Disorders of the Vestibular System

Common Neurologic Problems

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Disorders of the Vestibular System

Kenneth Lee Schunk, DVM*

The vestibular system is the sensory system that detects the position and the motion of the body in space by integrating information from peripheral receptors located in the inner ear. Normal function of the vestibular system is essential for the coordination of motor responses, eye movement, and posture. Disorders of the vestibular system are common in small animals and result in a clinical syndrome characterized by head tilt, ataxia with preservation of strength, and nystagmus . These signs may result from disorders of either the peripheral or central vestibular system. Because most diseases involving the peripheral vestibular system can be managed successfully compared with disorders affecting the central vestibular system, it is essential for the clinician to differentiate them based on the results of the neurologic examination and diagnostic tests. In addition, the specific cause of the vestibular dysfunction should be identified, because certain diseases require specific therapies and have different prognoses. FUNCTIONAL ANATOMY AND PHYSIOLOGY

The signs of vestibular dysfunction are better understood after a review of the functional anatomy and physiology of the vestibular system. 16. 18. 26 Anatomically, the peripheral vestibular system is composed of the membranous labyrinth, vestibular ganglion, and the vestibular portion of cranial nerve VIII. The central vestibular structures include the vestibular nuclei and the vestibular portion of the cerebellum. The inner ear, or labyrinth, is located within the petrous temporal bone and is made up of the bony labyrinth and the membranous labyrinth. The bony labyrinth consists of the vestibule, three semicircular canals, and the cochlea, and contains perilymph, which is a fluid similar in composition to cerebrospinal fluid (CSF). Within the bony labyrinth is the membranous labyrinth, which consists of communicating tubes made up of fine mem*Assistant Professor, Tufts University School of Veterinary Medicine, North Grafton, Massachusetts Veterinary Clinics of North America: Small Animal Practice-Vol. 18. No.3. May 1988

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KENNETH LEE SCHUNK

branes. The membranous labyrinth contains a viscous fluid called endolymph and consists of the saccule and utriculus, three semicircular ducts, and the cochlear duct. The vestibular receptors are located within the membranous labyrinth. The cochlea and cochlear duct are components of the auditory system. Endolymph is thought to be produced by the stria vascularis of the cochlear duct and is drained into the venous sinuses of the dura mater by the endolymphatic duct. 26 Perilymph, which may be secreted by arterioles located in the periosteum surrounding the labyrinth, is drained into the subarachnoid space by the perilymphatic duct. Abnormal production or drainage of either fluid may result in dysfunction of the entire labyrinth. Each part of the membranous labyrinth of the vestibular system contains a specialized receptor that is innervated by the vestibular portion of cranial nerve VIII. Within each semicircular duct, there is an enlargement called the ampulla, which houses the peripheral receptor. This receptor consists of a ridge of connective tissue (crista) and neuroepithelial cells covered by a gelatinous material (cupula). The neuroepithelium is composed of supporting cells and hair cells. Each hair cell has a large riumber of modified microvilli (stereocilia) and a single large cilium (kinocilium) projecting into the cupula. When the head undergoes angular acceleration or deceleration, the endolymph lags behind owing to inertia and pushes the cupula. This distortion of the cupula produces a potential in the hair cells that is detected in cranial nerve VIII. Deviation of the stereocilia toward the kinocilium results in stimulation of the hair cell; if the stereocilia is directed away from the kinocilium , the hair cell is inhibited., This arrangement results in directional sensitivity of the hair cell. Each semicircular duct on one side can be paired to a semicircular duct on the opposite side. Movement in one plane stimulates the activity of the hair cells of one duct and inhibits their activity in the opposite duct of the synergiC pair, resulting in a bilateral indication of head movement. The orientation of the three semicircular ducts permits movement of the head in any plane or angular rotation to be detected by these receptors . When the veloCity of movement is constant, receptors in the semicircular ducts are no longer stimulated. Therefore, the dynamic function of the semicircular ducts enables animals to detect motion of the head in space. In the utriculus and saccule, the principal receptor region is termed the "macula." It is composed of hair cells that project cilia into a gelatinous substance in which crystals of calcium carbonate (statoconia or otoliths) are embedded. Movement of the statoconia in relationship to the cilia of the hair cells results in stimulation of the vestibular neuronS. The macula of the utriculus is oriented in a horizontal direction, imd , the macula of the saccule lies in the vertiCal plane. Gravitational forces affect the position of the statoconia in relationship to the hair cells; therefore, the maculae are responsible for the sensation of the static position of the head relative to the force of gravity. In addition, linear acceleration or deceleration of the head is detected by the receptors within the utriculus. The vestibular nerve is composed of bipolar neurons with the cell bodies located in the vestibular ganglion. The dendritic Zone of the neurons is in synaptic relationship with the hair cells in the membranous labyrinth.

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The central processes course through the internal acoustic meatus and enter the rostral medulla at the cerebellomedullary angle. The majority of the axons terminate in the vestibular nuclei, which are located on either side of the dorsal part of the medulla adjacent to the lateral wall of the fourth ventricle. The remainder of the axons enter the cerebellum to terminate in the fastigial nucleus and flocculonodular lobe. Several efferent pathways of clinical significance project from the vestibular nuclei. The vestibulospinal tract descends in the ispilateral ventral funiculus and influences the activity of lower motor neurons via interneurons in the ventral gray column. 18 The interneurons are facilitatory to extensor muscles and inhibitory to flexor muscles on the same side; some interneurons cross to the opposite side and are inhibitory to the contralateral extensor muscles. The medial longitudinal fasciculus descends in the ventral funiculus to the level of the cranial thoracic spinal cord segments. The posture and motor activity of the trunk and limbs are coordinated with movements of the head via these spinal cord pathways. The vestibular nuclei also project axons rostrally to the nuclei of cranial nerves III, IV, and VI via the medial longitudinal fasciculus . This pathway permits changes in head posture to be accompanied by coordinated conjugate eyeball movements. In addition, some axons from the vestibular nuclei project to the emetic center in the reticular formation of the brain stem, whereas others terminate in the fastigial nucleus and flocculonodular lobe of the cerebellum. Several important neuroanatomic structures in close proximity to the vestibular system should be considered in animals with vestibular dysfunction. Involvement of these structures results in symptoms that can aid in the localization of lesions (see Table 1 and Fig. 1). The middle ear is often affected by diseases involving the vestibular apparatus in the inner ear. This may result in damage to the facial nerve and/or sympathetic innervation to the eyeball, which course through the middle ear. Important structures located in the brain stem in the region of the vestibular nuclei include the motor and sensory nuclei of cranial nerve V, the motor nuclei of cranial nerves VI and VII, the caudal cerebellar peduncle, the medial lemniscus, the descending motor pathways (rubrospinal, reticulospinal and corticospinal), and the vital centers within the reticular formation.

