PURPOSE. While excimer laser photorefractive keratectomy (PRK) may yield axial visual performance similar to that of the emmetrope, the geometry of the ablated cornea in relation to the pupil of the eye means that ray pencils entering the eye from off-axis object points may pass partly through ablated and partly through unablated cornea. The resultant unwanted simultaneous-vision bifocal effect would be expected to yield off-axis retinal images which are substantially blurred. The aims of this study were first to calculate the possible form of the off-axis retinal images and then to demonstrate that PRK-corrected myopes show worse visual performance in the peripheral retina than normal emmetropes.
METHOD. The likely magnitude of the blurring effects as a function of field angle, ablation zone diameter, pupil diameter and attempted myopic correction was calculated by ray tracing using standard eye models. To determine experimentally whether such off-axis blur could have measurable consequences, small-target static quantitative perimetry was carried out for groups of natural emetropes and patients who had undergone PRK for approximately -5.00 D of myopia.
RESULTS. The calculations show that, unlike the sharp axial image of a point object, retinal images in the mid-periphery are likely to be blurred to extend over the equivalent of at least a degree of visual field. The perimetric results show significantly higher thresholds for the PRK patients in the peripheral field (40-60 degrees), as compared to the natural emmmetropes.
CONCLUSIONS. Although under photopic conditions retinal image quality close to the visual axis for PRK patients is similar to that for natural emmetropes, it may be markedly worse in the peripheral field.