When the middle or external ear is responsible for hearing loss, such loss is due to a problem with the actual conduction or transmission of the sound waves from the external environment to the cochlea. Blockage of sound wave transmission may be due to occlusion of the external ear canal by ceruminous debris, tissue, or foreign bodies. These blockages may be congenital, as in the case of birth defects, or acquired. Conduction blockage in the middle ear may be due to rupture of the tympanic membrane, fluid or exudate, excessive tissue growth, foreign body, malformation of or damage to the ossicles, or stiffening of the ossicles with age.

If the inner ear auditory structures or auditory pathways are compromised, sensorineural deafness may result. Abnormalities of the cochlea, cochlear portion of the vestibulocochlear nerve, auditory pathways in the brain, thalamus, and cerebrum all have the potential to lead to a blockage of the auditory signal transmission and subsequent deafness. In all practicality, almost all cases of pure sensorineural deafness are due to a functional problem with the hair cells of the organ of Corti. There are no degenerative, congenital, or acquired conditions that specifically target the auditory pathways and projections in the brain.

Peripheral vestibular disease may be evident in young animals and attributed to a congenital malformation or degeneration of the inner ear structures. If the abnormality is bilateral, these animals may not have a head tilt or nystagmus; however, they will frequently have a symmetrical ataxia, a wide-based stance, and a side-to-side movement of the head in the horizontal plane. The vestibular disease may be present in association with deafness. The clinical signs will usually present when the animals begin to ambulate. They may consist of head tilt, nystagmus, strabismus, ataxia, circling, falling, rolling, and abnormal head movements. Clinical signs associated with the vestibular disease may resolve; however, a head tilt or head tilt that shifts from side to side may persist. Many of the reported animals with this congenital peripheral vestibular disease will compensate for the abnormalities in balance. It is thought that any compensation is centrally mediated, likely through visual mechanisms. If the animal is deaf, the hearing abnormality is permanent. The abnormality has been known to affect German Shepherds, English Cocker Spaniels, Doberman Pinschers, as well as Siamese and Burmese cats. In a report of Doberman Pinscher puppies affected from multiple litters from the same dam, a lymphocytic labyrinthitis was discovered on histological examination. An in utero viral infection was hypothesized to cause the degeneration. No other abnormalities have been documented in affected animals. There is no treatment for the condition.

Hypothyroid-associated neurologic dysfunction is well recognized. Specific involvement of the peripheral vestibular system may be the primary presenting clinical complaint. It is usually seen in older animals as an acute onset, nonprogressive presentation. Nearly all animals will have a head tilt and positional strabismus. Many animals also have decreased menace responses and decreased palpebral reflexes. More obvious signs of facial nerve paralysis may also be present. Other clinical signs may be an abnormal gait and circling. Brain-stem auditory evoked response (BAER) testing may be abnormal. There may be other clinical and clinicopathological signs of hypothyroidism (hypercholesterolemia, abnormal thyroid-stimulating hormone response). Hyperlipidemia may be a contributing factor. Labrador Retrievers with severe hyperlipidemia have been described with central and peripheral vestibular signs, tetraparesis, paraparesis, and facial nerve paralysis. Infarction may be an inciting or contributing factor. Other potential causes of vestibular disease must be ruled out, paying special attention to possible structural disease. Because the clinical signs associated with this disease may suggest central vestibular disease, the clinician should be certain a poor prognosis is not given to the owners of the animal without first evaluating and ruling out the possibility of hypothyroidism. The diagnosis may be challenging in that many animals are mistakenly thought to have euthyroid sick syndrome. With an adequate supplementation of levothyroxine, the majority of these animals will return to normal.

Neoplastic processes affecting the PNS may result in peripheral vestibular disease by compression or invasion of the vestibulocochlear nerve. Peripheral nerve tumors (i.e. schwannomas) may initially present as peripheral vestibular dysfunction; however, with time they can invade the brain stem. Tumors of the ear (e.g. ceruminous gland adenocarcinoma) may invade the middle and inner ear and cause vestibular disturbances. Other tumors—such as chondrosarcoma, osteosarcoma, fibrosarcoma, and squamous cell carcinoma—have the potential to invade adjacent tissue, resulting in vestibular disease. Treatment is directed at the primary lesion. Supportive treatment for vestibular dysfunction may also be necessary.

