What Bárány’s Caloric Test Might Have Overlooked: The Primary Factor May Be the Middle Ear Muscles

Abstract

The caloric test of vestibular function, originating from Bárány in the early 1900s, has conventionally been understood as a test of the effect of temperature on the horizontal semicircular canals of the inner ear. Warm water introduced into the external auditory meatus will, if the vestibular system is intact, cause back-and-forth beating of the eyes (nystagmus) in one direction; cold water will cause beating in the reverse direction. The text-book explanation is that the eye movements are caused by a thermal gradient across the horizontal canal, which in turn causes convection in the fluid within. The convective motion stimulates the vestibular hair cells, causing nystagmus, dizziness, nausea, and often vomiting. But here an alternative mechanism is proposed: warm or cold water causes the tensor tympani muscle in the middle ear to increase in tension (warm water) or decrease in tension (cold water), and in this way changes the force exerted by the ossicles on the inner ear fluids behind the oval window. Altered force on the stapes therefore means a change of hydraulic pressure inside the sealed labyrinth, and this pressure could directly stimulate hair cells within the inner ear -including the semicircular canals -and so generate nystagmus. If correct, this means the caloric test is really a test of the temperature sensitivity of the middle ear muscles, although the vestibular system still needs to be intact in order to register a positive response. The new hypothesis explains a range of anomalies surrounding the caloric test, and these are systematically reviewed.

The Acute Effects of Time-Varying Caloric Vestibular Stimulation as Assessed With fMRI

“…In order to explain Figure 4A, one must posit a mechanism that will produce a sharp square wave transition during a time when the change in applied temperature is on the order of 1 • C or less: effectively a constant temperature in terms of thermally mediated processes. The thermoconvection model provides a ready explanation, versus models invoking a direct temperature effect on neuronal firing rates (such as from the trigeminal nerve) or structures in the inner ear (Hood, 1989;Bell, 2019), and demonstrates that the primary source of BOLD response in this study is consistent with a vestibular origin. The lateralized cerebellar contrast in Figure 5 may reflect the documented connectivity between the vestibular nuclei and the posterior cerebellum (Barmack, 2003).…”

Cooling the Cochlea: Slowing Down Metabolism May Be a Way of Protecting Hearing from Surgical Trauma

“…The field has been left to just a handful of research groups. This contrasts with established clinical practice where thermal manipulation of the inner ear, such as with Barany's caloric test, has become an established procedure in the diagnosis of the vestibular organ [6,7]. At the same time, the neuroprotective effects of cryotherapy have been routinely used in intensive care-to protect the brains of concussed or oxygen-deprived patients and during brain surgery, for example [8].…”