AO Journal Group: Bi-thermal Caloric Test

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This article review was submitted Kamran Barin, Ph.D., with the Ohio State University Medical Center and a Contributing Editor for Audiology Online in the area of balance and dizziness.

Article:

Lightfoot, G.R. (2004). The origin of order effects in the results of the bi-thermal caloric test. International Journal of Audiology, 43, 276-282.

The website for the Journal is: www.internationaljournalofaudiology.com/

Review:

Previous studies (Furman and Jacob, Acta Otolaryngol, 113, 1993, pp. 3-10) had demonstrated an order effect in the bithermal caloric test. That is, there is a decrease of the peak caloric response from one irrigation to the next. This gradual reduction of caloric responses was attributed to the central adaptation mechanism. The most common order for caloric irrigations consists of first performing one irrigation in each ear with one temperature and then repeating the irrigation in the same order of the ears with a different temperature. In this case, the order effect will cause a systematic difference between the responses from the two ears. That is, the responses from the ear tested second tend to be weaker than the responses from the ear tested first. Furman and Jacob (1993) recommended two possible methods to eliminate this bias. One method was to modify the formula for calculating the caloric weakness to account for the difference. Another method was to reverse the order of the ears tested after changing the temperature.

Lightfoot (2004) proposed that the central adaptation was not the cause of this order effect and there was another reason. He tested 32 healthy subjects who underwent caloric irrigations. Caloric responses were measured simultaneously via electronystagmography (ENG) and videonystagmography (VNG). He discovered that the order effect was present in ENG but not in VNG. It turns out that the origin of the order effect is related to the gradual change of the corneoretinal potential (CRP). This change affects ENG, which uses CRP to measure eye movements, but does not have any effect on VNG.

The results of this study are important to those who perform vestibular function tests. Those who use VNG do not need to be concerned with the order effect. Users of ENG can minimize or eliminate the order effect by verifying the calibration before each irrigation and making adjustments when necessary.