Treatment of vestibular disorders with weak asymmetric base-in prisms: An hypothesis with a focus on Ménière's disease.

BACKGROUND: Regular treatments of Ménière's disease (MD) vary largely, and no single satisfactory treatment exists. A complementary treatment popular among Dutch and Belgian patients involves eyeglasses with weak asymmetric base-in prisms, with a perceived high success rate. An explanatory mechanism is, however, lacking. OBJECTIVE: To speculate on a working mechanism explaining an effectiveness of weak asymmetric base-in prims in MD, based on available knowledge. METHODS: After describing the way these prisms are prescribed using a walking test and its effect reported on, we give an explanation of its underlying mechanism, based on the literature. RESULTS: The presumed effect can be explained by considering the typical star-like walking pattern in MD, induced by a drifting after-image comparable to the oculogyral illusion. Weak asymmetric base-in prisms can furthermore eliminate the conflict between a net vestibular angular velocity bias in the efferent signal controlling the VOR, and a net re-afferent ocular signal. CONCLUSIONS: The positive findings with these glasses reported on, the fact that the treatment itself is simple, low-cost, and socially acceptable, and the fact that an explanation is at hand, speak in favour of elaborating further on this treatment.

Generalization of vestibular learning to earth-fixed targets is possible but limited when the polarity of afferent vestibular information is changed.

To maintain perception of the world around us during body motion, the brain must update the spatial presentation of visual stimuli, known as space updating. Previous studies have demonstrated that vestibular signals contribute to space updating. Nonetheless, when being passively rotated in the dark, the ability to keep track of a memorized earth-fixed target (EFT) involves learning mechanism(s). We tested whether such learning generalizes across different EFT eccentricities. Furthermore, we ascertained whether learning transfers to similar target eccentricities but in the opposite direction. Participants were trained to predict the position of an EFT (located at 45° to their left) while being rotated counterclockwise (i.e., they press a push button when they perceived that their body midline have cross the position of the target). Overall, the results indicated that learning transferred to other target eccentricity (30° and 60°) for identical body rotation direction. In contrast, vestibular learning partly transferred to target location's matching body rotation but in the opposite rotation direction. Generalization of learning implies that participants do not adopt cognitive strategies to improve their performance during training. We argue that the brain learned to use vestibular signals for space updating. Generalization of learning while being rotated in the opposite direction implies that some parts of the neural networks involved in space updating is shared between trained and untrained direction.