A condition of idiopathic vestibular disease is well recognized in dogs and cats. Cats of any age can be affected. Dogs tend to be older, and therefore the disease is often referred to as idiopathic geriatric vestibular disease. These vestibular episodes are often confused with vascular accidents (“strokes”) or seizures. The key to this diagnosis is the absence of any detectable structural, metabolic, or inflammatory disease, as well as lack of evidence of central vestibular disease. The onset is acute or peracute, and animals may present with signs of dysfunction ranging from a mild head tilt to severe imbalance and rolling. The clinical signs are usually unilateral, with a horizontal or rotary nystagmus (fast phase away from the side of the head tilt) and an asymmetrical ataxia. The ataxia may be confused with proprioceptive deficits. Animals with idiopathic vestibular disease maintain their strength and mental status. The vestibular signs can be so severe that the affected animals are often incapacitated, making the initial neurologic exam difficult, especially in cats. Most animals will improve rapidly, although complete recovery may take 2–3 wks in a typical case. Improvement is often appreciated within the first 72 hrs. In some animals, clinical signs may persist for up to 5 wks. Occasionally, a mild head tilt will persist after other clinical signs have resolved. The condition can be relapsing. It is thought that this syndrome may result from abnormalities with the endolymphatic fluid of the inner ear structures, a mild intoxication of the vestibular system, or autoimmune disease. There is no specific treatment for the disease. The use of corticosteroids has not resulted in more rapid clinical improvement. Care should be directed at supportive treatment, which may include sedatives, and therapy directed at relieving motion sickness. There is debate as to whether central signs (such as proprioceptive deficits) may be present with this clinical syndrome. Occasionally, in geriatric dogs with this disorder, pelvic limb conscious proprioceptive deficits will be demonstrated on neurologic examination. In many of these cases, the placing deficits are suspected to be due to chronic, subclinical disc protrusions unrelated to the vestibular disorder. However, until there is substantial improvement in vestibular function, or central nervous system (CNS) disease is ruled out, pelvic limb conscious proprioceptive deficits should be considered potential evidence of a central vestibular disorder.

The same toxic substances that may result in hearing deficits will affect the labyrinthine receptors of the vestibular system (see the “Diseases affecting the auditory system” section below). Animals will usually present with signs of unilateral vestibular disease, although bilateral involvement is possible. The most common compounds are antimicrobials that belong to the family of aminoglycosides, loop diuretics (e.g. furosemide, ethacrynic acid), and ear-cleansing compounds. If toxicity is recognized early, and the offending compound is discontinued, vestibular signs may resolve.

Although uncommon, trauma to the petrous temporal bone may cause unilateral vestibular signs. Disruption of the bony and membranous labyrinth will predominate; however, if the injury is severe, other adjacent structures may be affected. It is much more common to have associated brain-stem, cerebellar, or supratentorial signs with trauma to this region of the skull. Ipsilateral facial nerve or trigeminal nerve dysfunction may also result. If the injury is mild or confined to the petrous temporal bone, or the animal heals from associated areas of trauma (brain stem or cerebellum), the prognosis for recovery from peripheral vestibular disease is good, providing central compensating mechanisms are adequate.

Canine distemper virus (CDV) may cause a distemper encephalomyelitis. Older dogs that develop distemper encephalomyelitis may present with an altered gait and vestibular disease. These dogs usually have an adequate vaccination history, and a diagnosis of canine distemper encephalomyelitis can be difficult. Cerebrospinal fluid (CSF) may be characterized by a moderate mononuclear pleocytosis that is positive for the antibody directed at the CDV. It is important to realize that a breakdown in the blood–brain barrier (BBB) may result in a false positive test result due to a leakage of systemic antibody into the CSF. More specific information regarding diagnostic testing for CDV infection can be found in Chapter 7. Young dogs affected with distemper encephalomyelitis may recover from their systemic disease. In these cases, the resolution of vestibular signs may follow. Cats infected with the feline infectious peritonitis (FIP) virus will often develop vestibular signs. Cats diagnosed with FIP usually have the dry form of the disease. A diagnosis is based on history, potential exposure, clinical signs, and the demonstration of a high CSF antibody titer directed at the feline coronavirus (see Chapter 7). There is no treatment for the disease, and the prognosis is poor if the clinical signs are severe.

