[Thermographic detection of heat radiation in caloric vestibular function tests]

Since Bárány; caloric irrigations in the external ear canal have been used for unilateral stimulation of the peripheral vestibular system. However, the mechanism of heat transfer from the auditory canal to the vestibular organ is not completely known. From the physical point of view, three mechanisms may be discussed: heat conduction via the bone, convection via the middle ear gas, or radiation. Feldmann et al. (1991) singled out radiation as a very important factor in this regard. Using high-resolution thermography, we were able to "see" radiation almost directly in temporal bone experiments.

Using the system of infrared thermovision specially adapted for close-up studies, the effect of calorization can be observed and documented in colored planar thermograms. Fresh temporal bone specimens had to be prepared so as to permit simultaneous observation of the tympanic membrane and the medial tympanic wall.

Changes in temperature were readily visible during experimental caloric tests: turning blue indicated cooling and red indicated warming. In the caloric test with 44 degrees C or 30 degrees C water, changes in color of the eardrum appeared immediately. At the very same time, however, an area of the medial tympanic wall also changed color. This velocity of transfer cannot be attained by conduction or convection: heat radiation is the only possible explanation. This could only be demonstrated at the very onset of the reaction; subsequent thermograms became more and more diffuse. In this stage the heat transfer may also be effected by conduction and/or convection.

Thermography demonstrates that radiation is a very important factor in heat transfer; at least in the initial phase of calorization.