Image quality and visual performance in the peripheral visual field following photorefractive keratectomy.

Abstract

A theoretical and experimental study was performed to assess the possible effects of photorefractive keratectomy (PRK) on retinal image quality and thresholds in the peripheral visual field. Simple optical calculations suggest that although the quality of the retinal image at the fovea of the postoperative PRK eye may be comparable to that in an emmetropic eye, images in the peripheral field may be markedly worse, since peripheral ray bundles may pass partly through ablated and partly through unablated cornea, giving a simultaneous-vision bifocal effect. This would be expected to create an annular zone of confusion, so that light from two different directions in object space arrives at the same point on the retina. The position of this zone and its width are a function of the ablation zone geometry, the attempted correction, and the pupil diameter, but the major effects typically occur at field angles between about 40 degrees and 60 degrees. To explore this effect, Goldmann static quantitative perimetry was carried out along the 0 degree to 180 degrees meridian in six patients who had undergone myopic PRK and eight emmetropes. The results showed that thresholds for the PRK group at field angles from 40 degrees to 60 degrees were significantly higher than those of the emmetropic group (P < .01). These findings arise because the retinal images in the peripheral field of originally myopic, postoperative PRK patients are, in general, significantly degraded in comparison with those of emmetropes. Although under photopic conditions, retinal image quality close to the visual axis in patients who have been corrected by PRK is similar to that in emmetropes, it may be markedly worse in the peripheral visual field.