, head, trunk and extremities in relation to movement or static position via the caudal cerebellar peduncle to the cerebellum. ■ Influence of the vomiting center in the reticular formation as a consequence of motion sickness. STEP 1: DETERMINE THE NEUROANATOMIC LOCALIZATION Once a head tilt has been identified, the next step is to perform a complete neurologic examination of the patient to determine if the vestibular dysfunction is peripheral, central or paradoxical. Paradoxical vestibular disease refers to a distinct localization to the cerebellum: specifically, the caudal cerebellar peduncle or the flocculonodular lobe 2 that result in slightly different clinical signs than typical peripheral or central vestibular disease. In general, patients with peripheral vestibular disease present with normal (albeit sometimes anxious) mentation, no other cranial nerve deficits (except sometimes cranial nerve VII), occasionally Horner's syndrome and normal postural reactions. On the contrary, dogs with central vestibular disease are more likely to have a more dull mentation, conscious proprioception deficits and one or more cranial nerve deficits. Postural reaction deficits are the most important indicator of central disease; and deficits in cranial nerve V to XII are the most likely.2 Patients with paradoxical vestibular disease typically exhibit normal mentation, ipsilateral hypermetria, cerebellar ataxia and ipsilateral proprioceptive deficiencies but no other cranial nerve deficits (except occasionally a menace deficit); these patients exhibit a contralateral head tilt and lean opposite the side of the lesion.3 A more complete guide to individual areas of testing and their neuroanatomic localization can be found in TABLE 1 . STEP 2: DIFFERENTIAL DIAGNOSES TABLE 2 lists the differential diagnoses based on peripheral versus central vestibular disease. Below is a brief discussion of some of the more common diseases seen in clinical practice. In one study that evaluated 85 dogs that had MRI of the brain for the diagnosis of vestibular disease, 29% had peripheral vestibular disease, 46% had central vestibular disease and 25% had paradoxical vestibular disease.4 Ossicles Semicircular Canals Vestibular Nerve External Ear Canal Tympanic Membrane Tympanic Bulla (middle ear) Cochlea Cochlear Nerve TO B R A I N Sympathetic Nerve VII VIII FIGURE 2. Anatomy of the canine external, middle and inner ear. TABLE 1 Clinical Signs Associated With Localization PERIPHERAL CENTRAL PARADOXICAL Level of Consciousness Normal Normal or Impaired Normal Nystagmus Horizontal or rotary; >60 bps Horizontal, rotary or vertical; <10 bps Horizontal, rotary or vertical Head Tilt Ipsilateral Ipsilateral Contralateral Leaning/Falling Ipsilateral Ipsilateral Contralateral Postural Reactions Normal Ipsilateral Ipsilateral Cranial Nerve Abnormalities +/- Cranial nerve VII, +/- Horner's Syndrome Any Uncommon, +/- menace deficit Gait Normal or vestibular ataxia Vestibular +/- proprioceptive ataxia +/- Hypermetria vestibular or cerebellar ataxia

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