Peripheral and Central Vestibular Disorders in Dogs and Cats

Chapter 232

Peripheral and Central Vestibular Disorders in Dogs and Cats

The vestibular system maintains the body’s posture and balance. The vestibular system coordinates and communicates the position of the head in relation to the body and the body in relation to the ground. Vestibular disease is one of the more common presentations of an animal with neurologic disease, and it also can be one of the most terrifying for the owner to witness. This chapter reviews pertinent anatomy of the vestibular system, clinical signs of dysfunction, and some of the more common disease processes affecting the peripheral and central vestibular systems.

Vestibular Anatomy

A basic understanding of the neuroanatomy of the vestibular system is very helpful in neurolocalization, which aids in distinguishing the differential diagnoses and thus in selecting the diagnostic tests and treatments required.

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.

Clinical Signs of Vestibular Disease

Nystagmus can be physiologic or pathologic and usually has a fast and a slow phase. Physiologic, or naturally occurring, nystagmus can be elicited during the neurologic examination by testing the vestibuloocular reflex, as by moving the head in a lateral or medial direction. Physiologic nystagmus requires coordination between the vestibular system and cranial nerves III, IV, and VI and the medial longitudinal fasciculus. Some animals, such as the Siamese cat, have resting nystagmus, which is considered normal for the breed.

Pathologic nystagmus is the jerking eye movements seen with physiologic nystagmus but it occurs when the head is at rest. Pathologic nystagmus can occur when the animal is in a normal position, termed resting nystagmus, or when the animal is put in an unusual position such as on its back, termed positional nystagmus. Traditionally, the nystagmus is characterized by the direction of the fast phase, and in most cases this movement is away from the lesion. For example, nystagmus in which the fast phase is toward the left and the slow phase is toward the right is termed fast phase left and is suggestive of a left-sided lesion in most cases. This terminology can be confusing, and a way to help remember the direction is the mnemonic that the nystagmus is “running away” from the lesion.

Strabismus is an abnormal eye position. Strabismus may be spontaneous (occurring at rest) or positional. In vestibular disease strabismus is either ventral or ventrolateral and occurs on the same (ipsilateral) side as the lesion.

Ataxia describes an uncoordinated gait. Vestibular ataxia is characterized by turning, rolling, falling, or circling to the side of the lesion. Usually it is very asymmetric and generally the circles are very tight circles. Some animals may be unwilling to walk and some have a broad-based stance.

A head tilt often is present in vestibular disease and usually is toward the side of the lesion. Head tilts can range from very mild to quite severe. For reasons unknown, a head tilt often persists long term or indefinitely after resolution of the other clinical signs of vestibular disease.

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.

< div class='tao-gold-member'>

Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Peripheral and Central Vestibular Disorders in Dogs and Cats
Premium Wordpress Themes by UFO Themes