Intraocular scattering changes with age

Resumen

Driving at night, you may experience being blinded by the headlights of an oncoming car. This may also happen when looking at a low sun. Inhomogeneities in the ocular media can cause light scattering, resulting in a veil of light, called straylight, projected onto the retinal image. The Commission Internationale de l'Éclairage (CIE) defines straylight as disability glare. Straylight, however, manifests itself with more than glare alone. Contrast and color loss, hazy vision, and difficulty with face recognition against the light are some complaints people with increased straylight may experience. Even in young healthy eyes, a small part of the light entering the eye is scattered. The age-dependence of straylight in a healthy normal population has been extensively studied. It is now known that ageing and various ocular pathological conditions, such as cataract, can elevate straylight. Although straylight does not increase before the age of 40, some pathological conditions, such as congenital (early onset) cataracts, can increase it, sometimes to an extreme degree. Cataract-dependence of straylight has been studied in various cataract types; a significant difference in straylight has been shown to exist between cataract types. While straylight varies among different cataract morphologies, in some cases, strongly elevated straylight can be accompanied by good visual acuity. The main goals of this thesis were (1) to study in vivo straylight of ageing eyes with a focus on cataractous eyes; (2) to study the eligibility of certain optical functions and cataract morphology to be considered as reliable discriminators for establishing a surgical decision algorithm in the future. As general conclusions of this doctoral thesis, it should be noted that straylight and visual acuity seem to be quite autonomous. However, on average, some correlation exists. The rate of this dependency appeared to be a function of cataract morphology. These findings are in accordance with the literature and ensure that straylight is potentially an important measure for quality of vision and indicator for cataract surgery, along with visual acuity and cataract morphology. Another important finding of this thesis is the competence of visual Strehl ratio (VSMTF), an image quality metric derived from wavefront aberrations, in predicting visual acuity objectively in cataract eyes. The results show that the combination of mentioned optical-visual functions and lens morphology could be dependable ingredients for an algorithm to predict the optimal timing for performing surgery. Many studies have been conducted on the importance of considering straylight in such equation. This thesis confirms this notion. However, further investigation is needed to validate the liability of the VSMTF to predict the visual acuity in cataract population with taking the lens morphology into account. Moreover, the peculiar angular-dependence of straylight in pulverulent congenital cataract eyes deserves an extensive study in more subjects with more diverse types of optical scattering defects.