Abstract
Purpose::
Newborn human eyes show a wide range of refractive errors and require different visually guided adjustments to their growth rates in order to emmetropize during the first postnatal months. In particular, myopic newborns may need to restrain their axial growth relative to hyperopic newborns. Although nearly all myopic newborns achieve emmetropia, many subsequently develop juvenile-onset myopia (Gwiazda, J., et al. Clin Vision Sci 8:337-344, 1993; Howland, H. C., et al. Opt Soc Am 3:210-213, 1993). We examined, in tree shrews, whether eyes that restrain their early growth to achieve emmetropia are subsequently more susceptible to hyperopic defocus.
Methods::
We used plus lenses to slow early eye growth. To measure susceptibility we examined whether 2 hrs of emmetropia would block the myopiagenic effect of hyperopic defocus. One group (n=5) wore binocular +4 D lenses from 11 to 24 days of visual experience (VE) which caused the eyes to slow their growth and become "emmetropic" with the lenses in place. Then, from 24 to 45 days of VE, they wore the lenses for 2 hrs per day. Without the +4 D lenses, the animals initially were approximately +4 D hyperopic 22 hrs per day. A second group (n=3) had normal visual experience until 24 days of VE and then wore binocular -4 D lenses from 24 to 45 days of VE with the lenses removed for 2 hrs per day (also producing approximately +4 D of hyperopia for 22 hrs per day). One animal wore binocular +4 D lenses continuously from 11 to 45 days of VE.
Results::
The animal that wore the +4D lenses continuously compensated for the lenses and maintained "emmetropia" with the lenses in place. Group 1: Showed myopic progression in response to 22 hrs of hyperopic defocus per day (2 hrs of "emmetropia" did not block myopic progression). Group 2: As expected based on previous experiments (Shaikh, et al. OVS 76:308, 1999) this group did not show myopic progression in response to 22 hrs of hyperopic defocus per day (2 hrs of emmetropia did block myopic progression).
Conclusions::
In tree shrews, restraining early eye growth in order to emmetropize (Group 1) produces eyes with a higher susceptibility to subsequent hyperopic defocus compared to eyes that do not restrain their growth (Group 2). These data provide evidence that early adjustment of eye size during initial emmetropization may affect later susceptibility to myopiagenic stimuli, such as hyperopic defocus produced by underaccommodation to near targets during reading in children. Further, these data may help explain why some human eyes appear to be more sensitive to myopiagenic visual stimuli than most animal models predict.
Keywords: refractive error development • myopia • sclera