The vestibular system has 2 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 disease produces 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. It is important to differentiate central from peripheral disease because of the differences in treatment and prognosis. 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 extra-ocular 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 will 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 (post-rotational 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 is usually 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 is usually associated with visual 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 are usually 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 will 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 LMN strabismus, and can 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 will 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 are usually 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. There is no postural asymmetry. 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 ossea) 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 brainstem 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 VIth cranial nerves, are also indicative of central disease. Nystagmus may be a key to differentiating central from peripheral disease. Nystagmus occurs in most central 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. In one study 80% of affected cats (in the USA) were diagnosed in the months of July and August. Both dogs and cats have signs of peripheral vestibular involvement. Vomiting is occasionally seen in dogs. 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. Residual deficits such as mild head tilt may persist, and blindfolding or darkness will cause a re-occurrence of signs well after apparent recovery has occurred. It is important to distinguish this idiopathic benign disorder, which resolves spontaneously without therapy, from otitis media-interna, which requires vigorous therapy, and may produce recurrent or persistent signs. The idiopathic disease is characterized by a peracute onset of head tilt, asymmetrical ataxia, and horizontal or rotatory nystagmus, in the absence of facial paresis, Horner's syndrome, or signs of CNS involvement. An absence of otitis externa, in the presence of normal tympanic membranes, and normal radiographs of the temporal bones, further support this diagnosis. The cause of this idiopathic disorder remains undetermined. Prognosis for spontaneous recovery is good; however, recovery may require 2-4 weeks. Re-occurrence may be seen, especially in dogs, either on the same or the opposite side. There is no evidence that treatment of any type alters the course of the disease.

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. The majority of infections are caused by bacteria, including Staphylococcus spp., Streptococcus spp., Proteus spp., Pseudomonas spp., Enterococcus spp. and Escherichia coli. Occasionally, yeast infection (e.g., Pityrosporum spp. and Candida spp.) is observed. Rarely, fungal infection may be confined to the middle ear (e.g., Cryptococcus sp.). Foreign bodies such as grass awns may initiate inflammation and predispose to secondary bacterial infection. Varying degrees of vestibular dysfunction accompany labyrinthitis. Ipsilateral head tilt, nystagmus (usually rotatory), and ataxia are almost always present. Circling, falling, and rolling, may be seen in more severely affected animals. Ipsilateral facial paresis/paralysis and Horner's syndrome may occur. Because the facial nerve contains the parasympathetic preganglionic neurons that modulate lacrimal gland secretion, animals with labyrinthitis may have decreased tear production and develop ipsilateral keratoconjunctivitis sicca. Ipsilateral hemifacial spasms, resulting from "irritation" of the facial nerve, have been reported in dogs in association with otitis media. Deafness, resulting from involvement of the cochlear nerve, may accompany otitis interna. The diagnosis may be confirmed by otoscopic examination and skull radiography. Otoscopy may reveal otitis externa, and evidence of erosion or rupture of the tympanic membrane. Fluid in the middle ear may produce discoloration or bulging of the tympanic membrane. Inflammatory exudate or fluid should be submitted for culture and sensitivity testing. Fluid may be obtained by either aspiration or myringotomy. Radiographic examination of the temporal bones may reveal fluid within the tympanic cavity, or osteitis, sclerosis, or erosion of the tympanic bulla. CT images are more sensitive in outlining these alterations. The prognosis is usually favorable with prolonged oral antibiotic therapy, where selection is based on culture and sensitivity studies. In more chronic cases, surgical drainage of the middle ear may be required.

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. Clinical signs in affected dogs included head tilt, loss of balance, deafness, and difficulty and pain when eating or opening the mouth. Typically facial nerve involvement or Horner's syndrome is not present. Cholesteatomas may be responsible for extensive resorption and remodeling of temporal bone seen radiographically in some dogs.

DISEASES CAUSING CENTRAL VESTIBULAR DYSFUNCTION

Signs of peripheral vestibular disease, in the absence of deafness, have been observed in several breeds, including English cocker spaniels, German shepherd dogs, Tibetan terriers, and Burmese kittens. Severe head tilt, circling, and falling or rolling, may be noted from birth to 4 months of age. Nystagmus is rarely present. The cause is undetermined. Pathological lesions have not been demonstrated. Prognosis is guarded, as clinical signs may regress completely, re-occur, or remain static. There is no effective treatment.

A congenital condition characterized by early onset of deafness and vestibular dysfunction has been reported in Doberman pinscher puppies. Signs of vestibular disease become evident between birth and 10 weeks of age. Puppies improve with age however, relapses may occur. Deafness occurs in all affected puppies. Pathological examination confirms loss of auditory sensory hair cells in the cochlea, and otoconial abnormalities or absence in maculae. The disorder in Doberman pinschers may have an autosomal recessive mode of inheritance. A similar disorder has been identified in beagle and Akita puppies, and in Siamese kittens. Congenital nystagmus in the absence of vestibular disease occurs sporadically in puppies. The nystagmus is usually pendulous, and resolves spontaneously. It has also been seen in Belgian shepherds with incomplete development of the optic chiasm. Congenital pendulous nystagmus may be "normal" in Siamese cats, where it is associated with genetically-determined abnormalities of the lateral geniculate nucleus and optic pathways.

Polyneuropathy-neuritis

Unilateral or bilateral signs of peripheral vestibular disease may occur in association with facial paresis or paralysis, in mature dogs without evidence of otitis media-interna. Some affected dogs are hypothyroid or have a pituitary chromophobe adenoma. Thyroid hormone-replacement therapy has proved ineffective. Affected dogs have been observed without associated endocrinopathy, and have resolved spontaneously.

Trauma

Cranial trauma may result in signs of peripheral vestibular disease secondary to fractures of the temporal bone or tympanic bulla. Facial paralysis may accompany petrosal bone injury.

Degenerative

Prolonged therapy with aminoglycoside antibiotics may result in degeneration of the labyrinth receptors of the vestibular or auditory systems, or both.

Neoplasia

Neoplasms involving the temporal bone may produce peripheral vestibular disease, often with facial paralysis or paresis. Fibrosarcoma, osteosarcoma, chondrosarcoma, and squamous cell carcinoma have been reported. Squamous cell carcinoma and ceruminous gland adenocarcinoma may involve adjacent soft tissues. Squamous cell carcinoma has been reported to be is the most frequently occurring tumor affecting the middle ear of cats. Papillary adenomas and extension of adnexal or ceruminous gland tumors appear to be more common in dogs. Rarely, middle ear tumors may extend directly into brainstem. Neurofibromas of the vestibulocochlear nerve usually cause signs of unilateral vestibular disturbance prior to compression of brainstem. Inflammatory polyps have been seen in the middle ear of cats. Resection is recommended, however re-occurrence may take place.

Central Vestibular Disease

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. 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, or granulomatous meningo-encephalomyelitis). Neoplasms at these locations occur in animals of all ages; however, young dogs appear to be susceptible. Thiamine deficiency may produce a mild vestibular ataxia as the earliest sign of degeneration.