Correlation of Scheimpflug Densitometry Measurements of Corneal Haze With Disability Glare

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.

Inclusion Criteria:

- Age 18 years or greater

- Presence of corneal scar in one eye

- No clinical evidence of any other intraocular pathology

Exclusion Criteria:

- Clinical evidence of other ocular pathology (e.g. cataracts)

- Pediatric patients less than 18 years old

- Bilateral corneal pathology

- Presence of active corneal inflammation/infection

- Presence of degenerative corneal disease

- History of prior corneal surgery (e.g. corneal graft)