Reducing pre-existing corneal astigmatism

by Vanessa Caceres EyeWorld Contributing Editor

 

Refractive editor's corner of the world

In this month's inaugural refractive corner, I'd like to give some food for thought on the topic of astigmatism. Astigmatism is one of the most common conditions a cataract and refractive surgeon deals with. It's more common than dry eye and even more common than blepharitis. In fact, more than 70% of the adult population has more than 0.5 D of pre-existing astigmatism. Interestingly, our incidence of correcting pre-existing astigmatism varies by procedure. With laser vision correction (LASIK, PRK, LASEK), we correct any amount of astigmatism 100% of the time. If it's there, it's addressed. Why? To provide patients the best opportunity to see without being dependent on glasses or contacts lenses. That's the mission of today's refractive surgeon. With cataract surgery, however, there is still a transition of traditional surgeons converting to refractive cataract surgery. Yet time is moving things very quickly in that direction. Astigmatism treatments associated with cataract surgery vary from 0-40% or higher in 2011, depending on the practice and the individual surgeon's preference. A common question we ask ourselves is, "How much astigmatism warrants our attention?" It wasn't that long ago that FDA-sponsored clinical trials considered less than 1.5 D to be "not clinically significant." This was reduced to 1 D, then 0.75 D, and now many of our peers consider 0.5 D, especially in the setting of presbyopia-correcting IOLs, to be the cut-off point for clinically significant astigmatism. Our refractive colleagues would suggest any amount is too much, and surgical incisions and the astigmatism they induce need to be taken into account. Do they treat a tenth (0.1) of a diopter on wavefront testing with their LASIK or PRK procedures? They absolutely do, every patient, every day when indicated. As refractive cataract surgeons, we may soon evolve to that line of thinking as well. The paradigm is shifting and refractive cataract surgery is truly coming of age. I've asked three experts, Skip Nichamin, M.D., Richard Mackool, M.D., and Eric Donnenfeld, M.D., to share their thoughts and pearls on the treatment options for modern astigmatism procedures that can be performed at the time of cataract surgery (limbal relaxing incisions, penetrating limbal relaxing incisions, and arcuate incisions with the new femtosecond lasers). These techniques, in association with presbyopic, monofocal, or toric IOLs, should permit today's refractive cataract surgeons to provide their patients with the same degree of spectacle independence as our laser vision refractive colleagues.

Kerry Solomon, M.D., refractive editor

Nomograms used by Dr. Mackool when performing double penetrating incisions for astigmatism Source: Richard Mackool, M.D.Techniques aim to improve surgical outcomes, exceed patient expectations

There are different treatment approaches that surgeons can take when treating residual corneal astigmatism after presbyopia-correcting surgery. Here's an outline of three approaches—limbal relaxing incisions (LRIs), double penetrating incisions, and arcuate incisions with a femtosecond laser. The surgeons profiled here commonly use the approach that they describe.

LRIs
With patient expectations higher than ever, the goal of presbyopia-correcting surgery is to make the patient as emmetropic as possible, said Louis D. "Skip" Nichamin, M.D., medical director, Laurel Eye Clinic, Brookville, Pa. That said, LRIs can help surgeons and their patients achieve better refractive outcomes. Commonly used for 1-2 D of astigmatism, Dr. Nichamin believes that LRIs can correct a greater degree of astigmatism. "I've been an LRI advocate for a long time, and I'm comfortable correcting 2 or more diopters," he said. If the patient has greater than 3 D of astigmatism, he will use LRIs and a toric IOL or excimer ablation in a bioptics fashion. Dr. Nichamin believes that skepticism about the use of LRIs for higher degrees of astigmatism may relate to improper technique. He makes several moves to ensure proper technique. "LRIs take investment in technique," he said.