Dogs infected with the rickettsial agents causing Rocky Mountain spotted fever (RMSF) or ehrlichiosis may, among other clinical signs, develop vestibular dysfunction. The diagnosis of rickettsial infection will be based on potential exposure to ticks, hematological and biochemical abnormalities, clinical signs consistent with rickettsial disease, a single high titer, and/or a rising titer. Most cases will present between April and October, with the highest incidence in the month of June. RMSF is typically an acute onset infectious disease. Characteristic abnormalities may include anorexia, lethargy, fever, cutaneous lesions, ocular lesions, leukocytosis, anemia, thrombocytopenia, and other signs referable to a vasculitis. Vestibular signs may include a horizontal nystagmus and ataxia. Early recognition and treatment with doxycycline will often result in the complete resolution of the disease and vestibular signs.

Meningoencephalomyelitis resulting from primary bacterial infections is an uncommon occurrence in dogs and cats. When suspected, a CSF tap followed by antimicrobial culture and sensitivity, is indicated. A combination of antimicrobials should provide broad-spectrum antimicrobial activity against most organisms if a specific etiology cannot be determined. Animals with an inflammatory CSF sample may benefit from low doses of prednisone (0.5 mg/kg bid). The anti-inflammatory activity of the prednisone may alleviate some of the inflammation responsible for the clinical signs. The dose is low enough that it should not severely inhibit the immune response and may be discontinued early in the course of treatment (see Chapter 7). Other infectious organisms such as Toxoplasma gondii and Neospora caninum can infect the nervous system and result in clinical signs that may include vestibular disease. Other clinical signs such as myalgia (due to myositis), paresis, and multifocal systemic involvement may predominate. Chapter 7 contains more information on the diagnosis and treatment of toxoplasmosis and neosporosis. Fungal infections involving the nervous system may be isolated to neural tissue; however, most animals will show other systemic signs of fungal disease, including cutaneous, ocular, respiratory, and gastrointestinal disease. The most common fungal organism to involve the nervous system of dogs and cats is Cryptococcus neoformans. Coccidioidomycosis may be suspected in dogs with recent respiratory signs. Animals will often recover from a mild respiratory tract infection. In order to make this diagnosis, the animal’s history should indicate that it has been in the Southwest United States. Another fungal infection that usually affects multiple organs is blastomycosis. Multifocal disease, along with any indication for potential fungal infection (travel history, unresponsiveness to antimicrobials, etc.), should give the clinician cause to investigate a possible fungal infection with attempts at direct visualization of the organism from infected tissues, CSF analysis, and serology. The prognosis depends on the duration of the infection and the severity of clinical signs. Fluconazole is the antifungal of choice for cases of cryptococcosis and coccidioidomycosis. Very few animals show improvement with antifungal treatment for nervous system infection of blastomycosis. Disseminated protothecosis has been reported to cause vestibular signs in a dog. Prototheca wickerhamii and Prototheca zopfii have been identified as pathogenic species of algae. Although an uncommon infection, other clinical signs of multi-organ involvement are usually apparent. The prognosis for animals infected with protothecosis is poor.

