15 Head tilt and nystagmus
The vestibular system is responsible for balance and coordinating movements of the eyes, trunk and limbs with changes in head position. Head tilt and nystagmus are typically associated with unilateral vestibular disease and are relatively common presenting signs in canine emergency patients. Jerk nystagmus is typical of vestibular disease with the slow (pathological) phase towards the side of the lesion and the fast (corrective) phase away from the side of the lesion. Occasionally animals present with nystagmus due to cerebellar disease (paradoxical vestibular disease) or abnormalities in visual pathways. Other clinical signs associated with vestibular disease are listed in Box 15.1.
The central component of the vestibular system is located in the brainstem and cerebellum. Nystagmus in central vestibular disease can be horizontal, rotatory, vertical or positional, with the fast phase towards or away from the lesion. Affected animals may well have additional clinical signs that reflect brain involvement, such as reduced mentation from depression through to coma, or ipsilateral paresis and proprioceptive deficits.
The peripheral components of the vestibular system are sensory receptors in the inner ear and the vestibular portion of vestibulocochlear nerve. Nystagmus in peripheral vestibular disease can be horizontal or rotatory, with the fast phase away from the side of the lesion. Affected animals may have normal mentation or they may be markedly disorientated; signs of brainstem abnormality are not expected unless there has been extension of inner ear disease. Paresis and proprioceptive deficits should not occur.
Horner’s syndrome (third eyelid protrusion, pupillary constriction, drooping of upper eyelid, enophthalmos) and facial nerve deficits (including ipsilateral drooping of and inability to move ear and lip, widened palpebral fissure, absent blinking) may occur with peripheral vestibular disease but are typically not recognized in the idiopathic form.
Paradoxical vestibular disease refers to a syndrome of nystagmus, head tilt and circling due to cerebellar disease. Head tilt and circling occur contralateral to the side of the lesion and there are usually other more typical signs of cerebellar disease (e.g. head tremor, ipsilateral dysmetria).