The vestibular system has two main functions:1) To maintain the visual image by stabilizing the eyes in space during head movement, and 2) To stabilize the position of the head in space--thus ensuring that the position of the body is stable.

Clinical Signs of Vestibular Disease

Vestibular dysfunction causes varying degrees of loss of equilibrium causing imbalance and ataxia. Strength is not affected, and therefore paresis is not observed. As a rule, the disturbance is unilateral or asymmetrical, and the signs are those of an asymmetrical ataxia with preservation of strength. Unilateral vestibular signs may result from either central (brain stem) or peripheral (labyrinth) disease. Signs of vestibular disease include: falling, rolling, tilting of the head, circling, nystagmus, positional strabismus (deviation of one eye in some head positions), and an asymmetrical ataxia.

Nystagmus

Disturbed vestibular input to the neurons innervating extraocular eye muscles results in abnormal nystagmus. Nystagmus probably occurs at some time during all types of vestibular disease. Nystagmus is an involuntary rhythmic oscillation of the eyeball that nearly always affects both eyes equally. Typically, nystagmus consists of a slow phase in one direction and a fast phase in the other. It is customary to describe nystagmus in terms of the fast phase, despite the fact that in most cases the slow phase may be directed towards the affected side. Nystagmus tends to occur early in the course of peripheral vestibular disease, and to disappear later.

Physiological nystagmus may be induced in normal animals. It occurs with normal turning of the head from side to side, or up and down (vestibular in origin), or after rotation (postrotational nystagmus). If nystagmus occurs when the head is stationary, and there is no rotation or movement of the surroundings, it is called spontaneous nystagmus. Spontaneous nystagmus usually is pathological in origin and may be horizontal, rotatory, or vertical in direction. If nystagmus occurs only when the head is placed in an unusual position (e.g., laterally or dorsally), it is known as positional nystagmus. Nystagmus that consists of eye movements of the same velocity in each direction is termed pendulous nystagmus, and is not of vestibular origin. Pendulous nystagmus usually is associated with visual pathway deficits.

Abnormal Posture and Ataxia

Loss of co-ordination between head, trunk, and limbs, results in loss of balance. This may result in a head tilt. The trunk may fall, or even roll, to one side. The trunk may be flexed laterally. Animals tend to circle. These usually are circles with a small radius. It may be possible to elicit mild hypertonia and hyperreflexia in the limbs on one side.

An animal will often fall when attempting to shake its head. Vision will assist an animal to compensate for a vestibular system deficit. Blindfolding an animal with a vestibular lesion may accentuate the clinical signs.

Postural Reactions--Strabismus

When the head is extended in a tonic neck reaction, the eyeballs should remain in the center of the palpebral fissure in dogs and cats. This often fails to occur on the side of a unilateral vestibular disturbance, and results in a ventrally deviated eyeball. Occasionally, in vestibular disease, an eyeball is noticed to deviate ventrally or ventrolaterally without extension of the head and neck. This appears as a lower motor neuron strabismus, and may be corrected by moving the head into a different position, or by inducing the patient to move its eyeballs to gaze in different directions. This is referred to as vestibular strabismus. The ventrally deviated eyeball is on the side of the vestibular lesion. Occasionally, the opposite eyeball may appear to be deviated dorsally.

Paradoxical Central Vestibular Syndrome

Unilateral lesions of the peripheral vestibular system produce a head tilt towards the side of the lesion. With few exceptions, the same occurs with lesions of the central components of the vestibular system. Exceptions to this rule are therefore termed "paradoxical". Some unilateral lesions of the central vestibular pathways, especially unilateral involvement of the flocculonodular lobe of the cerebellum or the supramedullary part of the caudal cerebellar peduncle, produce a head tilt and ataxia directed toward the side opposite to the lesion, and a nystagmus with the fast component towards the side of the lesion. Such lesions usually are space-occupying lesions. Usually these lesions will produce postural reaction deficits or additional cranial nerve abnormalities on the affected side, which aid in determining on which side a lesion is located.