SYMPTOMS OF VESTIBULAR DISORDERS Unilateral Peripheral Disease The most common symptoms in animals with unilateral peripheral vestibular dysfunction include a head tilt toward the side of the lesion, nystagmus, and asymmetric ataxia with preservation of strength. The animal will usually lean, drift, and fall toward the affected side. Circling in the same direction as the head tilt may also occur. There may be a curvature of the trunk with the concavity directed toward the side. of the lesion. The animal may lean or walk along a wall on the affected side for additional support. Infrequently, animals with acute peripheral vestibular dysfunction

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Table 1. Clinical Data of Neural Structures Anatomically Related to the Vestibular System

0:.. PERIPHERAL VESTIBULAR SYSTEM

Structure

Anatomic Location

Function

Clinical Signs

Facial nerve

Middle ear

Motor innervation to muscles of facial expression; nasal, palatine and lacrimal glands; and sublingual and mandibular salivary glands

Sympathetic innervation of eyeball

Middle ear

Motor innervation to smooth muscles of periorbita and eyelids and dilator muscle of pupil

Ipsilateral inability to close the eyelid or move the lip or ear; decreased/absent tear production; hemifacial spasm (blepharospasm, elevation of the ear and lip with caudal displacement of the labial commissure) Ipsilateral enophthalmos, ptosis, miosis, protrusion of third eyelid (Horner's syndrome)

Facial nerve

Internal acoustic meatus and cerebellomedullary angle Pons and medulla

Same as above

Same as above

Motor innervation to muscles of mastication and sensation of the head

Motor nucleus of cranial nerve VI

Rostral medulla

Motor nucleus of cranial nerve VII

Rostral medulla

Motor innervation to lateral rectus and retractor bulbi muscles Motor innervation to muscles of facial

Caudal cerebellar peduncle

Rostral medulla

Ipsilateral atrophy of the muscles of mastication and facial hypalgesia! analgesia Ipsilateral medial strabismus and inability to retract the globe Ipsilateral inability to close the eyelid or move the lip or ear Ipsilateral ataxia with dysmetria (hypermetria)

Rubrospinal, reticulospinal, and corticospinal upper motor neuron tracts

Pons and medulla

Medial lemniscus

Pons and medulla

Reticular formation

Pons and medulla

CENTRAL VESTIBULAR SYSTEM

Motor and sensory nuclei of cranial nerve V

expression

General proprioception, special (vestibular) proprioception and extrapyramidal motor input to the cerebellum Function in the initiation of voluntary movement, maintenance of muscle tone and control of muscular activity associated with visceral function Conscious proprioception input to the thalamus Control of respiratory and cardiac functions, control of vomiting, swallowing and excretion

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Ipsilateral paresis/paralysis with upper motor neuron signs

Contralateral ataxia (mild) with loss of conscious proprioception Abnormalities in cardiac rate and rhythm, respiratory pattern, urination, defecation and swallowing

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To Flo.ecullonc,dular

Cranial Nerve

Petrous Temporal Bone

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Tympanic Bulla

Sympathlc Innervat ion to Eyeball Cranial Nerve

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CCP Caudal Cerebellar Peduncle ML = Medial Lemniscus MLF = Medial Longitudinal Fasciculus UMN Upper Motor Neuron

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Vestibulospinal Tract

5 = Molor and Sensory Nuclei of Cranial Nerve:J[ 6 = Molor Nucleus of Cranial Nerve1/[ 7 = Motor Nucleus 01 Cranial Nerve:llII

Figure 1. Diagrammatic representation of the vestibular system and related structures.

will roll to the side of the head tilt. Because vision enables the animal to partially compensate for the vestibular dysfunction, blindfolding the patient will usually make the ataxia and head tilt more obvious. There is a decrease in extensor tone of the limbs on the affected side and an increase in extensor tone of the contralateral limbs. The abnormal posture and ataxia are due to the loss of activity of the vestibulospinal tract ipsilateral to the lesion. Nystagmus is an involuntary rhythmic oscillation of the eyeballs that often has a fast and a slow phase; nystagmus should be described by the direction of the fast component. Vestibular or physiologic nystagmus occurs in a normal animal when the head is moved rapidly in any direction. During movement of the head, the eyes move away from the direction in which the head is turning and then quickly jerk back toward that direction . Therefore, the fast phase of the nystagmus is toward the direction in which the head is moving. Postrotatory nystagmus can be elicited normally in smaIr animals by rotating the animal rapidly in one direction then suddenly stopping. During the initial acceleration of the animal, nystagmus occurs and the fast phase is in the direction of the rotation. Once the velOCity of the rotation is constant, nystagmus does not occur. When the rotation is suddenly stopped, postrotatory nystagmus with the fast component opposite the direction of the rotation will be present for a short time. Postrotatory nystagmus can be used clinically in small animals to test the functional integrity of each peripheral vestibular apparatus. The duration of the postrotatory nystagmus

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should be approximately the same when the response to spinning the animal in both directions is compared. The postrotatory nystagmus is reduced or absent when the animal is rotated in the direction opposite to the side of a peripheral receptor lesion. Animals with unilateral peripheral vestibular disease often have ·horizontal or rotary nystagmus with the fast component away from the side of the lesion. The nystagmus may be observed when the head is in its normal position (spontaneous nystagmus) or when the head is fixed in an abnormal position (positional nystagmus). The direction of the nystagmus should not change regardless of the position of the head. In patients with unilateral peripheral vestibular disorders, this abnormal nystagmus usually disappears within 4 to 7 days owing to compensation dependent on functional central vestibular components. A concomitant eyelid contraction may be observed with the spontaneous nystagmus in some animals. Positional or vestibular strabismus can be seen in most animals with unilateral vestibular disease. The eyeball ipsilateral to the lesion is deviated ventrally; this can best be seen when the head is straightened and the nose is elevated. The eyeball can move in all directions and there is no paralysis of the extraocular muscles. The vestibular strabismus and abnormal nystagmus result from the vestibular dysfunction and its influence on cranial nerves III, IV, and VI. The postural reactions such as hemistanding, hopping, placing, and conscious proprioception are normal except for the righting response . Because it is often difficult to evaluate the postural reactions critically in animals with severe ataxia, serial neurologic examinations should be performed. The segmental spinal reflexes and sensory examination are usually normal; mild hypertonia and hyperreflexia may be present in the limbs contralateral to the vestibular lesion. Vomiting may occur with any vestibular dysfunction but is more common in dogs with acute peripheral vestibular disorders. This occurs because of the vestibular connections to the vomiting center in the brainstem reticular formation . Bilateral Peripheral Disease Animals with bilateral vestibular disease exhibit a symmetric ataxia and a tendency to stagger or fall to either side. The ataxia may resemble cerebellar ataxia; however, there is no intention tremor or hypermetria. The patient often has a crouched posture and is reluctant to walk. Head tilt and abnormal nystagmus are not present, and normal vestibular nystagmus and postrotatory nystagmus cannot be elicited. The~e is often a marked side-to-side head movement that is characteristic of bilateral vestibular dysfunction . The animal may also be deaf if the disease involves the auditory receptors. Central Vestibular Disease Diseases affecting the vestibular nuclei, fastigial nuclei, or flocculonodular lobes of the cerebellum result in symptoms that are similar to those seen in peripheral vestibular disorders. In addition to the vestibular symptoms, these animals usually exhibit signs indicative of brain-stem or

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Table 2. Neurologic Signs: Peripheral vs Central Vestibular Disease PERIPHERAL VESTIBULAR DISEASE

CENTRAL VESTIBULAR DISEASE

Head tilt

Head tilt

Nystagmus-Horizontal or rotary; fast phase directed away from the side of the lesion; direction not altered with changes in head posture

Nystagmus-Horizontal, rotary, or vertical; fast phase directed toward or away from the side of the lesion; may change direction

Gait-Asymmetric ataxia with leaning, falling, circling, and rolling

Gait-Asymmetric ataxia with leaning, falling, circling, and rolling; ipSilateral hypermetria; ipsilateral hemiparesis or tetraparesis; cerebellar ataxia; conscious proprioceptive deficits

