The main function of the vestibular system is to adapt the position of the eyes and the body with respect to the position and movement of the head.
Vestibular syndrome is a disturbance of the body’s balance system. It is frequently unilateral, but it can bilaterally affect the vestibular systems. This disorder should be differentiated into peripheral and central causes. The symptoms of peripheral and central vestibular dysfunction can overlap, therefore comprehensive general and neurological examinations can often help differentiate between the two.
Animals with acute vestibular disease can show anorexia or vomiting and salivation due to the unbalance, as the vomiting centre, located within the medulla, receives afferent input from the vestibular nuclei.
During a neurological examination, a common abnormality of the posture at rest in those with vestibular syndrome is head tilt, which results from the loss of anti-gravity muscle tone on one side of the neck. It is characterised by abnormal rotation of the head along the median plane, resulting in one ear being carried lower than the other. It is important to make sure that one eye is also lower than the other because ear diseases can cause a dropped ear without head tilt.
Unilateral vestibular dysfunction will often cause a head tilt. The direction of the head tilt is towards the lower ear and often towards the affected side. The most common exception to this is a lesion affecting the cerebellum (cerebellar peduncle or flocculonodular lobe), which causes central vestibular signs with head tilt contralateral to the lesion. This is called a paradoxical head tilt. In case of a bilateral dysfunction, the head tilt is replaced by wide head excursion.
Unilateral vestibular dysfunction will often cause a head tilt. The direction of the head tilt is towards the lower ear and often towards the affected side
Head tilt should be differentiated from head turn, which is characterised by turning of the nose towards the shoulder and is often accompanied by body turn (pleurothotonus). The median plane of the head does not change, and this is often associated with a forebrain lesion.
In cases of central vestibular syndrome, mentation can be altered, and the animal can show signs of depressed consciousness due to an involvement of the ascending reticular activating system (ARAS) in the brainstem.
Vestibular ataxia is another clinical sign of vestibular syndrome, and it is defined as an uncoordinated gait. The animal can also show a reluctance to move and have a crouched posture, a wide-based stance and a tendency to lean, fall or roll to the same side as the head tilt. Rolling and falling can be very distressing for the owners and the animal and in some cases they can be mistaken for seizures.
Circling can be associated with vestibular dysfunction, and it is usually towards the side of the lesion unless it affects the cerebellum, in which case the circling is contralateral (as with the head tilt). In the case of vestibular disease, circling is often in tight circles (while wide circles are more common with a forebrain disorder) and there is a tendency to fall.
Postural reactions are abnormal in the case of central vestibular syndrome; however, there is evidence of paresis and ipsilateral abnormal postural reactions due to involvement of the upper motor neuron (UMN) pathways to the limbs.
Nystagmus is a physiological response to the movement of the head that permits sustained viewing of a scene (physiological nystagmus). It can be observed while testing the oculocephalic reflex by moving the head of the animal slowly from side to side. When testing the oculocephalic reflex, jerk nystagmus is characterised by involuntary rhythmic movement of the eyes, with a slow phase in one direction (opposite to the head direction) and a quick phase in the other (same direction as the head rotation). If it occurs when the head is at rest, nystagmus is associated with underlying disease (pathological nystagmus).
The direction of the nystagmus (vertical, horizontal or rotatory) can help differentiate between central and peripheral lesions
Pathological nystagmus can be spontaneous (present with a normal head position) or positional (present when the head is in certain positions, eg when the animal is lying on its back or the head is lifted in a sitting patient). The fast phase of the nystagmus is always away from the lesion, therefore the slow phase is towards the lesion. A useful way to remember this is that the slow phase is the pathological phase and the fast phase is corrective or compensatory. By convention, nystagmus is described by reference to the fast phase. It develops from the dysfunction of the pathways responsible for integrating vestibular input with the extraocular eye muscles.
The direction of the nystagmus (vertical, horizontal or rotatory) can help differentiate between central and peripheral lesions, as vertical nystagmus and nystagmus that changes direction with different head positions can often indicate a central lesion. It is worth noting that in Siamese, Birman and Himalayan cats, there is a form of pendular nystagmus (small oscillations of the eyes with no fast or slow component) caused by a congenital abnormality of the visual pathways.
