Objective and independent measurement of scattering caused by the cornea and crystalline lens

Abstract

meritxell.vilaseca@upc.eduThere are several methods that provide information on the intraocular scattering. Procedures based on the observation of the lens through the slit lamp from which a gradation of the state of every cataract is assessed (e.g. lens opacities classification system III ‐ LOCS III) are the most widely used in clinics. Nevertheless, they provide information related to the back‐scattered light and not forward scattering, which is responsible for the degradation of vision. To do so, a procedure that assess retinal straylight using a device based on a psychophysical compensation‐comparison approach was proposed, in which the subject's task is a forced‐choice between two half fields to decide which one flickers more intensely. From these measurements, a psychometric function is fitted and used to determine the straylight compensation level log(s) on the basis of a few stimuli responses.As an alternative approach, some attempts have been made to use the double‐pass as a means of objectively estimating forward scattering. The double‐pass technique is a procedure that is able to capture the complete optical information of the eye, including the effect of higher‐order aberrations and intraocular scattering within a small visual angle. Artal et al. proposed the Objective Scatter Index (OSI) calculated from double‐pass images for estimating scattering with the intention to grade cataracts. This technique was also used successfully to account for scattered light in patients with keratitis and uveitis, and those undergoing refractive surgery. More recently, a new Frequency Scatter Index (FSI3) based on the analysis of double‐pass retinal images in the frequency domain has also been suggested, as a means of minimizing the impact of optical aberrations on the results obtained.Nevertheless, all of the former methods provide information on the total scattered light in the eye, which consists of the combined contributions originating from different ocular structures. In this regards, we recently proposed a technique for the objective and independent assessment of scattering caused by the cornea and the lens based on the analysis of the contrast of the third and fourth Purkinje images. The technique was preliminarily validated first by using artificial eyes with different levels of corneal and lens scattering; second, it was validated in eyes wearing customized contact lenses to simulate corneal scattering and eyes with nuclear cataracts. Finally, it was tested on a larger population of eyes with cataracts and corneal disorders to prove its clinical usefulness.In this work, we review all the above‐mentioned techniques that have been proposed for objective intraocular scattering assessment, putting especial emphasis on the last one based on Purkinje images, and compare their results and clinical performance.