Positional ventral strabismus

Positional ventral strabismus

Cranial nerve VII (if middle ear is involved)

Cranial nerves V, VI, VII, IX, X or XII may be involved

Horner's syndrome

Horner's syndrome not reported

Mental status-Alert, disoriented

Mental status-Disoriented, may show varying degrees of depression

cerebellar dysfunction. The most common clinical signs indicating that the vestibular disorder is central in origin are the following: "central" nystagmus; upper motor neuron paresis and general proprioceptive ataxia, which usually involve the liinbs ipsilateral to the lesion in the medulla; cranial nerve deficits other than .cranial nerve VII (cranial nerve VII involvement is commonly seEm with middle ear disease and does not necessarily imply a centrallesibn); and cerebellar a~axia. Vertical nystagmus, nystagmus with the fast phase directed toward . the head tilt, or nystagmus that changes direction when the position of the head is altered indicates a lesion in the central vestibular pathways. However, nystagmus resulting from disorders of the central vestibular system maybe in any direction, including nystagmus with the fast phase directed away from the head tilt. The findings on the rieurologic examination should enable the clinician to determine whether the lesion is peripheral or central. A summary of the neurologic signs differentiating peripheral from central vestibular disease is presented in Table 2. Paradoxical Central Vestibular Disease In most animals with ceritral vestibular dysfunction, the head tilt and asymmetric ataxia are toward the side of the lesion ; Occasi09ally, however, unilateral lesions involving the cerebellar medulla and the caudal cerebellar peduncle result in a head tilt and ataxia directed toward the opposite side. These are referred to as parado)(ical signs. The lesions causing paradoxical vestibular signs are .usually destructive mass lesions such as neoplasms or granulomatous meningoencephalitis. These lesions often affect the general proprioceptive system to the cerebellum, producing a general proprioceptive ataxia with dysmetria and postural reaction deficits on the same. side as the lesion. These signs can be useful in determining the side of the lesion.

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KENNETH LEE SCHUNK Primary complaint (head tilt, asymmetric ataxia, nystagmus)

~ Database (history, physical and neurologic examinations, complete blood count, / serum chemistry profil, Peripheral vestibular lesion

Central vestibular lesion'

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Radiographs of bulla and examination of tympanic membrane

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Abnormal

Normal

Rule our 1. Idiopathic vestibular disease 2. Otitis interna 3. Congenital vestibular disease 4. Polyneuropathy 5. Ototoxicity (drug-induced)

Myringotomy, culture, cytology, ± biopsy

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CSF analysis and skull radiographs

Normal

l Rule out 1. Neoplasia 2. Storage disease

Abnormal

l Rule out 1. Inflammation 2. Neoplasia

Rule out 1. Otitis interna 2. Neoplasia 3. Trauma

'The diagnoses of thiamine defiCiency and trauma are usually determined by other methods. tComputed tomography is indicated in many animals to rule out the diagnosis of neoplasia.

Figure 2. Diagnostic approach for the evaluation of animals with vestibular dysfunction.

ANCILLARY DIAGNOSTIC INVESTIGATIONS Following a detailed history and physical and neurologic examinations, the clinician should be able to determine whether the lesion involves the peripheral or central vestibular system. In order to arrive at the exact etiology of the vestibular dysfunction, a variety of ancillary diagnostic tests are often needed. Although hematologic and serum biochemical tests frequently do not contribute to the diagnosis, they should be part of the database in animals with vestibular disease. Additional diagnostic tests depend on whether the animal has symptoms indicating involvement of the peripheral or central vestibular pathways (Fig. 2). Animals with signs compatible with a peripheral lesion should undergo radiographic examination of the tympanic bullae and bony labyrinth under general anesthesia. Ventrodorsal, right and left lateral obliques, and rostrocaudal (open-mouth) views should be evaluated to detect changes in the petrous temporal bones and tympanic bullae. While the patient is under anesthesia, the external ear canal, tympanic membrane, and pharynx should be carefully examined. Samples for culture may be obtained from the horizontal ear canal if indicated. If necessary, exudate can be suctioned from the external ear canal using a Frazier suction instrument prior to examining the tympanic membrane. Only low, negative pressure should be used to avoid injury to the tympanic membrane. The tympanic membrane, which is divided into two-parts, should be visualized. The pars flaccida is the smaller, triangular portion that appears as an opaque pink membrane; it is located dorsocranial to the pars tensa. The pars tensa

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is normally pearl gray in color and translucent; the manubrium of the malleus can be visualized behind this portion. If indicated, a myringotomy using a 20-gauge, 3V2-inch spinal needle can be performed. The needle should be inserted through the edge of the eardrum at the 6 o'clock position, and samples should be obtained for culture and cytology. If fluid cannot be aspirated from the middle ear cavity, .25 to .50 ml of sterile saline can be flushed into the cavity and then aspirated and used for culture and cytology. This same procedure can be performed when the tympanic membrane is ruptured. The pharynx should also be examined in animals with peripheral vestibular dysfunction, because some diseases may spread from the pharynx to the middle and inner ears via the auditory tube. In addition, the bullae can be palpated through the pharynx. The normal bullae are rounded, smooth, and symmetric. . If a central vestibular lesion is suspected following neurologic evaluation, CSF analysis, skull radiographs, and computed tomography (CT) of the head may be necessary to determine the underlying cause. The CSF analysis should include a determination of the pressure, protein concentration, total red and nucleated cell counts, and differential nucleated cell count. If there is an increase in the total nucleated cell count, the CSF should also be cultured. Even though survey radiography of the skull is often unrewarding for most diseases, it is a noninvasive diagnostic test that allows the recognition of osseous abnormalities of the skull. Lateral, ventrodorsal or dorsoventral, and frontooccipital views are routinely evaluated; oblique views may be useful in particular cases. Currently, the most valuable method for accurate detection and localization of intracranial lesions in small animals is computed tomography. In addition, CT may proVide definitive information useful in the analysis of the type of lesion. It has been shown to be effective clinically in the detection of caudal fossa lesions in the dog. 28 In addition to these diagnostic investigations, brain-stem auditoryevoked response (BAER) and tympanometry have been used in evaluating animals with vestibular dysfunction. The BAER is an averaged recording of neural activity following the external application of click stimuli. 7 , 15, 36 The BAER can be used to evaluate brain stem as well as auditory function. Tympanometry uses impedance audiometry to evaluate the integrity of the tympanic membrane and functional condition of the middle ear. i7, 32 Conditions that alter the normal tympanograph inclqde middle ear effusion, tympanic membrane scarring or .rupture, occlusion of the auditory tube, and disarticulation of the auditory ossicles.