Strabismus, wherein the eye on the affected side deviates ventrally or ventrolaterally, can also be observed with vestibular syndrome. Strabismus is often positional (induced when the head is rotated dorsally) but can also be spontaneous (always present). It is due to a disturbance of the vestibular input.
Clinical signs typical of vestibular syndrome can also be present with other conditions. Head tilt and nystagmus, for example, have been described with thalamic infarcts due to possible damage to adjacent midbrain regions. Head tilt can also be observed with asymmetrical focal cervical lesions.
The vestibular system
The vestibular system is composed of proprioceptors within the inner ear (petrous temporal bone), the vestibulocochlear nerve (CN VIII), four brainstem nuclei and the cerebellum. The CN VIII is the only cranial nerve that does not exit the skull. The proprioceptors (saccule, utricle and semicircular canals) detect the position and movement of the head and then send impulses to the neurons of the vestibular ganglia. These project to the four vestibular nuclei through the internal acoustic meatus, where this information is also integrated and connected to the nuclei of the cranial nerves responsible for eye movement. They also project to the ipsilateral extensor muscles of the limbs (ipsilateral facilitation of extensors and contralateral facilitation of flexors in the muscles of the limbs) and the ipsilateral flocculonodular lobe of the cerebellum via the caudal cerebellar peduncles.
The vestibular system is composed of proprioceptors within the inner ear (petrous temporal bone), the vestibulocochlear nerve (CN VIII), four brainstem nuclei and the cerebellum
These pathways allow the vestibular system to control the position of the eyes, trunk and limbs according to the position and movements of the head. A final pathway will ascend to the cerebrum, providing a conscious awareness of the body’s position in space.
There are several clinical signs that can help the practitioner in diagnosing and localising a vestibular lesion. The presence of a head tilt and abnormal nystagmus are indicative of a disease affecting the vestibular system. Head tilt, circling, leaning, falling or rolling occurs in the direction of the affected side due to the relative facilitation of extensors on the normal side and the lack of facilitation of extensors on the side of the lesion.
In the case of peripheral vestibular syndrome, the clinical signs are:
- Head tilt
- Circling, falling or rolling
- Alert or mild disorientation
- Horizontal or rotary nystagmus where the direction is not altered with head position and fast phase is away from the lesion
- In a case of middle or inner ear disease, Horner’s syndrome and/or facial nerve paralysis or spasm can also be present
The peripheral vestibular system includes the sensory receptors in the membranous labyrinth of the inner ear and the vestibular portion of CN VIII. These structures are within the petrous temporal bone.
The common signs for central vestibular disease are:
- Head tilt
- Circling, falling or rolling
- Patient is depressed, stuporous or comatose
- Horizontal, vertical or rotary nystagmus where the direction may be altered with head position and the fast phase may be towards or away from the lesion
- Deficits of CNs V to XII
- Paresis and proprioceptive deficits
The central vestibular system includes the nuclei and pathways located within the brainstem and cerebellum.
Paradoxical vestibular disease is due to the involvement of the cerebellum, more specifically the flocculonodular lobe or caudal cerebellar peduncle. Head tilt and circling occur contralateral to the side of the lesion. Postural reactions can be abnormal, and they are often ipsilateral to the lesion but contralateral to the head tilt and circling. It can be associated with other cerebellar signs such as head tremors, truncal swaying or ipsilateral dysmetria.
Paradoxical vestibular disease is due to the involvement of the cerebellum, more specifically the flocculonodular lobe or caudal cerebellar peduncle
Bilateral vestibular disease is characterised by head sway from side to side, loss of balance on both sides and symmetrical ataxia with a wide-based stance. Physiological nystagmus cannot be elicited, and positional and spontaneous nystagmus are absent.
It is important to try to localise disease to the central or the peripheral vestibular system as the differential diagnoses and the prognosis vary considerably depending on whether the vestibular deficits are central or peripheral in origin. However, an accurate localisation within the vestibular system may not always be possible, especially in the absence of obvious central vestibular deficits such as abnormal postural reactions. In these circumstances, advanced imaging (MRI and CT scans) and further tests (such as CSF tap and analysis and myringotomy) can be key in the diagnostic process.