Corneal light scattering following excimer laser surgery

Abstract

Introduction Increased corneal scattering is assumed to be a considerable side-effectof refractive surgery due to the postoperative healing response after photoablativesurface treatments. Clinically, corneal scattering is associated with haze development.Corneal haze is subjectively evaluated method based on the observation of the lightbackscattered on the cornea as seen under a slit lamp. However, corneal light scatteringaffects the retinal image quality only by its forward scattering component. The scopeof this thesis is to evaluate the the effect of corneal light scattering on the retinal imagequality, specifically in relation to the morphological changes in the cornea followingexcimer laser surgery.Methods Initially, the optical mechanisms of the image formation on the retina arediscussed. Following, the optical parameters of the corneal morphology are characterizedto form a theoretical model describing the forward light scattering in the cornea.Simulation of this theoretical model is done by a physical model of light scatteringon microspheres. This model is evaluated both by psychophysical and optical measurements.Further, we compared the use of two psychophysical and three opticalmethods to evaluate both the amount of light scattered in the cornea and the angulardistribution of the scattered light.Results Based on histological data of stained corneal samples and in vivo confocalmicroscopy, the morphologic corneal changes addressed to the wound healing responseare induced by the activation of keratocytes, inhibiting a newly formed unorganizedcollagen layer, a scar tissue called the foam layer. The theoretical models developed in this thesis reveal the presence of scattering particles that cause a local distortion of theincoming wavefront. The point spread function of this scattered wavefront is stronglyforward distributed with a full width at the half maximum of approximately 20 minutesof arc. After refractive treatments, the light scattering increases until 1 month aftertreatment, reducing to a long term moderate level of increased light scattering after6 months. Moreover, we derived that where, in a healthy cornea approximately 20percent of the incoming light is scattered, this value increases up to 70 percent forcorneas with marked haze. Furthermore, psychophysical data suggest that the amountof light scattered over angles from 5 to 10 degrees increases in a similar extend asobserved in the ageing eye.Conclusion The wound healing response of the cornea on photorefractive ablationtreatments result in an increase of corneal scattering. The light distribution of thisscattered light is narrowly forward peaked. Correlation of the foam layer with amountof forward light scattering suggests that the increased corneal scattering followingrefractive surgery mainly originates on this foam layer. Therefore, a correct postoperativecare of the cornea following excimer laser surgery is detrimental for an optimalretinal image quality. The effect of light scattering in the eye results on a lowering ofthe contrast sensitivity and an increased disability glare around light sources in nightvision.