AbstractAnalysis of results obtained from caloric tests showed that, on average, a significant sequential response decline was present for the four caloric responses. The effect of this decline was to produce an 'apparent 1 canal paresis of 4% of the second ear to be irrigated. There was no significant effect upon the calculation of directional preponderance. Also, differences in both maximum slow phase velocity and the latency of this maximum were found to exist between the warm and cold caloric responses. Although the difference of nystagmus intensities may be explained by the difference in the magnitude of the respective water temperatures, there remained a possible influence of a stimulus-related adaptation effect upon the response latencies.
Regarding pathways and mechanisms of heat transfer during caloric stimulation of the labyrinth, a study of horizontal serial sections of human temporal bones showed that the most direct anatomical route from the external auditory meatus to the lateral semicircular canal is through the air of the middle ear cleft. The bony posterior-inferior extension of the external meatus, which has been generally accepted as the main route for caloric stimulation, was found to be a highly cellular structure and was not seen in serial section until a level was reached which was well below that of the lateral limb of the semicircular canal.
Additionally, temperature measurement in isolated temporal bones showed that convection within the middle ear cleft was a significant heat transfer mechanism in caloric stimulation. The mathematical model incorporating temperature profiles based on those from cadaveric temporal bones and which also included a term describing adaptation gave good prediction of caloric primary nystagmus. The true mechanism of vestibular adaptation was however considered to be stimulus related and would therefore be more accurately modelled by a non linear term.
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