Dry Eye Disease and Diminished Visual Acuity

Dry Eye Disease and Diminished Visual Acuity

Dry Eye Disease and Diminished Visual Acuity
By Gary N. Foulks, MD, FACS

It makes intuitive sense to believe that decay in the quality of the eye’s first refractive surface, the tear film, would have a negative effect on visual acuity. Indeed, most dry eye researchers have long held that the condition impacts vision but have lacked a means to quantify that visual loss.
   
For that reason, it is exciting to look at several recent research papers that test interblink interval visual acuity decay (IVAD) in an attempt to quantify the impact of dry eye disease on visual acuity. While the test incorporates a standard measure of visual acuity, its specialized design enables it to detect decreases in visual function related to tear film instability that standard visual acuity testing misses. As tear film instability is associated with dry eye disease, a simple means to measure dry eye-related changes in acuity could prove useful in several ways.

IVAD Design
   
In theory, the rapid breakup of the unstable tear film typical of dry eye disease should cause visual acuity to decay readily following blink. Except in extreme cases, however, standard measures of visual acuity (such as Snellen testing) don’t register this effect. This is because most patients with dry eye disease can compensate for the instability of their tear films by increasing their rate of blink—thus demonstrating “normal” visual acuity on standard tests. The IVAD, however, measures visual acuity between blinks, which reveals dry eye patients’ true visual impairment.
   
Similar in design to a Japanese computer-based system called functional visual testing, the IVAD test makes use of an optotype (“c”) that is presented on a computer screen at patients’ best corrected visual acuity (BCVA) during interblink intervals. Presented at the patient’s response rate, the optotype increases in size in accordance with a decrease in visual acuity, thereby measuring the decay in visual acuity between blinks.1
   
Tests with the IVAD indicate that tear film instability may be one reason that patients with dry eye disease complain of visual impairment. In testing this correlation, researchers have observed that patients with central corneal staining are able to maintain their BCVA for less time between blinks than those without central staining.2 Additionally, tests with the IVAD confirm that visual acuity is more seriously impacted later in the day, which confirms the common anecdotal description of dry eye disease as having diurnal variation in which symptoms worsen as the day progresses.3

Potential Applications
   
By measuring loss of visual function, the IVAD may allow researchers and clinicians to better establish the severity of dry eye disease in individual patients. Since most algorithms for treating dry eye disease add therapies in a stepwise fashion based on disease severity, the IVAD may one day help to guide clinical treatment by providing an objective measure of acuity impairment and, by extension, disease severity.
   
To date, studies with the IVAD have been presented at meetings but not published in peer-reviewed journals. However, if IVAD testing is eventually validated to the point where it is accepted for use in peer-reviewed studies, there is potential to use this technology as an outcome indicator for measuring the effectiveness of new drugs and/or treatment interventions. This would be most welcome, as demonstrating the value of new dry eye therapeutics to the FDA’s satisfaction is very difficult with current efficacy measures.
   
The FDA has been understandably reluctant to allow new outcome measures that are not well validated, and there remains much more work to be done to gain acceptance for the IVAD. However, the technology should prove useful for establishing the severity of dry eye disease, and it will be interesting to see if it is eventually used to determine the effectiveness of dry eye treatments.

Gary N. Foulks, MD, FACS, is the Arthur and Virginia Keeney professor of ophthalmology, University of Louisville, Louisville, KY, and is editor-in-chief of The Ocular Surface.

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References
1. Walker PM, Ousler III GW, Workman DA, et al. Visual Function in Normals Compared to Patients Diagnosed with Dry Eye as Measured by the Inter-blink Interval Visual Acuity Decay (IVAD) Test. ORA Clinical Research and Development. Presented at ARVO 2007 in Fort Lauderdale, FL, from May 6-10.
2. Ousler III GW, Durham T, Brazzell K, et al. A Correlation between Central Corneal Staining and Visual Function in Patients Diagnosed with Dry Eye. Poster presented at ARVO 2007 in Fort Lauderdale, FL, from May 6-10.
3. Walker PM, Ousler III GW, Schindelar MR, et al. Diurnal Variation of Visual Function and Corneal Keratitis in Patients with Dry Eye. ORA Clinical Research and Development. Poster presented at ARVO 2007 in Fort Lauderdale, FL, from May 6-10.