Fig. 11.12. Caloric testing of vestibular function. Fixation is excluded, e.g., by means of Frenzel glasses. With the head reclined, the external auditory canal on one side is irrigated with water at 44°C. This warms the endolymph in the lateral sector of the horizontal canal, whereas the endolymph in the medial sector of the horizontal canal remains unaffected. The warmer fluid in the lateral sector tends to rise, setting up a weak circulation within the canal. This tiny circulation suffices to bend the ciliae on top of the sensory cells of the cupula. With a healthy vestibular apparatus, nystagmus is generated with its slow phases in the same direction as the circulation of the endolymph in the right stimulated canal tibular organ is damaged, the afferent signal (in case of a stationary head the resting activity) expected to come from both sides comes from the left side only. This disequilibrium is interpreted by the brain to mean that the head is turning to the left. This causes a feeling of dizziness, comparable to the feeling that a healthy observer has when stepping off a carousel. The environment appears to be turning in the direction opposite to that of the slow phase, and a sensation of nausea is often present.
Fortunately, the nystagmus caused by damage to one vestibular organ will be inhibited within a few days by adaptation within the CNS. After adaptation, the diagnosis of a unilateral loss of vestibular function can still be established by caloric testing: Warm-water lavage of the external auditory canal on the diseased side should in a normal case cause slow nystagmus phases to the opposite side. With loss of function, however, no nystagmus is elicited (■ Fig. 11.12).
Bilateral loss of vestibular function is not accompanied by any disequilibrium of the vestibular afferents from both sides, so there will be no nystagmus. However, there will be loss of vestibular reflexes. Consequently, the patient will experience illusory movement of the environment (oscil-lopsia) during any rotational movement of the head. Therefore, for example, the patient cannot read the station clock when running to catch a train; the patient has to stop moving. Typical causes of bilateral loss of vestibular function include Meniere's disease and chronic use of aminoglycoside antibiotics. In addition, disease within the brainstem, such as multiple sclerosis, can damage the VOR, resulting in oscillopsia.
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