Based on neurologic examination findings, a vestibular lesion can be localized to the peripheral or central vestibular system.1 This distinction is important since the differential diagnoses, work-up, treatment, and prognosis in patients with peripheral vestibular dysfunction are different from those in patients with central vestibular dysfunction.
Here's how to first determine whether a vestibular problem stems from the inner ear or the brain and then how to pinpoint the underlying cause and administer appropriate treatment.
PERIPHERAL VS. CENTRAL VESTIBULAR ANATOMY
The vestibular system maintains balance and coordinates the position of the head, eyes, neck, and limbs in space.
The central vestibular system comprises eight vestibular nuclei in the brainstem (four on each side), two caudal cerebellar peduncles, and the flocculonodular lobe of the cerebellum (Figure 2).1
SIGNALMENT AND HISTORY CLUES
Although rare, puppies may have a congenital peripheral or central vestibular disorder.3,4 Vestibular dysfunction due to neoplasia is more common in older dogs, although a primitive neuroectodermal tumor may cause vestibular signs in a young dog.
Vestibular signs are often acute in onset regardless of the lesion's location. Metronidazole toxicosis as a cause of vestibular signs may be supported by a history of metronidazole treatment. Dosages as low as 67.3 mg/kg/day in a dog and 58 mg/kg/day in a cat have been documented to cause vestibular signs.5,6 I have diagnosed metronidazole toxicosis in a dog that received 40 mg/kg/day. A recent ear infection may precede peripheral vestibular dysfunction. A systemic disease such as hypothyroidism or hyperadrenocorticism—possibly uncovered through a physical examination and laboratory testing—may cause peripheral or central vestibular dysfunction.
IS IT PERIPHERAL OR CENTRAL VESTIBULAR DYSFUNCTION?
Classic neurologic signs
The hallmark neurologic signs of vestibular dysfunction, whether peripheral or central, are1
Head tilt, vestibular ataxia, and strabismus are most commonly ipsilateral to, or on the same side as, the vestibular lesion.
Identifying animals with chronic vestibular dysfunction can be challenging because they commonly adapt to their imbalance. Blindfolding the animal or examining the animal in a dark room or walking stairs is challenging for the vestibular system and may result in eliciting a spontaneous nystagmus or a head tilt that was not present initially. Also, placing the animal in dorsal recumbency may induce a spontaneous nystagmus in an animal that has adapted to its chronic vestibular dysfunction, regardless of whether the lesion is peripheral or central.
Assessing postural reactions
The neurologic examination may be challenging in very ataxic animals. Often extra support and a nonslippery surface are needed to assess postural reactions. In my experience, the paw replacement test (previously called conscious proprioception) is the most reliable test in vestibular patients. Testing hopping, wheel barrowing, and hemiwalking are not recommended in animals presenting with clinical signs of vestibular dysfunction since the movement frequently exacerbates the ataxia and discomfort of the animal, and the results of the tests may be difficult to interpret. If the patient is severely ataxic, it may not be possible to stand the patient up to test postural reactions. In these cases, the patient may adapt to the vestibular dysfunction and testing may be easier after a day or two.
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
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
Peripheral vestibular dysfunction
Recommended diagnostic procedures in patients determined to have peripheral vestibular dysfunction include
Idiopathic vestibular dysfunction is diagnosed after ruling out all other causes of peripheral vestibular dysfunction.
Central vestibular dysfunction
Recommended diagnostic procedures in patients determined to have central vestibular dysfunction include
The thick bones of the tympanic bullae frequently cause artifacts on CT, which prevent good visualization of the brainstem. For this reason, CT is not a good diagnostic tool for imaging animals with central vestibular dysfunction. The CSF tap is preferably done after the MRI, since the likelihood of cerebellar herniation is greater with a lesion in the caudal fossa (in the area of the brainstem and cerebellum) than with a lesion in the rostral fossa. If cerebellar herniation is present, CSF collection may carry a higher risk of death and should not be performed.
TREATMENT AND PROGNOSIS
Treat all patients with vestibular dysfunction with supportive care. In some severe cases of rolling, the patient may need to be strapped down to a board. Soft padding of the cage is recommended to prevent the patient from injuring itself when rolling or falling. Patients that are vomiting or are inappetent may feel better if treated with an antihistamine such as meclizine.
General treatments and prognoses for selected causes of vestibular syndrome include
Signs of vestibular dysfunction are easily recognized, but it can be difficult to differentiate peripheral from central dysfunction since a neurologic examination is challenging in severely ataxic patients. However, the diagnostic work-up, treatment, and prognosis may differ between diseases resulting in peripheral vs. central dysfunction, so the challenge is worth accepting.
Helena Rylander, DVM, DACVIM (neurology)
Department of Medical Sciences
School of Veterinary Medicine
University of Wisconsin
Madison, WI 53706-1102
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