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CANINE VESTIBULAR DISEASE (CVD)

• Vestibular Clinical Signs

  • Head tilt - ipsilateral

  • Leaning/falling/tilting - ipsilateral

  • Ataxia

  • Vomiting and salivation -usually more prominent in peripheral disease

  • Nystagmus - labelled according to fast phase that is away from lesion

  • Strabismus - ipsilateral

Vestibular Diseases in Animals
by R.M. Clemmons, DVM, PhD 
Associate Professor of Neurology & Neurosurgery

All veterinary species suffer from various forms of vestibular disease. Many of which require only recognition, while others represent significant diagnostic challenges. Although there are a number of diseases which can affect the vestibular system, generally we can break them down anatomically into peripheral and central disorders. With certain exceptions, peripheral diseases bear a better prognosis in most species than central vestibular disease. Partially due to this concern, vestibular diseases represent a large number of neurologic referrals. Often, it is only reassurance that the problem will pass that is necessary. Recognition of when to intervene is as important as when not too. 

The cardinal signs of unilateral vestibular disease are head tilt, nystagmus (spontaneous abnormal eye movements), circling (toward the lesion in "tight" circles), and incoordination. This is because the vestibular system is an important part of the CNS balance control system. In order for animals to know how they are oriented in space, three neural systems must be functioning. The vestibular system, through the stimulus-response of the hair cells in the semicircular canals, reacts to angular acceleration and deceleration. The visual system allows the animal to focus on the horizontal and vertical, orienting in space. Finally, gravity is detected by pressure receptors in the skin, orienting the animal on up and down. While the vestibular system is very important, it requires at least 2 of these orienting systems to function for the animal to negotiate within its environment. This can be important with vestibular disease, since, in acute disease, the nystagmus prevents the eyes from focusing on the horizon, effectively eliminating spacial orientation. 

The anatomic structures involved in the vestibular system include the hair cells in the saccule and utricle (containing the semicircular canals), the vestibular portion of CN VIII, the vestibular nuclei in the brainstem and the flocculonodular lobe of the cerebellum. The vestibular nuclei send fibers forward in the medial longitudinal fasciculus (MLF) which coordinates ocular movements, projects fibers to the spinal cord as the vestibulospinal tract and descending MLF, projects fibers to the cerebellum, and sends fibers to various structures in the brainstem including the emetic center. Involvement of any of the portions of the vestibular system will result in signs of dysfunction. Most lesions result in loss of function and, hence, are ablative in nature. The signs develop due to the imbalance existing between the normal and abnormal sides. 

The nystagmus seen in vestibular disease can be helpful in localizing the disease process. While horizontal and rotatory nystagmus can be seen with disease anywhere within the vestibular system, vertical and positional nystagmus are almost exclusively seen with central vestibular diseases. Moreover, horizontal nystagmus from peripheral vestibular disease oscillates with the fast-phase away from the direction of the head tilt. With central vestibular disease (particularly of the cerebellum), however, the fast-phase is toward the lesion. So although horizontal and rotatory nystagmus are not specific for peripheral disease, they are compatible with it. Vertical and positional nystagmus suggest the lesion is within the CNS and indicate the need for a thorough neurologic work-up. 

Vestibular diseases can be classified into three major disease processes: idiopathic vestibular disease, inner ear disease, or central vestibular disease. The former 2 represent common forms of peripheral vestibular disease which need to be separated from each other and from central vestibular disease. 

Idiopathic Vestibular Disease: 

All cranial nerves have the potential to develop specific syndromes which are clinically classified as idiopathic disorders. This is probably due to the fact that each cranial nerve represents a unique developmental anatomy from their respective brachial arches. This also gives them an unique antigenic signal allowing very specific immune attack upon them. Idiopathic vestibular disease represents one of these cranial nerve syndromes. 

Clinically, idiopathic vestibular disease presents as an acute onset of vestibular signs with severe imbalance, due to its sudden onset and the severe nystagmus which is associated with the onset of the disorder. Since the eyes are unable to fix on the horizon and the vestibular mechanism is defective, there is severe vertigo. This often results in the rolling and rolling described by the owners. This can be mistaken for a seizure, which it is not. During the early phases of idiopathic vestibular disease, the patient often experiences nausea to the point of frequent vomiting and inappetence. The head tilt will be toward the side of dysfunction and the nystagmus will be horizontal or rotatory with the fast-phase away from the head tilt. If supported, there are no other neurologic deficits and proprioception is normal. 

