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JT Siegwart, TT Norton; When Viewing Distance Is Controlled, Which Lens Power Competes Most Effectively to Slow Myopic Compensation to a -5 D Lens? . Invest. Ophthalmol. Vis. Sci. 2002;43(13):185.
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Purpose: To examine in tree shrews, under controlled viewing conditions, whether brief periods of plus lens wear are more effective than a plano lens in slowing the development of myopia induced by wearing a -5 D lens. Methods: Tree shrews fully compensate for a monocular -5 D lens within 11 days by increasing the axial length in the treated eye. Removing the -5 D lens for 45 min/day (in the home cage) reduces compensation by about half. To examine the relative effectiveness at reducing compensation for the -5 D lens, different power lenses were substituted for the -5 D lens (45 min/day) while the animals were restrained with no visual stimuli closer than 1 m. Substituted lens powers were: -3 D (n=3), plano (n=4), +3 D (n=4), +5 D (n=4), and +10 D (n=3). In some animals, the absence of significant accommodative changes during the lens substitution period was demonstrated with an eccentric infrared photorefractor. Cycloplegic refractive measures (retinoscopy and Nidek autorefractor) and A-scan ultrasound measures were made at the end of the 11-day period. Results: Animals that wore a plano lens during the 45 min substitution period developed almost no axial elongation or myopia in the treated eye relative to the control eye. Animals wearing either plus or minus lenses did develop axial elongation and myopia, with the -3 D and +3 D lenses producing similar results. View OriginalDownload SlideView OriginalDownload Slide Conclusions: When viewing distance is restricted to ≷1 m during a 45 min lens substitution period, the primarily myopic defocus produced by a plus lens seems less effective than the mostly focused images produced by a plano lens at reducing compensation for a -5 D lens worn the rest of the day. These results suggest that, in tree shrews, clear images are a stronger stimulus than myopic defocus for slowed axial elongation.
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