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Sally McFadden, Dennis Tse, Chi-ho To, Christine Wildsoet; Ocular growth guided by dual focal fresnel lenses requires an intact optic nerve. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4038.
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Hyperopic defocus imposed with a negative lens induces myopia from excessive ocular growth while positive lenses cause ocular growth to slow. When negative and positive powers are combined 50:50 in a Fresnel lens, ocular growth is matched to the average imposed defocus. We asked whether this apparent integration occurs in eyes with sectioned optic nerves.
47 guinea pigs underwent either optic nerve section (ONS) or sham surgery at 3 days of age in one eye. The treated eye then wore either a single vision lens (-5 D or 0 D) or a Fresnel lens composed of two powers in equal ratios (-5/+5 D, -5/0 D, +5/0 D) for 11 days (from 6-16 days of age). Refractive error and axial ocular dimensions were measured before and after treatments. Interocular differences in refractive error and axial length after treatment are reported.
After ONS, eyes with -5 D lenses became excessively myopic (-6.4 ± 1.6 D, p = 0.006) and elongated (150 ± 22 µm, p = 0.02) while there was little effect of ONS with no defocus (0 D: -1.4 ± 1.7 D, p = 0.08 and 87 ± 103 µm, p = 0.02). After sham surgery, eyes wearing -5/+5 D Fresnel lenses responded to the average imposed defocus (Refractive Error: -0.8 ± 0.7 D, p = 0.4; Axial Length: 65 ± 13 µm, p = 0.2), while after ONS, the same -5/+5 D lenses induced myopia (-5.2 ± 1.5 D) and excessive elongation (151 ± 36 µm). Similarly, ONS eyes wearing -5/0 D or +5/0 D lenses failed to integrate the dual defocus, and the degree of myopia and ocular elongation was similar to that in animals wearing -5/+5 D or -5 D lenses (-5/0 D: -5.8 ± 1.2 D; +5/0 D: -7.2 ± 1.3 D; F(3,38) = 0.3, p = 0.8).
An intact optic nerve is required for the integration of imposed optical defocus. Therefore, either ONS rapidly alters retinal processing and causes this failure, or neural processes beyond the eye contribute to the inhibition of mammalian eye growth and are required for the ocular growth response to dual focal lenses.
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