Development and Understanding of Caloric Responses; An Overview of the First Century

The aim of this study is to explore the evolution of knowledge regarding caloric responses. In 1906, Robert Bárány published his original research on the caloric test and his convection theory to explain the direction of nystagmus. In 1914 Dr. Bárány was awarded the Nobel Prize in Medicine and Physiology for publishing his discovery of the caloric response and his convection explanation for its occurrence in 1906. Subsequent publications by others suggested different physiological theories, including the effect of temperature on the vestibular nerves, and pressure changes in the ampulla related to temperature expansion, but another suggestion that was advanced was the possibility that there was an otolithic modulation of the response. Technical developments have allowed quantitative measurement of the caloric response using electronystagmography; caloric assessment consists of measuring the slow phase velocity of nystagmus and uses the Phillipszoon-Jongkees equation (difference over sum) to detect pathology. The caloric test has become an entrenched assessment in the evaluation of patients with vestibular complaints. As predicted by vestibular scientists, caloric tests conducted on the first day or two of space flight (using an air stimulus in orbital flight) showed minimal response. However, after several days, the velocity of the caloric response returned to nearly the same magnitude as pre-flight levels in the astronauts.

Bárány’s convection theory was accepted as the explanation for the fact that with the lateral semicircular canal at right angles to horizontal (30 degrees head up) the response to caloric irrigation was greater with the patient supine (face up) than prone (face down). The convection theory was questioned when caloric responses in the appropriate direction were documented during spaceflight. It seems that there are many factors affecting the underlying convection caloric response that has been advanced for the last century.

This overview of the evolution of the caloric response over the past century suggests that while we have quantified tests of different vestibular structures, and that quite often the test abnormalities are in keeping with a patient’s signs and symptoms, sometimes this is not the case. This reflects our poor understanding of the exceedingly complex caloric/otolithic interaction that has been shown to exist.