Chapter Neurology
Introduction
Vestibular disease is a common neurological presentation in small animal patients, characterized by disturbances in balance and orientation. Understanding the distinction between central (brainstem or cerebellar) and peripheral (inner ear or vestibulocochlear nerve) lesions is crucial for accurate diagnosis, prognosis, and treatment planning. For veterinarians preparing for the Membership in the Australian and New Zealand College of Veterinary Scientists (MANZCVS) in Small Animal Medicine, recognizing the key clinical and diagnostic hallmarks of each form of vestibular dysfunction is a core competency.
Overview of Vestibular Anatomy and Function
1.1. Peripheral Vestibular System
• Structures
– Located in the inner ear (semicircular canals, utricle, saccule) and the vestibulocochlear nerve (Cranial Nerve VIII).
– Responsible for detecting rotational and linear acceleration to maintain balance and coordinate eye movements.
• Transmission
– Sensory information travels via the vestibulocochlear nerve to the vestibular nuclei in the brainstem.
1.2. Central Vestibular System
• Brainstem and Cerebellar Connections
– Vestibular nuclei in the medulla oblongata (brainstem) integrate sensory input and coordinate motor responses.
– Flocculonodular lobe of the cerebellum modulates vestibular tone and fine-tunes posture.
• Pathways
– Ascending tracts to extraocular nuclei (III, IV, VI) facilitate coordinated eye movements (vestibulo-ocular reflex).
– Descending tracts (vestibulospinal) help maintain posture and balance.
2. Clinical Signs of Vestibular Disease
2.1. General Signs Shared by Peripheral and Central Lesions
• Head Tilt
– Usually toward the side of the lesion (ipsilateral head tilt).
• Nystagmus
– Spontaneous or positional, involving horizontal, rotary, or vertical eye movements.
– Vestibulo-ocular reflex disturbances can be observed.
• Ataxia
– Staggering gait; animal may circle or lean toward the affected side.
• Strabismus
– Deviation of the eye (ventral or ventrolateral) due to abnormal vestibulo-ocular control.
2.2. Additional/Distinct Signs
• Peripheral Lesions
– Possible Horner’s syndrome if middle ear sympathetic fibers are impacted (ptosis, miosis, enophthalmos, protrusion of the nictitating membrane).
– Often an acute onset, sometimes accompanied by facial nerve paralysis if the facial nerve (CN VII) is affected in the middle ear.
• Central Lesions
– Other cranial nerve deficits (e.g., CN V, VI), altered mentation, or postural reaction deficits.
– Vertical or changing nystagmus (direction may change when head position changes).
– Ipsilateral proprioceptive deficits if the lesion affects adjacent sensory or motor tracts in the brainstem.
3. Differentiating Central from Peripheral Vestibular Disease
3.1. Key Distinguishing Features
Paradoxical Vestibular Syndrome:
• Occurs with lesions in specific areas of the cerebellum or caudal cerebellar peduncle.
• The head tilt appears contralateral to the lesion due to disrupted inhibitory cerebellar influence on the vestibular nuclei.
3.2. Diagnostic Imaging
• Magnetic Resonance Imaging (MRI)
– Modality of choice for evaluating the brainstem and cerebellum (central lesions).
– Can also help identify middle/inner ear pathology such as otitis media/interna.
• Computed Tomography (CT)
– Useful in detecting bony changes associated with otitis media/interna, skull trauma, or neoplasia.
• Radiography
– May reveal bulla thickening or fluid opacities in cases of chronic middle ear disease, but less sensitive than advanced imaging.
3.3. Other Diagnostic Tests
• Otoscopic Examination
– Important for peripheral disease suspected from chronic otitis externa extending into the middle ear.
• Cerebrospinal Fluid (CSF) Analysis
– Indicated for central disease suspicion, looking for inflammation or infectious agents.
• Infectious Disease Titers
– E.g., Toxoplasma gondii, Neospora caninum, and fungal serology if central infectious disease is suspected.
• Advanced Tests for Inflammatory Brain Disease
– PCR, antibody tests, or specialized imaging techniques as needed.
4. Associated Conditions and Etiologies
4.1. Peripheral Vestibular Disease Etiologies
• Otitis Media/Interna
– Bacterial infection ascending from the external ear canal or hematogenous spread.
• Idiopathic Vestibular Syndrome (Geriatric Vestibular Disease)
– Acute onset in older dogs; spontaneous improvement typically occurs within days to weeks.
