Abstract
Purpose: :
Hyperopia increases the risk for age-related macular degeneration (ARMD). Light damage in the photoreceptors is a suspected but unproven risk factor for ARMD. In the present study we measured the retinal light levels in a model schematic eye for refractive errors between ± 6.0D. We hypothesize that the protective effect of myopia in ARMD may be related to lower intensity of light forming the macular image.
Methods: :
We constructed a mechanical schematic eye using PMMA contact lenses for the cornea, PMMA intraocular lenses (IOL) for the crystalline lens, and tap water for the aqueous and vitreous. The refractive components, anterior chamber depth and axial length were adjusted to create a range of refractive errors using population-based ocular biometric values from the Reykjavík Eye study. Each refractive state was verified with a Nikon Retinomax K-Plus auto-refractor. The pupil diameter and anterior chamber depth were constant at 3mm and 3.2 mm respectively. A circular array of 5 LEDs powered by a regulated DC power supply served as a luminous target positioned 40 cm from the cornea. Measurements of light power (µW for 632 nm) in the retinal image plane were made with an Ophir Nova power/energy meter before and after correction of refractive errors using standard trial lenses.
Results: :
As refractive error changed from -5.50D to +5.87D the light intensity at the retina increased linearly, for a total change of ~15% (84µW to 96.7µW). Following correction of the refractive error, the light intensity increased to 43.3% (which also respected a linear function). This was due to a reduction in light intensity at the macula for myopic eyes and an increase in light intensity for hyperopic eyes >+3.0D.
Conclusions: :
Our findings support the hypothesis that the protective effect of myopia for ARMD is related to decreased macular light levels in these eyes compared to hyperopic eyes.
Keywords: age-related macular degeneration • refraction • radiation damage: light/UV