Regional Variation in Susceptibility to Myopia From Partial Form Deprivation in the Guinea Pig

Abstract

Purpose: : Myopia arises from excessive ocular elongation and can be induced by form deprivation (FD) in a growing eye. The control of ocular growth is thought to be contained within the eye, because FD still occurs when the optic nerve is cut¹; and if only half the eye is deprived, only that part of the eye elongates and becomes myopic^2,3. We sought to discover whether all parts of the mammalian visual field were equally susceptible to partial FD.

Methods: : Guinea pigs were raised with partial diffusers over one eye from 7-14 days of age to induce local FD. Diffusers covered the superior (S), inferior (I), nasal (N), or temporal (T) visual fields (VF) (n=37). At 14 days of age, refractive error was mapped in different sectors of the VF under cycloplegia. Eyes were cut on a freezing microtome in either the vertical (for S and I) or horizontal (for T and N) plane, and eye shape compared to untreated eyes from digital images of the frozen sections.

Results: : Partially deprived eyes became relatively myopic and elongated in the deprived region, but the S-VF was relatively resistant to local change, while the I-VF became excessively myopic and elongated (p<0.001). This asymmetry was not present in the N and T regions. The difference between the two eyes in refractive error and vitreous chamber depth in the deprived region was -7.5, -1.6, -5.6, -5.8D and 137, 90, 20, 45µm for I, S, N and T deprived VF respectively. The resistance of the S-VF to developing local FD myopia was not because of space limitations for growth. Refractive error and ocular shape also changed in the opposite direction in the I and S non-deprived VF (-2.1, +4.8D and +72, -41 µm respectively).

Conclusions: : Partial FD of the mammalian eye results in local myopia suggesting that the mechanisms within the retina can be evoked in a spatially restricted manner. However, the sensitivity to FD varies in different retinal regions. We speculate that the resistance of the S-VF to FD and the excessive myopia in the I-VF may be related to the asymmetric distribution of blue and green cones in corresponding retinal regions.¹McFadden SA, Wildsoet C. IOVS, 2009, 50: E-Abstract 1620.²McFadden SA. IOVS, 2002 43: E-Abstract 189.³Smith EL et al. IOVS, 2009, 50:5057-5069.

Keywords: myopia • refraction • color vision