C.W. Dewey, DVM, MS, DACVIM (Neurology), DACVS
The vestibular system is responsible for the maintenance of posture and balance. It is closely linked with the cerebellum in this function. What is most important for purposes of clinical usage is that you understand the concept of how the vestibular (primarily) and auditory system operates. In general, the receptors are located within a membranous labyrinth, which in turn is encased by the bony labyrinth. The dendritic zones of bipolar neurons for the vestibular neurons (located within the vestibular ganglia within the petrous temporal bone) are in synaptic contact with specialized "hair cells" (the hairs are actually stereocilia) in specific receptors. These receptors in include the crista ampullaris, and maculae for vestibular function. Movement of endolymph in certain directions within the membranous labyrinth will deflect the hair cells and cause either an excitatory or inhibitory synaptic discharge that will be transmitted via the vestibulocochlear nerve toward the brain. Once an impulse is generated along CN VIII, this information is carried to the brain stem at the level of the cerebellomedullary angle. For vestibular axons, connections are made here with the vestibular nuclei (on either side of the 4th ventricle) as well as the cerebellum. The vestibular nuclei give rise to descending vestibulospinal tracts, the most important of which clinically is the lateral vestibulospinal tract (supplied primarily via lateral vestibular nuclei). When activated, this tract will cause ipsilateral facilitation of extensor musculature. Another clinically important vestibular tract is an ascending tract called the medial longitudinal fasciculus (MLF). This tract coordinates eye movements with head movements by synapsing with the CN LMNs of III, IV and VI.
Vestibular Function and Dysfunction
One thing to keep in mind is that there is a peripheral vestibular apparatus on each side of the head for a reason. I like to think of these peripheral components as primarily excitatory, so that the loss of one side will lead to relative hyperactivity from the other side. Tonic activity from both sides of the head generally keeps us all from falling over constantly. In the normal patient, rotating the head to one side will stimulate the vestibular system on that side. This will lead to a jerk nystagmus with a fast phase to the direction the patient is being turned. The fast phase going in the direction of the turn (stimulation direction) is really the only thing about vestibular function you need to memorize. Stimulation of one side of the vestibular apparatus will also lead to excitation of extensor muscles on the side being stimulated. This should make some inherent sense. If a dog or a cat starts to fall to the right, the animal will extend the limbs on the right side to catch itself; also, the neck extensors will be activated and the head will be directed away from the direction of the fall. In this example, the right side was stimulated. The result was the body and head being deviated toward the left side. This dog would also display fast phase nystagmus to the stimulated side (right side). Consider that damaging or removing the left peripheral vestibular apparatus is the same as exciting the right side. In a patient with a left-sided peripheral vestibular lesion, you would then expect a fast phase nystagmus away from the lesion side (i.e., to the right), a head tilt to the left (same side as the lesion), and potentially falling/rolling to the left side as well. It is also common to observe an ipsilateral ventrolateral strabismus or eye deviation due to interference with ascending vestibular pathways (i.e., MLF).
Paradoxical vestibular syndrome refers to the phenomenon in some patients with central vestibular lesions (typically involving the cerebellum) in which the head tilt is away from the lesion side and the fast phase of the nystagmus is directed toward the lesion side. A paradox implies a contradiction-something opposite to what is expected. So, if you have a patient with clinical signs of central vestibular disease and the head tilt is on the opposite side of where you have localized the lesion, you should immediately think of this syndrome as a possibility. The cerebellum is an inhibitory structure-it constantly inhibits activity of other parts of the CNS (including the vestibular nuclei). Compare this with the tonically excitatory or facilitating effect of the peripheral vestibular apparatus. If a patient has a mass lesion affecting the right side of the cerebellum, this will lead to a disinhibition (loss of the constant cerebellar inhibition) of the vestibular nuclei activity on that side, which is the same as excitation. This will then lead to a left-sided head tilt and a fast-phase nystagmus directed to the right. So a right-sided cerebellar lesion may lead to a left-sided head tilt with fast phase nystagmus to the right, similar to what would occur with a left-sided peripheral vestibular lesion. But, how would you know the difference? You could only tell the difference if you found evidence of a right-sided central vestibular lesion (like decreased proprioceptive placing, cranial nerve deficits, etc.) that initially seems contradictory to the left-sided head tilt.