The diagnosis of idiopathic vestibular disease is tentatively made by the presence of acute clinical signs in the absence of other physical findings. The minimum data base include physical examination, otoscopic examination and neurologic examination. The lack of findings (other than the peripheral vestibular signs) supports the diagnosis. The signs of idiopathic vestibular disease are regressive, meaning that they disappear without treatment over time. As such, the fact that the disease is self-limiting is how the final diagnosis is achieved. The nystagmus will usually improve or disappear all together within 3-5 days of the onset. The patient will, then, improve in their imbalance and be more able to function normally. This improvement will continue until minimal deficits will remain. It is possible that there will be a residual head tilt. If the head tilt persists beyond 6 months following the onset of signs, it is likely to be permanent. 

There is no treatment which will hasten the recovery from idiopathic vestibular disease. Corticosteroids probably do not offer an effective treatment. On the other hand, since idiopathic vestibular disease may represent an immune disease, anti-oxidant steroids (such as Solu Medral) may decrease severe symptoms. During the early phases, anti-vertigo drugs might make the patient more comfortable. Generally, I use diphenhydramine at 2-4 mg/kg every 8 hours as needed. Diphenhydramine is a centrally active anticholinergic, antihistamine which helps reduce vertigo and nausea. Assuming that the regressive course becomes evident, then I monitor the patient periodically for the signs of continued improvement. 

Antidotal evidence suggests that idiopathic vestibular disease may represent toxicity to eating certain strains of lizards. Owners often notice the cat with a lizard in its mouth just prior to the onset of clinical signs. However, experimental feeding of the suggested lizard species to cats does not lead to the disease. It is still possible that laboratory conditions do not mimic field conditions. On the other hand, idiopathic vestibular disease occurs in many animals and in animal species where exposure to lizards plays no role in the condition. It is most likely that idiopathic vestibular disease is an immune-related condition affecting the unique antigens presented by the vestibular nerve. It can recur and is often more severe on recurrence. 

Inner Ear Disease: 

Many different problems result in inner ear disease; however, the clinical signs caused by these diseases are similar, indicating the location of the disease rather than the specific cause. These signs are those of peripheral vestibular dysfunction, including head tilt, nystagmus, circling and imbalance. On the other hand, since the diseases which cause inner ear disease are usually slower in evolution, these signs are generally less severe than with idiopathic vestibular disease. In addition to the vestibular signs, there are also varying degrees of facial nerve dysfunction and often Horner's syndrome. Anatomically, the facial nerve and the sympathetic fibers heading to the eye pass near the inner ear in the osseous petrous temporal bone. Damage of these neural structure, in addition to the damage of the vestibular nerve is a hallmark for inner ear disease. It is possible to affect both the facial and vestibular nerves together in the calivarium, but it is rare to see Horner's syndrome from central nervous system disease. As such, Horner's syndrome suggests that the disease in process is in the peripheral C8-T2 nerve roots, the vagosympathetic trunk, the inner ear or within the orbit. When Horner's syndrome is seen in combination with vestibular disease and facial nerve disease, the location must be in the peripheral vestibular system in the region of the osseous petrous temporal bone. 

The signs of facial nerve dysfunction include paresis or paralysis of the muscles of facial expressions (lack of ear movement, lack of blink and lack of buccal muscle reaction on palpation). This leads to deficiency of the vibrissa reaction, decreased to absent menace response and diminished to absent palpebral response. In addition, the facial nerve supplies parasympathetic innervation to the lacrimal gland of the eye. As such, peripheral facial nerve disease can lead to diminished tear production in the eye on the affected side. This can be rather catastrophic in inner ear disease where the facial nerve dysfunction results in the inability to close the eye, while also decreasing tear production. As such, every dog with inner ear disease should have a Schirmer's tear test run on the eyes and appropriate treatment instituted if tear production is deficient. 

Horner's syndrome varies among species. In small animals the ocular signs predominate, including myosis, ptosis and enophthalmos. In horses, the signs of Horner's syndrome are expressed primarily as excessive sweating on the affected side of the face. In cattle, there is a lack of sweating on the muzzle of the affected side. 

The most common cause of inner ear disease in all species is secondary in inner ear infection. Most of these represent bacterial extension from otitis media which can arise from hematogenous spread from bacteremia or from spread up the eustachian tube to the middle ear. Luckily, these infections, once recognized, can often be successfully treated. Other causes of inner ear disease may not be treatable, including fungal infections and neoplasia. Therefore, it is generally best to "treat-for-the-treatable" when dealing with inner ear disease, using appropriate antibiotic therapy. 

