Abstract
Purpose
An accurate vestibulo-ocular reflex is important for gaze stability, and is adaptable through modification, based on stable and repeated vestibular and visual feedback. The optical power of eye glasses changes the ocular rotation needed to view an object due to the prismatic effect. Depending on the diopter of a lens, eyes have to rotate through fewer or more degrees to correct for refractive change. We aim to determine how multifocal lenses, such as progressives, affect needed ocular rotation and ocular gain based on location of the lens in which an object is viewed. Differing ocular gains within the same field of vision will create non-stable visual feedback possibly delaying vestibular recovery.
Methods and materials
Needed ocular rotation, and ocular gain to view an object 30° lateral to midline were calculated across different points in the vertical axis of a representative progressive lens.
Results
In a progressive lens with superior correction of + 1 D and inferior correction of + 5.5 D, needed ocular rotation to view an object 30° lateral to midline would differ by 4.015°, with difference in ocular gain of 0.1336, indicating a 13.027% change in ocular gain, based on superior verse inferior viewing.
Conclusions
Progressive lenses have variable areas of VOR gain. This lack of consistent vestibulo-visual feedback may lead to prolonged compensation time in patients with vestibular injury.
1
Introduction
Damage to the vestibular system is associated with patient morbidity secondary to the vertigo, visual disturbances, and balance disorders created. These dizzy symptoms result in seven million doctor visits per year in the United States . Vestibular symptoms are most commonly encountered after a unilateral vestibular loss, creating static and dynamic asymmetries within the system. A static imbalance of vestibular tone is present even in the absence of head movements and quickly decreases within the first few days after insult . Long term dynamic imbalance is related to asymmetry of the vestibulo-ocular reflex (VOR) . The VOR stabilizes gaze during rotational movements, producing compensatory eye movements in the direction opposite of head rotation and at an equal angular velocity. For the image to remain stable on the retina, ocular gain, defined as (angular eye velocity/angular head velocity), must equal one. When ocular gain is not equal to one, such as after labyrinthine insult, retinal slip occurs and an individual experiences vestibular symptoms .
Recovery from vestibular damage and change in ocular gain relies on plasticity for motor learning based on visual stimuli . Stable and repeated slips of images on the retina initiate neural mechanisms of VOR plasticity . This principle provides the rationale behind vestibular rehabilitation such as Cawthorne–Cooksey exercises, in which a series of head movements combined with eye-movements is done to maximally stimulate the vestibular system . The eventual goal of these exercises is to produce VOR gain recovery and reduce retinal slip, thus reducing vestibular symptoms . In monkey labyrinthectomy models, animals that were unable to experience visual input, due to cortical lesions or dark environments, were unable to correct their VOR .
Situations other than vestibular damage present similar challenges that require adaptation. Experimental interventions involving optical devices have revealed impressive adaptation of the VOR. The most extreme of these optical devices is the dove prism, a device which results in complete reversal of left and right ( Fig. 1 ). Jones demonstrated in cats that prolonged dove prism usage of several weeks resulted in a decrease in ocular gain and, most impressively, reversal of direction of the reflex . Additionally, in human subjects wearing reversing prisms for several weeks, VOR reversal was also observed, so that head rotation caused ocular rotation in the same direction . While subjects stated moderate to severe nausea, and abnormal irritability the first few days, these complaints resolved after seven to ten days . This reversal of VOR phase was functionally significant as the subjects were even able to eventually play sports while wearing the dove prisms.
