Correlation of Scheimpflug Densitometry Measurements of Corneal Haze With Disability Glare

Overview

Disability glare is described as "halos" or "starbursts" around bright sources of light that can cause discomfort and reduce vision. The cornea is the clear "window" at the front of the eye, but certain conditions such as a previous infection can leave a scar. Corneal scars can cause disability glare by scattering and spreading incoming light instead of allowing it to focus on the back of the eye (retina) to get a crisp image. In this study, the corneal scar will be analyzed using a new device that measures scar density (Pentacam), and a relationship with disability glare will be made. This can help us further understand disability glare and make better decisions in the future on when to treat these scars to help patients see better.

Disability glare is a phenomenon commonly described as starbursts and halos that results in difficulty seeing clearly in the presence of bright "blinding" light. It can appear as if a veil of light is cast over the world outside. Optically, this occurs when incoming light is scattered in the eye instead of being focused on the retina. This is called straylight, and it diffusely illuminates the retina which causes desensitization of the photoreceptors and reduces the contrast of the retinal image (Lombardo \& Lombardo, 2010). The main sources of scatter in the human eye are opacities in the clear ocular media, primarily due to diffusion and loss of transparency in the cornea and lens, as well as within the retina. While the lens is the largest contributor to light scatter (especially with cataract formation and aging), opacification of the cornea (e.g. scars or haze) can similarly cause intraocular light scattering, resulting in disability glare and decreased contrast sensitivity (Fan-Paul et al., 2002). Due to the subjective nature of disability glare and contrast sensitivity, it is fairly difficult to develop a reliable objective way to measure and quantify this phenomenon. One of the most commonly used clinical tests for disability glare is the Brightness Acuity Tester (BAT), which is described as an ice cream scooper 60 mm in diameter with a 12 mm hole in the center. With the patient viewing a visual acuity chart through the hole, a uniform luminance at three different settings-high (white sand beach), medium (clear day), low (overhead lighting)-is used to induce glare and simulate the various real-life conditions. The drop in visual acuity with the various glare settings is used as a measure of disability glare. The BAT test has been found to be a reliable predictor of outdoor visual acuity (clinically validated for use in measuring disability glare secondary to cataracts), is ubiquitously available, and is easy to administer. To further and more accurately reflect real life situations, it can be used in conjunction with a contrast sensitivity chart rather than the regular black-on-white Snellen letter chart (Aslam et al., 2007). Contrast sensitivity can be tested using letter-based charts (e.g. Pelli-Robson letter sensitivity chart) that consist of letters of the same size but with decreasing contrast, or contrast gratings, which are based on sinusoidal waves of light (e.g. Vistech MCT-8000 or FACT). Opacification of the cornea in the form of scars or haze is a natural response to a wide array of pathological insults (e.g. infection, degeneration, corneal dystrophies) and can result in light scatter and glare. Measurement of corneal haze was previously crudely performed via standard slit-lamp examination and human measurement. Recently, an automated, non-invasive, and objective method to measure corneal scar density was introduced using Scheimpflug imaging. This Pentacam device (Oculus Optikgerate GmbH, Wetzlar, Germany) consists of a rotating camera that captures images of the cornea at an angle, and analysis of these anterior segment images can be employed to quantify scattered light in various diameters and depths of the cornea. The generated maps of the amount of scatter in different regions of the cornea are called corneal densitometry maps, and they are displayed along with maps of corneal topography (elevation) and pachymetry (thickness). Normative values for Scheimpflug densitometry have recently been established (Dhubhghaill et al., 2013). Measurements have also been obtained after various surgical procedures to analyze their impact on corneal clarity. This includes analysis of corneas after refractive surgery such as LASIK (Cennamo et al., 2011; Fares et al., 2012) and PRK (Takacs et al., 2011), as well as various forms of corneal transplantation (Koh et al., 2012; Bhatt et al., 2012; Arnalich-Montiel et al., 2013; Ivarsen \& Hjortdal, 2013) and collagen cross-linking (Gutierrez et al., 2012; Greenstein et al., 2010). Furthermore, it has been used to characterize pathological conditions including infectious keratitis (Otri et al., 2012; Orucoglu et al., 2014) and congenital corneal opacities (Elfein et al., 2013). However, no functional correlation has been made with these anatomical maps, and the relationship between scattered light as measured by densitometry and its effect on visual function such as contrast sensitivity and disability glare has not been well-characterized.

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