Heat radiation during caloric vestibular test: thermographic demonstration in temporal bone experiments

Since Barany's descriptions in 1906, cold and warm irrigations of the external ear canal have been used for unilaterally testing vestibular excitability. The fluid kinetics within the endolymph have been studied thoroughly, whereas relatively few published articles deal with the mechanisms of heat transfer from the external to the internal ear. Even though heat conduction via the bone seemed to be well established in the textbooks, Feldmann and co-workers found heat radiation to be a very important factor. Using a rather uncommon method, we tried to make this heat radiation more apparent: in temporal bone experiments, temperature distribution was followed by thermography. Temporal bone specimens were prepared in such a way that heat distributions became visible after experimental caloric irrigations of the external ear canal. Temperature changes could well be verified by changes in coloration of the 2-dimensional thermograms. Conclusions were drawn from registrations performed in 2 projections, 90 degrees to each other, allowing cautious statements about 3-dimensional distribution. The velocity of heat transfer at the very onset of the reaction can only be explained by radiation, whereas later, other mechanisms, such as conduction via the bone or convection via the middle ear gas, may have some influence. Our results support Feldmann's findings and underline the significance of radiation in the heat transfer mechanism.