Tear-film dynamics by Fourier analysis in double-pass images

Abstract

Introduction. The double-pass method allows an integral evaluation of the retinal image quality, taking into account the degradation due to both optical aberrations and intraocular scattering [1].Objective. The objective of this work was to analyse the dynamics of the double-pass image (DPI) as a measure of the tear-film stability.Methodology. A study of the dynamics of the DPI during tear-film rupture was carried out in two groups of voluntaries, a control group with normal subjects (n = 4) and a study group where we sought to simulate dry eye conditions in normal subjects using disposable contact lenses (n = 3), which quickly degrades the tear-film [2]. DPI series were recorded every 0.5 seconds while the subject avoided blinking as long as possible. From the DPIs, a new scattering metric called FDSI (Frequency Domain Scatter Index), was computed based on the Fourier analysis of these images, and the tear-film stability was analyzed by the ratio (FDSI-FDSI0) / FDSI0, where FDSI is the measurement in each image and FDSI0 is the measurement for the first image after the last blink.Results. In the control group the ratio (FDSI-FDSI0) / FDSI0 remained steady for all the time that the subject could avoid blinking (about 40 seconds), with a mean value of 0.1; while in the study group the ratio showed a significant increase before 10 seconds (p <0.0001, t-test) in all subjects, showing a sigmoidal growth.Discussion. Our results are consistent with those found in previous works, in which the decrease in tear-film break-up time in normal subjects with contact lenses was measured [3,4]. However, our data show a sigmoidal growth not found in previous works. The applied methodology allows a better description and analysis of the tear-film dynamics. In this sense, different parameters of the curve fitted to our data (like slope and inflection point) could correlate with the tear break-up time and would allow a clustering of the results. This opens new horizon to study the dynamics of the tear-film during the lifetime of the contact lens, which seems to depend on a complex adaptation process between the type of contact lens (soft, rigid gas permeable, disposable) and the surface of the eye.