Peripheral vs Central Vestibular Dysfunction
Attempting to ascertain whether a dog or cat with vestibular dysfunction has a peripheral or central lesion is important for several reasons. One reason is that the differential diagnosis list is quite different for peripheral vs central vestibular disorders. Also, central vestibular disorders are generally more challenging and tend to be more serious compared with peripheral disorders. In general, any deficit in a patient with vestibular dysfunction that cannot be explained by a peripheral lesion should be regarded as evidence of central vestibular disease.
Dogs and cats with peripheral vestibular disease typically display head tilt, spontaneous (resting) nystagmus, strabismus, and ataxia. Falling and rolling may also be observed. Other cranial nerves that can be affected because of their proximity to CN VIII in the petrous temporal bone are CN VII and the sympathetic innervation to the eye (i.e., Horner's syndrome). Patients with peripheral vestibular lesions have preservation of strength and have normal proprioceptive positioning responses. Because there is no brain involvement, there will be no alteration of consciousness. Nystagmus tends to be either horizontal or rotary, vertical nystagmus usually indicating central vestibular disease.
In addition to the clinical features associated with peripheral vestibular dysfunction, certain abnormalities are indicative of a central vestibular lesion. These include signs of cerebellar dysfunction (e.g., intention tremor), proprioceptive deficits, mentation changes, vertical nystagmus, and cranial nerve deficits other than VII, VIII and Horner's syndrome. Nystagmus tends to be positional with central lesions as well. However, positional nystagmus also occurs with chronic peripheral vestibular disorders.
In a comparative clinical study of dogs with peripheral or central vestibular disease, various specific clinical signs were evaluated to determine whether or not they were predictive of a peripheral vs. central lesion. It was found that nonambulatory tetraparesis was significantly more likely to occur in dogs with central vs. peripheral vestibular disease, and that both resting nystagmus and veering/leaning to one side were significantly more common in dogs with peripheral vestibular disease compared with dogs with central vestibular lesions. The number of beats per minute (BPM) of resting nystagmus was significantly higher in the peripheral (median rate = 90 BPM) vs. central (median rate = 0 BPM) group of dogs. A resting nystagmus rate of ≥ 66 BPM was found to be highly specific (95%) and sensitive (85%) for the diagnosis of peripheral vestibular disease. Overall, there was an inverse correlation between duration of clinical signs and rate of resting nystagmus. There were no significant differences between the two groups of dogs for either degree of head tilt or rate of post-rotatory nystagmus.
Peripheral Vestibular Disorders
Compared with central vestibular disorders, this is a pretty short list. Several breeds of dogs and cats have been reported with a congenital vestibular disorder, sometimes in association with deafness. Hypothyroidism has been associated with peripheral vestibular disease in dogs. Neoplasia of the ear canal (e.g., ceruminous gland adenocarcinoma) can lead to peripheral vestibular disease. You will encounter acute to peracute idiopathic vestibular disease in both dogs and cats. In cats, this disorder occurs at any age (often young adults), but tends to be seasonal (July–Sept). In dogs, this is seen in older patients and is referred to as geriatric vestibular syndrome. The most common cause of peripheral vestibular disease in dogs and cats is otitis media/interna. Toxins and trauma are occasionally the cause of peripheral vestibular disease in dogs and cats.
Central Vestibular Disorders
The list of central vestibular disorders is fairly extensive. It includes degenerative disorders (e.g., lysosomal storage diseases), congenital anomalies (e.g., intracranial arachnoid cyst), neoplasia (e.g., meningioma, glioma), inflammatory/infectious disease (e.g., GME), ischemic/vascular disease (e.g., cerebellar infarct), toxins (e.g., metronidazole) and trauma.