Abstract
Purpose::
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.
Methods::
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).
Results::
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.
Conclusions::
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.
Keywords: refractive error development • sclera • myopia