Functional Neuroanatomy of the Vestibular System

The vestibular system is a special proprioception system responsible for the proper maintenance of posture, orientation and balance of the head and trunk, and position of the eyes in relation to head position or movement. In addition this system coordinates activity with portions of the cerebellum. Neuroanatomically and functionally, the vestibular system can be divided into peripheral and central components.

Peripheral Component

The membranous labyrinth and the vestibular portion of the vestibulocochlear nerve are the peripheral vestibular components in the inner ear. The membranous labyrinth is a series of fluid-filled chambers and tubes, including the cochlea, which manages auditory function, and the utricle, saccule and semicircular canals, which manage vestibular function. The vestibulocochlear nerve comprises sensory neurons of the 8th cranial nerve (CN VIII). CN VIII has bipolar neurons; those functioning in the vestibular system are located in the vestibular ganglion and those functioning in the auditory system are located in the spiral ganglion. These ganglia are located within the bony labyrinth of the petrous temporal bone close to the facial nerve (CN VII) and sympathetic innervation to the face. The function of the utricle and saccule is to detect gravity and linear acceleration and that of the semicircular canals is to detect head rotation.

Central Component

The central part of the vestibular system includes structures within the brainstem and cerebellum. The vestibular portion of CN VIII connects to the vestibular nuclei sited in the medulla oblongata of the brainstem, and to neurons in the rostral portion of the cerebellum. The vestibular nuclei are connected to the nuclei of the oculomotor (CN III), trochlear (CN IV) and abducens (CN VI) nerves and aid in control of eye movements. The vestibular nuclei are also connected to the neurons in the spinal cord via descending pathways which adjust tone in the muscles of the neck, trunk and limbs to oppose gravity to maintain posture.

The first step to diagnose cases with vestibular system abnormalities is localisation of the lesion: determining whether the lesion is in the peripheral or central vestibular system is critically important for making differential diagnosis and predicting prognosis.

Coordination of Eye Movements

In the healthy animal, head rotation induces a compensatory eye movement in the opposite direction to the initial head movement. This coordination of eye and head movement is called the vestibulo-ocular (or oculo-cephalic) reflex, and stimulated from information acquired in the semicircular canals. Information then enters the appropriate motor nuclei of the CN III, IV and VI to control the extraocular muscles. For example, if the head of the animal is turned to the left, movement of the endolymph in the left horizontal semicircular canal increases the activity of the hair cells; in contrast the endolymph activity in the right horizontal semicircular canal is decreased. These responses result in movements of the eyes toward the right and are termed the slow phase of the vestibulo-ocular reflex. If the head is further turned continuously after initiating the slow phase, this slow phase is interrupted by corrective fast movement of the eyes in the same direction of head turning (the 'fast phase' of the vestibulo-ocular reflex). Following the fast phase, the slow phase is resumed. A series of slow and fast phases is called nystagmus.

Clinical Signs of Vestibular Disease

Damage to either the peripheral or central vestibular system leads to various vestibular dysfunctions including head tilt, nystagmus, strabismus, ataxia and gait disturbance.

Head Tilt

Head tilt is a condition of loss of antigravity muscle tone on one side of the neck, usually due to unilateral vestibular dysfunction. As a result, the head tilts towards the loss of muscle tone. In cases with peripheral vestibular lesion, head tilt is towards the side of the lesion. For example, left inner ear disease (i.e., otitis media interna) causes left head tilt. A unilateral central vestibular lesion can cause a head tilt to either side. Bilateral vestibular lesions usually do not cause a head tilt; however, because of the loss of antigravity muscle tone on both sides the affected animal may not be able to hold the neck in a normal position.

