Peripheral Vestibular System

The peripheral vestibular system is located within the inner ear and consists of receptors and the vestibular portion of cranial nerve VIII (vestibulocochlear nerve). The bony labyrinth, which is located in the petrous temporal bone, consists of the semicircular canals, vestibule, and cochlea and is filled with perilymph. The membranous labyrinth, filled with endolymph, is located within the bony labyrinth and is composed of the semicircular ducts, utricle, saccule, and cochlear duct. The semicircular ducts are located within the semicircular canals, the utricle and saccule are located within the vestibule, and the cochlear duct is located within the cochlea. The crista ampullaris is located in the semicircular ducts and responds to acceleration, deceleration, and rotation. Hair cells in the crista ampullaris respond to displacement of the endolymph. The dendrites of cranial nerve VIII synapse on these hair cells and are stimulated when they deflect.

Maculae are receptors located in the utricle and saccule that respond to linear movement. The surface is covered by hair cells that trigger an action potential in cranial nerve VIII when they are deflected. The receptors provide continual tonic input to cranial nerve VII, which functions to maintain normal posture of the head and body.

Central Vestibular System

The central vestibular system consists of the vestibular nuclei located in the medulla and the projections to the spinal cord, brainstem, and cerebellum. Spinal cord projections from the vestibular nuclei exert facilitatory and inhibitory effects on ipsilateral extensors and flexors, respectively. These projections also inhibit contralateral extensors. Brainstem projections relay information to the motor neurons of cranial nerves III, IV, and VI (oculomotor, trochlear, and abducens nerves, respectively) via the medial longitudinal fasciculus. These projections are responsible for normal eye position and physiologic nystagmus. Other brainstem projections are directed to the vomiting center within the reticular formation. Cerebellar projections travel via the caudal cerebellar peduncle to the vestibulocerebellum to maintain coordination.

Peripheral Vestibular Disease

Peripheral vestibular disease is characterized by any combination of vestibular ataxia, head tilt, strabismus, and nystagmus. With peripheral disease the nystagmus can be in any direction, but vertical nystagmus is extremely rare in peripheral disease and should be considered central in origin. Many believe that nystagmus associated with peripheral disease should not change in character or direction, but nothing definitive has been published regarding this finding. Nystagmus can be spontaneous or positional.

It is important not to misinterpret vestibular ataxia as proprioceptive deficits. Animals with peripheral vestibular disease often have quite dramatic clinical signs, and examining them thoroughly can be challenging. It is important to support the animal’s weight during proprioception to prevent orthopedic disease or weakness from interfering with the ability of the animal to replace the paw to a normal position.

Bilateral vestibular disease can be confused easily with a more central condition because the clinical signs often are symmetric. Animals with bilateral vestibular disease tend to walk low to the ground in a crouched position and often fall to both sides. Lateral head excursions are common with bilateral vestibular disease. A head tilt and vestibuloocular reflex often are absent.

Central Vestibular Disease

Central vestibular disease has several hallmark features that can distinguish it from peripheral vestibular disease (Table 232-1). Central lesions may be accompanied by a change in mentation; for example, the animal may be overly dull or obtunded. If abnormalities in other cranial nerves, besides cranial nerves VII and VIII, are found on neurologic examination, then a central lesion should be considered. Proprioceptive deficits often are present with central lesions and are characterized by decreased placing or hopping on the side ipsilateral to the lesion. Any form of nystagmus, including vertical or changing nystagmus, may be present with central lesions. The other signs listed for peripheral vestibular disease, such as ataxia, head tilt, and strabismus, also can be seen with central lesions.