December 2018

REFRACTIVE

Research highlight
Weighing thin LASIK


by Maxine Lipner EyeWorld Senior Contributing Writer




Corneal topography of a subject who had LASIK with a thin cornea. The corneal flap thickness and the residual stromal bed thickness values are 114 µm and 273 µm in the right eye and 104 µm and 293 µm in the left eye.
Source: Li Wang, MD

What 5-year study results say

Patients with thin corneas can still ultimately do well with LASIK, according to Li Wang, MD, associate professor of ophthalmology, Baylor College of Medicine, Houston. Results from a study1 published in the International Journal of Ophthalmology indicate that when the stromal bed thickness is 50% or more when compared to the original thickness, at the 5-year mark LASIK appears safe.
The study considered how patients with thin corneas fared in terms of ectasia. “Before we do LASIK, we screen as best as we can to exclude patients with keratoconus or who are at risk for ectasia,” Dr. Wang said. “Thin corneas are one of those risk factors, so that’s why we don’t have a lot of eyes with thin corneas.”
While in the United States there aren’t many LASIK patients with thin corneas, the population in China is slightly different, Dr. Wang said, adding that the latter tend to be younger and want LASIK. In some cases, they opt for a partial LASIK treatment that doesn’t involve full correction, unlike in the United States where patients usually want the full correction or opt for a different procedure.

Studying thin corneas

Investigators in the retrospective study reviewed charts from 1998 to 2010 and identified 339 patients who met the inclusion criteria of having undergone LASIK with thin corneas more than 5 years prior. “We contacted those patients and asked them to come back for a follow-up, and 89 returned and participated in this study,” she said.
One unique part of the study is that it considered the percentage of tissue altered (PTA), Dr. Wang said, adding that investigators were aware that with the Santhiago paper,2 a PTA of 40% or more was the most significant variable associated with the development of ectasia. “In our group of patients, we found that 56.6% had PTA of more than 40%,” she said.
Douglas Koch, MD, professor and Allen, Mosbacher, and Law Chair in Ophthalmology, Cullen Eye Institute, Baylor College of Medicine, who also took part in the study, pointed out that a key purpose of the investigation was to open up the conversation of who might be at risk for ectasia. He likewise stressed that PTA was an important factor here. “More than half of patients had more than 40% of the tissue altered,” Dr. Koch said. “But the residual stromal bed was at least 50% of the original thickness.” Another important factor, in his view, is that all but two eyes had a postoperative bed thickness of 250 microns or more.
Dr. Wang noted that in those two cases the residual stromal bed was less than 250 microns, yet at the 5-year mark neither had developed ectasia. “Those eyes are fine,” she said, adding that no eyes in the study had developed ectasia during this timeframe.

Clinical considerations

Dr. Koch agrees with prior studies that show that abnormal topography and young age are other critical factors to consider in terms of forestalling ectasia. “We’re struggling with a condition that luckily is uncommon, and we want to expand this conversation about who might be at risk,” Dr. Koch said.
To help patients avoid ectasia, Dr. Koch urges practitioners to do their calculations in a manner that ensures that they have at least a 300-micron bed. “It’s certainly much easier to do that calculation with femtosecond-created flaps,” he said.
In cases where the patient is young, Dr. Koch tends to push them toward PRK instead of LASIK. “If they’re over about a –7 or a –8 D and they’re under the age of 25, I often recommend that they stay in their contact lenses if they’re functioning well until they get beyond that age,” he said. “Then we can make sure that, as they get older, there isn’t any topographic evidence of ectatic potential in the corneas.”
Dr. Koch hopes that practitioners come away from the study with the realization that corneal thickness alone is not a risk factor for ectasia. However, corneal thickness in conjunction with other factors such as abnormal topography, young age, and the removal of too much tissue is a risk factor.
Dr. Wang pointed out that study results showed that LASIK is safe in thin corneas with 5-year follow-up. “But more studies are warranted,” she said.
Dr. Koch stressed that many patients were lost to follow-up. “Maybe some of them had ectasia. We certainly can’t prove that a thin cornea is completely safe.” He recommends that if a practitioner has any concerns otherwise about an eye, consideration be given to surface ablation or refraining from surgery.

References

1. Song YW, et al. Long-term safety of laser in situ keratomileusis in eyes with thin corneas: 5-year follow-up. Int J Ophthalmol. 2018;11:1227–1233.
2. Santhiago MR, et al. Association between the percent tissue altered and post-laser in situ keratomileusis in eyes with normal preoperative topography. Am J Ophthalmol. 2014;158:87–95.

Editors’ note: Dr. Koch has financial interests with Carl Zeiss Meditec (Jena, Germany) and Johnson & Johnson Vision (Santa Ana, California). Dr. Wang has financial interests with Alcon (Fort Worth, Texas) and Hoya (Tokyo, Japan).

Contact information

Koch
: dkoch@bcm.edu
Wang: liw@bcm.edu

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