Vestibular specialists use caloric irrigation as a test of labyrinthine (particularly horizontal semicircular canal) sensitivity. The results have historically been used to make judgments about the functional status of an entire labyrinth. More recent test modalities have shown that many of those assumptions were incorrect. Rotational chair and vHIT testing have taught us that an ear with an abnormal caloric response might be functioning normally at different speeds, and/or a different plane of movement than that tested with caloric irrigation.
A second historical assumption is that the reflex response to caloric stimulation is rudimentary. A simplistic description of caloric testing might be: The warm air or water in the ear canal warms up the inner ear fluid on one side. The fluid expands, causing fluid movement on one side, but not the other. This creates an asymmetry resulting in nystagmus, which we can measure to see if both sides are equally responsive.
From a physiological standpoint, this asymmetry is detected at the level of the vestibular nuclei, when the left and right nuclei recognize different stimulation. It would be nice if it were that simple in real life, but the vestibular nuclei are responsible for a lot more than just agreeing or disagreeing with each other. Here is a short excerpt (condensed and paraphrased) from my most recent book describing the neural projections of the vestibular nuclei.:
“Changes in electrical potentials in the labyrinths are delivered to the vestibular nuclear complex via the vestibulocochlear nerve (cranial nerve VIII). Neural projections arise from the hair cells of the labyrinthine sensors described above, join with the cochlear nerve as they pass through the internal auditory canal, then extend to the vestibular nuclei.( 1) The vestibular nuclear complex consists of four main sections that are described by anatomic location. The superior VN is primarily a way station for ocular reflexes in response to stimulation of the semi-circular canals. The Lateral NV is primarily responsible for its contribution to vestibulo-spinal reflexes. The Medial VN plays a large role in coordination of eye, head and neck movements with some commissural connections to the contralateral VN. Finally, the Descending VN is primarily responsible for integrating vestibular signals from both labyrinths and coordinating with the cerebellum ( 2 ). The VN does not only receive projections from the vestibular nerve. There are reciprocal projections from other sensory systems such as the proprioceptive sensors in the neck, trunk and limbs, as well as neural structures such as the cerebellum ( 3 )”
It is the last line from the above excerpt that brings us to our topic. What else does the brain do with the information received from the vestibular nuclei? Is there evidence of a reciprocal relationship with the neck, trunk, limbs and cerebellum? Stop in after holiday break to continue this discussion.
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