Abstract
Abstract: :
Purpose: To learn more about the visual guidance of recovery in tree shrews by examining the effect of lenses worn during recovery from an induced myopia. Methods: Starting on day 16 of visual experience (Treatment Day 1) tree shrews wore a monocular –5 D lens. After 11 days of lens wear, the recovery phase was started by replacing the –5 D lens with no lens (n=4), a plano lens (n=2), a +2 D lens (n=4) or a –2 D lens (n=3). Awake non–cycloplegic autorefractor measures were made daily for 22 days and then less frequently until 44±2 days of treatment. Exponential curves were fitted to the individual compensation and recovery data to determine the time constants. The refractive endpoints and time constants for the different groups were compared. Results: All tree shrews compensated for the –5 D lens, producing (mean±SD) 4.4±0.7 D of myopia, relative to the control eye, after 11 days of –5 D lens wear, measured with the lens removed. The time constant was 4.4±1.9 days (range 1.3 – 6.6 days). The extent and rate of recovery was dependent on the lens the animal wore during recovery. No–lens and plano–lens animals all recovered so that the recovering eye was similar to the control eye by the end of the recovery period though the plano–lens recovery eyes showed a slight myopic shift (treated–control, –1.1±0.6 D.) In the no–lens animals the difference was 0.1±0.4 D. The recovery time constant in these groups was 5.2±3.3 days. Eyes wearing a –2 D lens recovered until they refractively matched the control eyes while the lens was in place (–0.7±0.3 D), so that when the lens was removed they were myopic relative to the control eyes. Eyes that wore a +2 D lens during recovery showed variable results. One animal recovered until the refractive state while wearing the lens matched the control eye. The recovered eye was 1.6 D hyperopic with the lens removed. Three animals recovered until the eyes were 2.0±1.2 D myopic with the lens in place. With the lens removed, the refractive state matched the control eyes. Conclusions: Recovery from induced myopia in tree shrews must not be guided solely by visual cues. Tree shrew eyes recovering from induced myopia appear to be more tolerant of being myopic than being hyperopic. Some +2 D–lens recovery eyes did not continue to slow their elongation rate until they became emmetropic with the lens in place. These eyes tolerated being "myopic", perhaps because they had returned to the same axial length as their fellow control eyes. In contrast, the –2 D–lens recovery eyes, with the –2 D lens in place, were refractively the same as the control eyes. They did not continue recovery until they matched the control eyes (with no lens), apparently to avoid being hyperopic while wearing the lens.
Keywords: emmetropization • myopia • refractive error development