The vestibular system is responsible for maintaining normal body position and coordination. The vestibular system is divided into two portions, the peripheral vestibular system (inner ears, vestibulocochlear nerve) and the central vestibular system (brainstem). There are also vestibular structures in the cerebellum (fastigial nucleus, flocculus, nodulus) and caudal cerebellar peduncle. Dysfunction in these areas of the cerebellum typically causes paradoxical central vestibular signs (see below). With classic vestibular dysfunction, the head tilt and vestibular ataxia are usually toward the side of the lesion and the fast phase of nystagmuus is away from the side of the lesion (nystagmus “runs away” from the lesion). Postural reaction deficits are always ipsilateral to the lesion with vestibular dysfunction. Be careful during the early stage of severe, acute vestibular dysfunction because the nystagmus will occasionally look vertical, but it’s actually rotary. In addition, postural reaction testing may be difficult in moderately to severely affected patients. Try to give the patient at least 24 hours before definitively calling a patient central vestibular and/or considering euthanasia (unless other clear signs of central disease are present). Many patients with severe vestibular dysfunction will improve and even return to normal. This is definitely a “don’t judge a book by its cover” situation.
Paradoxical central vestibular dysfunction
At times, the abnormalities noted on exam are suggestive of central vestibular disorder (e.g., vertical nystagmus, postural reaction deficits), but the signs do not follow the “rules” listed above. This is called paradoxical central vestibular dysfunction and is due to disease in the cerebellum or caudal cerebellar peduncle. The paradox occurs because the head signs suggest a lesion on one side of the body, while the postural reaction deficits indicate the other side. Believe the postural reaction deficits as they are always ipsilateral to the lesion in patients with vestibular dysfunction. Occasionally, animals with cerebellovestibular dysfunction will have an absent menace response with intact vision (ruling out an optic nerve lesion) and intact palpebral reflex (ruling out a facial nerve lesion). As with postural reaction deficits, absent menace response is always ipsilateral to the lesion in patients with cerebellovestibular dysfunction.
Case example: A 10-year-old MC Greyhound is presented to you with an acute onset of non-progressive clinical signs of right head head tilt, vestibular ataxia, resting & positional left rotary nystagmus, and left-sided postural reaction deficits. In this patient, the head tilt and nystagmus suggest a right-sided lesion while the postural reaction deficits suggest a left-sided lesion. The neuroanatomic diagnosis for this patient would be left paradoxical central vestibular.
Bilateral peripheral vestibular dysfunction
Otitis interna is the most common cause of bilateral peripheral vestibular dysfunction, but bilateral signs certainly can be observed in patients with other conditions (e.g., hypothyroidism). These patients often are presented with signs of vestibular dysfunction, including vestibular ataxia (often to both directions) and horizontal or rotary nystagmus. Many of these patients do not have a head tilt, or they have a head tilt that intermittently changes sides. Patients often walk low to the ground or crouched and will have wide side-to-side lateral head excursions.
Vestibular dysfunction due to thalamic disease
Just to confuse things even more, central vestibular signs occasionally occur secondary to a thalamic lesion, most often due a thalamic infarct. This is not very common. It is more important to remember the basic rules of localizing peripheral vs. central dysfunction.