Other telltale signs of peripheral vestibular dysfunction
Peripheral vestibular dysfunction is caused by pathology in the inner ear, which often occurs together with a middle ear problem.
The sympathetic innervation to the eyes travels near the middle ear in dogs and through the middle ear in cats. The facial
nerve exits the brainstem in the same dural sheath as the vestibular and the cochlear nerves, travels in the facial canal
of the petrous temporal bone, and continues near the middle ear to exit the skull through the stylomastoid foramen just caudal
to the tympanic bulla. Therefore, findings of both facial paralysis and Horner syndrome ipsilateral to a head tilt are highly
suggestive of peripheral vestibular dysfunction on that side.
Other telltale signs of central vestibular dysfunction
To diagnose central vestibular dysfunction, other signs of brainstem dysfunction are useful to localize the lesion to the
central nervous system. Postural reaction deficits are present in most dogs with central vestibular dysfunction. Mentation
change—often mild to moderate obtundation (due to the reticular formation being affected)—is also seen in about half of cases
of central vestibular dysfunction. Other cranial nerve deficits may be present (reduced facial sensation [CN V], facial nerve
paralysis [CN VII], reduced gag reflex [CN IX], reduced tongue movement [CN XII]). In one study of 20 dogs with central vestibular
dysfunction, 95% of the dogs had abnormal postural reactions, 45% of the dogs had mentation change, and 60% of dogs had cranial
nerve deficits other than CN VIII.7
Hemiparesis may be difficult to detect but is only present in central vestibular dysfunction. The direction of the nystagmus
in animals with central vestibular dysfunction is usually not helpful. However vertical nystagmus is more common in animals
with central vestibular dysfunction than in animals with peripheral vestibular dysfunction. The clinician should be careful
localizing a lesion based on direction of the nystagmus, since peracute peripheral vestibular dysfunction may present with
vertical nystagmus and mixed horizontal and rotary nystagmus.
In paradoxical vestibular syndrome, in which the signs of head tilt and ataxia are to the side opposite the lesion, if horizontal nystagmus is present, the
fast phase is toward the lesion, and abnormal postural reactions are ipsilateral to the lesion. This form is most often caused
by a lesion in the caudal cerebellar peduncle or the flocculonodular lobe of the cerebellum.1
Once the lesion has been presumptively localized to the peripheral or central vestibular system, a list of differential diagnoses
can be made. Table 2 lists underlying causes of peripheral and central vestibular dysfunction.
Table 2: Differential Diagnoses in Dogs and Cats with Vestibular Dysfunction
Peripheral vestibular dysfunction
Differential diagnoses in patients with signs of peripheral vestibular dysfunction include otitis interna, hypothyroidism,
middle or inner ear neoplasia, nasopharyngeal polyps, old dog vestibular dysfunction, idiopathic feline vestibular dysfunction,
trauma, and aminoglycoside toxicosis. Primary secretory otitis media in cavalier King Charles spaniels may affect hearing
and also sometimes causes peripheral vestibular signs.8,9
In a study including 27 dogs with peripheral vestibular dysfunction, 67% (n=18) of the dogs had abnormalities in the middle
ear; 41% (n=11) had magnetic resonance imaging (MRI) changes compatible with a diagnosis of otitis media, and 26% (n=7) had
MRI abnormalities compatible with middle ear neoplasia.10 In addition, 7% (n=2) had lesions in the cranial cavity but outside the brain parenchyma found on MRI. Seven dogs (26%)
did not have any abnormalities found on MRI.
Central vestibular dysfunction
Differential diagnoses in dogs with signs of central vestibular dysfunction include intracranial intra-arachnoid cyst, Chiari-like
malformation, syringomyelia, hypothyroidism, hyperadrenocorticism, primary or secondary neoplasia, infectious or inflammatory
disease, vascular insult, and metronidazole toxicosis.5,11-16