Vestibular disease

Anatomy and function

The major function of the vestibular or balance system is the transmission of information from the inner ear balance organ to the brain about the position of the head in the three dimensional space. In this way brain is able to give orders to the muscles about the muscle tone, their needed contraction or relaxation so that body is staying in the upright position and is not falling. Balance system is in tight connection also with the innervation of the eyes as when the head is moving the eyes are following without delay. The receptor cells of the vestibular system are located in the inner ear and are very close to the hearing organ. When the animals head is moving the impulse is generated in the receptor cells of the balance organ in the inner ear and along the vestibulocochlear nerve are reaching the vestibular nuclei located in the brainstem. The receptor and the vestibulocochlear nerve are the so called peripheral part of the vestibular system and the brainstem nuclei are the central part of the vestibular system.

Diseases

Vestibular-balance system diseases are frequent in dogs and cats. The most frequent are acquired diseases. It was mentioned before that vestibular system has two parts: peripheral and central. Both parts of the system might be disturbed and cause balance problems for the animals. Independently where is the disease process the animal is usually showing head tilt, rhythmical movements of the eyes (nystagmus), abnormal positioning of the eyes (strabismus) and uncoordinated gait or tendency to drift and fall to one side. In addition the central vestibular disease will frequently cause signs of weakness or paralysis as well as depression of the mental status. By performing the neurological examination it can be decided if the lesion is in the inner ear (peripheral) or in the brainstem (central). It is important to distinguish between the two localizations as the diseases of the brain frequently are harder to treat and the diseases of the inner ears are having much better prognosis.

The most frequent diseases of the inner ears are the inner ear infections which come from the external/middle ear infections although the latter do not necessary have to be easily detectable. The inner infections are caused by bacteria usually and can be successfully treated with long term antibiotic therapy. Also some medicine administered topically (from the external ear to the middle ear if the tympanic membrane is damaged) or systematically (in the form of tablets through the bloodstream) can cause damage to the inner ear structures, therefore dog owners should never administer any medication in to the dog or cat ear without consultation with the doctor. Traumas, tumors and other diseases can affect the peripheral vestibular system but hey are rather rare. In contrary the second most frequent condition affecting the peripheral vestibular system is so called idiopathic vestibular disease the exact mechanism of which is not completely understood. As the disease is frequently observed in older dogs it is sometimes called old dog vestibular disease or geriatric vestibular disease. The typical for this disease is that the balance problems are usually getting better with time without any therapy but might have a relapses. Another cause for the disturbance in the balance system might be the hypothyroidism, therefore the thyroid function should be checked in older dogs with vestibular disease in order to distinguish between the idiopathic and the disease caused by thyroid gland dysfunction. The inherited form of the vestibular disease has been observed in several breeds of dogs (German Shepherds, Cockerspaniels, Dobermans etc) and in cats. The symptoms start to be visible when the animal starts to walk, they usually gradually improve with time as animal learns to compensate the deficits with the help of the viual and the proprioceptive systems.

The central vestibular diseases are usually caused by anomalies, vascular lesions, traumas, inflammatory-infectious diseases and tumors. Therefore it is extremely important to distinguish between the peripheral and the central diseases as they have different origin, therapy and prognosis.

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