EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
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EW CATARACT 28 November 2011 What we know, what we don't know, and what we can do about it I t's always concerning to hear surgeons say, "I don't know; it's a mystery," when speaking about a medical phenomenon. Physicians live and breathe facts and data on who develops a condition and why and what can be done to treat it. But with negative dysphotopsia, the answers to many of those questions are puzzling. "We don't really know what it is, and we don't understand it very well," said James Davison, M.D., cataract and refractive specialist, Wolfe Eye Clinic, which has loca- tions throughout Iowa. "It only seems to happen in people who have perfect surgery. Everything else looks absolutely perfect and then they have this difficulty." This difficulty, or negative dys- photopsia (ND), comes in the form of a black line, a parenthesis out of the patient's peripheral vision often described as a dark shadow. "The typical thing they will say is to envision a horse with blinders," said Samuel Masket, M.D., clinical professor of ophthalmology, Jules Stein Eye Institute, David Geffen School of Medicine, University of California, Los Angeles. "That's what it looks like to them. They can't see to the side." What causes ND? The maddening thing about ND is doctors haven't been able to nail down why this phenomenon hap- pens or to whom it will happen. There are plenty of theories about what causes ND and what to do about it, but not much has been proven clinically. Different physi- cians have different stories. For example, Kevin Miller, M.D., professor of clinical ophthal- mology, Jules Stein Eye Institute, Los Angeles, believes ND is directly re- lated to the square-edge shape of the implanted IOL. "It has something to do with the square edge," said Dr. Miller. "With rounded-edge lenses, no one ever complained about this kind of thing." Dr. Miller believes ND occurs be- cause light coming into the pupil from the temporal field of vision causes the problem. When light hits the square edge of the IOL on the nasal side of the optic, the edge acts like a plano mirror, reflect- ing light off the edge. This light then bounces back to the temporal side of the retina, causing one of several positive dysphotopsias, and doesn't get through the edge to illu- minate a patch of nasal retina, thus casting an arc-shaped shadow over the area. He explained this theory in the August 2005 issue of the Journal of Cataract & Refractive Surgery (31:1488-1489). "The interesting thing is we started seeing this with the square- edge silicone lenses, although it was- n't as bad with the silicone as it was with acrylic," he said. "So it has something to do with the square edge and a little bit to do with the material." Likewise, Dr. Davison believes ND is lens-shape related, although he's unable to pinpoint precisely why. "I think it has something to do with the shape of the lens we put in the eye versus the shape of the lens we take out," he said. "I think we probably saw things like this before the square edge, but the square edges seem to make for more fre- quent complaints. There might be some internal reflections; there might be some sort of edge effect. Some papers show that you can still have ND even if you put in a round- edge IOL." Dr. Masket, on the other hand, believes ND is generated from the overlap of the anterior capsulorhexis onto the anterior surface of the IOL. He wrote about this and the surgical techniques to quell it in the July issue of JCRS (37:1199-1207). "The most important thing about ND is that it only occurs with the lens in the bag," Dr. Masket ex- plained. "It can occur with any lens in the bag. I think it's still controver- sial whether some lenses are more prone than others. People often point to the AcrySof lens [Alcon, Fort Worth, Texas] as being signifi- cantly causal, but they overlook the fact that more than half the lenses implanted in the world are the AcrySof lens." Patient patterns Just as no one fully understands why ND occurs, doctors are unable to an- ticipate who will become sympto- matic. Drs. Masket and Miller have picked up on a couple of trends, but evidence is antidotal for now. "When we look at the demo- graphics of patients with this condi- tion, there is kind of a blip on the screen where there will be more women in their 50s than any other gender or age group," Dr. Masket said. "I won't say that's hard data, but that's pretty much what we see. It's odd. It hasn't been carefully ana- lyzed." Dr. Miller notices a comparable personality thread throughout his ND patients. "I think the pattern is the more observant the patient, the more technical she is, the more engineer- like, the more she is likely to notice it," he said. In terms of who won't become symptomatic, Dr. Davison hasn't seen ND in patients who are ex- tremely nearsighted. Dr. Masket has never had an ND complaint from a child, although he hasn't done a large number of pediatric cataract cases. He also stressed ND has never been reported in a sulcus-placed lens or with an anterior chamber lens. "The only thing I've proven clinically is if you take the lens out by Faith A. Hayden EyeWorld Staff Writer Uncovering the mystery of negative dysphotopsia