Nystagmus

A series of slow and fast phase of the rhythmical eye movement is called nystagmus. Several variations of direction including horizontal, vertical and/or rotational nystagmus can be observed. The direction of nystagmus is defined as the direction of fast phase. Physiological nystagmus describes the normal vestibulo-ocular reflex. In contrast, pathological nystagmus occurs in animals with vestibular disease and can be classified into spontaneous nystagmus and positional nystagmus. Spontaneous nystagmus is the nystagmus which occurs when the head is in a normal stationary position. The direction of spontaneous nystagmus in animals with acute unilateral peripheral vestibular lesion is horizontal or rotational nystagmus. The fast phase is directed opposite to the side of lesion. For example, if the animal had left vestibular disease, the animal may show fast eye movement towards the right, and this is called nystagmus to the right side. Nystagmus of peripheral origin often disappears a few days after its appearance because of compensation by other balance systems (cerebellum, vision), but sometimes it can be induced by altering the head position. Nystagmus of central vestibular origin may be vertical, horizontal or rotational. Therefore spontaneous vertical nystagmus suggests that the lesion is in the central nervous system. Positional nystagmus can be seen when the head is placed in unusual positions, such as upside down. Therefore the examiner should try to induce positional nystagmus by positioning animals in lateral and dorsal recumbency. It is impossible to localise the lesion by assessment of positional nystagmus, although the direction of nystagmus sometimes changes in direction in animals with a central lesion. Usually bilateral vestibular diseases do not show spontaneous and positional nystagmus.

Ataxia and Gait Disturbance

Either peripheral or central vestibular dysfunction can be the cause of ataxia and gait disturbance.

Strabismus

Strabismus is an abnormal position of the eyes. Ventral or ventolateral strabismus can be seen in animals with vestibular disease when the head is raised and the neck is extended (positional strabismus), or spontaneously. Usually strabismus is seen on the same side as the lesion.

Vomiting and Salivation

Some animals show vomiting and salivation secondary to vestibular diseases.

Clinical Localisation of Peripheral Versus Central Vestibular Disease

From the clinical signs mentioned above, it may be difficult to localise the vestibular lesion because most of the clinical signs can be seen in association with lesions affecting either the peripheral or central components. The most important key to determine the lesion location is to identify other clinical signs as compatible with only components of the peripheral vestibular system or that could only be explained by a central lesion (see Figure 1). An accurate history and physical and neurological examination are essential to success.

Figure 1. Neurological signs of peripheral vs central vestibular dysfunction.

Peripheral Vestibular Lesion

Near the peripheral vestibular system, there are CN VII and sympathetic innervations. Therefore key clinical signs in animals with peripheral vestibular diseases may be facial nerve paresis and/or Horner's syndrome (ptosis, enophthalmos, miosis and protrusion of the third eyelid) ipsilaterally to the clinical signs of vestibular dysfunction. Animals do not show proprioceptive deficits due to peripheral vestibular dysfunction.

Central Vestibular Lesion

The single strongest sign of central vestibular disease is the presence of conscious proprioceptive deficits. The examiner must be cautious not to mistake ataxia for proprioceptive deficits, and to perform an accurate neurological examination. In addition, central vestibular lesions may cause neurological signs and cranial nerve deficits other than deficits of CN VII and CN VIII, such as abnormal mental status including depression, stupor or coma; cerebellum deficits including intentional tremor and dysmetria (hypermetria); behavioural changes and seizure; other cranial deficits. One specific clinical sign of central vestibular disease in association with vestibular function is vertical nystagmus.

Peripheral Vestibular Diseases

 Otitis media-interna. Otitis is the most common cause of peripheral vestibular disease in the dog and also common in cats. Bacteria themselves may cause dysfunction and toxin produced by the bacteria may affect to the inner ear.

 Idiopathic vestibular syndrome. The second most common cause of peripheral vestibular disease in dogs, and is also well recognised in cats. There is no age tendency in cats, but dogs tend to be older (~12 years old). Therefore sometimes the disease is known as idiopathic geriatric vestibular disease. Diagnosis of idiopathic vestibular syndrome is by exclusion of other diseases. Proprioception and other cranial nerves should be normal.

 Nasopharyngeal polyps. Inflammatory polyps derive from the lining of the tympanic cavity or auditory tube and are commonly seen in cats aged 1–5 years old.

Central Vestibular Diseases

Any diseases, including neoplastic, inflammatory, toxin ingestion and miscellaneous, which involve the brainstem or cerebellum have the potential to cause vestibular dysfunction. Differential diagnosis should be made according to other clinical signs, including age and speed of onset of the clinical signs.