Bilateral Vestibular Disease

Bilateral peripheral vestibular disease with complete loss of function is characterized by symmetrical ataxia and loss of balance of either side, with strength preserved. Postural asymmetry is not present. A characteristic "side-to-side" head movement often accompanies these signs. Abnormal nystagmus is not observed, and with bilateral destruction of the receptor organs, normal vestibular nystagmus cannot be elicited by head movement or caloric testing.

Peripheral Vestibular Disease

Peripheral lesions involve the middle and inner ear. Middle ear (bulla tympanica) lesions usually produce head tilt (ipsilateral to the lesion) only, in the absence of other signs. Horizontal or rotatory nystagmus also may be seen. Inner ear disease, which actually involves the receptors and vestibular nerve within the petrosal bone, usually produces other signs in addition to the ipsilateral head tilt--falling, rolling, circling, nystagmus, positional strabismus, asymmetrical ataxia. Horner's syndrome (miosis, ptosis, enophthalmos) of the ipsilateral eye may be present with either middle or inner ear disease in dogs and cats, because the sympathetic trunk passes through the middle ear in close proximity to the petrosal bone.

The facial nerve may be affected in inner ear disease, as it courses through the petrosal bone in contact with the vestibulocochlear nerve. The primary characteristics of unilateral peripheral vestibular disease are: asymmetrical ataxia without deficits in postural reactions, and a horizontal or rotatory nystagmus that does not change in direction with different head positions. The fast phase of the nystagmus is directed away from the affected side.

Central Vestibular Disease

Any signs of brain stem disease in association with vestibular signs indicate that central involvement is present. The most frequent differentiating feature is a deficit in postural reactions, as central vestibular lesions most often result in paresis or loss of conscious proprioception. Alterations in mental status, or deficits in Vth or VIIth cranial nerves, also may be indicative of central disease. Nystagmus may be a key to differentiating central from peripheral disease. Nystagmus occurs in most central vestibular syndromes, and appears to be a permanent deficit. It is a positional nystagmus; therefore it may be present in some head positions (with respect to gravity), but not in others. Also the nystagmus may vary in direction with change in head position. Vertical nystagmus in any head position is most consistent with central vestibular disease.

DISEASES CAUSING PERIPHERAL VESTIBULAR DYSFUNCTION

Idiopathic Vestibular Disease

This is an acute vestibular syndrome of cats of all ages and older dogs. There is no evidence of inflammatory disease in affected animals. The signs appear suddenly, and often result in severe dysfunction and inability to stand and walk. In a few days the affected animals tend to stabilize and improvement continues for several weeks

Otitis Interna (or Labyrinthitis)

Labyrinthitis refers to inflammation of the inner ear that results in dysfunction of the membranous labyrinths. This disorder is almost always an extension of otitis media. Retrograde infection may occur via the Eustachian tubes. Another source of infection of middle ear structures is hematogenous spread. Medial extension of middle ear infection to involve meninges may occur, especially in cats.

Aural Cholesteatoma

Aural cholesteatoma may accompany otitis media. A cholesteatoma is a form of epidermoid cyst. It appears as a laminated structure composed of layers of keratin, and rests on a fibrous stroma of inflammatory granulation tissue. The masses may form from pockets of the tympanic membrane, which became adherent to the inflamed middle ear mucosa.

DISEASES CAUSING CENTRAL VESTIBULAR DYSFUNCTION

Neoplasia

Neoplasms of the cerebellomedullary angle affect the vestibular system. Neoplasms may be located at the surface of the parenchyma (e.g., meningioma, neurofibroma, medulloblastoma, choroid plexus papilloma or malignant lymphoma), or may be located within the parenchyma (e.g., glioma). Neoplasms at these locations occur in animals of all ages; however, young dogs appear to be susceptible.

Infection/Inflammation

Any cause of meningo-encephalitis may result in involvement of central vestibular structures. Reported causes include canine distemper, feline infectious peritonitis, toxoplasmosis, cryptococcosis and granulomatous meningo-encephalomyelitis. Aberrant parasitic migration may produce severe signs of vestibular disturbance.