Otolith Function: Basis for Modern Testing
The major challenge in developing a robust test of otolith function, particularly with regard to linear vestibulo-ocular reflex (LVOR) and perceptual measures, is to find a way in which graded lesions are reflected in graded response properties and abnormalities. The ability of the vestibulo-ocular reflex (VOR) to compensate and adapt to dysfunction and pathology presents formidable challenges for registering localizing clinical findings, whether in the angular vestibulo-ocular reflex (AVOR), the LVOR, or both. Based on a variety of considerations, various forms of eccentric rotation seem to provide the most convenient, and potentially the most useful, means to generate motion profiles from which otolith function can be directly assessed. Both translational and tilt responses can be recorded depending on the stimulus profile. The near-centric version is particularly enticing because of the ability to study one labyrinth at a time, much like calorics. In that case and in others in which the tilt-LVOR is prominent, measures of the perceived visual vertical are useful and by all accounts similar to ocular torsion. The latter does hold the important advantage of being an objective measure, requiring no intervention on the part of the patient. The translational-LVOR can be derived from eccentric rotation responses with the head displaced forward as well as backward, while viewing near targets in hopes of generating a large addition or subtraction (even inversion) of an otherwise AVOR-driven reflex. These considerations provide an impetus to pursue improved methods of quantifying otolith function in a clinical population. The sobering caveat is that the diagnosis of total unilateral vestibular loss presents little challenge either clinically or by classic testing (e.g., calorics), and yet most of our efforts in developing quantifiable measures of dysfunction over the years have yielded results that are modest and hardly compelling.
otolith vestibulo-ocular reflex eccentric rotation calorics