• Ototoxic Drugs
– Aminoglycoside antibiotics, chlorhexidine ear flushes, etc.
• Neoplasia
– Ear canal tumors, paraganglioma, or other masses impacting the middle/inner ear structures.
4.2. Central Vestibular Disease Etiologies
• Neoplasia
– Primary brain tumors (meningioma, choroid plexus tumor) or metastatic lesions in the cerebellopontine angle or brainstem.
• Inflammatory or Infectious Encephalopathies
– GME (Granulomatous meningoencephalomyelitis), necrotizing encephalitides, viral or fungal infections.
• Vascular Events
– Cerebellar infarcts or ischemic strokes affecting the vestibular nuclei or associated tracts.
• Trauma or Hemorrhage
– Direct brainstem injury from skull fracture or coagulopathies.
5. Treatment Considerations
5.1. Peripheral Vestibular Disease Management
• Identify and Address Underlying Cause
– Treat otitis media/interna with appropriate antibiotics (guided by culture), possibly surgery (bulla osteotomy) if severe.
• Supportive Care
– Antiemetics, antivertigo medications (e.g., meclizine) for symptomatic relief.
– Fluid therapy, nursing care to prevent falls or motion sickness.
• Prognosis
– Generally good if the underlying cause is resolved.
– Idiopathic or geriatric vestibular disease often shows improvement within 1–2 weeks.
5.2. Central Vestibular Disease Management
• Disease-Specific Interventions
– Neoplasia: surgical resection, radiation therapy, or chemotherapy where indicated.
– Inflammatory/Immune-Mediated: immunosuppressive corticosteroids or other immunomodulatory agents.
– Infectious: targeted antimicrobial, antifungal, or antiparasitic therapy based on etiology.
• Supportive Care
– Control of intracranial pressure, seizures, or concurrent neurological deficits.
– Hospitalization for intensive monitoring in severe cases.
• Prognosis
– Highly variable, depending on lesion type, location, and treatability (e.g., benign vs. malignant tumor, extent of inflammation).
6. Prognosis and Follow-Up
6.1. Monitoring Clinical Improvement
• Watch for reduction in head tilt, resolution of nystagmus, and improvement in gait stability.
• Postural reactions and other neurological deficits (especially central) may take longer to recover or may remain permanent in advanced disease.
6.2. Diagnostic Re-Evaluation
• Repeat imaging to assess lesion response (e.g., tumor shrinkage, resolution of inflammation).
• Periodic CSF taps if inflammatory conditions are present.
• Continued otoscopic exams and imaging if the middle ear or bulla was involved.
Conclusion
Differentiating central from peripheral vestibular disease is essential for accurate diagnostic and therapeutic strategies. Key clinical cues (e.g., presence of vertical or changing nystagmus, altered mentation, cranial nerve deficits beyond VII, or proprioceptive deficits) strongly suggest a central lesion, while Horner’s syndrome or isolated facial nerve involvement often indicate a peripheral cause. Thorough diagnostic workups, including advanced imaging, otoscopic evaluation, and possible CSF analysis, enable targeted treatments—ranging from antibiotics for otitis media to immunosuppression for encephalitis or surgical intervention for neoplasia. Mastery of these concepts is vital for clinicians preparing for MANZCVS in Small Animal Medicine, ensuring optimal patient outcomes in vestibular disorders.
Suggested References
Lorenz, M. D., Coates, J. R., & Kent, M. (2010). Handbook of Veterinary Neurology (5th ed.). Saunders.
Garosi, L. S. (2013). Neuroanatomical diagnosis: Brain. In Platt, S. R., & Olby, N. J. (Eds.), BSAVA Manual of Canine and Feline Neurology (4th ed.). BSAVA.
Sturges, B. K. (2011). Canine idiopathic vestibular disease. Clinician’s Brief, 9(2), 45–48.
Lowrie, M. (2015). Vestibular signs and diseases affecting the vestibular system in dogs and cats. Veterinary Record, 177(15), 383–389.
de Lahunta, A., Glass, E., & Kent, M. (2014). Veterinary Neuroanatomy and Clinical Neurology (4th ed.). Saunders.
Gonçalves, R., & Carrera-Justiz, S. (2017). Clinical approach to central vestibular disease. Veterinary Clinics of North America: Small Animal Practice, 47(1), 89–102.
