Dizzy or "ataxia," meaning lack of coordination, is a sensory dysfunction. The 3 forms of ataxia based on neuroanatomical localization and clinical purposes are: vestibular, cerebellar and sensory (spinal or general proprioceptive) ataxia. This lecture focuses on vestibular disease.

The vestibular system detects gravity, linear acceleration and rotational movement of the head, and controls the muscles involved in maintaining equilibrium, positioning the head and regulating eye movements. The two functional components of the vestibular system are: the peripheral portion in the inner ear (membranous labyrinth and CN 8 vestibular portion) and the central portion in the brainstem (CN 8 vestibular portion synapses on the 4 pairs of vestibular nuclei and on cerebellar neurons, via medial longitudinal fasciculus (MLF) connecting to CN III, IV, VI nuclei to control eye movements, and descending pathways to the spinal cord grey matter to provide tone to extensor muscles of the neck, trunk and limbs which oppose gravity to maintain upright posture. Vestibulo-ocular reflex (VOR) should be present in all alert normal animals. This requires normal function of the vestibular system components, normal medial longitudinal fasciculus (MLF) bilaterally, and normal neurons in the oculomotor (CN III), trochlear (CN IV) and abducens (CN VI) nuclei (Fig. 1).

Figure 1. Coordination of eye movement in horizontal vestibulo-ocular reflex (VOR) When the head is moved horizontally, primary afferents from the horizontal semicircular canals project to specific neurons in the vestibular nuclei. Most of these cells send excitatory signal through the MLF to the contralateral abducens nucleus (lateral rectus muscle), and to the ipsilateral oculomotor nucleus motor neurons (medial rectus muscle). Inhibitory signals are also sent from the vestibular nuclei to the ipsilateral abducens nucleus. This allows movement of both eyes in one direction, and this slow phase helps to stabilize images on the centre of the retina during head movement. This if followed by the fast phase (eyes jerk to the centre of the orbit in the direction of head movement).

When a dizzy cat is presented, we need to make a neuroanatomical diagnosis and differentiate between the 3 forms of ataxia (see chart below).

Diagnostic Work-Up

Take history to determine the onset of signs (sudden or insidious), progression (deterioration, static or improving), previous history of trauma, otitis externa, use of topical (e.g., chlorhexidine, iodine, ear cleansers) or systemic medication (e.g., aminoglycoside antibiotics especially in high doses and therapy for longer than 14 days, or in patient with reduced renal function), or other illnesses (renal insufficiency).

Perform Neurological Exam

Ideally, locate the lesion to the peripheral or central vestibular system. However, it is not uncommon that the patient in extreme distress and disorientation that localization is impossible. In addition, some animals with central vestibular lesions may not demonstrate signs of brainstem involvement and appear clinically identical to those with peripheral vestibular disease.

Ancillary Diagnostic Tests Include

Examination of the external ear canal and oropharynx: to look for inflammatory polyp, or bulging/ruptured tympanic membrane. However, an intact tympanic membrane does not rule out otitis media-interna (OMI).

Diagnostic Imaging

Radiography of the tympanic bullae requires GA and should include: dorsoventral, oblique lateral, and rostroventral-caudodorsal open-mouthed views. Abnormal findings for OMI may include increased opacity within the tympanic cavity and sclerosis of the bullae. However, "normal" radiographic finding does not exclude OMI. Cross-sectional imaging such as MR and CT provides greater anatomic detail and thus a more sensitive method of imaging the middle and inner ear than radiography. CT is able to identify proliferation, sclerosis or lysis of the tympanic bulla and the presence of soft/fluid tissue density within the bulla suggestive of (OMI). In one study, subclinical middle ear disease is relatively frequent in cats undergoing CT imaging of the head thus interpret in light of clinical picture. For idiopathic feline vestibular disease, diagnosis requiring exclusion of other causes by MR/ CT, and a normal cerebrospinal fluid (CSF) analysis. Lysis of the tympanic bulla, petrous temporal bone and calvaria with soft-tissue proliferation are seen on CT in neoplastic cases. MR is superior in detecting otitis interna based on absence of normal signal intensity from the labyrinthine fluid on T2W images. MR is also able to identify meningeal enhancement on T1W post-contrast images suggestive of OMI with intracranial extension. Not to mention that the brainstem and cerebellum can be readily imaged by MR, unlike CT in which beam-hardening artifact obscure examination of the caudal fossa. MR finding (T2W/FLAIR sequences) is characteristic for thiamine deficient cats: symmetrical hyperintense lesions within the vestibular nuclei and other brainstem nuclei.

Serology or CSF Analysis

Cerebrospinal fluid may provide further clues if concurrent inflammatory CNS disease exist. Isolation of infectious organism on CSF may be seen in central extension of OMI cases. However, CSF result can be normal in some inflammatory CNS diseases such as MUE. For cryptococcal encephalitis, the diagnosis is by identifying organism on CSF, or detection of cryptococcal capsular antigen in serum or CSF. In feline toxoplasmosis, a 4-fold increase in IgG antibody in serum and/or CSF is diagnostic, or high IgM antibody levels in serum or CSF is supportive. A positive PCR for the detection of coronavirus RNA in tissues and fluids (e.g., CSF) is suggestive of feline infectious peritonitis meningoencephalitis (FIPV).

Myringotomy to obtain fluid from the middle ear for culture and sensitivity tests: use a 22-gauge spinal needle passed through an otoscope. Attach a 5-cc syringe to the needle and gently aspirate. Warn the owners that the vestibular signs may be exacerbated temporarily following the procedure.

If available, brainstem auditory evoked response (BAER) testing can estimate peripheral (i.e., assess deafness) and brainstem auditory function.

Tissue biopsy: may be required in cases suspicious of FIPV-immune-histochemical staining for coronavirus antigen in paraffin-embedded tissues (requires biopsy), and identifying the Toxoplasma gondii organism in tissue is diagnostic.

Examples of common feline vestibular diseases

References

References are available upon request.