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WHAT BÁRÁNY’S CALORIC TEST MIGHT HAVE OVERLOOKED: THE PRIMARY FACTOR MAY BE THE MIDDLE EAR MUSCLES

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.