LTK studies show treatment holds promise for hyperopia, presbyopia and retreatments

Refinements in technique and patient selection are credited with improving outcomes.

---Concentric rings of laser spots result after corneal treatment with LTK.

Increasing the steepness of a flat cornea has proven a significantly greater challenge to refractive surgeons than flattening one that is too steep, but clinical studies of non-contact Ho:YAG laser thermokeratoplasty (LTK) suggest that it may provide patients with hyperopia or presbyopia as an effective surgical corrective option.

Non-contact Ho:YAG LTK uses heat generated by the absorption of infrared laser energy to thermally modify stromal collagen - it shrinks collagen without necrosis. These pockets, or spots, then alter the corneal curvature. Usually two concentric rings of 6 mm and 7 mm, and consisting typically of eight equally spaced spots, are created. More aggressive variations of LTK, designed for higher degrees of hyperopia, use 16 spots per ring.

Penetration depth of the laser energy is an important point to consider in LTK. The thermal effect of the treatments is most pronounced on the surface of the cornea and decreases deeper in the stroma. Typically, stromal haze extends to 60% to 70% of the corneal thickness.

Recent test data suggest that concern among doctors over regression following LTK is subsiding. Previously, surgeons worried that regression would negate the benefits of the therapy, but since a revision in treatment protocol that excludes younger patients and introduced a corneal drying technique, regression is less frequent.

"We now know that the age factor is very important," said Frank J. Goes, MD, in private practice in Antwerp, Belgium. "Young patients invariably will regress. From age 40, we have excellent results."

A variety of studies are underway in North America, South America and Europe to evaluate Ho:YAG LTK for hyperopia, presbyopia, retreatments and enhancements. All LTK procedures described in this article were performed with the Sunrise Technologies (Fremont, U.S.A.) Sun 1000 Corneal Shaping System holmium laser. The Sunrise system generates pulses of infrared light with 100 µs of duration. Spot diameters are about 600 µm. Technomed GmbH (Baesweiler, Germany) also manufactures an Ho:YAG laser, the 25 Plus.

Hyperopia

For the treatment of simple, low-degree hyperopia, Hugo Daniel Nano, Jr., MD, and Sergio Muzzin, MD, in private practice in Buenos Aires, Argentina, found that LTK appears to be a safe and effective treatment option, though one that may require retreatment in the event of poor patient selection.

Drs. Nano and Muzzin studied 182 hyperopic eyes of 116 patients, with a mean age of 50 years and average refraction of +2.5 D. All patients had corneal power no greater than 45 D, and corneal pachymetry not thicker than 550 µm. Patients who had undergone previous corneal refractive surgery and those with corneal disease were excluded from the study.

In Dr. Nano's protocol, from one to three rings, made up of eight spots each, were created with 240 mJ bursts of Ho:YAG energy. Patients were followed for 12 months after surgery.

At the 12-month follow-up, all patients were 20/40, with an average gain of three lines of Snellen acuity. Spherical equivalent had improved to +1.5 D at 6 months, and further to +1.25 at 12 months. Dr. Nano said that about 17% of patients required retreatment. Of patients requiring retreatment, 44% were retreated with photorefractive keratectomy (PRK) or laser in situ keratomileusis, and 56% were retreated with a second application of Ho:YAG.

"Regression is decreased in our study, because I have found that corneal thicknesses greater than 500 µm and curvatures greater than 45 D tend to foreshadow higher rates of regression," Dr. Nano said. "We select the patients very carefully."

Drying technique

David C. Brown, MD, in private practice in Ft. Myers, U.S.A., reported that based on expanded phase 2 results, the procedure appears safe and effective for patients with hyperopia of 2.5 D or less. A corneal drying technique has made outcomes more stable and decreased the chances of regression, Dr. Brown reported.

With the drying technique, the eye is pre-treated with a topical anesthetic drop, and after it is numbed for 11 minutes, a lid speculum is placed in the eye and it is air dried for 3 minutes

"This has apparently made quite a bit of difference in outcomes," Dr. Brown said. "Inappropriate drying can result in undercorrection."

As of April, there are 177 eyes with complete 3-month data, and 103 eyes with 6-month data. All patients had hyperopia of between 0.75D and 2.5 D, uncorrected visual acuity of 20/30 or worse, with a stable refraction. All were treated with two rings of eight spots, with diameters of 6 mm and 7 mm.

In terms of efficacy, more than 80% of patients in the 3- and 6-month groups achieved uncorrected visual acuity of 20/40 or better, compared with 21% preoperatively. While no patients were seeing 20/25 or better uncorrected preoperatively, 55% of patients were able to see at this level at 3 or 6 months postoperatively.

Dr. Brown reported that predictability with the Sunrise holmium laser is excellent. Sixty-one percent of patients at the 3-month follow-up point are within 0.5 D of emmetropia. At 6 months, 62% of patients are within the same range. Eighty-nine percent of patients at the 3-month follow-up point are at 1 D of emmetropia and 87% at 6 months are within 1 D.

Regression has not been an issue in Dr. Brown's patient group.

"It has been said that this treatment is not stable, and that it tends to regress, but that is not the case here," he said. According to U.S. Food and Drug Administration (FDA) criteria, a change of 1 D or more between two refractions taken at least 3 months apart in 95% of all cases constitutes regression. In Dr. Brown's study, 100% of patients in both groups experienced less than 1 D change in refraction, and 81% less than 0.5 D. "The stability is really quite remarkable," he said.

