February 2013

 

COVER FEATURE

 

Refractive/astigmatism

Experts differ on corneal astigmatism correction in cataract surgery


by Erin L. Boyle EyeWorld Senior Staff Writer

   
Dual Scheimpflug image

Dual Scheimpflug image showing more than 3 D of WTR astigmatism on the anterior cornea and 0.65 on the posterior cornea (which has an ATR refractive effect) Source: Douglas Koch, M.D., and Wang Li, M.D.

Total corneal astigmatism correction during cataract surgery could be either by eliminating it or leaving slight with-the-rule astigmatism

Corneal astigmatism correction in cataract surgery should achieve zero residual astigmatism, some experts say, but there is also a theory that patients might benefit from one quarter to one half a diopter (D) of with-the-rule astigmatism because of against- the-rule drift.

Jack T. Holladay, M.D., clinical professor of ophthalmology, Baylor College of Medicine, Houston, said the goal in corneal astigmatism correction should be to eliminate it completely.

"The idea that you should leave a little with-the-rule and against-the-rule are old myths that come from articles written about 10 or 15 years ago," said Dr. Holladay. "It's not true. Residual astigmatism is like any other aberration. The best vision and the best result are with zero residual astigmatism and with- or against-the-rule are not beneficial. They blur the image, particularly if you don't wear glasses."

The ultimate goal in patient management for total corneal astigmatism correction in cataract surgery is both short-term and long-term patient satisfaction, said Douglas D. Koch, M.D., professor and the Allen, Mosbacher, and Law Chair in ophthalmology, Cullen Eye Institute, Baylor College of Medicine. He said a key step in achieving that goal is determining patients' needs. "If we go to the assumption that most patients want to see clearly at some distance without glasses, and therefore have a significant reduction of their astigmatism, the goal in my mind would be a small amount of with-the-rule astigmatism, around 0.25 D or at most 0.5 D, the reason for that being there's a long-term against-the-rule shift that takes place. If you leave patients with just a little bit of with-the-rule astigmatism, that will enable them to maintain a relatively small amount of astigmatism over a long period of time," Dr. Koch said. "We should also recognize that occasionally patients do well with myopic astigmatism, which gives them greater depth of focus, but at the expense of clear vision at any distance. It is difficult to predict those who might like this, so I rarely recommend it," he said.

Patient age

Dr. Koch has been researching corneal astigmatism and toric IOL selection in cataract surgery cases. He has developed a nomogram that incorporates the mean posterior corneal astigmatism in eyes with either with-the-rule astigmatism or against-the-rule astigmatism and the effect of against-the-rule drift that happens with aging. Warren E. Hill, M.D., East Valley Ophthalmology, Mesa, Ariz., said that he follows Dr. Koch's recommendation of leaving patients with one quarter to one half a D of with-the-rule astigmatism as the final operative goal.

"I think that's a very good strategy. Typically what happens for the older patients is that they may gain a little against-the-rule astigmatism over time, so if you leave them with some with-the-rule astigmatism, they'll always be changing toward something better [rather] than away from what it is that they want," said Dr. Hill. "Younger patients are completely different. They're going to drift toward against-the-rule over time, but it may take decades. We're very good at taking care of the older patients, but the younger patients still have some questions that need to be answered." Dr. Koch said that it could be argued that with an 85-year-old patient, with-the-rule astigmatism is not needed, as there is not likely to be significant change over the course of the patient's life. "On the other hand, an 85-year-old could live to 95, and you could argue that a 50-year-old will have a much greater chance of against-the-rule shift, so you might leave them more with-the-rule," he said. "My philosophy is, most of these folks, if they want their astigmatism corrected, they want it corrected so they have good vision now," Dr. Koch said. "So the goal in my mind is to leave them with just a little bit, just enough that they will have great uncorrected vision, and then you can deal with the against-the-rule shift as it takes place in a 50-year-oldif it takes place in 15 years, then you can treat it at that time. But I think the 50-year-old would be disappointed if you left him or her at 1 D of with-the-rule astigmatism with blurry vision planning that far ahead, unless that patient's a special kind of patient and really understands that concept."