First, "as with any astigmatism correction, the treatment has to be centered precisely over the steep meridian," he said. Additionally, the incisions have to be perpendicular to the corneal surface. Quality knives are key to well-done LRIs, and Dr. Nichamin believes the incisions are best made with a thin diamond blade. "If it's beveled, the incision may not be deep enough," he said. Old radial keratotomy knives, steel blades, and double-edge blades do not usually perform well, he said. A thin diamond blade designed for LRI can prevent regression and help produce predictable incisions, he said. Surgeons should also be sure to measure the cornea before making an incision. "A 550-micron incision doesn't fit everyone," he said.

LRIs are a valuable part of a surgeon's technique toolkit, Dr. Nichamin said. "I think they have gotten a bad reputation because not everyone understands their subtleties," he said.




LRI being performed for with-the-rule astigmatism Source: Louis D. "Skip" Nichamin, M.D.



Penetrating limbal relaxing incisions (PLRI)
Richard Mackool, M.D., Astoria, N.Y., jokingly says that PLRIs are "so easy a caveman can do it." "If you can make a phaco incision, you can do this and do it reproducibly," he said. He noted that PLRIs obviously achieve reliable depth compared with LRIs.

Dr. Mackool will use PLRIs for up to 2 D of astigmatism. The incisions are 2 mm in length, start just inside the conjunctiva, and their width varies depending on the amount of astigmatism. He creates them just before viscoelastic material is removed at the end of a case, with the I/A handpiece in the eye (foot pedal position one). "You never have to do stromal hydration, the incisions self-seal immediately," he said. He calculates incisions with the assistance of his nomogram.

A backup plan is key if you make the incision and the astigmatism doesn't improve, Dr. Mackool said. "Don't do what you did before," he said. "The cornea is talking to you and saying that it wants to stay in that shape." In that situation, particularly if the patient has a multifocal IOL, Dr. Mackool recommends treating the astigmatism using PRK. This is advantageous over LASIK, which can cause more high-order aberrations, he said.

Nomograms used by Dr. Mackool when performing double penetrating incisions for astigmatism Source: Richard Mackool, M.D.


Arcuate incisions with the femto laser
Laser refractive cataract surgery gives surgeons a tool that can help improve outcomes when treating astigmatism, said Eric D. Donnenfeld, M.D., clinical professor of ophthalmology, New York University Medical School, New York. Dr. Donnenfeld uses the LenSx Laser (Alcon, Fort Worth, Texas) to create arcuate incisions that help correct residual astigmatism. The incisions created by the laser are more predictable than manually created incisions, Dr. Donnenfeld said. First, they do not vary in depth, architecture, or wound length, he said. "The laser creates reliable and reproducible wounds that give surgeons a precise incision that is more self-sealing and reduces the induced cylinder, which is difficult to predict with a manual blade," he said.

Additionally, the use of the laser helps to create uniform depth and curvature. Dr. Donnenfeld uses the laser at a 9-mm optical zone, from the center of the cornea, and at 85% of the depth of the cornea based on real-time optical coherence tomography measurements. He also opens incisions to about 75% of their length, and the following day, he will open and adjust to a greater degree depending on the patient's post-op refraction. Dr. Donnenfeld will comfortably treat 2 D of astigmatism with arcuate incisions. With the help of arcuate incisions and the ORange intra-operative aberrometer (WaveTec Vision, Aliso Viejo, Calif.), Dr. Donnenfeld routinely gets cylinder down to 0.25 D or less, which is far better than he regularly achieved with manual incisions. Dr. Donnenfeld uses the DONO nomogram available at www.

lricalculator.com. He reduces the calculations from that nomogram by 30% because the incisions made via the laser are more predictable and reliable. Dr. Donnenfeld plans to release a new nomogram soon.

Editors' note: Dr. Donnenfeld has financial interests with Alcon, Allergan (Irvine, Calif.), and Bausch & Lomb (Rochester, N.Y.). Dr. Mackool has a financial interest with Alcon. Dr. Nichamin has a financial interest with Bausch & Lomb.


Contact information
Donnenfeld: 516-446-3525, eddoph@aol.com
Mackool: phacodr@aol.com
Nichamin: 814-849-8344, ldnichamin@aol.com