discovering dry eye

The True Story Behind Tear Film Break-up

BY CAROLYN BEGLEY, OD, MS & RON TUTT, OD, MS
May 1999

Do you have patients who complain of blurry vision even though you can't find a refractive cause for their complaint? When questioned, they often describe their vision as foggy or steamy, but clearing temporarily after a blink. These symptoms typically become worse by the end of the day or with computer use. Snellen visual acuity for these patients is often excellent (20/15 or 20/20) and their lenses fit well and appear to be clean. So what's causing their complaints?

Tear Film Break-up

In clinical practice, we don't consider the changeable nature of the tear film when we measure visual acuity. We instruct patients to view a high contrast Snellen chart and ask them to read the smallest line possible. Because blinking isn't controlled during the test, patients can blink as often as necessary to reconstitute the tear film over the surface of their eye or contact lens. Tear film break-up is monitored from behind the slit lamp biomicroscope, but its effect on vision usually isn't considered.

The tear film contributes greatly to vision. The refractive index difference between the curved surface of the tear film and the air creates approximately two-thirds of the refractive power of the eye. Traditionally, we have been taught that the cornea has this power, but the refractive index difference between the tear film and the cornea is small.

Since the tear film serves as a powerful optical surface, its break-up must have visual consequences. To investigate these effects, we, along with two other researchers from the Indiana University School of Optometry, Dr. Bradley and Dr. Thibos, analyzed the changing contrast of fundus images videotaped while the eyes were held open for one minute or more. Changes seen in the fundus image are equivalent to the patient's vision changes.

Figure 1 shows the fundus of a soft contact lens wearer who held his eyes open for 50 seconds. The first image (0 sec) was taken just after the blink. Details of the nerve head and vessels are clearly visible. After 30 seconds, the fundus image became hazy and after 50 seconds, the details of the fundus were obscured. Tear film break-up was also monitored and occurred concurrently with the changes in the fundus image. The appearance in the fundus image after 30 to 50 seconds without blinking showed just how poor the patient's vision became. By 50 seconds, the retinal image lost 60 percent of its contrast and the patient showed an 80 percent loss of contrast sensitivity using the Pelli-Robson Chart.

FIG. 1: Fundus images from a soft contact lens wearing patient who held his eyes open for 50 seconds. The fundus was videotaped using a slit lamp biomicroscope and a mounted +90D lens.

The changing fundus images in Figure 1 illustrate what patients mean by foggy or blurry vision. As the tear film breaks up on the surface of the eye or contact lens, it produces light scatter, which is similar to viewing objects through a fog.

Blinking reconstitutes the tear film and smooths the surface of the eye, thus improving vision temporarily. But visual tasks, like using a computer, require concentration, which inhibit blinking. Therefore, performing these tasks increases the likelihood that break-up will occur, causing reports of foggy or blurry vision.

Dr. Begley is an associate professor at the Indiana University School of Optometry. Dr. Tutt is a member of the Armed Forces Optometric Society, AOA and a student member of the AAO.