Eyeworld

EW FEATURE 48 Patient satisfaction after the procedure is a benefit. "It is rare to experience an un- happy patient," Prof. Barrett said. He added that he was pleased to have this thought backed up in Dr. Zhang's study, which showed the overall satisfaction score is higher in monofocal patients. "I wasn't surprised to see [the re- sults], but it was nice to see a head- to-head comparison," he said. "This is a good study because it's a prospective group, and, overall, it's a well-conducted study. There were a lot of parameters across a broad range of features." While traditional monovision aims for –2 D of myopic defocus, which offers excellent near vision, Prof. Barrett warned that amount of separation may not be tolerated by every patient. He prefers, instead, to target about 1.25 D of myopia. "At that level, you can be al- most certain that a patient will not experience problems of disassocia- tion between the two eyes," Prof. Barrett said. Lower levels of myopic defocus also will preserve contrast sensitivity and stereoacuity. "However, at that lower level, you don't get quite the same ability to read up close," Prof. Barrett said. "That patient will require, in some cases, spectacle correction for read- ing a book for prolonged periods. It's a small percentage of the time be- cause intermediate vision is so good. The trade-off is that you can be much more certain of patient ac- ceptance." With the smaller levels of my- opic defocus being used, some could contest the use of the term monovi- sion, Prof. Barrett said. "It's a poor description because patients have binocular vision and it doesn't describe the situation very well," he said. "The two eyes are used synergistically." J.E. "Jay" McDonald II, M.D., Fayetteville, Ark., said he prefers monovision because it preserves the complete visual pathway and sig- nals. "The issue is that when you use a multifocal lens you decrease the energy level of light by about 18%, and on each retina you have a super- imposed, out-of-focus image that varies depending on the pupil size," Dr. McDonald said. "That is true of all diffractive multifocal lenses." Giving up that much energy in the eye is problematic, especially if the eye later becomes symptomatic with macular degeneration, glau- coma, or vein occlusions. "With monovision, if you lose the sight in one eye or partial sight in one eye, you haven't given away any of the person's visual potential because of loss of optics," Dr. McDonald said. The weaknesses of the monofo- cal approach are much the same as those of the multifocal approach, he said. "You have to hit your targets," Dr. McDonald said. "You have to clean up refractive errors that are off more than 0.5 D of either sphere or cylinder." Dr. McDonald, too, keeps the separation of the two eyes small. "We are down to –1.25 D," he said. "At that difference, the only true difference between what you see in the focused eye and the out- of-focus eye is some loss of higher spatial frequencies in the out-of- focus eye, but in the mid and low spatial frequencies, 90% of the same information is there. With monovi- sion, you don't see with one eye and suppress and then see with the other eye. You actually see with both eyes." Dr. McDonald said there are a small number of people who are strongly right- or left-eye dominant. Those people might have trouble adjusting to monovision. Dr. McDonald and colleagues are in the February 2011 PRESBYOPIA June 2011 The great continued from page 47 continued on page 50

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