When examining a patient with vestibular signs, it is crucial to collect details of signalment and history, as they provide key information regarding the patient. Details of onset and progression of clinical signs, vaccination status, history of trauma, previous ear diseases, use of ototoxic drugs and the presence of other clinical signs, for example, can guide the clinician through the diagnostic process. A thorough general examination is also key when investigating comorbidities which can affect the choice of further tests and/or have an impact on the procedures, such as choice of anaesthesia.
As previously mentioned in the first article of this miniseries on the neurolocalisation and clinical signs of vestibular syndrome, it is essential that you try to localise the disease to the central or peripheral vestibular system, as differential diagnoses and prognosis vary considerably depending on its origin. However, an accurate localisation may not always be possible, especially in the absence of obvious central vestibular deficits. In these circumstances, advanced imaging (MRI and/or computed tomography (CT) scan) and further tests (cerebrospinal fluid (CSF) tap and analysis or myringotomy) can be vital in the diagnostic process.
Clinical investigation
A complete blood count, biochemistry and thyroid profile should ideally be carried out on all patients with vestibular signs, to rule out any metabolic or inflammatory causes of the clinical signs. Blood tests will also be useful as a preanaesthetic screening in case further tests are needed.
Neurodiagnostic investigations for peripheral vestibular disease
In the case of otitis, myringotomy should be performed to sample the fluid for cytology evaluation, bacterial culture and antimicrobial sensitivity
In cases of peripheral vestibular disease, otoscopic examination under general anaesthesia is recommended to rule out otitis media/interna. In the case of otitis, myringotomy should be performed to sample the fluid for cytology evaluation, bacterial culture and antimicrobial sensitivity. A thorough pharyngeal examination should be carried out in cats to rule out pharyngeal polyps. Radiographs of the tympanic bullae can help identify changes typical of otitis, including sclerosis of the external acoustic meatus and tympanic bulla. A CT or MRI scan of the head should be conducted to further investigate the middle/inner ear, especially in non-conclusive cases or ones that do not respond to initial treatment. If results are normal, idiopathic vestibular syndrome can be considered.
Neurodiagnostic investigations for central vestibular disease
Advanced imaging of the brain (MRI) is the preferred diagnostic test to rule out structural abnormalities in animals with central vestibular signs. CSF collection and analysis are recommended to support the neurodiagnostic process, but only in the absence of raised intracranial pressure signs on the MRI. A cisternal tap can alter the intracranial pressure. If this is raised due to meningoencephalopathy, a secondary life-threatening brainstem herniation (transtentorial or foraminal) can occur.
A brainstem auditory evoked response (BAER) test can be used to evaluate the cochlear component of CN VIII, which is responsible for hearing. This electrodiagnostic test is performed on awake or lightly sedated animals and can help to differentiate between peripheral and central vestibular disease.
Differential diagnosis and treatment
The main differentials for peripheral vestibular disease include congenital vestibular disease, hypothyroidism, otitis media/interna, nasopharyngeal polyps, idiopathic vestibular disease, tumours of the middle/inner ear, toxicity (aminoglycosides, topical iodophors, chlorhexidine) and trauma to the middle ear. On the other hand, the main differentials for central vestibular disease are neurodegenerative disease, lysosomal storage disorders, Chiari-like malformation, intra-arachnoid cysts, thiamine deficiency, metronidazole toxicity, meningoencephalitis, brain tumours, head trauma and cerebrovascular disease. There are no typical diseases affecting the central vestibular system, but there are some disease processes that seem to primarily affect the central vestibular structures.
There are no typical diseases affecting the central vestibular system, but there are some disease processes that seem to primarily affect the central vestibular structures
The most common causes of peripheral vestibular signs are otitis media/interna and idiopathic vestibular disease, and the most common causes of central vestibular disease are neoplasia and infection/inflammation.
Peripheral vestibular diseases
Otitis media/interna
Inflammatory otitis media/interna is one of the most common causes of peripheral vestibular signs. Due to an involvement of the sympathetic supply within the petrous temporal bone, this can be accompanied by facial paralysis, Horner’s syndrome and neurogenic keratoconjunctivitis sicca. Otitis media/interna can develop from otitis externa of the Eustachian tube or haematologically. However, a non-infectious cause of otitis media has been described in Cavalier King Charles Spaniels without otitis externa (primary secretory otitis media), which is caused by the accumulation of a viscous plug in the middle ear.
In cases of otitis externa, a general examination can reveal typical clinical signs. Otoscopic examination and imaging of the tympanic bullae via radiographs, CT or MRI is the preferred diagnostic modality. Otitis externa can be cleaned with a gentle saline flush, and the integrity of the tympanic membrane should be checked. Fluid in the middle ear can be collected via myringotomy and submitted for cytology, culture and sensitivity.
