Vestibular Syndrome in Dogs
■Differential diagnoses of peripheral vestibular disease
The above section should enable the veterinarian to distinguish a peripheral lesion of the vestibular system from a central one. Table 2 lists the most frequent differential diagnoses of peripheral and central causes of vestibular syndrome. All general practitioners should be able to diag-nose peripheral vestibular disease, and a short discussion on the three most common etiologies of peripheral vestibular syndrome is worthwhile. It is the author’s opinion that animals with a central lesion should always be referred to a specialist.
Otitis media/interna is the underlying etiology in approximately 50% of patients with peripheral vestibular disease (4). Otitis media by itself does not cause loss of vestibular function, but the anatomical proximity of middle and inner ear means that otitis media is often associated with otitis interna. By comparison, it is rare to have otitis interna without involvement of the middle ear, so when otitis media occurs with simultaneous peripheral vestibular failure, this indicates that the inner ear is involved in the inflammatory process. Note that it is not inevitable that otitis externa will also be present, since infection can reach the middle and inner ear through hematogenous spread or by ascending from the pharynx through the Eustachian tube into the middle ear. Aside from peripheral vestibular failures, otitis interna/ media can, as noted above, involve two other nerves; the facial nerve, lying within the wall of the tympanic bulla, and fibers of the sympathetic nerve, running through the middle ear to the eye. Hence an otitis media/ interna can lead to a facial paralysis and Horner’s syndrome; other cranial nerves should not be affected. On the basis of the frequent causal relationship between middle and inner ear inflammation as well as otitis fluid obtained should be subjected to cytology and bacterial culture.
Imaging is a useful diagnostic tool and otitis media/ interna can frequently be identified with conventional radiography, although this requires careful positioning under general anesthesia (Figure 6). Inflammatory secretions may obscure the tympanic bulla and proliferative or destructive changes to the bulla wall (in the form of wall thickening or lysis) may be visualized (Figure 7). Mineralization of the external auditory canal may be noted as a result of chronic otitis externa. Normal radiological appearance does not exclude otitis media/ interna, and in such cases cross-sectional imaging techniques (computed tomography, magnetic resonance imaging) may be necessary.
Therapy for otitis media/interna involves at least six weeks of antibiotics, ideally supported by bacterio-logical culture and sensitivity, although if this is not available either clindamycin, cephalosporin, potentiated sulfonamides or fluoroquinolones may be the drug of choice. Should the patient not respond to therapy, or if the animal exhibits severe signs at initial presentation, surgery, in the form of a bulla osteotomy or interventional otoendoscopy, is often required.
■Idiopathic vestibular syndrome
This is an acute vestibular disorder arising in older dogs, typically at least 10 years of age. The clinical signs are re-lated only to the peripheral vestibular system and are often marked compared to other causes of vestibular disease: extreme ataxia (sometimes with an inability to walk), head tilt and horizontal or rotatory nystagmus. Diagnosis is via signalment, careful clinical observation and exclusion of other causes. There is no specific therapy, but sometimes externa, diagnosis of a peripheral vestibular syndrome should always prompt an otoscopic examination. This should allow the integrity of the eardrum and its color to be verified. If the eardrum is not visible due to secretions or cerumen, the ear should be flushed with a warm saline solution until the eardrum can be seen; a cleansing agent should not be used. If there is suspicion that the middle ear is infected, e.g., the eardrum may appear yellowish due to pus behind it, a myringotomy (puncture of the eardrum) should be performed (Figure 5) using a spinal needle (0.7 x 75 mm) and syringe with otoscopic monitoring; the puncture should be made in the caudal aspect of the eardrum to avoid damage to the auditory ossicles. If no secretion can be extracted this way, the middle ear can be flushed with 0.5 mL of sterile saline and the liquid aspirated; any secretion and the aspirated (based on comparison with the human situation) intravenous fluids and drugs such as propentofyllin (which have been shown to enhance blood flow) may be considered. Occasionally, centrally-acting anti-emetics (e.g., maropitant, promethazin) may be necessary if the patient does not eat due to nausea. As a rule, signs improve within a few days, with reduced intensity of the nystagmus being the first indication of recovery, and although clinical signs generally disappear within 3-4 weeks the head tilt may remain permanently despite resolution of all other symptoms. However, even the head tilt can ameliorate over time as the brain seeks to compensate for the malposition caused by the vestibular damage.
Hypothyroid-associated vestibular disorders exclusively affect older dogs, but general indicators of thyroid insufficiency (sluggishness, obesity, coat alterations, polydipsia) may not be present, and acute onset vestibular signs may be the sole manifestation of disease. Although there are various theories to explain the link between hypothyroidism and vestibular disease, there is currently no conclusive proof as to the exact etiology in this situation. This is generally a peripheral vestibular disorder, although oc-casionally a central syndrome may be seen (5). In some cases, the vestibular signs are combined with other failures of the peripheral nervous system such as limb paresis, facial paralysis and laryngeal paralysis.
The diagnosis is made on determination of decreasedfT4 and increased TSH levels. The TSH stimulation test, using genetically produced human TSH, is often not performed in veterinary practice due to cost, but is gen-erally regarded as the gold standard for diagnosis and should be performed in cases of doubt. Clinical signs frequently improve with thyroid supplementation (levo-thyroxine at 20 μg/kg q12H; maximum 0.8 mg q12H), and recovery often occurs within a few days of commencing treatment, although it can take weeks or even months on therapy for improvement to be noted.
4. Schunk KL, Averill DR. Peripheral vestibular syndrome in the dog: A review of 83 cases. J Am Vet Med Assoc 1983;182:1354-1357.
5. Vitale CL, Olby NJ. Neurological dysfunction in hypothyroid, hyperlipidemic Labrador retrievers. J Vet Intern Med 2007;21:1316-1322.