Two treatment methods successfully correct astigmatism

The cross-cylinder method and ablation of both meridians have tissue-sparing qualities.

Issue: March 1, 2000

ByMichelle Stephenson

ORLANDO, Fla. — Recent studies have found that two refractive surgery procedures successfully correct astigmatism.

The first procedure is laser in situ keratomileusis (LASIK) using the Visx (Santa Clara, Calif.) Star SmoothScan S2 cross-cylinder method. “This procedure involves the bitoric or the cross-cylinder method using the positive and the negative cylinders together. We chose to evaluate this technique because of its tissue-sparing qualities,” said Renato Augusto Neves, MD, at the annual meeting of the American Academy of Ophthalmology (AAO) held here.

Cross-cylinder method

---Progressive change of dioptric power of the astigmatism from one axis to the other.

Dr. Neves and his colleagues prospectively collected data on 40 consecutive eyes on which the cross-cylinder method was used. Preoperative data showed a spherical equivalent (SE) of +1 D to +5.5 D and a cylinder of 1.5 to 4.5. Patients’ ages ranged from 35 to 63 years, and manifest refraction was used on all patients.

Approximately 95% of patients were within ±1 D of intended correction. Ninety percent were within ±0.5 D, and all patients were within ±1.5 D. Additionally, 88% had 20/32 or better uncorrected vision, 75% had 20/20 uncorrected vision and more than 90% had 20/40 or better uncorrected vision. Regarding best corrected vision, no lines were lost, and four eyes gained one line. No severe complications were seen.

“The cross-cylinder method was found to be faster and shallower, and there was no difference between doing the negative cylinder and the positive cylinder first. However, we usually do the negative first and then the positive,” he said.

“LASIK provided rapid visual and refractive outcomes after 3 months, and it is important to continue to follow these patients,” he said.

Ablation of both meridians

---Inadequate transition on the steep meridian preoperatively: -6.00-4.00 x 5°.

Another method for correcting astigmatism involves the ablation of both meridians in LASIK and photorefractive keratectomy (PRK).

“Until now, all of the correction has been done on only one meridian. However, astigmatism remains outside of the optical zone. This remaining astigmatism causes optical aberration as the pupil dilates, and the red ring in the mid-periphery decreases optical quality. When we have this kind of astigmatism, we can correct half on the minus meridian and the other half on the other meridian. That creates better surfaces,” said Paolo Vinciguerra, MD, at the AAO meeting.

According to Dr. Vinciguerra, the advantage of this treatment is a better postoperative cornea shape, which means less regression and better visual quality. “Additionally, tissue is spared because we split part of the ablation on the outside of the center. That is especially used in LASIK. We are not overcorrecting the oblique meridian,” he said.

The majority of 32 patients who were followed for at least 1 year had reduced astigmatism. “There were two exceptions in two patients where we just made an error in putting the data in the computer. All patients increased uncorrected visual acuity. However, interestingly, the mean increased close to two lines between the best corrected visual acuity before and after the treatment. The standard deviation was very low,” he said.

The vector analysis showed a steady result that is uncommon in this magnitude of correction.

“This is really useful in LASIK where we are sometimes confined between two walls: one due to the thickness of the flap and the other one due to the untreated stroma we want to lift,” he said.

Pioneering work

---Fellow eye (-6.00 -4.00 x 5°) treated with cross cylinder.

According to Stephen A. Updegraff, MD, who discussed these two studies, “Dr. Vinciguerra’s pioneering work in cross-cylinder ablation is one of the most exciting things that has come down the pike.”

In modern excimer laser surgery and astigmatism treatment, the cornea is typically flattened. “However, when we treat astigmatism in cross-cylinder, we treat the astigmatism in different meridians. That’s how we preserve the overall depth and improve the quality or shape of the cornea,” Dr. Updegraff said.

The current minus cylinder ablations done in the U.S. induce hyperopia by flattening over the visual axis, go deeper in the stroma because the ablation is in one meridian and leave an oblong optical zone that can cause optical aberration especially in low light settings, Dr. Updegraff said.

“Other determinants of quality of vision after LASIK have to deal with the transition zone where the laser treatment tapers out to the untreated area of the cornea. Anterior chamber depth also plays a role in side effects, such as spherical aberrations and night vision problems,” he said.

“These two studies looked at cross-cylinder ablations with two different patient populations. Dr. Vinciguerra looked at myopic astigmatism. He also not only used cross-cylinder, dividing the cylinder into 50% to the steep and flat meridians, but he also used a splitting technique that is essentially a multizone technique for the spherical component. This is critical in increasing the taper of the optical zone and improving quality outcomes,” Dr. Updegraff said.

Hyperopic astigmatism

Dr. Updegraff then commented on Dr. Neves’s study of hyperopic astigmatism. “The most important feature of his study is that the patients did not lose best corrected vision. With hyperopic treatment, the ability to do a plus cylinder ablation is critical. If all you have is minus cylinder treatment, such as in the U.S., with hyperopia, then it’s a ‘push me, pull me’ in that you try to steepen the cornea for the hyperopia then flatten it with the cylinder treatment. This leads to unpredictable results, possibly loss of best corrected vision.

“There is no question that cross-cylinder ablation reduces the treatment depth for patients who are highly astigmatic. The question is, does this improve quality of vision? To date, there hasn’t been a randomized study randomizing myopic cylinder treated with cross cylinder versus those treated with minus cylinder only. And does it really reduce striae? We’re not sure, but it certainly makes sense that it would. Topographically, the optical zones are larger and smoother with cross cylinder,” Dr. Updegraff said.

He added that a randomized study needs to be done between eyes, with patients evaluating contrast sensitivity and glare, and wavefront evaluation. It also is important to look at refractive outcomes of the two separate techniques to determine how accurate the treatments are for the astigmatism component.

“There is no question that we all want to achieve a prolate, smooth, aspheric surface. The optics of the cross-cylinder ablation are allowing us to understand what we need to do to the cornea to obtain quality outcomes. One day, we’ll have nomograms and software in the laser computers so we won’t have to calculate this ourselves. But I envision all of these things coming together with cross-cylinder to really come up with a custom ablation, or custom cornea,” he said.

For Your Information:

Renato Augusto Neves, MD, can be reached at Av. Brasil 1368 Sao Paulo, 01430-001, Brazil; (55) 11-852-6716; fax: (55) 11-306-11911. Dr. Neves has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.

Paolo Vinciguerra, MD, can be reached at Via Ripamonti 205, Milan 20141, Italy; (39) 02-5521-1388; fax: (39) 02-5741-0355. Dr. Vinciguerra did not disclose whether he has a direct financial interest in any of the products mentioned in this article or if he is a paid consultant for any companies mentioned.

Stephen A. Updegraff, MD, can be reached at 1607 9th St. N, St. Petersburg, FL 33704; (727) 822-4287; fax: (727) 822-0657. Dr. Updegraff did not disclose whether he has a direct financial interest in any of the products mentioned in this article or if he is a paid consultant for any companies mentioned.

Visx Inc. can be reached at 3400 Central Expressway, Santa Clara, CA 95051; (800) 246-8479; fax: (408) 773-7055.