The external ear canal should not be flushed with topical products, as a leak into the middle ear via a ruptured tympanic membrane can worsen neurological signs
Treatment for otitis involves four to six weeks of systemic antibiotics, based on the results of the bacterial culture and sensitivity. Cephalosporin, amoxicillin/clavulanate or a fluoroquinolone are suitable choices. The external ear canal should not be flushed with topical products, as a leak into the middle ear via a ruptured tympanic membrane can worsen neurological signs. In case of a lack of response to treatment, bulla osteotomy is recommended for surgical drainage and debridement.
Prognosis is favourable, but some neurological deficits can persist due to permanent neural damage. In some cases, especially if the treatment is unsuccessful or delayed, otitis media/interna can spread from the inner ear to the cranial cavity.
Nasopharyngeal polyps
A nasopharyngeal polyp is inflammatory tissue that grows into the middle ear or nasopharynx from its origin in the auditory tube or tympanic cavity. They are common in cats between one and five years old, and they can be accompanied by upper respiratory signs and dysphagia. Otoscopy and pharyngeal examination under general anaesthesia can help to visualise the polyps. Radiographs can show occlusion of the nasopharynx and changes in the tympanic bulla, but CT and MRI scans can give more detailed information on the areas involved.
Removal of the polyp is recommended via ventral bulla osteotomy. The prognosis of nasopharyngeal polyps is good.
Idiopathic vestibular syndrome
The clinical signs of idiopathic vestibular syndrome are often the same as peripheral vestibular disease, but they are not accompanied by involvement of the facial nerve or the sympathetic system. They are normally unilateral and quite severe at onset. Dogs are normally affected at an older age (geriatric vestibular disease), but cats can be affected at any age. The cause of idiopathic vestibular syndrome is unknown, and diagnosis is based on the exclusion of other potential causes.
Supportive care should include intravenous fluids if the animal is vomiting and/or not eating, motion sickness treatment and support to walk and avoid injury
Clinical signs should improve within two to three days with supportive care, though this may be slightly longer in cats, and should resolve in four weeks. Supportive care should include intravenous fluids if the animal is vomiting and/or not eating, motion sickness treatment and support to walk and avoid injury. Mild ataxia and head tilt can persist, and the clinical signs can relapse.
Central vestibular diseases
Brain tumours
Meningiomas and choroid plexus papillomas are the most common brain tumours affecting the caudal fossa in dogs, but any primary or secondary neoplastic process that affects the brain can involve the vestibular system. Diagnosis is achieved with an MRI scan of the brain.
Where possible, the treatment for brain tumours affecting the vestibular system is surgical removal. Glucocorticoids can be used to reduce peritumoral oedema. Chemotherapy or radiotherapy can also be considered depending on the tumour type, behaviour and localisation.
Meningoencephalitis
Inflammation of the brain and the meninges is normally acute and progressive, often with asymmetrical signs. Neurological signs associated with meningoencephalitis reflect the areas involved and are not always accompanied by systemic signs, such as hyperthermia. However, neck pain can reflect the presence of meningitis. Common infectious causes of meningoencephalitis in dogs are distemper, Toxoplasma gondii and Neospora caninum. The most common non-infectious cause is meningoencephalitis of unknown origin (MUO). In cats, common infectious causes are coronavirus (usually a dry form of FIP) and Toxoplasma gondii.
Ophthalmological examination can reveal signs of uveitis or the fundic changes typical of some inflammatory diseases. An MRI scan of the brain can reveal changes typical of inflammatory conditions, while CSF analysis often reveals increased total nucleated cells and protein concentration. However, CSF analysis can be normal if the infection does not involve the meninges or if the animal has been treated with corticosteroids. Together with CSF culture, antigen titres or polymerase chain reaction (PCR) analysis, cytological evaluation can help distinguish between the different infectious agents.
Treatment of central vestibular syndrome as a result of meningoencephalitis depends on the primary cause of the clinical signs
Treatment of central vestibular syndrome as a result of meningoencephalitis depends on the primary cause of the clinical signs. After investigations have been carried out, antiprotozoal treatment (clindamycin or trimethoprim/sulphonamide) can be started while the results of the CSF analysis are pending. Once infectious diseases have been ruled out, treatment with corticosteroids (at anti-inflammatory or immune-suppressive doses) and/or other immunomodulatory drugs (cytosine arabinoside, ciclosporin and azathioprine) should be started.
Prognosis depends on the cause and severity of the neurological presentation. MUO, protozoal and fungal infections can relapse, and there can be residual neurological abnormalities due to permanent damage to the affected areas of the brain.
