discovering dry eye
A New Look at Tear Break-Up
BY CAROLYN G. BEGLEY, OD, MS, & GRAEME WILSON, OD, PHD
Tear break-up time (TBUT) is the time required for the first disruption in the fluorescent image of the tears to appear. Usually a TBUT value of less than 10 seconds is used as a cut-off in differentiating between dry eye and normal eye conditions.
A good quality tear film will show a high TBUT because it spreads uniformly over the surface of the eye and does not disrupt or break-up easily. Conversely, a poor quality tear film will disrupt quickly, and the TBUT will be very low. However, we know from a number of well-controlled clinical studies that TBUT is a sensitive, but not specific technique. This means that most dry eye patients have a low TBUT, but many other patients who don't complain of dry eye also show a low TBUT. Therefore, TBUT fails to differentiate between dry eye and normal eye conditions. In order to understand any clinical test, it's necessary to examine the manner in which it is performed.
Evaluating the TBUT Test
There are two potential problems with the first step in this technique, which involves wetting a disposable fluorescein strip and instilling the dye onto the surface of the eye. If preserved saline is used, the surfactant nature of many preservatives may affect the quality of the tear film. Secondly, the strip is often wetted with a large drop of saline, which may add a quantity of fluid to the eye and thus dilute the natural tears.
After instilling fluorescein dye, the TBUT is measured. The first spot of break-up is the typical endpoint of the test. However, after the first spots of tear disruption, dry areas continue to form, which provide information about the stability of the tear film.
There are several steps you can employ to improve tear break-up as a clinical test. Use nonpreserved saline to wet the strip to avoid the effect of preservatives on the quality of the tear film.
The quantity of fluid instilled with the dye should also be controlled. This can be accomplished by carefully shaking the fluorescein strip to remove excess fluid or by using the new Dry Eye Test (D.E.T.) developed by Akorn Laboratories in conjunction with Dr. Donald Korb. The D.E.T. is a disposable fluorescein strip that is greatly reduced in size in order to deliver a minimum of fluid to the eye. Taking these precautions should minimize the impact of the test on tear film stability.
A New Approach
Examining tear break-up beyond the first disruption in the tears has been suggested by several researchers. Dr. Salih Al-Oliky and his colleagues examined tear break-up in dry eye and normal patients. Rather than measuring just the TBUT, they continued to monitor tear break-up in the corneal area over which the tear disruption occurred. They termed this comparison of corneal area versus time the Dry Area Growth Rate, or DAGR. Dry eye patients showed a rapid disruption of tears over the corneal surface, yielding a high DAGR, whereas normal patients showed a low DAGR.
Clinicians can easily incorporate the DAGR technique into practice. Instead of stopping at the first break in the tears, watch to see how quickly other areas of break-up appear in the tear film and make an estimate of their size. A tear film showing a low TBUT that continues to quickly disrupt over the surface of the eye is of poorer quality than one that doesn't. Fluorescein tear break-up can be a powerful technique if you minimize dye instillation errors and fully exploit the potential information from the test.
Dr. Begley is an associate professor at the Indiana University School of Optometry and is also a member of the Graduate Faculty.
Dr. Wilson is a professor at Indiana University School of Optometry and a member of the International Society for Contact Lens Research.