DISORDERS OF THE VESTIBULAR SYSTEM Peripheral Vestibular System

Idiopathic Benign Vestibular Disease in Dogs. The most common cause of unilateral peripheral vestibular disease in geriatric dogs is idiopathic benign vestibular disease (IBVD). Even though this disease may occur in

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dogs of any age, the mean age of onset is 12.5 years. 35 There is no known breed or sex predilection. Idiopathic benign vestibular disease is characterized by a peracute onset of unilateral peripheral vestibular signs. The degree of ataxia exhibited is usually moderate (able to stand, but with occasional stumbling and falling) or severe (unable to stand or walk without assistance). Circling and rolling are not common. The nystagmus almost always has a rotary component. Facial nerve dysfunction and Horner's syndrome are not present in this disease. Approximately 40 per cent of the dogs will experience transient vomiting near the onset of their vestibular dysfunction. The diagnosis of IBVD is made by eliminating other causes of peripheral vestibular dysfunction. Radiographs ofthe tympanic bulla and petrous temporal bone and examination ·of the tympanic membrane under general anesthesia are needed to rule out other etiologies. It is important to distinguish IBVD from otitis media-interna because the latter disease requires long-term antibiotic therapy and may have recurrent or residual signs. Dogs with a peracute onset of unilateral peripheral vestibular signs and without a history of trauma are diagnosed as having IBVD if physical abnormalities of the tympanic membrane and radiographic changes within the tympanic bulla and petrous temporal bone are not present. In patients in which CSF analysis has been performed, the results have been normal. Specific therapy is not recommended. Corticosteroids do not alter the clinical course of the disease and are not administered. Because the vomiting usually supsides within the first 24 to 36 hours, antimotion drugs are not necessary. The prognosis for recovery is good to excellent. Spontaneous nystagmus usually disappears in the first 3 to 4 days. Positional nystagmus in the same direction as the spontaneous nystagmus can often be detected for · an additional few days. Within the first S to 7 days, the ataxia usually improves to the extent that the dog is able to witlk without falling but leans and drifts to the affected side. The animal's gait continues to improve over the next 2 weeks. The head tilt usually resolves within the first month. Following recovery, the dog may exhibit a mild, transient disturbance of balance when stressed. Recurrent attacks are not common, but attacks may occur on the same or opposite side. The etiology of IBVD is unknown. Histologic examination of the labyrinth, vestibular nerve and ganglion, and central nervous system has not revealed any lesions. Abnormal production, circulation, or absorption of the endolymphatic fluid or a neuritis of the vestibular portion of crani;:tl nerve VIII is a possible cause. 18 Experimental labyriqthectomy produces an identical clipical syndrome with spontaneous recovery due to compensation thaf is deplOlndent on central vestibular structures. 14 . Feline Idiopathic Vestib1,llar Neuropathy. Idiopathic vestibular neuropathy is a fairly common disease in adult cats of any age. There is no breed or sex predilection. The disease may have a higher incidence in the summer and early fall. 18 . The clinical syndrome of a peracute onse.t of unilateral peripheral vestibular dysfunction is almost identical to the idiopathic disorder in dogs, with the exception that vomiting is rarely observed in cats. Occasionally,

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animals with bilateral vestibular involvement may be seen. All diagnostic tests, including physical examination of the tympanic membrane and radiographs of the petrous temporal bone and tympanic bulla, are normal. Spontaneous recovery is usually complete within 2 to 3 weeks; however, transient vestibular attacks with recovery in 1 to 3 days have been observed in cats. No therapy is recommended, and the prognosis is excellent. Otitis lnterna. A common cause of unilateral peripheral vestibular dysfunction in both dogs and cats is otitis interna. Otitis interna is usually caused by a bacterial infection that extends from the middle ear. It has been shown experimentally that the round window membrane, which separates the middle and inner ear, becomes more permeable to macromolecules with otitis media. 31 Vestibular signs occur when the middle ear inflammation affects the function of the membranous labyrinth. Most cases of otitis media are caused by bacteria and are secondary to otitis externa. Extension of disease from the pharynx via the auditory tube may also result in otitis media. Hematogenous spread of infection to either the inner or middle ear appears to be uncommon. The most common bacteria isolated are Staphylococcus spp., Streptococcus spp., Esherichia coli, Pseudomonas spp., Enterococcus spp., and Proteus spp. Other potential causes include yeasts (Pityrosporon spp., Candida spp.), parasites, and foreign bodies such as grass awns. Otitis interna may occur at any age. In one report in dogs, the mean age of onset was 8.5 years with a range of 6 months to 18 years. 35 Although otitis interna may be seen in any breed, animals predisposed to chronic otitis externa may have an increased risk. In a recent study reporting 41 dogs with otitis media-interna, 73 per cent were males; no reason was proposed for this sex distribution. 35 The rate of onset is variable, with signs appearing acutely or subacutely over a period of 2 to 3 weeks. Along with the vestibular symptoms, the owner may observe head shaking, pawing or rubbing of the ear, and an inflammatory discharge from the external ear canal. The vestibular signs are consistent with unilateral peripheral vestibular dysfunction, but the ataxia is often mild (able to walk without falling but leaning and drifting to the affected side), and nystagmus may not be present. Transient, intermittent signs of ataxia and head tilt may be reported by the owner. Vomiting may occur in dogs but is less common than in the idiopathic disease. Physical examination often reveals otitis externa. Animals with otitis media may show pain when pressure is applied in the region of the tympanic bulla. A draining fistula located caudal to the ramus of the mandible may occur secondary to chronic otitis media. In addition to the vestibular signs, facial paresis or paralysis, hemifacial spasm', and/or Horner's syndrome may be seen on neurologic examination. Facial nerve involvement was present in over half of the dogs with otitis media-interna in one report.35 Facial paralysis may be accompanied by decreased tear production and keratitis sicca, because the facial nerve contains the parasympathetic preganglionic neurons that produce lacrimal gland secretion. Unilateral hearing deficits may be present but are difficult to detect without performing specialized tests. . Bilateral otitis interna may occasionally be seen with or without

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Figure 3. Radiographic appearance of two dogs with otitis media-interna (rostrocaudal views). A, Increased opacity of the left tympanic bulla. B, Increased opacity and thickening of the left tympanic bulla.

involvement of the facial nerve. Signs of bilateral vestibular disease will be present if the involvement of the vestibular system is symmetric. However, most animals have asymmetric vestibular dysfunction resulting in some asymmetry of the vestibular signs. Total loss of hearing may occur with bilateral otitis interna. The diagnosis of otitis interna secondary to otitis media is based on the presence of physical or radiographic evidence of non-neoplastic, nontraumatic disease in the middle and inner ears. With the animal under general anesthesia, radiographs of the tympanic bulla and petrous temporal bone should be evaluated. Radiographic evidence of otitis media-interna includes increased density within the tympanic bulla and exostosis and sclerosis of the bulla and petrous temporal bone; normal detail of the bony labyrinth may be obscured (Fig. 3). The radiographs may be normal in acute infections. It is not uncommon to find bilateral radiographic changes in dogs with signs of unilateral otitis media-interna. 35 Radiographic evidence of bone lysis is uncommon in animals with inflammatory middle and inner ear disease; its presence should suggest the possibility of neoplasia. Following radiographs, the external ear canal, tympanic membrane, and pharynx should be examined. If signs of otitis externa are present, specimens for culture should be taken from the horizontal ear canal. After suctioning any debris from the external canal, the tympanic membrane should be identified. Vigorous flushing of the outer ear canal to remove debris is not recommended, because this procedure may result in perforation of the ear drum. If necessary, normal saline can be instilled to facilitate cleaning of the ear canal. Most animals with otitis media-interna have an abnormalappearing or ruptured ear drum. The tympanic membrane may appear opaque and hyperemic; fluid in the middle ear may cause the membrane to bulge laterally. Myringotomy followed by saline irrigation and aspiration of the middle ear cavity should be performed if there are radiographic changes involving the bulla or abnormalities of the tympanic membrane.