It is thought that granulomatous meningoencephalomyelitis (GME) is a primary autoimmune disease. This disease may represent an immune response to an as yet unidentified organism. There are disseminated (multifocal), focal, and ocular forms of the disease. The distinction between autoimmune disease and neoplasia has been debated. This disease typically affects middle-aged, small-breed dogs. Brain-stem signs including vestibular dysfunction are the primary signs associated with the disease; however, clinical signs would be referable to that portion of the nervous system that is most severely affected. In the vast majority of cases, a definitive diagnosis of GME can only be made on a postmortem examination of affected tissue. The histologic lesions consist of perivascular granulomas comprising primarily dense accumulations of inflammatory cells. Accumulations of histiocytes, lymphocytes, and plasma cells in the nervous system result in compression and invasion of surrounding tissue. A presumptive diagnosis may be made based on CSF analysis, by ruling out infectious etiologies, and by a demonstration of focal or diffuse contrast-enhancing lesions on CT or MR examination. Lesions may not be evident on CT/MR images in GME cases. On CSF examination, there is usually a mixed, predominantly mononuclear, pleocytosis and moderately elevated protein. Most animals will initially show a response to systemic corticosteroids; however, the remission of disease is usually short-lived. Radiation therapy may provide an effective treatment option for dogs with focal GME. Improved survival times for GME have been obtained in recent years with the use of several new treatment options, including procarbazine, cyclosporine, and cytosine arabinoside (see Chapter 7).

Metronidazole has been associated with vestibular disease. The onset is acute and usually occurs when animals receive high doses of the medication for a long duration. However, signs of metronidazole toxicity have been seen with treatment for as little as 1 wk and as long as 4–6 wks. There is much debate as to the toxic dose of metronidazole in the dog. Both dogs and cats are susceptible to metronidazole toxicity, and the toxicity is well recognized in humans. Clinical signs of metronidazole toxicity usually show up 7–12 days after continuous high doses (greater than 60 mg/kg/day), but they have also been reported to occur at lower doses. The exact mechanism of the toxicity is unknown. It has been hypothesized that it may interfere with RNA synthesis or possibly inhibit the actions of the primary inhibitory neurotransmitter in the cerebellum and vestibular systems; gamma-aminobutyric acid (GABA). The onset of clinical signs is likely dependent on the dose the animal is receiving. However, clinical signs can also develop after long-term, low-dose therapy. They may consist of generalized ataxia, nystagmus, anorexia, and vomiting. In severe cases, altered mental status, seizures, and opisthotonus may be present. Cats are typically affected by altered mental status and supratentorial dysfunction (seizures, blindness, and ataxia). Clinical signs will usually resolve within 1–2 wks after discontinuation of the drug. Clinical recovery has been shown to be more rapid in dogs that were treated with injectable and oral diazepam compared to those that were not treated. In a treated group, dogs receiving an average dose of 0.43 mg/kg of injectable or oral diazepam every 8 hrs for 3 days had an average recovery time of 38.7 hrs compared to an average recovery time of 11.6 days in those that were not treated. Some changes to the CNS may result in permanent neurologic deficits. Although a very effective antimicrobial, metronidazole is not innocuous. It is recommended that the dosage schedule be individualized for every animal, dependent upon the intended target of the drug. For most applications of metronidazole, a total daily dose of 30 mg/kg body weight (e.g. 10 mg/kg body weight, q 8 hrs) is sufficient.

Degenerative processes rarely specifically affect the central vestibular system. However, hereditary polioencephalomalacia results in degeneration of central vestibular pathways (thalamocortical pathway), such as the caudal colliculus, vestibular nuclei, and cerebellar nuclei, amongst other areas of the brain and cervical spine. Cases have been reported in Australian Cattle Dogs and a Shih Tzu. A heritable, rapidly progressive lethal necrotizing encephalopathy similar to human Leigh syndrome (a mitochondrial encephalopathy) has also been reported in related American Staffordshire Terrier dogs. These dogs presented at 6–8 wks of age with clinical signs of neurologic dysfunction referable to the central vestibular system (ataxia, head tilt, intention tremor, strabismus, and nystagmus). This disorder is distinct from another mitochondrial encephalopathy that affects this breed (L-2-hydroxyglutaric aciduria). Vestibular dysfunction may result secondary to ischemic changes or direct trauma to the vestibular pathways. Hydrocephalic animals may have vestibular deficits that are part of the constellation of clinical signs that are commonly present (see Chapter 7). Metabolic disturbances—such as hepatic encephalopathy, altered electrolyte balance, or renal disease—may also cause secondary vestibular signs; however, these abnormalities would not likely cause isolated alterations in vestibular function. Rather, they would result in multifocal signs, of which vestibular signs may be a part.

