Dry Eye's Impact on Vision
Dry Eye Dx and Tx
Dry Eye's Impact on Vision
By William Townsend, OD, FAAO
As ocular healthcare providers, we triage or treat conditions that pose a threat to life, sight, or vision. It's easy to forget that we do not treat glaucoma to lower pressure, but to preserve vision. The DEWS Report (Lemp, 2007) defines dry eye as, “… a multifactorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability…” I emphasize “visual disturbance” because in evaluating dry eye, we can be so focused on objective tests that we fail to appreciate that blurring may be the symptom that most impacts patients' lives. We need to understand how dry eye influences vision.
Visual Symptoms of Dry Eye
Begley and coworkers (2001) investigated the prevalence of symptoms in contact lenses wears and non-contact lens wearers. Visual disturbance was one of the most commonly reported symptoms; 67 percent of contact lens wearers and 57 percent of non-contact lens wearers reported alterations in vision. They reported increased blurring in the evening; this pattern was significantly more apparent in contact lens wearers. There may be several explanations for this phenomenon.
Tutt et al (2000) evaluated the optical and visual impact of tear film breakup. Refraction normally occurs at the interface between tears and air, but local changes in tear film thickness may lead to an irregular air/tear interface that degrades optical quality. When tears break up, the cornea becomes the refracting surface; unfortunately, it is optically inferior to tears. This study evaluated degradation of image quality after the blink in soft lens wearers and non-wearers. Their results demonstrate progressive degradation of image quality after the blink in both groups of subjects. Lohmann et al (1993) showed that the corneal surface devoid of tears is an undulating, irregular surface that has poor optical characteristics. These studies demonstrate that an adequate tear film is essential for excellent refractive characteristics.
Ocular dryness impacts refractive qualities in other ways. Chen et al (2009) used confocal microscopy to evaluate the corneas in controls and in individuals who had confirmed dry eye findings and related their findings to key clinical severity parameters. They found that the mean area of opacified epithelial cells was increased in dry eye subjects. Analysis of their data established significant correlation between the area of opaque corneal epithelial cells, symptoms of blurred vision, and best-corrected visual acuity. They concluded that surface irregularity in dry eye is a major cause of blurred vision.
Contact lenses impact the dry eye in several ways; they divide the tear film into a pre- and post-lens layer. The surfaces of lenses, especially soft lenses, are prone to develop dry spots after a few seconds and ultimately to develop surface desiccation. Lens wearers present with decreased visual performance compared to non-lens wearers. Visual changes in contact lenses can be caused by alterations in hydration or by changes in the pre-lens tear film. In clinical practice we observe tear film breakup, which results in reduced contrast sensitivity in the middle- to high-range frequencies (Timberlake, 1992).
From Research to Practice
These studies represent a fraction of the literature addressing the impact of dryness on quality of vision. We must consider ocular surface disease in the differential diagnosis of patients who complain of blurred vision.
Armed with the knowledge that soft lens patients are especially susceptible to blurring from dryness, we should prescribe materials that exhibit excellent wetting characteristics, or in some cases refit in a GP material. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #185.
Dr. Townsend practices in Canyon, Texas and is an adjunct faculty member at UHCO. He is an advisor to TearLab Corporation, has received research funds from Alcon and B+L and is an advisory panel member of Alcon, B+L, and Inspire Pharmaceuticals. You can reach him at email@example.com.
Contact Lens Spectrum, Issue: April 2011