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T. T. Norton, J. T. Siegwart, Jr.; Similarities Between Myopia Induced in Tree Shrews With Form Deprivation, Minus Lenses, and Darkness. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1531.
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We recently found that continuous darkness produces axial elongation and myopia in visually experienced tree shrews (IOVS 47:4700, 2006). As is the case with myopia induced with form deprivation or minus lenses, dark-induced myopia is due to vitreous chamber elongation with no change in corneal curvature. We asked whether dark-induced myopia also involves an increase in the biomechanical property, "creep rate" in the sclera, and whether the myopia can be blocked by brief daily periods of light.
Creep rate experiments: Starting at 24 days of normal visual experience (VE), tree shrews were treated with continuous darkness for 4 days (n=2) or 11 days (n=4) after which scleral creep rate was measured using 30 min periods with tensions of 1, 3 and then 5 grams. Interrupted dark experiments: Three groups of tree shrews were treated with darkness for 11 days that was interrupted daily with light for 15 minutes (n=5), 1 hr. (n=4), or 2 hrs (n=1). These were compared with a group of tree shrews (n=5) treated with continuous darkness for 11 days (reported previously).
Creep rate experiments: Continuous darkness for 4 or 11 days produced an increase in scleral creep rate (41% and 54% @ 3 gm tension) above untreated, age-matched normal animals. Increases have been found after the same durations of form deprivation (70% and 159%) and minus lens treatment (95% and 48%) (Vision Res, 39:387, 1999). Interrupted dark experiments: Continuous darkness produced a (mean ± SEM) myopic shift of -4.3 ± 0.5 D. 15 minutes of light per day did not block dark-induced myopia in 2 animals, partially blocked it in 2 animals, and completely blocked it in 1 animal (group refractive shift = -3.0 ± 0.9 D). 1 hr. of light per day produced partial blocking in all 4 animals (group refractive shift = -2.2 ± 0.8 D). 2 hrs of light per day mostly blocked myopia development in the one animal tested (refractive shift = -1.6 D). This pattern is similar to that found using short intervals of unrestricted vision to block deprivation- and lens-induced myopia.
In tree shrews, dark-induced myopia shows key similarities to form deprivation and hyperopic defocus-induced myopia. Understanding how three very different visual stimuli (no light, diffuse light, and hyperopic defocus) all have a similar myopiagenic effect may shed light on the retinal signals and scleral responses that control axial elongation and refractive state.
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