Though only a small number of Dr. Brown's patients have reached the 12-month follow-up point, 100% are within FDA acceptable regression boundaries.

He reported no safety issues or side effects with the treatment protocol.

"This treatment carries the unfortunate legacy of the pre-air drying era, with associated regression," he said. "People do not realize that this is a totally different technique now."

Expanding indications

Alan Aker, MD, in private practice in Boca Raton, U.S.A., reported that new test pattern applications of LTK with the Sunrise laser suggest that treatment of patients with up to 4 D of hyperopia is possible.

At a clinic in the Dominican Republic, Dr. Aker successfully completed a two-ring, 16-spot variation of conventional two-ring, eight-spot LTK on 10 eyes, extending the effective treatment range up to 4 D. Other than doubling the number of spots, treatment protocols and power settings were identical to those used during treatment of patients with hyperopia under 2.5 D.

"What we did was apply eight spots, then rotate the beam and apply a second eight spots in each of the treatment zones," Dr. Aker said. "In all, we are treating the patient with 32 spots instead of 16."

Follow-up for the 10 eyes treated with this newer technique is between 5 and 9 months.

"At 3 months, patients held greater than 2 D of correction and, in some cases, up to 4 D," Dr. Aker said. "Regression, we have found, ceases at the 6-month point, and from that point there is a flat line in terms of regression."

Presbyopia

Another possible application of LTK may come in the treatment of presbyopia. Because the corneal collagen of patients over age 40 seems to regress less following surgery than it does in younger patients, a number of physicians, including Douglas D. Koch, MD, of Houston, believe that LTK also is a promising treatment for presbyopia.

In a five site, FDA-endorsed study of 60 eyes, Dr. Koch said LTK effectively induces monovision in patients with presbyopia. Patients in the study must be a least 40 years old, have a preoperative refraction ranging from +0.75 to -0.25, uncorrected near vision of 20/50 or worse and stable refraction with no corneal disease or previous corneal surgery.

Presbyopia patients were treated with two rings of eight spots.

One-month data on the initial group of six patients showed good postoperative uncorrected near acuity. Patients in the subgroup ranged from 20/50 to 20/100. Postoperatively, all had achieved 20/40 or better.

Spherical equivalent measurements taken before and after surgery also suggest the treatment is effective. Average preoperative measurements of +0.29 were -1.65 D at 1 week and -1.52 D at 1 month.

"So, preliminary findings are that this looks to be an effective treatment," he said. "But there is still a lot to be done."

Necessary for the success of LTK for presbyopia is a clear understanding of monovision on the part of the patient.

"Before they are given monovision, we provide patients a contact lens trial, so they can see what their vision will be like once the procedure has been completed," Dr. Brown said.

Retreatments and enhancements

Dr. Goes has used LTK as a retreatment tool for 72 eyes that had previously undergone PRK for hyperopia and myopia, as well as for those that have received radial keratotomy, which resulted in a hyperopic shift.

Following retreatment, a correction of more than 1 D was attained in 44% of patients, and a correction of more than 2 D was attained in 19%. Twenty-eight percent of eyes did not improve. Average hyperopia decreased from +1.9 D to +0.6 D. In all, 4% of patients lost some best corrected visual acuity.

"The technique seems most effective for the treatment of overcorrected myopic PRK because of the thinner cornea and/or the absence of Bowman's membrane in these eyes," Dr. Goes said. "In these cases, we have to make sure that we do not use too much energy because, otherwise, we get over-reactions. The results here were good, but not that great."

Overcorrected myopic PRK patients received only one ring of eight spots. "It is too dangerous otherwise," he said. "With two rings, there will certainly be overcorrection."

For the retreatment of PRK for hyperopia, Dr. Goes said the outcomes are less impressive. As a retreatment tool, the predictability of LTK decreases. "At best, the predictability is 50% ±1 D," he said.

Papers described in this article were originally presented at the 1998 Meeting of the American Society of Cataract and Refractive Surgery in San Diego.

For your information:

• Hugo Daniel Nano Jr., MD, can be reached at Marcelo T. de Alvear 1418, Buenos Aires 1060, Argentina; ++(1) 717-4000 or ++(1) 816-7227; fax: ++(1) 717-3556 or ++(1) 811-9636. Dr. Nano has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• David C. Brown, MD, can be reached at 4101 Evans Ave., Ft. Myers, FL 33901-9310 U.S.A.; ++(941) 939-3456; fax: ++(941) 936-8776. Dr. Brown is on the Scientific Advisory Board of Sunrise Technologies Inc.
• Alan Aker, MD, can be reached at the Aker-Kasten Surgical Eye Institute, 1445 N.W. Boca Raton Blvd., Boca Raton, FL 33432 U.S.A.; ++(561) 338-7722; fax: ++(561) 338-7785. Dr. Aker has a direct financial interest in Sunrise Technologies Inc. He is not a paid consultant for any companies mentioned in this article.
• Frank J. Goes, MD,can be reached at the Antwerp Surgical Center, Willem Klooslaan 6, 2050 Antwerp, Belgium; ++(3) 219-3925; fax: ++(3) 219-6667. Dr. Goes has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
• Douglas D. Koch, MD, can be reached at Baylor Eye Consultants, One Baylor Plaza, Ste. NC 200, Houston, TX 77030-2703 U.S.A.; ++(713) 798-6443; fax: ++(713) 798-3027. Dr. Koch has a direct financial interest in the Ho:YAG laser. He is a paid consultant for Sunrise Technologies Inc.