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Inflammatory polyps are common in cats with otitis media-interna. They originate within the middle ear or auditory tube secondary to chronic inflammation. 22 The polyp can often be visualized on otoscopic examination near the level of the tympanic membrane. Grossly, inflammatory polyps have a smooth or lobulated glistening surface and are pale pink-gray in color. The radiographic findings in cats with inflammatory polyps are similar to the changes seen in animals with otitis media-interna alone; occasionally, enlargement of the tympanic bulla is present, suggesting an expansile lesion. It is important to rule out the presence of a polyp in cats with otitis. media-interna, because surgical excision is necessary in their treatment. The treatment of otitis interna depends on the physical and radiographic findings. A medical approach is used initially unless radiographs reveal a moderate or marked increased density within the tympanic bulla, suggesting the presence of a large amount of exudate or an inflammatory polyp. The majority of cases of otitis media-interna can be treated successfully with long-term (4 to 6 weeks) systemic antibiotics; recurrences are common when shorter courses of antibiotics are administered. Selection of the antibiotic is based on cultures from the external and middle ears . If culture and sensitivity results are not available, treatment with chloramphenicol (25 to 50 mg per kg repeated every 8 hours) or cephalexin (30 mg per kg repeated every 8 hours) has been used successfully. Long-term administration of chloramphenicol may cause anorexia, weight loss, and blood dyscrasias in cats and should be used cautiously. Prolonged administration of aminoglycosides systemically should be avoided, because it may cause degeneration within the vestibular and auditory systems; the ototoxic signs may be difficult to detect owing to the existing disease. 29. 38. 39 Although not recommended by the author, systemic corticosteroid therapy may be used within the first 5 to 7 days to decrease inflammation within the vestibular labyrinth. 16 If otitis externa is present, topical ear medication is indicated based on the results of the culture. Even though ototoxicity may occur more easily with a ruptured tympanic membrane, topical gentamicin has been used in such cases without apparent clinical side effects. 31 Thiabendazole (Tresaderm) and miconazole (Conofite) are topical antifungal agents that are useful in the treatment of Pityrosporon spp. Nystatin (Mycostatin) may be effective in cases of candidiasis and Pityrosporon spp. Topical medication should be continued for a minimum of 2 to 3 weeks. Lateral ear resection may be necessary in some animals with chronic, recurrent otitis externa. A major indication for surgical exploration and drainage of the middle ear is lack of response to aggressive medical therapy. In chronic otitis media, the tissue lining the bulla becomes metaplastic and liyperplastic, resulting in resistance to antibiotic administration.8 Because myringotomy and aspiration of the middle ear cavity do not provide adequate drainage of the tympanic bulla, animals with large amounts of exudate within the middle ear may require surgery. It should be emphasized that the majority of animals with otitis media-interna do not have significant amounts of middle ear exudate and can be successfully treated with antibiotics. Other indications for surgical exploration of the middle ear in~lude the presence of an inflammatory polyp and osteomyelitis with lysis of the tympanic bulla

654

KENNETH LEE SCHUNK

or petrous temporal bone. The surgical procedure of choice is a ventral bulla osteotomy, which allows good visibility and exposure for biopsy, debridement, and drainage. I. 8. 23 At the time of surgery, samples for culture and sensitivity and histologic examination should be obtained. Ventral drainage is established by suturing a tube into the bulla; this tube can be used to lavage the middle ear with antibacterial solutions. Because the tympanic cavity of the cat contains a septum that divides the bulla into two compartments, part of this septum must be removed to facilitate drainage of both compartments. I The drain is usually maintained for a minimum of 5 to 7 days. It is important that proper medical therapy be used in conjunction with surgical intervention. The treatment for inflammatory polyps is surgical resection. If the polyp can be visualized in the external ear canal, it can be grasped with a hemostat and removed by traction; lateral ear resection may facilitate removal of the polyp through the outer ear canal. Because inflammatory polyps usually originate within the middle ear, ventral bulla osteotomy with excision of the polyp is the preferred method of treatment. Horner's syndrome is occasionally seen following surgical removal of an inflammatory polyp. In general, the prognosis for animals with otitis interna is good if properly treated. Recurrences following antibiotic therapy are occasionally observed. Animals with osteomyelitis involving the tympanic bulla or petrous temporal bone have a guarded prognosis. Extension of infection from the inner ear to the brain stem is a rare sequela. When this occurs, the prognosis is guarded to poor despite CSF culture and aggressive systemic antibiotic therapy. The most common residual vestibular symptom following treatment is a head tilt, which may not resolve completely. Facial nerve paralysis and Horner's syndrome are often permanent sequelae of otitis media; artificial tear preparations will be necessary if keratitis sicca is present because of the facial nerve involvement. Reduced auditory function may be permanent in some cases but will not be clinically significant unless the deficit is bilateral. Neoplasia. Tumors originating in the osseous bulla or bony labyrinth, such as osteosarcomas, fibrosarcomas, and chondrosarcomas, and those arising from adjacent soft tissues, such as squamous cell carcinomas and ceruminous gland adenocarcinomas, may result in peripheral vestibular dysfunction. 16. 24 The majority of these tumors are diagnosed in middle-aged to older dogs and cats; no breed or sex predilection exists. The rate of onset of neurologic signs may be acute or subacute over a period of several weeks. On physical examination, discharge from the external ear canal, pain on opening the mouth, and a palpable mass in the region of the tympanic bulla may be present. In addition to signs referable to unilateral peripheral vestibular dysfunction, facial weakness and/or Horner's syndrome are often seen on neurologic examination. These tumors usually result in lysis of the tympanic bulla or petrous temporal bone, which can be visualized on routine radiographs .(Fig. 4) . A ventral bulla osteotomy should be performed on animals with evidence of bony lysis in order to obtain a biopsy of the lesion. Because these tumors are locally

DISORDERS OF THE VESTIBULAR SYSTEM

655

Figure 4. Radiographic appearance of a dog with a fibrosarcoma involving the left middle and inner ear with lysis of the left osseous bulla (rostrocaudal view).

invasive, total resection is not possible in the majority of cases. Depending on the type of tumor, chemotherapy and/or radiation therapy may be beneficial. However, the prognosis remains poor at this time. Neurofibromas and neurofibrosarcomas of cranial nerve VIII are rare in the dog and cat. 16 This diagnosis should be considered in adult animals with slowly progressive unilateral peripheral vestibular symptoms that are unresponsive to medical therapy. Initially, radiographs of the middle and inner ear are normal, but lysis of the petrous temporal bone may be seen later in the clinical course. Evidence of ipSilateral facial nerve involvement may develop as the tumor enlarges. Eventually, growth of these tumors along the vestibulocochlear nerve may result in compression of the brain stem. Computed tomography of the middle and inner ear and skull may aid in the diagnosis. Although surgical excision may be possible in the early stages, .there are no reports in animals to document this. Trauma. Head trauma may result in damage to the peripheral vestibular structures and a peracute onset of unilateral vestibular symptoms in an animal of any age. On physical examination, the region of the tympanic bulla may be painful on palpation, and hemorrhage, with or without rupture of the tympanic membrane, may be visualized on otoscopic examination. The severity of the vestibular signs is variable; some animals may be severely ataxic and roll continuously to the affected side. Facial paresis and/or Horner's syndrome may accompany the vestibular dysfunction. Serial neurologic examinations should be performed in order to differentiate peripheral from central vestibular trauma. The diagnosis is based on the history, physical examination findings, and radiographs of the middle and inner ear, which may demonstrate fractures of the petrous temporal bone or tympanic bulla.

