Vestibular disease is a disorder of the vestibular system — the sensory apparatus responsible for detecting head position and movement and for maintaining balance and spatial orientation. In horses, the vestibular system includes peripheral components within the inner ear (the semicircular canals and otolith organs) and central components in the brainstem and cerebellum. Disease affecting either component produces a recognizable clinical syndrome characterized by head tilt, circling toward the affected side, ataxia (uncoordinated movement), and nystagmus (involuntary rhythmic eye movement).
Peripheral vestibular disease, arising from the inner ear, is more common than central forms and is generally associated with better recovery. Causes include otitis interna (inner ear infection), trauma to the temporal bone, idiopathic (“sudden” origin with no identifiable cause) vestibular dysfunction, and in some cases middle ear polyps or neoplasia. The characteristic finding is a horizontal or rotary nystagmus with the fast phase directed away from the lesion side, combined with a head tilt toward the lesion.
Central vestibular disease, involving the brainstem, is more serious. Equine protozoal myeloencephalitis (EPM), caused by Sarcocystis neurona, is among the most common causes of central vestibular signs in North American horses. Other causes include toxic plant or drug exposure from plants or drugs affecting the central nervous system, hepatic encephalopathy, and space-occupying lesions. Central disease may be accompanied by depression, facial nerve deficits, and other signs of brainstem dysfunction that help distinguish it from peripheral forms.
Diagnosis involves a thorough neurological examination, CSF analysis, EPM serology, and advanced imaging when available. Many horses with peripheral vestibular disease recover with supportive care; central disease prognosis depends on the underlying cause and the speed of intervention.