The eye blink is vital to the maintenance of normal tear film physiology, ocular surface integrity and visual acuity.2 Lasting only about a quarter of a second, each blink facilitates the drainage of tears, excretion of lipids from meibomian glands, elimination of debris in the tear film and spreading of lipids across the tear film. The cornea requires this constant nourishment, as extended periods of non-blinking lead to a progressively thinner tear film. This can cause dry spots over the cornea, causing surface damage that is manifested as keratitis.3
Since tear film break-up time (TFBUT) was first described as a clinical entity at Schepens Eye Research Institute in the late 1960s, it has been the primary diagnostic tool in dry eye disease. Traditionally, reference values of TFBUT in normal (≥10 seconds) and dry eye (<10 seconds) subjects were determined using large volumes of fluorescein dye (20µL to 40µL), but a more physiologically accurate threshold of five seconds (similar to the average time between blinks) has been established for dry eye subjects, using smaller more controlled quantities of 5µL of fluorescein.
Despite improvements in both the specificity and sensitivity of TFBUT, its use as a primary clinical endpoint is flawed for a few key reasons. First, the calculation of TFBUT assumes a static and invariable blink rate, an unreasonable expectation even in the most controlled clinical environment. Secondly, tear film break-up is recorded upon the first observation of a single dry spot on the cornea without consideration of the area or magnitude of the drying. Finally, TFBUT does not account for the five patterns of tear film break-up, which have been identified by our group.
In an effort to more adequately measure the symptomatology of dry eye, researchers have developed a parameter known as symptomatic break-up time (SBUT). It is a measure of the time it takes patients to first report a sensation of ocular awareness after they blink twice then stare straight ahead. The outcome shows variable reproducibility when compared with TFBUT, with 72% of patients experiencing ocular discomfort within one second after their recorded TFBUT. However, SBUT may help augment the assessment of mild cases of dry eye, in which an irritated symptomatology is often present without clinical signs (i.e., decreased TFBUT).4
Recently, investigators have found direct correlation between shorter inter-blink interval (IBI) and shorter TFBUT. An analysis by the group ORA, Inc., found that dry eye patients have a higher blink rate than non-dry eye controls.5-7 In accordance with these findings, drugs that are currently being evaluated for dry eye, such as ecabet sodium, have shown a positive effect on blink rate.
Other researchers, however, have failed to find a statistically significant correlation between IBI and TFBUT.8 This may be explained by the fact that as the disease intensifies, dry eye patients begin to experience progressive corneal desensitization. Conversely, those dry eye patients who still experience corneal sensation may exhibit a rapid blink rate in response to continual irritation.
In the future, blinking may serve as a measure in the diagnosis and treatment of dry eye. Because dry eye patients may have a lower IBI in order to compensate for a decreased TFBUT, maintain VA and alleviate irritation, blink rate may serve as a valuable clinical endpoint in the future of dry eye management.
1. Brewitt H, Sistani F. Dry eye disease: the scale of the problem. Surv Ophthalmol. 2001;45 Suppl 2:S199-202.
2. Carney LG, Hill RM. The nature of normal blinking patterns. Acta Ophthalmol (Copenh). 1982 Jun;60(3):427-33.
3. Nakamori K, Odawara M, Nakajima T, et al. Blinking is controlled primarily by ocular surface conditions. Am J Ophthalmol. 1997 Jul;124(1):24-30
4. Abelson MB, Ousler GW, III, Welch D, Nentwig B. Symptomatic break-up time (SBUT) in dry eye patients with normal vs. low corneal sensitivity. Invest Ophthalmol Vis Sci. 2009 May;50(5):4675.
5. Tsubota K, Hata S, Okusawa Y, et al. Quantitative videographic analysis of blinking in normal subjects and patients with dry eye. Arch Ophthalmol. 1996 Jun;114(6):715-20.
6. Yap M. Tear break-up time is related to blink frequency. Acta Ophthalmol (Copenh). 1991 Feb;69(1):92-4.
7. Walker PM, Ousler GW, III, Lane K, et al. A comparative investigation of spontaneous blink rates across a series of tasks in dry eye and control patients; IOVS. 2008. ARVO E-Abstract 5317.
8. Schlote T, Kadner G, Freudenthaler N. Marked reduction and distinct patterns of eye blinking in patients with moderately dry eyes during video display terminal use. Graefes Arch Clin Exp Ophthalmol. 2004 Apr;242(4):306-12.