656

KENNETH LEE SCHUNK

Treatment of animals with trauma to the peripheral vestibular system is mainly supportive. Animals with severe ataxia and rolling may require mild sedation and a padded cage to prevent further injury. Diazepam (Valium), 2.5 to 10 mg given orally or intramuscularly every 6 to 8 hours, has been used safely in these patients. If the trauma involves only the peripheral vestibular system, the prognosis is good, because compensation, which is dependent on functional vestibular components of the brain stem and cerebellum, will occur. Significant improvement within the first 3 to 5 days occurs in most animals with injury to the labyrinth. With the exception of a mild head tilt, all vestibular signs usually resolve within 2 to 3 weeks . Congenital Peripheral Vestibular Disorders. Congenital vestibular disorders are characterized by varying degrees of peripheral vestibular dysfunction in purebred animals from birth to 3 months of age. 16. 18 Unilateral congenital disease has been reported in German Shepherd, Doberman Pinscher, and English Cocker Spaniel puppies, and Siamese and Burmese kittens. The most consistent sign observed is head tilt, although some animals may exhibit ataxia, circling, or rolling. Abnormal spontaneous nystagmus is usually not seen. The diagnosis is made by ruling out other causes of peripheral vestibular dysfunction; ancillary tests, including examination of the tympanic membrane and radiographs of the petrous temporal bone and tympanic bulla, are normal. The majority of these animals improve and are asymptomatic within 2 to 4 months; occasionally, the head tilt persists and circling may be observed when the animal is excited. Deafness may accompany the vestibular signs and is usually permanent. No effective therapy is available. Clinical signs of bilateral peripheral dysfunction and deafness have been reported in Beagle and Akita puppies . When the animals begin to walk, ataxia and bobbing movements of the head are observed. The cause of congenital vestibular disorders in animals is unknown. In cases that have been examined histologically, no evidence of inflammation, degeneration, or malformation has been foupd in the labyrinth or central nervous system. A hereditary basis is suspected for these disorders. Toxicity. Aminoglycoside antibiotics are known to cause degeneration within the vestibular and auditory systems. 29: 38. 39 This side effect usually occurs with prolonged high doses or when they are administered to animals with impaired renal function. Even though dihydrostreptomycin, neomycin, gentamicin, kanamycin, and vancomycin usually affect the auditory system, the vestibular system is also susceptible. In cats, streptomycin more commonly affects the vestibular receptors. Clinical signs may relate to either unilateral or pilateral peripheral vestibular dysfunction. Deafness may occur with or without vestibular symptoms. Any animals receiving these drugs should be monitored for loss of hearing or malfunction of the vestibular system. If the drug is discontinued soon after the onset of the vestibular signs, the animal may compensate and recover; however, deafness is often irreversible. The signs are due to degeneration of the hair cells of the vestibular receptors in the inner ear. Degenerative changes are also present in the flocculonodular lobe and fastigial nucleus of the cerebellum . Polyneuropathy. A polyneuropathy of unknown cause involving cranial

DISORDERS OF THE VESTIBULAR SYSTEM

657

nerves VII and VIII has been observed in adult dogs. Unilateral or bilateral peripheral vestibular signs with facial paresis or paralysis characterize this disorder. Clinically, the animal initially exhibits symptoms of unilateral facial and vestibulocochlear nerve dysfunction. In most cases, symptoms of bilateral involvement become apparent within 2 to 3 weeks. Ancillary tests, including examination of the tympanic membranes, radiographs of the tympanic bullae and petrous temporal bones, and CSF analysis, are normal. It has been reported that hypothyroidism has been present in some cases; however, appropriate thyroid replacement therapy is usually not beneficial. 18 Compensation with resolution of the peripheral vestibular symptoms occurs over a period of several weeks. Improvement in facial nerve function may occur slowly, but residual weakness is common. Central Vestibular System

Inflammation (Meninogoencephalitis). Inflammatory diseases of the CNS are commonly associated with clinical signs relating to involvement of the vestibular structures within the brain stem and cerebellum. The causes of meningoencephalitis resulting in central vestibular symptoms in the dog and cat are canine distemper, feline infectious peritonitis, granulomatous meningoencephalitis, cryptococcosis, toxoplasmosis, rickettsial diseases (ehrlichiosis and Rocky Mountain spotted fever), and bacterial infections (Table 3). 3. 9. II . 13.21.34 Inflammatory diseases may be seen in animals of any age but are seen less frequently in older animals. There is no breed or sex predilection. Most inflammatory diseases are characterized by an acute or subacute onset and are progressive . The neurologic signs are variable; characteristically, there is evidence of diffuse or multifocal disease based on findings of neurologic examination. In addition to the central vestibular signs, the animal may exhibit limb weakness , cervical pain, cerebellar ataxia, cranial nerve deficits, blindness, and personality changes. There mayor may not be systemic signs of the disease. The diagnosis of inflammatory CNS disease is based on finding diffuse or multifocal neurologic deficits on examination and compatible abnormalities on CSF analysis . Most cases of meningoencephalitis have an increased number of nucleated cells and protein concentration in the CSF; the CSF pressure may also be elevated. The number and types of nucleated cells vary with the cause of the inflammatory disease. Culture and sensitivity shoulq be performed on CSF having more than five nucleated cells per cubic millimeter. In addition to CSF analysis, skull radiographs, BAERs, electroencephalograms, and CT scans may be useful in the evaluation of the animal. In some cases, it is not possible to identify the specific cause of the inflammatory disease. The treatment and prognosis depend on the cause of the disease. Most cases of inflammatory disease have a guarded to poor long-term prognosis . Residual neurologic deficits are common following therapy. Neoplasia. Primary or secondary brain tumors located in the caudal fossa are often associated with central vestibular symptoms in the dog and cat. Meningiomas, choroid plexus papillomas, and neurofibromas are tumors that are located at the surface of the brain and compress the brain as they

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Table 3. Inflammatory Diseases oJthe CNS Causing Central Vestibular Symptoms DISEASE

INCIDENCE

CLINICAL SIGNS

DIAGNOSTIC TESTS

COURSE/PROGNOSIS

Relatively common; occurs Multisystemic signs (respiraExamination of buffy coat for Acute/subacute and protory, gastrointestinal); multiin young dogs canine distemper inclusions; gressive; poor prognosis focalldiffuse CNSsigns (seidetection of viral antigen from conjunctival· or respirazures., ataxia, myoclonus, paraparesis, tetraparesis) tory epithelium using immunofluorescence; CSF-slight increase in lymphocytes (1060/mm3); slight increase in protein (IgG with specific anti-CDV .activity), specific . neutralizing antibody against CDV; CSF may be normal Subacute/chronic and Uncommon;' occurs in rna"': No systemic signs; multifocal/ CSF (as above) diffuse CNS signs (parapature dogs slowly progressive; poor resis, ataxia, tetraparesis, prognosis loss of vision) Feline infectious perito- Relatively common; occurs Effusive and noneffusive forms CBC (leukocytosis, anemia); Acute, subacute, or chronic with systemic signs; CNS inhyperglobulinemia; analysis nitis in cats of an y age and progressive; poor volvement more common of peritoneal and pleural . prognosis fluids; CS·F -moderate to with noneffusive form; multifocalldiffuse CNS signs marked increase in WBCs (ataxia, seizures, hype res(polymorphonuclear and . thesia, paraparesis); ocular mononuclear), slight to marked increase in protein; lesions ± serology CSF-slight to marked inGranulomatous meningo- Common; usually occurs in Multifocalldiffuse CNS signs Acute to chronic and proencephalitis young and middle-.aged (ataxia, facia\. paresis, cercrease in WBCs (mainly gressive; guarded/poor mononuclear), slight to modvical pain, visual and pupillong-term prognosis dogs .erate increase in protein lary abnormalities, paresis,