Thiamine deficiency is a nutritional disorder that can affect the vestibular system. The deficiency may result from a diet that is deficient in thiamine, an all-cooked diet, or a diet that contains large amounts of thiaminase (fish viscera). Thiamine is necessary for the completion of the citric acid cycle and therefore is particularly important in tissues that rely on glucose for energy (brain). Thiamine deficiency primarily results in bilaterally symmetrical areas of necrosis, spongiosis, and hemorrhage in the brain stem. In cats, the vestibular nuclei are often affected, resulting in vestibular signs. Cerebellar signs consisting of ataxia, tremors, and absent menace responses may also be seen. Because the heart is also very dependent upon glucose as a fuel source, it too may be affected. Bradycardia, tachycardia, and arrhythmias may be appreciated with thiamine deficiency. If diagnosed early, many animals will respond to parenteral administration of thiamine hydrochloride (vitamin B₁). Many animals will have a complete resolution of clinical signs if treated early.

Hypothyroidism has been associated with central vestibular dysfunction in a limited number (14) of dogs. Whether the pathophysiology of hypothyroid-related central vestibular dysfunction involves infarction, abnormal neurotransmitter release, or some combination of these and other factors is unknown. All dogs responded favorably to thyroid supplementation.

Intracranial epidermoid cysts, while technically tumors, are not neoplastic. They are commonly found at the cerebellomedullary junction but may also arise within the fourth ventricle. It is hypothesized that they arise from fragments of epidermoid tissue at an abnormal location due to a defect in neural tube closure. While the incident occurs around 3–5 wks of gestation, the clinical signs may not be apparent until adulthood. It may be challenging to differentiate intracranial epidermoid cysts from other intracranial cysts with advanced imaging; however, characteristics particular to cystic lesions in the brain with poor or no contrast enhancement in the area of the cerebellar medullary junction and fourth ventricle should be suspected. These lesions have also been described in other areas of the brain. There is one report of a fourth ventricular cholesterol granuloma in a dog.

Vascular disease is associated with central vestibular signs. Transient ischemic attacks (TIA) and cerebrovascular accidents (CVA) or strokes are widely recognized in the human population. MR imaging has aided greatly in the detection of CVA events in dogs, particularly in the region of the brain supplied by the paired rostral cerebellar artery and the paired caudal cerebellar artery, both of which arise from the basilar artery. The rostral cerebellar artery supplies the rostral portion of the cerebellar hemispheres, the vermis, and the dorsolateral brain stem. The caudal cerebellar artery provides blood supply to the caudoventral portion of the cerebellum and the lateral medulla oblongata. Ischemic events involving both these arteries have been documented in veterinary medicine. TIAs are a diagnostic challenge considering that even with advanced imaging techniques, such as diffusion-weighted imaging using MRI, the diagnosis is often presumptive based primarily on the time course of the clinical signs (usually less than 24 hrs in humans) and lack of MRI findings. Central vestibular signs may be paroxysmal and are more commonly observed in patients with hypertension, heart disease, hypercoagulable conditions, diabetes mellitus, and hypothyroidism. However, a significant number of documented CVAs have no underlying cause and are therefore defined as “cryptic” or “cryptogenic.” Prognosis for complete recovery with TIAs is good to excellent, whereas that of CVAs is relatively good to excellent but significantly more dependent on the degree of damage to the affected portion of the brain. Time to recover and supportive care of the patient should be encouraged.

Due to the close anatomical relationship between the auditory and vestibular receptors, abnormalities of hearing may be observed in conjunction with clinical signs of peripheral vestibular disease. As previously mentioned, a presumptive diagnosis of deafness is easier to make if bilateral disease is present.