The minimum data base for diagnosis of inner ear disease includes physical and neurologic examination, Schirmer's tear test, otoscopic examination (with culture of the external ear canal, if indicated), pharyngeal examination, CBC and urinalysis. If there is evidence of cardiac murmur, then cardiac ultrasound should be performed. Skull radiographs are then necessary to evaluate the degree of change in the osseous structures of the inner ear. This will be helpful in making the diagnosis and in monitoring the response to treatment. 

The treatment of bacterial inner ear infection must consider the fact that the disease represents bone infection. As such, the antibiotic must be able to penetrate bone, develop good tissue concentrations (including the blood-ear barrier) and, preferably, be bactericidal in action. Many veterinarians use enrofloxacin as their antibiotic of choice. I find that enrofloxacin is great for treating gram negative infections in the lung, but it may not reach tissue concentrations within neural structures like the inner ear. It needs additional help to do this. Therefore, if I do not use my treatment of choice, I will add a sulfa drug to enrofloxacin to take advantage of the synergistic effect of sulfa drugs. My antibiotic regime of choice is cephalosporins and sulfa drugs (sulfadimethozine) in combination. This meets the criteria for the ideal therapy for inner ear disease. It is excellent in treating gram positive bacteria, which are the most common organisms infecting the inner ear. Since this is a bone infection, the treatment must be continued for 6-8 weeks, minimum. The most common cause of treatment failure is not treating long enough. 

Central Vestibular Disease: 

Whenever anything else is seen other than the signs above, one must consider the likelihood that the problem is due to central vestibular disease. Additional cranial nerve deficits, proprioceptive deficits and motor deficits indicate brainstem damage affecting the vestibular nuclei and sensor and motor pathways which course through the vestibular region of the brainstem. In addition, the nystagmus seen in central vestibular disease will often be vertical or positional in nature, supporting the location of the disease process within the brainstem or cerebellum. If there is whole body and head tremors, the lesion is likely to be within the flocculonodular lobe of the cerebellum. While diseases which affect the peripheral vestibular system are usually good diseases; that is, diseases which regress without treatment or which respond to appropriate antibiotic therapy, most central vestibular diseases carry a less optimistic prognosis. 

The major causes of central vestibular disease are inflammatory/infectious diseases or neoplasia. Organophosphate intoxication, liver disease (with metabolic brainstem degeneration) and thiamine deficiency can occasionally result in central vestibular disease (depending upon the species of animal), but these causes are far less than the inflammatory or neoplastic causes. In dogs, canine distemper virus, granulomatous meningoencephalitis, toxoplasmosis, neosporidiosis, aspergillosis, cryptococcosis, steroid-responsive meningoencephalitis, Lyme's disease, Rocky Mountain spotted fever and ehrlichiosis are the most common inflammatory and infectious diseases recognized. In the cat, FeLV, FIP, and cryptococcosis are the most common infectious diseases. Any of the primary brain tumors can occur in dogs, while only meningiomas are common in cats. Cats who are not eating and stressed can easily develop thiamine deficiency and this should not be overlooked in treating sick cats with vestibular signs. 

Diagnosis of central vestibular disease involves the minimal data based for inner ear disease, but must be expanded to include a chemistry profile, a CSF tap and analysis (including species specific titers) and, often, advanced brain-imaging techniques, such as MRI examination. Since CSF cytology is important in assessing central vestibular disease and advanced imaging techniques are needed, central vestibular disease crosses "the referral line", the point in assessing disease which may require the interaction or interpretation of a neurologist. 

The treatment and prognosis for central vestibular disease depends upon the cause. In neoplasia, biopsy may help determine whether radioablative surgery might be useful. Unfortunately, the brainstem is not an area amenable to conventional neurosurgery. In small animals, bacterial infections causing central vestibular disease is uncommon. Rickettsial infection is also rare. In cats, cryptococcosis may respond to therapy whether with remission or control of the neurologic signs. In dogs, fungal diseases usually progress in spite of vigorous treatment. Toxoplasmosis may be controllable for a period in the dog and treatable in the cat. Canine distemper virus infection may run its course and stop or be chronic and progressive. FeLV and FIP infections are generally, rapidly progressive. Granulomatous meningoencephalitis (GME) will respond temporarily to corticosteroid therapy, but ultimately progress. Steroid-responsive meningoencephalitis can be controlled with medication for long periods in the dog. Finally, organophosphate intoxication and thiamine deficiency may respond to appropriate therapy. 