Canine distemper encephalomyelitis

seizures); ± fever

TREATMENT

Supportive-antibiotics (for secondary infection), anticonvulsants, ± corticosteroids

± Corticosteroids

Supportive-immunosuppressive drugs (corticosteroids, cyclosphosphamide)

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Cats-Upper respiratory signs; Cytologic identification of organism in nasal exudate, multifocalldiffuse CNS signs CSF, or tissue; CSF-slight (seizures, ataxia, paresis); ocular signs (granulomatous to marked increase in WBCs (polymorphonuclear and chorioretinitis, optic neuritis) Dogs-Multifocalldiffuse CNS mononuclear; eosinophils may be seen), slight to signs (ataxia, paresis, facial paresis, cervical pain, seimarked increase in protein; latex agglutination test to dezures); ocular signs tect capsular antigen; culture of organism Focal or multifocalldiffuse CNS CBC (leukopenia in acute Relatively uncommon in Toxoplasmosis signs (paresis, ataxia, seistate); biochemical abnormaladult dogs; rare in cats ities; CSF-slight to moderzures); multisystemic signs ate increase in WBCs (gastrointestinal, respiratory); (mononuclear and polymorocular signs (retinochoroiditis); myositis phonuclear), slight to moderate increase in protein; serology; identification of organism in tissue CSF-moderate to marked inBacterial meningoUncommon; occurs in dogs Multifocalldiffuse CNS signs crease in WBCs (mainly (ataxia, paresis, depression, encephalitis (Staphyloand cats cranial nerve deficits, hyperpolymorphonuclear); slight coccus sp., Pasteurella to marked increase in proesthesia); focal CNS signs if sp.) abscess is present; ± multitein; aerobic and anaerobic culture of CSF; ± blood systemic signs and fever culture Multisystemic signs; CNS signs CBC (thrombocytopenia, panUncommon in dogs Ehrlichiosis uncommon (hyperesthesia, cytopenia); hyperglobulinecranial nerve deficits, ataxia, mia; serology; CSF-slight increase in WBCs (mainly paresis) mononuclear), slight increase in protein; identification of organism in tissue aspirates Multisystemic signs; CNS signs CBC (thrombocytopenia); bioRocky Mountain spotted Uncommon in dogs variable (depression, seichemical abnormalities; fever zures, ataxia, paresis, hyperCSF-slight increase in WBCs (polymorphonuclear esthesia) and mononuclear), slight increase in protein; serology; direct fluorescent antibody test of infected tissue

Fungal-cryptococcosis (others rarely affect CNS)

Relatively uncommon; occurs in adult dogs and cats

Acute to subacute and pro- Amphotericin B, flucytogressive; guarded/poor sine, ketoconizole prognosis

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Subacute/chronic and progressive; guarded/poor prognosis

Sulfonamides (sulfadiazine) and pyrimethamine; clindamycin

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Acute/s1!bacute and progressive; guarded/fair prognosis

Antibiotics based on sensitivity tests

Acute to chronic and progressive; guarded/fair prognosis depending on the stage of the disease

Supportive-tetracycline

Acute/subacute and progressive; fair/good prognos is if treated early

SUPPOl·tive-tetracycline

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660

KENNETH LEE SCHUNK

enlarge. Tumors that are located within the parenchyma and infiltrate neural tissue include astrocytomas, medulloblastomas, and metastatic neoplasia. In the dog, meningiomas and choroid plexus papillomas or carcinomas located near the cerebellomedullary angle are the most common tumors resulting in central vestibular dysfunction. 2. 41 Caudal fossa brain tumors in cats are rare. 30. 40 Despite the occurrence of medulloblastomas and choroid plexus papillomas in young dogs, the majority of tumors located in the caudal fossa occur in dogs over 5 years of age. 10. 19. 41 Breed predilection has been reported, with brachycephalic breeds having a higher incidence of gliomas and dolichocephalic breeds having a higher incidence of meningiomas. 10 Choroid plexus papillomas are found most frequently in non brachycephalic breeds. 19 Neurologic signs associated with caudal fossa tumors are usually progressive over several weeks to months. Asymmetric tetraparesis, ataxia, nystagmus, head tilt, deficits of cranial nerves V, VI, VII, IX, and X, and an altered level of consciousness are common symptoms in affected animals. Paradoxical central vestibular signs are occasionally observed with these tumors. The diagnosis is based on the signalment, history, finding of localizing neurologic deficits, and results of ancillary diagnostic tests. Routine thoracic and skull radiographs, CSF analysis, and CT scan of the head are useful in the diagnosis of these tumors. Thoracic radiographs should be obtained prior to other investigations; evidence of a primary thoracic tumor or metastatic pulmonary disease may be seen. Although skull radiographs are normal in most cases, they may identifY a primary tumor of the skull or show proliferative or lytic changes in the bone adjacent to a primary brain tumor. Cerebrospinal fluid analysis often reveals an increase in CSF pressure and/or protein concentration; however, an increased number of nucleated cells has been reported in approximately 40 per cent of dogs with primary brain tumors.4 Tumor cells are rarely encountered in the CSF, and the CSF analysis may be normal in some patients. Computed tomography is the most reliable diagnostic test in an animal suspected of haVing a caudal fossa tumor.28 It allows one to determine the location of the tumor as well as to define the size and extent of the mass. The treatment of caudal fossa tumors has been for the most part palliative and noninvasive. Corticosteroids are used to reduce increased intracranial pressure and peritumoral edema. Surgical excision or partial removal is important in the management of most brain tumors but is difficult in the majority of caudal fossa tumors. Radiotherapy using external beam, megavoltage radiation has been demonstrated to be effective in the treatment of brain tumors in small animals and is currently the recommended method of therapy.37The role of chemotherapy and immunotherapy in management of CNS tumors is not known at this time. Despite the increased survival time following radiation therapy, the prognosis for animals with caudal fossa tumors remains guarded to poor. Thiamine Deficiency. Thiamine or vitamin BI defiCiency occurs sporadically in cats and dogs. It occurs most commonly in cats fed all-fish diets containing thiaminase, a thiamine-splitting enzyme naturally present in

