The vestibular system has two main functions
1.
To maintain the visual image by stabilizing the eyes in space during head movement, and
2. To stabilize the position of the head in space - thus ensuring that the position of the body is stable.
Clinical Signs
Vestibular dysfunction causes varying degrees of loss of equilibrium causing imbalance and ataxia. Strength is not affected,
and therefore paresis is not observed. As a rule, the disturbance is unilateral or asymmetrical, and the signs are those of
an asymmetrical ataxia with preservation of strength.
Unilateral vestibular signs may result from either central (brain stem) or peripheral (labyrinth) disease. Signs of vestibular
disease include: falling, rolling, tilting of the head, circling, nystagmus, positional strabismus (deviation of one eye in
some head positions), and an asymmetrical ataxia.
Nystagmus
Disturbed vestibular input to the neurons innervating extra-ocular eye muscles results in abnormal nystagmus. Nystagmus probably
occurs at some time during all types of vestibular disease. Nystagmus is an involuntary rhythmic oscillation of the eyeball
that nearly always affects both eyes equally. Typically, nystagmus consists of a slow phase in one direction and a fast phase
in the other. It is customary to describe nystagmus in terms of the fast phase, despite the fact that in most cases the slow
phase may be directed towards the affected side. Nystagmus tends to occur early in the course of peripheral vestibular disease,
and to disappear later.
Physiological nystagmus may be induced in normal animals. It occurs with normal turning of the head from side to side, or
up and down (vestibular in origin), or after rotation (post-rotational nystagmus). If nystagmus occurs when the head is stationary,
and there is no rotation or movement of the surroundings, it is called spontaneous nystagmus. Spontaneous nystagmus usually
is pathological in origin and may be horizontal, rotatory, or vertical in direction. If nystagmus occurs only when the head
is placed in an unusual position (e.g., laterally or dorsally), it is known as positional nystagmus. Nystagmus that consists
of eye movements of the same velocity in each direction is termed pendulous nystagmus, and is not of vestibular origin. Pendulous
nystagmus usually is associated with visual pathway deficits.
Abnormal Posture and Ataxia
Loss of co-ordination between head, trunk, and limbs, results in loss of balance. This may result in a head tilt. The trunk
may fall, or even roll, to one side. The trunk may be flexed laterally. Animals tend to circle. These usually are circles
with a small radius. It may be possible to elicit mild hypertonia and hyperreflexia in the limbs on one side.
An animal will often fall when attempting to shake its head. Vision will assist an animal to compensate for a vestibular system
deficit. Blindfolding an animal with a vestibular lesion may accentuate the clinical signs.
Postural Reactions - Strabismus
When the head is extended in a tonic neck reaction, the eyeballs should remain in the center of the palpebral fissure in dogs
and cats. This often fails to occur on the side of a unilateral vestibular disturbance, and results in a ventrally deviated
eyeball. Occasionally, in vestibular disease, an eyeball is noticed to deviate ventrally or ventrolaterally without extension
of the head and neck. This appears as a lower motor neuron strabismus, and may be corrected by moving the head into a different
position, or by inducing the patient to move its eyeballs to gaze in different directions. This is referred to as vestibular
strabismus. The ventrally deviated eyeball is on the side of the vestibular lesion. Occasionally, the opposite eyeball may
appear to be deviated dorsally.
Peripheral Vestibular Disease
Peripheral lesions involve the middle and inner ear. Middle ear (bulla tympanica) lesions usually produce head tilt (ipsilateral
to the lesion) only, in the absence of other signs. Horizontal or rotatory nystagmus also may be seen. Inner ear disease,
which actually involves the receptors and vestibular nerve within the petrosal bone, usually produces other signs in addition
to the ipsilateral head tilt - falling, rolling, circling, nystagmus, positional strabismus, asymmetrical ataxia.
Horner's syndrome (miosis, ptosis, enophthalmos) of the ipsilateral eye may be present with either middle or inner ear disease
in dogs and cats, because the sympathetic trunk passes through the middle ear in close proximity to the petrosal bone.
The facial nerve may be affected in inner ear disease, as it courses through the petrosal bone in contact with the vestibulocochlear
nerve. The primary characteristics of unilateral peripheral vestibular disease are: asymmetrical ataxia without deficits in
postural reactions, and a horizontal or rotatory nystagmus that does not change in direction with different head positions.
The fast phase of the nystagmus is directed away from the affected side.
