Abstract

Bilateral vestibular disease has not been addressed often in the literature, suggesting that it is rare in occurrence or is not appropriately recognized. Since bilateral vestibular cases have a significant ataxia component to them, they can be misdiagnosed as unilateral vestibular disease or cerebellar disease unless a complete neurological examination recognizes the head excursions and the reduced to absent oculovestibular response. This case report of bilateral vestibular disease in a cat documents the radiographic and CT findings and records the unique neurological examination findings via video imaginery. A final diagnosis of bilateral otitis media caused by Pseudomonas infection and ear mite infestation was made and the cat improved with treatment. However a certain amount of ataxia and disorientation remained.

History

A 3-year-old male domestic shorthair cat was referred for progressive disorientation, ataxia and a crouched posture of 3 weeks duration. The cat had been treated with oral enrofloxacin for 15 days (5 mg/kg/d) for an "ear infection" with no improvement.

Physical and Neurological Examinations and Laboratory Findings

Physical examination was normal apart from bilateral ear canal exudates suggestive of inflammation and the presence of Otodectes cyanotis in both ears, confirmed by external ear canal cytology examination.

Neurological examination revealed an ambulatory but disoriented, ataxic cat, with pendulous movements of the head. The cat had an abnormal oculovestibular response characterized by an almost absence of rotation-induced nystagmus as well as an almost complete absence of physiological nystagmus. The extensor postural thrust reaction was moderately slower than normal, but still present. The cat had no other neurological deficits.

See the two neurological examination videos below:

The hematology results showed mild neutrophilia (12.4 x 10³ cells/µL; reference range, 2 to 9 x 10³ cells/µL). Blood biochemistry values were within reference ranges. Serological tests for FIV/FeLV, Cryptococcus and Toxoplasma gondii were negative.

Diagnostic Imaging Findings and Interpretation

Diagnostic radiographs of the skull included open mouth and oblique views of the bullae. The "open-mouth" view revealed a thickened (about 2-3 mm) and irregular osseous wall of the right tympanic bulla (Figure 1). The oblique views (Figures 2 and 3) showed a slightly increased attenuation to the right middle ear cavity (Figure 2) compared with the left side (Figure 3).

A computerized axial tomography (CT) scan of the head demonstrated bilateral thickening and sclerosis of both tympanic bullae walls and increased attenuation in both lumens consistent with soft tissue / fluid inside the middle ear cavities (Figure 4). The main differential diagnoses for these diagnostic findings included bilateral otitis media with extension to the inner ears and inflammatory polyps. Neoplasia was considered unlikely because of the bilateral nature of the findings, the age of the cat, and the lack of bony destruction.

Click on any image to see a larger view

	Figure 1. Open mouth view of tympanic bullae. Note slight sclerosis of right tympanic bulla osseous wall (arrow).
	Figure 2. Oblique lateral view of right tympanic bulla. Compared to the left tympanic bullae (figure 3), there is a slight increase in density, consistent with soft tissue/liquid, within the tympanic bullae.
	Figure 3. Oblique lateral view of the left tympanic bulla is normal.
	Figure 4. Transverse CT image of the skull, showing both bullae. Arrows note abnormalities of a soft tissue/liquid density inside both bullae. Middle ear cavities should show air density.

Treatment

The cat was immediately treated for the ear mites with topical application of Fipronil (2 drops in each ear). A myringotomy was performed by passing a spinal needle through a sterilized otoscopic speculum and flushing the tympanic bulla with a small amount of sterile saline (0.9% NaCl) solution. Culture and sensitivity of the contents of each middle ear cavity⁶ yielded Pseudomonas spp in both ears, sensitive to enrofloxacin. After six weeks of treatment with enrofloxacin, the cat showed a significant improvement. Head excursions improved but did not completely resolve, and the cat seemed to be less disoriented.

Discussion

The vestibular system (VS) is responsible for the body's balance. It can be divided into two separate regions: the peripheral and the central vestibular systems. The peripheral vestibular system consists of the inner ear receptors (labyrinth) inside the petrous temporal bone, and the vestibular portion of the VIII cranial nerve. The central vestibular system comprises the vestibular nuclei inside the brain stem and the flocculonodular lobes of the cerebellum.¹

The vestibular system coordinates gaze and head position in order to keep a stable field of view. It is responsible for orientation of head and body with respect to gravity. Therefore, the most common signs of unilateral vestibular disease are head tilt, loss of balance, circling, falling, rolling (vestibular ataxia) and spontaneous nystagmus. In bilateral vestibular disease, loss of balance occurs but because the disease is bilateral in nature, there is no preferential falling to one side nor is there a preferential head tilt. And instead of having nystagmus, bilateral vestibular cases typically lack spontaneous nystagmus and have little to no physiologic nystagmus, as noted in the case presented here. Bilateral vestibular cases have horizontal head excursions, a crouched posture and an abnormal oculovestibular response, (an absence of normal nystagmus or eye movement as the head is rotated from side to side),¹ such as demonstrated by this case.

A central lesion that affects bilateral vestibular nuclei in the brain stem would necessarily involve and damage a large area of the brain stem, including structures such as the reticular formation as well as ascending and descending motor and sensory pathways. Thus a bilateral vestibular brain stem lesion would result in changes in mentation, weakness of movement, proprioceptive deficits, and other cranial nerve signs. Bilateral vestibular signs without major mental changes and proprioceptive deficits are almost always due to peripheral vestibular disease.

The most common causes of peripheral vestibular signs in the cat are extension of outer and middle ear infections and idiopathic vestibular disease.² The diagnosis of otitis media is based on otoscopic examination, which may reveal an incomplete or abnormal tympanic membrane, and/or diagnostic imaging findings.³ To confirm changes compatible with otitis media, radiographic imaging requires at least 3 views (dorsoventral, open-mouth and oblique lateral) on an anesthetized or deeply sedated patient.⁴ Even so, radiographs are less sensitive than computerized tomography (CT) for bullae disease, as CT is able to differentiate between tissues that differ in physical density by less than 1%. CT scanning also provides cross sectional images of the body, avoiding superimposition of images of structures outside the area of interest.^5,6 Due to the low number of CT slices necessary for diagnosing middle ear disease, the costs of both techniques are comparable.⁴

Magnetic resonance imaging (MRI) has also been reported of great diagnostic value in ear diseases in dogs and cats.^2,8 Although being a more expensive procedure and less sensitive to tissue calcification and bone proliferation, MRI has proven to be more sensitive than CT in lesions involving the central vestibular system because it has a higher sensitivity for subtle changes in soft tissue. Lesions of the caudal fossa can be missed on CT scans because of the scatter artefact induced by the petrous portion of the temporal bone.^8,9 Thus, MRI scans may provide supplementary information in those animals where an accurate diagnosis cannot be established by CT scans.¹⁰

References

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8.  Garosi LS, Dennis R, Penderis J, et al. Lamb CR. Results of magnetic resonance imaging in dogs with vestibular disorders: 85 cases (1996-1999). JAVMA 2001; 218 (3): 385-391.

9.  Jones JC. Neuroimaging. In: Vite CH (Ed.). Braund's Clinical Neurology in Small Animals: Localization, Diagnosis and Treatment. International Veterinary Information Service, Ithaca NY (www.ivis.org), 2004; A3234.0102

10. Allgoewer I, Lucas S, Schmitz SA. Magnetic Resonance Imaging Of The Normal And Diseased Feline Middle Ear. Vet Radiol Ultrasound 2000; 41 (5): 413-418.