Etiology

• Central or peripheral nervous system lesions:
  • Central:
    • Head trauma   CNS: brain trauma  .
    • Infectious meningoencephalitides   Equine viral encephalitides  .
    • Metabolic encephalopathies   Liver: hepatoencephalopathy  .
    • Focal brainstem disease, eg protozoal myeloencephalitis   CNS: Equine protozoal myeloencephalitis (EPM)  , parasite migration   CNS: verminous encephalomyelitis  .
    • Mycotoxicosis   Mycotoxicosis  , eg Dallis grass toxicity.
    • Space-occupying lesions, eg abscess, tumor.
  • Acute skull fracture, secondary to temporohyoid osteopathy.
  • Peripheral:
    • Head trauma   CNS: brain trauma  .
    • Polyneuritis equi  Polyneuritis equi  .
    • Temporohyoid osteoarthropathy   Temporohyoid joint: osteoarthropathy  .
    • Otitis media/interna   Ear: otitis externa / otitis media  .
    • Idiopathic.
    • Lightening strike.

Specific

• Otitis media/interna   Ear: otitis externa / otitis media  .
• Temporohyoid bone fusion prior to skull fracutre (chronic problem).

Pathophysiology

• The vestibular system maintains balance, and orientation of the trunk, limbs and eyes with respect to the position and movements of the head.
• The vestibular system comprises a receptor organ within the inner ear (within the petrous temporal bone) and the vestibulocochlear nerve (VIIIth cranial nerve).
• The facial nerve (VIIth cranial nerve) is closely associated with the vestibulocochelar nerve and the petrous temporal bone, and they enter the internal auditory meatus together.
• Vestibular disease usually results in disturbed equilibrium and ataxia, without paresis.
• Disorders are usually unilateral.
• Extension of the suppurative process in otitis media   Ear: otitis externa / otitis media  into the adjacent facial canal and internal auditory meatus or temporohyoid joint disease    Temporohyoid joint: osteoarthropathy  may   →    sudden fracture of the petrous bone    →   acute signs of vestibular and facial nerve dysfunction.
• Extension of the infection through the internal acoustic meatus can   →    focal suppurative meningitis, fever and depression; complicating the signs and worsening the prognosis.
• Idiopathic vestibular syndrome may be the result of transient disease of the vestibular nerve, eg viral or immune-mediated neuritis or labyrinthitis.

Timecourse

• Central and visual accommodation often occurs with time, and the horse can return to its former use, although signs of vestibular disease can still be stimulated by blindfolding (Rombergs test).
• Idiopathic vestibular syndrome: can be very acute in onset and full recovery occurs in 2-3 weeks without treatment. 
• Tremorgenic mycotoxocosis: full recovery occurs a few days to several weeks following early removal of the affected fodder.