Adaptation in the oculomotor response to caloric irrigation and the merits of bithermal stimulation

Caloric irrigation of the ear is a familiar clinical technique for the investigation of vestibular function that has the advantage of allowing each inner ear to be examined separately. However, it also has the disadvantage that the heating effect lasts for a period of 10-20 minutes, with the result that it is normally necessary to leave a period of at least 10 minutes between successive irrigations. This prolonged heating effect is not immediately apparent from the induced nystagmus, which normally decays within a period of 3 minutes, even in darkness. In the first part of this article, evidence is presented to show that the premature decay of eye movement can be attributed to the effect of adaptation, which is known to operate during other forms of vestibular stimulation. Experiments in which the subject was repeatedly re-orientated with respect to gravity counteracted this adaptation effect and thus revealed the continuation of the underlying thermal stimulus. In the second part of the article, an irrigation technique is described in which an attempt is made to avoid the prolonged heating effect by administering a bithermal caloric stimulus. The effectiveness of this technique is reviewed on the basis of data from previously published trials.