Changes in Eye Shape During the Development of Form Deprivation Myopia in the Guinea Pig

Abstract

Purpose: : Human myopic eyes elongate centrally, are prolate in shape and show relative peripheral hyperopia. The latter has been suggested as a possible cause of central axial elongation. We asked how the mammalian eye changes shape during the development of myopia by studying the relationship between central and peripheral changes in growth, the relative sequence of changes in each, and how these changes differed between the vertical and horizontal meridians.

Methods: : 32 guinea pigs were form deprived (FD) from 5 days of age for either 1, 2 or 3 weeks. At the end of the rearing period, refractive error was mapped in different sectors of the visual field under cycloplegia. Eyes were subsequently enucleated, quickly frozen and cut on a freezing microtome in either the vertical or horizontal plane. Eye shape was analyzed from digital images of the frozen sections, and the area surrounding the optic nerve and cup was calculated from the characteristic inflection pattern.

Results: : The eye developed myopia rapidly with the refractive error in the central visual field relative to the fellow eye changing from -6.8D to -7.3D between 1 and 2 weeks of FD. After 1 week of FD, the FD eye elongated dramatically about the optic nerve head (a 203 um difference in the vitreous chamber) combined with increases in mid- nasal (53 um); superior (199 um) and inferior (157 um) retina. No significant change occurred in the temporal retina. With longer periods of FD, the elongation at the optic nerve head was maintained while the peripheral retina started to shrink, losing the prior increases in its vitreous chamber and shrinking by up to -100 um in the temporal retina. The area adjacent to the optic cup increased 2 fold after 1 week of FD (0.27 Vs 0.54 mm², p < 0.0001).

Conclusions: : During the development of myopia, the eye elongates initially and specifically at the optic nerve and in the adjacent cup zone, and in the superior and inferior retina. Relative peripheral shrinkage, particularly in temporal retina, occurs subsequent to these changes implying that they are consequence not a cause of the central axial elongation.

Keywords: myopia • refractive error development • shape and contour