discovering dry eye
Tear Film Break-Up and Vision
BY NIKOLE HIMEBAUGH, OD & CAROLYN
G.BEGLEY, OD, MS
The tear film is the first refractive surface of the eye. The refractive index difference between the curved surface of tear film and air contributes about two-thirds of the refractive power of the eye. When the tear film breaks up across the cornea, the optical quality of the eye suffers greatly. Researchers at Indiana University are investigating the impact of these optical effects on vision.
The researchers in Dr. Larry Thibos's lab at the Indiana University School of Optometry used a Hartmann-Shack wavefront sensor to study the optical aberrations caused by tear film break-up. The wavefront sensor images hundreds of points of light across the pupil and measures their deviations from normal.
Figure 1 shows data from one patient before (top row) and after (bottom row) tear break-up. The fluorescein image after tear break-up shows tear disruption centrally and peripherally. Corresponding images from the Hartmann-Shack wavefront sensor depict a regular arrangement of points before tear break-up, but a distorted image after break-up. Wavefront aberration maps can be constructed to show the relative thickness of the tear film. These maps clearly demonstrate a smooth optical surface before tear film break-up and a highly aberrated surface afterwards. Wavefront maps can also be used to simulate loss of vision. It is clear from the two simulated retinal images that this patient's vision was very poor after tear break-up, corresponding to approximately 20/100 Snellen visual acuity.
Figure 1. Optical effects of tear film break-up: Top row of images was captured immediately after a blink; bottom row of images was obtained after subject held the eyes open for 40 seconds.
Patients describe the poor vision occurring with tear break-up as foggy or steamy that clears up temporally with a blink. This is because tear film break-up causes light scatter, in addition to other optical aberrations. Scatter creates the appearance of viewing objects through a thick fog, as the simulated retinal image in Figure 1 shows. With a blink, the tear film smooths the surface of the eye, improving vision until tear break-up occurs again.
Visual disturbances are a common symptom among dry eye patients and contact lens wearers, presumably because both groups experience tear break-up between normal blinks. Visual tasks requiring concentration, such as computer use, inhibit blinking and may exacerbate these complaints. Clinicians should be aware that Snellen visual acuity may not be representative of the vision experienced by these patients in their daily lives.
Dr. Begley is an associate professor at the Indiana University School of Optometry and is also a member of the Graduate Faculty.
Dr. Himebaugh is a graduate student at Indiana University School of Optometry, studying optical and visual effects of tear film breakup.