discovering dry eye
A Closer Look at Tear Break-up
BY CAROLYN BEGLEY, OD, MS, AND DEBRA RENNER, OD
The problem with tear break-up time (TBUT) is that it shows a poor correlation with dry eye symptoms. Dry eye has become a symptom-based disorder because we have no gold standard clinical test for its diagnosis. Many of our standard clinical tests don't measure the cause of dry eye symptoms. We may need to develop new clinical tests to diagnose dry eye or re-evaluate existing tests.
TBUT is usually considered a test for quality of the tear film. Unfortunately, it measures only the initial stability of the tear film and does not provide information about later events in tear break-up. If you want to examine how well the tears coat the corneal surface, continuing to watch other areas of break-up appear in the tear film may furnish valuable information.
In order to observe and objectively measure tear break-up over time, researchers at the Indiana University School of Optometry have developed relative tear thickness maps using the intensity of fluorescence over the corneal surface. Figure 1a shows a sample image taken from a videotape of tear break-up. The subject was asked to hold his eyes open as long as possible, but to blink when necessary. This videotape fluorescein image occurred just before the subject blinked. The frame was converted to a digital grayscale image to produce the relative tear thickness color map in Figure 1b. Areas of tear break-up (dark blue) were defined as having the same amount of fluorescence as the background. In this way, an objective measurement of tear break-up could be determined over time.
Figure 1a. Tear break-up. Figure 1b. Relative tear thickness map.
Most of the subjects in the study experienced tear break-up while holding their eyes open as long as possible. However, many subjects did not feel any sensation until the areas of break-up were relatively large. Then subjects typically reported a stinging or burning sensation and immediately blinked. We repeated this "staring contest" three times for each eye to observe the pattern of break-up over time. Many subjects showed repeated tear break-up over the same corneal areas and reported similar sensations during the trials.
Subjects also filled out symptom questionnaires before and after testing. Almost all subjects became more symptomatic after testing. Typical dry eye symptoms such as discomfort and soreness increased about the same amount in both dry eye and normal patients. This suggests that tear break-up can cause symptoms of ocular irritation and dry eye. Given that information, we checked subjects for corneal staining and found little or no surface damage that could account for the change in symptoms. The subject in Figure 1, for example, showed no corneal staining after the experiment. The cause of the increased ocular symptoms after tear break-up remains unknown, but is under study by our group at Indiana University. Further examination of the effects of tear break-up may unlock the secrets of dry eye and help us determine the connection between dry eye symptoms and signs.
Dr. Begley is an associate professor at the Indiana University School of Optometry and is also a member of the Graduate Faculty.
Dr. Renner is the Contact Lens Resident at the Indiana University School of Optometry. Her interests include lens fitting and dry eye research.