So, while central vestibular disease has many causes, in the absence of specific disease processes, there is limited hope for successful treatment and that hope is often based upon the response of the animal to medical management. The medical management is largely based upon the responsiveness of the disease process to corticosteroids. In other words, there are many causes of central vestibular disease, but often only one treatment approach. If the client understands this, it is possible to treat central vestibular disease without a specific diagnosis, realizing that the response to therapy can suggest whether the disease was "good" or "bad". The treatment approach that I use when a specific diagnosis cannot be made (either because the patient is too ill to undergo the diagnostic test or the client cannot afford them) is to treat with corticosteroids (usually oral prednisolone) and antibiotics (doxycycline and sulfadimethozine). The prednisolone will reduce inflammation while the doxycycline can help control bacterial and rickettsial disease while the sulfadimethozine may help control protozoal infection. I take a more pro-active approach in cats, since toxoplasmosis and cryptococcosis might be treatable. Therefore, I prefer to always perform CSF tap and analysis in cats with central vestibular disease, particularly when they also exhibit active chorioretinitis. I also add parenteral thiamine therapy when treating cats. 

We have made important strides in understanding the breadth of central vestibular diseases. There are new promising approaches which may help treat more of the diseases than we have previously been able to treat. New antifungal drugs offer hope in controlling CNS fungal infections. More potent cytotoxic drugs may help diminish the effects of GME in the dog. Computer-Assisted Radioablative surgery offers hope in treating brainstem tumors. On the other hand, we have a long way to go. These new methods are expensive and not always available to every pet owner. We are, however, investigating whether natural medicine might be useful adjunctive therapies in many vestibular diseases. These approaches might reduce the cost and improve the outcome and prognosis for many patients. 

© University of Florida 1997 

Vestibular Syndrome

There is a syndrome, variously referred to as peripheral vestibular syndrome (the current "preferred name"), geriatric vestibular syndrome and idiopathic vestibular syndrome. This disorder is more common in older dogs and thus the name geriatric vestibular syndrome -- but it can occur in middle aged dogs, too, so the name was changed. Idiopathic just means "happens for no known cause" -- so it is a good name but not the preferred one. It does sum up the situation well, though. For some reason dogs can suddenly develop vestibular disease. The problem seems to be due to inflammation in the nerves connecting the inner ear to the cerebellum (which controls balance and spatial orientation). It usually lasts between a couple of days and three weeks. A few dogs have residual signs beyond this time, such as a head tilt. This disease normally affects dogs that seem normal up until the signs appear. Then there is sudden loss of balance with many dogs unable to even stand up. Rythmic eye motion known as nystagmus is usually present. Dogs may be nauseous from the "sea sickness" effect of vestibular disease. Most dogs will not eat or drink unless hand fed or given water by hand because they have a hard time with the fine motor movements necessary to eat or drink from a bowl. As long as they are nursed through this condition almost all dogs will recover. There is no known treatment. Some dogs do have relapses but most do not. 

Peripheral vestibular disease can be confused with anything that will cause cerebellar damage or inner ear disease. Inner ear infections are probably the most common cause of similar symptoms and if recovery does not progress satisfactorily it is a good idea to do whatever testing seems necessary to rule out inner ear problems, such as ear examination and X-rays. Cancer affecting the cerebellum, the peripheral nerves to the cerebellum or the inner ear can cause similar signs. In golden retrievers lymphoma is a common cancer problem that can cause CNS signs. Trauma is a possible problem that could be confused with peripheral vestibular syndrome if brain damage occurs. Granulometous meningoencephalitis (GME). Infarcts (blood clotting leading to lack of circulation in part of the brain) occur in some dogs. If the damage to the brain is minimal then recovery may occur quickly. If the damage is severe, recovery may not occur at all. I do not know the incidence of infarcts affecting the brain in dogs but I think it is pretty low. 

Even when dogs do not recover fully from peripheral vestibular syndrome they normally have a good life. They adjust to residual problems like head tilts and do not seem all that bothered by them. If progress towards recovery is not evident, then the other disorders mentioned above need to be considered. 

VESTIBULAR DISEASES OF CATS AND DOGS

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-abismus 

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. 


Richard A. LeCouteur, BVSc, PhD, DACVIM (Neurology), DECVN 
University of California, Davis
Davis, CA, USA
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