DISORDERS OF THE VESTIBULAR SYSTEM

661

many species of fish. Thiamine deficiency has also been observed in cats affected with a chronic illness in which the animal is anorexic and does not receive vitamin supplementation. Processed foods for dogs and cats can be made deficient in thiamine because the vitamin can be destroyed by heating at 212°F or above. 25 Thiamine deficiency refers to a deficiency of thiamine diphosphate, the coenzyme form of the vitamin. The activities of three thiaminedependent enzymes involved in energy utilization (transketolase, pyruvate dehydrogenase , and alpha-ketoglutarate dehydrogenase) are abnormal in affected animals. 6 . 12 The major clinicopathologic changes include decreased erythrocyte transketolase activity and increased concentrations of lactate and pyruvate in the blood and CSF.33 Clinical signs may develop within 2 to 4 weeks in animals being fed a diet deficient in thiamine .25 The clinical syndrome in cats is characterized by anorexia followed by cerebellovestibular ataxia and pupillary dilation with sluggish pupillary light reflexes. At this stage, "seizures," characterized by ventroflexion of the head and forelimb extension, can often be induced by handling the animal. The terminal signs, which develop within 2 to 4 days, include semicoma, persistent crying, opisthotonus, and death. Clinical signs of thiamine deficiency in dogs are rare and have been reported to include depression, weight loss, spastic paraparesis, ataxia, torticollis, and recumbency. IS. 33 Generalized motor seizures, coma, and death rapidly follow. The diagnosis of thiamine deficiency is based on the history, clinical signs, and response to therapy in the majority of cases. Decreased blood thiamine levels and erythrocyte transketolase activity can be found in affected animals but are rarely used clinically. In the early stages of the disease, prompt administration of thiamine usually results in significant improvement within 24 hours and complete remission of signs in a few days. Five to 50 milligrams of thiamine are given intramuscularly or intravenously each day. Oral supplementation with foods high in thiamine, such as brewer's yeast, may be included in the therapy. The treatment of animals in the terminal stage of the disease is often unrewarding. Pathologic CNS findings are similar in dogs and cats. The lesions consist of bilaterally symmetric spongy change, necrosis, and hemorrhage in the periventricular brain-stem gray matter, including the caudal colliculi, lateral geniculate nuclei, oculomotor nuclei, and vestibular nuclei. 12. IS . Trauma. Head injuries are common in small animals and may result in damage to the central vestibular system . Neurologic sigrys usually reach their maximum severity within hours of the trauma. Serial neurologic examinations of the patient are important in the localization of lesions, detection of progressive CNS deterioration, and determination of the prognosis. Special attention should be given to the level of consciousness, brain-stem reflexes, respiratory pattern, and motor responses of the animalY Animals with central vestibular symptoms foJlowing head trauma usually have neurologic deficits associated with damage to nearby brainstem structures. Clinical signs associated with damage to the nuclei of

662

KENNETH LEE SCHUNK

cranial nerves V, VI, VII, IX, and X, cardiac arrhythmias, chaotic respirations, alteration in the level of consciousness, and abnormal motor responses are frequently observed. The degree of brain-stem injury and prognosis are determined by the severity of these signs. The diagnosis of cranial trauma is based on the history, physical examination findings, and radiographic evidence of skull injury such as fracture lines or depressed bone fragments. Computed tomography may be useful in identifying subtle fractures or intracranial hemorrhage. The management of animals with head injury should initially involve the treatment of life-threatening non-neural injuries. The goals of therapy are to reduce brain swelling, decrease intracranial pressure, and maintain perfusion and tissue oxygenation. 20.27 A patent airway should be established, and the animal should be placed in an oxygen-rich environment with its head elevated. Intravenous fluids should be administered carefully because overhydration may exacerbate brain edema. Although the role of corticosteroids in the management of brain injury remains unknown, they are commonly used in an attempt to reduce brain edema. The author uses dexamethasone at an initial dosage of 2 mg per kg intravenously; this is followed by a dosage of 0.2 mg per kg intravenously every 6 to 8 hours for 2 to 4 days. Hyperosmolar agents, such as mannitol, are only used in animals with severe neural dysfunction or in those displaying rapid deterioration. Nursing care and supportive therapy are extremely important in the management of these patients. The prognosis for animals with central vestibular trauma varies depending on the extent of brain-stem involvement. Many animals improve and become functional pets over a period of several weeks to months. Residual neurologic deficits may be seen in some animals. Storage Diseases. Storage diseases are a group of inherited diseases of dogs and cats characterized by a deficiency in the activity of a lysosomal enzyme. The enzyme deficiency disrupts the normal metabolic degradation of lipids, glycoproteins, glycogen, or mucopolysaccharides. This results in the lysosomal accumulation of a specific substrate normally hydrolyzed by the enzyme. The stored material eventually alters cellular function and produces cellular death. Even though the enzyme deficiency is present in many cells of the body, clinically the majority of animals manifest neurologic signs at a young age. Storage diseases resulting in vestibular and/or cerebellar dysfunction include G M 1 gangliosidosis, G M2 gangliosidosis, sphingomyelinosis, mannosidosis, glucocerebrosidosis, and globoid cell leukodystrophy. s.12 Specific enzyme deficiencies are known to occur i.n certain breeds and are usually inherited as autosomal recessive traits. Affected animals are normal at birth but often manifest slowly progressive neurologic symptoms before they are 1 year of age. The neurologic signs will vary depending on the specific enzyme deficiency; multifocal signs are not uncommon. Many affected animals fail to grow as rapidly as their littermates and may show evidence of multisystem involvement, such as corneal clouding, hepatomegaly, skeletal dysplasia (that is, deformities of the vertebral bodies and coxofemoral joints), and facial abnormalities (that is, frontal bossing). The diagnosis of lysosomal storage disease may be suspected from the

DISORDERS OF THE VESTIBULAR SYSTEM

663

signalment, history, and physical and neurologic examinations. An antemortem diagnosis of many of these disorders can be confirmed by enzymatic analysis of peripheral blood leukocytes or skin fibroblast cultures. Laboratories with the capability for lysosomal enzyme assay should be consulted concerning the specimen requirements. Biopsy and histologic examination of affected tissues, including portions of the nervous system, may aid in making the diagnosis. Fresh tissues, such as lymph node, liver, kidney, and brain, that are stored at - 20°C can be analyzed biochemically to determine the nature of the stored material. Because storage diseases are progressive and no treatment is available, the prognosis is grave. Efforts shbuld be directed toward the detection of heterozygotes and the elimination of these animals from the breeding population. Although clinically normal, heterozygotes or carriers can be identifieq by determining the level of the specific enzyme in their h:mkocytes. Heterozygotes have an enzyme level that is intermediary between normal and affected animals. A breeding program can then be structured to eliminate the carriers. .

SUMMARY Disorders of the vestibular system are common in veterinary practice and result in a clinical syndrome characterized by head tilt, asymmetric ataxia, and nystagmus. These signs may occur with lesions involving the peripheral or central vestiJ:mlar structures. Careful neurologic evaluation of the animal arid knowledge of surrounding neuroariatomic structures and their functions enable the clinician to differentiate between peripheral and central disorders, Conditions resulting in peripheral vestibular symptoms include idiopathic syndromes, otitis interna, trauma, neoplasia, druginduced ototoxicity, arid congenital disorders. Radiographs of the petrous temporal bone and tympanic bulla and examination of the tympanic membrane are useful in determining the specific etiology. Central vestibular dysfunction may result from inflammatory CNS diseases, neoplasia, trauma, thiamine deficiency, and storage diseases. Cerebrospinal fluid analysis, skull radiographs, and computed tomography are diagnostic tests· useful in animals with central vestibular symptoms. Once the etiology is identified; specific therapy may be provided and a more accurate prognosis can be determined.

REFERENCES 1. Ader PL, Boothe HW: Ventral bulla osteotomy in the cat. JAm Anim Hosp Assoc 15:757, 1979 2. Andrews EJ: Clinicopathologic characteristics of meningiomas in dogs. J Am Vet Med . Assoc 163: 151, 1973 3. Bailey CS, Higgins RJ: Characteristics of cerebrospinal fluid associated with canine granulomatous meningoencephalomyelitis: A retrospective s~udy. J Am Vet Med Assoc 188:418, 1986 . 4. Bailey CS, Higgins RJ: Characteristics of cisternal cerebrospinal fluid associated with

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5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

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