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Xiaoying Zhu, Josh Wallman, Sally A McFadden; Non-visual factors influencing emmetropization in chicks. Invest. Ophthalmol. Vis. Sci. 2016;57(12):No Pagination Specified.
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Visually based treatments for myopia progression are largely based on animal models and human studies which show that eye growth is influenced by post-natal visual experience. Treatment options for school-aged myopia aim to change visual experience but none appear to completely halt or reverse the progression pattern. Since the size of all of our internal organs are closely maintained during development and adulthood, we hypothesize that, beside visual experience (“defocus-factor”), eye growth may also be influenced by an internal, homeostatic mechanism ("size-factor") that prevents the eye from deviating from its normal size. We studied the role of the size factor in the chick lens-induced myopia model.
(1) To study if the hypothesized size-factor alone can guide the recovery from eye growth perturbations, chicks wore a +7 D or –7 D lens over one eye for 4 to 7 days, then the lens was removed and the chicks were kept in either normal light or darkness for 2-3 days. (2) To study the interaction of the size- and defocus-factors, chicks first wore a –5 D or –7 D lens on one eye for 7 days (to generate longer eyes), then a –10 D and –15 D lens, respectively, for 4 more days. After the step up in lens power, the size- and defocus-factors would be working in the opposite directions, with the size-factor acting to decrease eye growth (since the eyes were already longer than normal) and the defocus-factor acting to further increase eye growth to compensate for the hyperopic defocus. As a control, another group of chicks wore a –10 D or –15 D lens from the beginning for 11 days.
(1) While kept in darkness (in the absence of the defocus-factor), all chicks eyes showed robust recovery from prior hyperopia or myopia similar to those that recovered in normal light, suggesting that the size-factor is involved in eye growth. (2) After compensating for –5 D or –7 D lenses, chicks eyes did not further elongate to compensate for –10 D and –15 D lenses, respectively; whereas they fully compensated for these higher-powered negative lenses if worn from the beginning, suggesting that the size-factor prevents the eye from further elongating.
Non-visual factors relating to prior growth state or possibly binocular innate yoked growth mechanisms contribute to the control of eye growth in chicks and may provide an opportunity and/or be necessary to target in order to maximize the effects of myopia treatments in humans.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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