Variability and habituation of nystagmic responses to hot caloric stimulation of normal subjects. Evidence that this test may be inapplicable to monitoring drug-induced vestibular toxicity.

Experiments were performed on 25 otoneurologically "normal' subjects to evaluate the hot caloric test as a screening test for aminoglycoside vestibular toxicity. Using portable equipment under non-ideal conditions, it was found that there was a large inter-subject variability in nystagmic response and that, instead of a random test-retest variability, a systematic variation in response occurred on repeated caloric stimulation with water at 44 degrees C. A response decline (habituation) evident in both the maximum slow phase velocity and the maximum frequency occurred at second test, although the inter-test interval ranged from 24 to 72 hours. After a 3-month interval with no intervening tests, the mean value of the maximum frequency reverted back to the original level. However, there was still a significant reduction in maximum slow phase velocity at this time. Some individuals had a sustained reduction in both parameters. Hence it is concluded that the hot caloric test, used under the conditions described in this study, is not a suitable serial screening method for aminoglycoside vestibular toxicity. The reproducibility of this test under other conditions, or any other caloric test, should be established in normal subjects before employing it as a serial screen for aminoglycoside vestibular toxicity.