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I.G. Morgan; Brief Periods of Exposure to Myopic Defocus Block the Effects of Prolonged Periods of Hyperopic Defocus on Axial Elongation in the Chick . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1988.
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Purpose: To determine if short, infrequent exposure to myopic defocus produced by (+) lenses can prevent the axial elongation of the chicken eye induced by imposed hyperopic defocus using (-) lenses. Methods: Hyperopic defocus was induced by placing -5D PMMA contact lenses over the eyes of 5 day-old chickens using Velcro dots. Chickens were divided into groups that were given 1 hour of exposure to +10D lenses every day, or every second or third day, or normal vision for the same period of time, or had the -5D lenses maintained all the time. Continuous wear of the -5D lenses was interrupted between noon and 1pm. After 10 days, axial lengths were measured using an A-scan ultrasound. In parallel experiments, chickens were exposed to +10 lenses for 1 hour, and sulphate incorporation into scleral glycoaminoglycans was monitored by incubating scleral buttons with 35S-sulphate in vitro. Results: As expected, continuous exposure to -5D lenses produced increased axial elongation. If the continuous exposure was interrupted with normal vision, there was little suppression of axial elongation. However, replacement of the -5D lenses with +10D lenses every day produced complete suppression of eye growth. Large effects were also detected with replacement of the lenses every second day, but little effect was produced by lens replacement every third day. This suggests that brief exposure to myopic defocus can produce significant inhibition of eye growth for at least two days. Consistent with this, after 1 hour exposure to +10D lenses, sulphate incorporation into scleral glycosaminoglycans was suppressed for around 2 days. Conclusion: These results show that exposure to +10D lenses for as little as one hour can produce almost complete inhibition of axial elongation for around 2 days, and can outweigh the growth-promoting effects of 11 hours/day of hyperopic defocus. The inhibition produced by +10D lenses appears to be stronger than that produced by normal vision, suggesting that the mechanisms involved may be distinct.
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