EyeWorld India June 2013 Issue

32 EWAP CAtArACt/IOL June 2013 Determining visual outcomes by Michelle Dalton EyeWorld Contributing Writer With all the improvements in technology, is measuring vision by Snellen alone enough these days? U sing a letter chart to measure vision is cheap, easy, and quick. If a patient reads the letters on the chart correctly and at a reasonable visual acuity, then it’s likely the rest of the visual system is functioning correctly as well. But if a patient cannot read those letters well, what then? “Being able to see the letters on a chart in a really high contrast situation is relatively easy to do even if the visual quality is poor,” said Uday Devgan, MD , in private practice, Los Angeles, Calif., USA, and chief of ophthalmology, Olive View–UCLA Medical Center. “Snellen is, and will remain, the standard. But it’s not the be-all and end-all. It’s just a small portion of what makes up a person’s vision.” Character recognition on a letter chart (as proposed by Snellen in 1862) should still be the standard for “visual acuity” measurement, said August Colenbrander, MD , the Smith- Kettlewell Eye Research Institute and California Pacific Medical Center, San Francisco. “Visual acuity alone is not enough to define vision, but neither is visual field, or contrast sensitivity, or color vision, or dark adaptation, or any other parameter,” he said. “Is Snellen acuity assessment going anywhere? No. But is it adequate? No,” said George O. Waring IV, MD , assistant professor of ophthalmology, and director of refractive surgery, Medical University of South Carolina. “They’re important, but they’re not enough. We need better objective analyses that describe quality of vision and functionality.” Snellen may be a “very strong indicator of vision,” but “real world vision is very different and impacts directly on quality of life,” said Eric D. Donnenfeld, MD , partner, Ophthalmic Consultants of Long Island, Rockville Centre, NY, USA, and clinical professor of ophthalmology, NYU School of Medicine, New York, NY, USA. “Vision is a complex phenomenon,” Dr. Colenbrander said. “But if you want to capture that complex phenomenon in a single measurement of visual acuity, Snellen remains the best.” Improving visual outcome measurements Dr. Waring said contrast sensitivity is “the most important thing we measure. In our clinic, we measure it under varying conditions to determine what a person’s functional vision is.” One of the most common complaints he addresses is from patients who have difficulty driving at night or during other low contrast circumstances, and reading charts alone cannot address those quality of vision issues. “A number of my early cataract and dysfunctional lens syndrome patients have stopped driving at night because of difficulty with vision in this low contrast circumstance, but they may still have very good best corrected Snellen acuity,” he said. Contrast sensitivity has become Dr. Donnenfeld’s “gold standard” for vision testing “because it measures vision under adverse circumstances such as glare and scotopic conditions.” He noted almost all of the current laser and IOL evaluation studies are incorporating contrast sensitivity testing to evaluate quality of vision. “There are plenty of situations that can affect contrast sensitivity—a really bad ocular surface, irregular cornea, cataract, the rings in a multifocal lens, higher order aberrations, etc. There are all kinds of layers of what could happen,” Dr. Devgan said. “We are working on advanced functional vision testing modalities and more widespread acceptance of the other objective measures of quality of vision and functionality,” Dr. Waring said. Evaluating the ocular surface “can also be a strong indicator of contrast sensitivity,” Dr. Donnenfeld said. “I use topography on these patients to look for dropout or significant irregularity.” What to use Dr. Colenbrander has designed a series of mixed contrast sensitivity charts that mix letter chart acuity with contrast sensitivity. Other tests are “useful to find some of the many causes of contrast loss,” but his cards can measure the consequence of contrast loss. Figure 1. Densitometry dysfunctional lens Source: George O. Waring IV, MD