With-the-rule astigmatism

Dr. Koch said that he finds with-the-rule astigmatism cases (as measured on the anterior corneal surface) require lower amounts of correction per unit diopter of astigmatism because of the against-the-rule refractive effect on the back of the eye. He bases this on data from a recently published study in which posterior corneal astigmatism was measured with a dual Scheimpflug device1 and from analysis of clinical results with toric IOLs. "These patients often don't need much, and they need a lot less than we used to think. That applies to relaxing incisions and it applies to toric lenses," he said. Dr. Holladay said that Dr. Koch and others have found that those with-the-rule astigmatism cases need a different amount of cylinder than against-the-rule cases.

"[Dr. Koch's] observation has been that whether it's with-the-rule or against-the-rule astigmatism, for some reason, they end up with more or less residual," Dr. Holladay said. "The question is, why?" he said. "What I've told [Dr. Koch] is, we've got to pin down what the reason for that is, if it's to make sense. it's possible that it may be due to the fact that the with- and against-the-rule astigmatism have different posterior corneal astigmatisms, and if that's true, we should be able to see that with Scheimpflug devices like the Pentacam [Oculus, Lynnwood, Wash.] and the Galilei [Dual Scheimpflug Analyzer, Ziemer, Port, Switzerland] and be able to show that it's a result of the posterior surface. That's one possibilitythat the posterior cornea might have an effect, and that's what he believes. But we've looked at the Pentacam and Galilei and that's not supported yet."

Other reasons could exist, Dr. Holladay said, including the fact that, regardless of whether a horizontal or vertical cataract incision is made in what location, patients drift in the direction of against-the-rule with age. "The other possibility is that when you put an implant in, that implant is never parallel or perpendicular to the visual axis; it may be tilted a little bit and that tilt induces a small amount of astigmatism. That's under investigation right now. In other words, that's [Dr. Koch's] observation, but there's no mechanism yet that's been confirming that that observation may be correct. The reason for that observation is still up in the air," Dr. Holladay said.

Against-the-rule astigmatism

Corneal astigmatism correction article summary

According to Dr. Koch, against-the-rule astigmatism cases need more adjustment than with-the-rule cases because the posterior cornea "increases the amount of against-the-rule astigmatism." "So in using a toric IOL, you want to go up at least one half D of increased correction for the against-the-rule patient, and in terms of relaxing incisions for the against-the-rule patient, yes, they are more likely to need it, even if there's a small amount of against-the-rule," he said.

The difficult part of against-the-rule astigmatism cases is that relaxing incisions must not be made too long, he said. If they are, they can create dryness and foreign body sensation because of incised corneal nerves.

"That astigmatism, in my mind, is more challenging to treat," Dr. Koch said. "I more often will go to a toric lens and do a relaxing incision in those, whereas more often in with-the-rules, for amounts up to 1 D, I do nothing and will not use a 1 D toric IOL until anterior corneal with-the-rule astigmatism is 1.7 D."

Dr. Holladay said in some cases, despite the best planning, the outcome is still not as desired. In those cases, his Holladay IOL Consultant has a Toric Back Calculator tab that provides physicians with a second chance.

"It allows the surgeon to take the observed axis of the lens and the refraction and by observed axis, you look in with the slit lamp, you line up the slit beam and you say, that lens is at 45 degrees. And then, if you refract the patient, with those two bits of information, I can calculate for you exactly how much you need to rotate that lens to the perfect position," he said.

He also recommended that physicians look at the post-op refraction and observed axis or post-op refraction and K reading.

"When you do end up with an outcome that's not on the button, you use that Toric Back Calculator to find out how much you need to rotate it to get it to the right position. That helps a lot, and it tells you what the residual astigmatism is," he said.

Reference

1. Koch, D., Ali, S.F., Weikert, M.P., Shirayama, M., Jenkins, R., and Wang, L., Contribution of posterior corneal astigmatism to total corneal astigmatism. J Refract Surg. 2012: 38: 2080-2087

Editors' note: Dr. Hill has financial interests with Alcon (Fort Worth, Texas). Dr. Holladay is the developer of the Holladay IOL Consultant programs. Dr. Koch has financial interests with Alcon, Abbott Medical Optics (Santa Ana, Calif.), OptiMedica (Sunnyvale, Calif.), and Ziemer.

Contact information

Hill: 480-981-6111, hill@doctor-hill.com
Holladay: holladay@docholladay.com
Koch: 713-798-6443, dkoch@bcm.tmc.edu

Experts differ on corneal astigmatism correction in cataract surgery Experts differ on corneal astigmatism correction in cataract surgery
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