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Eye Care Archive (Expert Forum)
cataract power calculations for monocular vision
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cataract power calculations for monocular vision
by dorisb, Jan 11, 2007 12:00AM
I'm 70, nearsighted, astigmatism. My left non-dominant eye cataract was operated on 12/12/06 with relaxing incisions. My surgeon recommended against monocular vision but, for many years, I very successfully had a distance contact lens in my right eye and a near contact lens in my left eye. The surgeon finally agreed (verbally) to provide a fixed focus IOL (Alcon SN60WF) set for “near” in my left eye (a 2.50 correction).
The left eye results are a mixed bag. My distance vision is now extraordinary but I can't see close up. My left eye is now not quite 20/40. The power he chose was 13.0. My file pages with IOL power calculations indicate the baseline SN60WF was an 11.94 power (Nov 2005) and 12.5 (Aug 2006). So, an IOL with a 13.0 power improved my vision only by 0.5 to 1.0. I really needed a power of 14 or 15.
My astigmatism went from 1.75 to 0.75. He was shooting for zero but he said, and I understand, that that's part art, part science. He also said that the amount of astigmatism affects my ability to see near and far. He said the healing process takes 4-6 weeks. The right dominant eye will be operated on 01/23/07. The surgeon offers two possibilities: 1) set my right eye for near (but the rule of thumb is that the dominant eye is supposed to be for distance), or 2) set my right eye for distance and later change the IOL in my left eye to the proper one. But that means another operation although he was reassuring that it's a simple matter to change IOLs. I am leaning toward choice #2, even if that means another operation (scary). What do you recommend?
The left eye results are a mixed bag. My distance vision is now extraordinary but I can't see close up. My left eye is now not quite 20/40. The power he chose was 13.0. My file pages with IOL power calculations indicate the baseline SN60WF was an 11.94 power (Nov 2005) and 12.5 (Aug 2006). So, an IOL with a 13.0 power improved my vision only by 0.5 to 1.0. I really needed a power of 14 or 15.
My astigmatism went from 1.75 to 0.75. He was shooting for zero but he said, and I understand, that that's part art, part science. He also said that the amount of astigmatism affects my ability to see near and far. He said the healing process takes 4-6 weeks. The right dominant eye will be operated on 01/23/07. The surgeon offers two possibilities: 1) set my right eye for near (but the rule of thumb is that the dominant eye is supposed to be for distance), or 2) set my right eye for distance and later change the IOL in my left eye to the proper one. But that means another operation although he was reassuring that it's a simple matter to change IOLs. I am leaning toward choice #2, even if that means another operation (scary). What do you recommend?
Doctor's Answer
by Forum-OD-MP, Jan 11, 2007 12:00AM
well i wont really "recommend" anything. thats hard to do over the internet. i try to avoid it. but i will tell you that most patients have a very difficult time changing their distance vision/near vision eye. if you were happy with monovision for many years with your left eye as the near-vision eye...relatively few patients IMO can/could "switch" and have their right eye be the near vision eye. thats difficult to do. kind of like learning to drive a stick shift (manual transmission) car with your left hand on the wheel and the right hand on the stick shift and driving that way for 20 years...then suddenly having to drive a car with the right hand on the wheel and shifting with the left. is it POSSIBLE? of course. you might even be really good at making that switch. but most people would find making that switch very difficult.
Like you, I also had high myopia with significant astigmatism. Consequently, every cataract surgeon I consulted warned me that my surgical outcome would be difficult to predict. This was because the results of limbal relaxing incisions are unpredictable, reducing astigmatism by about 50% on average, with wide variation. And any remaining astigmatism would affect the quality of vision at all IOL powers, thereby reducing the accuracy of those IOL Master power calculations. Two of the surgeons I saw actually suggested that I would probably benefit from some post-surgery laser vision enhancement (in my case PRK, since my eyes are borderline dry.) But I was very lucky in surgery, and my results far exceeded my expectations.
Perhaps the astigmatism issue was a factor in the unexpected outcome of your left eye. In any case, I suggest that you proceed with the distance correction on your right eye as scheduled. I guess it's subjective, but I'd consider 20/20 or 20/25 distance vision to be an excellent result. (IMO, 20/30 would be acceptable, but "not quite 20/40" would be less than satisfactory for monovision.) Then allow enough time for healing before evaluating the outcome. Any residual astigmatism you have might (or might not) be bothersome. Your doctor could easily simulate what your vision would be like with different corrections. Based on this evaluation you could decide which would work best, an IOL exchange in your left eye to improve near vision (but would not eliminate astigmatism) or some laser vision enhancement in one or both eyes (which could correct spherical error and eliminate astigmatism.)
However, he is now strongly recommending making my right dominant eye the near vision eye, saying I should do just fine. This is because, for about ten days now, I have been wearing eyeglasses with no lens over the left eye and have been reading relatively comfortably (relying on my eyeglass-corrected right eye), although it does feel slightly weird at times.
I wonder if the fact that I spend more of my time at the computer or otherwise reading than at distance-viewing activities would argue for his recommendation? That is, I'd be using my dominant eye for my more dominant activity?
So I'm going to take your very excellent advice to have the distance correction in my right eye and then later evaluate what's needed.
Thanks so much for your very clear advice as well as the reasoning on which you base it.
1) Almost 20/40 vision is not quite good enough for driving without correction in most states. You'd probably need glasses/contact lens for driving, watching movies and many other activities, which defeats the purpose of having monovision.
2) If your right eye has significant astigmatism, the surgical outcome of that eye might also deviate significantly from the target refraction. Conceivably, you could wind up needing glasses/contact lens for near/intermediate vision