EyeNet Magazine


 
Refractive
Is Custom Ablation Living Up to Its Promise?

By Lori Baker Schena, Contributing Writer


 
 

Two years after the FDA approved the first wavefront-driven excimer laser for custom ablation,1 the debate continues on whether this much-touted technology has lived up to the initial excitement associated with its introduction.

Daniel S. Durrie, MD, director of refractive surgery at Durrie Vision in Overland Park, Kan., reported that 60 percent of his LASIK treatments now involve wavefront custom ablation. “It would be 100 percent of my patients if the procedure was approved for hyperopes. Not only are my custom ablation patients more satisfied in terms of BCVA, but also they experience an improvement in contrast sensitivity and low contrast acuity. Custom ablation has become the standard of care in my practice because of these results.”

On the other side of the debate is Chris J. Cabler, MD, a refractive surgeon in private practice in Houston. “Presently, I don’t believe that custom LASIK represents value for most patients. I have yet to determine any significant difference in quality of vision in those patients who undergo traditional [wavefront-adjusted] LASIK and those who have custom [wavefront-guided] LASIK. I believe custom LASIK has been oversold. The technology is not all it is cracked up to be.”

Why are some refractive surgeons and patients reporting less than satisfying results with a technology that holds so much promise? Dr. Durrie pointed to some surgeons’ lack of familiarity with the technology. Others have referenced issues related to the accuracy of the laser and eyetracker. Yet for Steven E. Wilson, MD, director of corneal research at the Cleveland Clinic Foundation’s Cole Eye Institute, the issue is related to how visual data are processed in the central nervous system. He is concerned about our lack of understanding concerning the role of neural processing and adaptation on visual acuity.2

Nagging Questions About Brain Plasticity
“Wavefront technology can objectively measure low- and high-order aberrations within the optical system from the tear film to the retinal surface,” Dr. Wilson said. “However, it doesn’t take into account how visual information gets processed in the retina and into the brain.”

Dr. Wilson pointed to evidence of this central nervous system influence, including patients who have problems adapting to spectacle prescriptions no matter how minor the change. “In individuals who have trouble adjusting to refractive surgery, one can’t help but think that there may be an issue of variations in the brain’s plasticity,” he said.

He added that the complex interrelationships between optical system aberrations and compensatory central nervous system function may explain how some people naturally adapt to astigmatism, coma, trefoil and other optical aberrations. “For example,” Dr. Wilson said, “just because we detect coma in a patient doesn’t mean that the patient is sensing—especially if he was born with it. Instead, it is likely that some of it has been compensated for during development of the central nervous system.” The implications for wavefront custom ablation may be profound, prompting more questions than answers.

Do patients who experience persistent problems with night glare and halos following LASIK and PRK, with modest levels of aberrations, actually have less capacity to centrally process and adjust to changes in visual input? Will some patients who have limited central nervous system plasticity believe their quality of vision is actually reduced after correction of a wavefront-detected aberration?

“We cannot answer these questions with the technology available today,” Dr. Wilson said. “Yet if we want to improve the results from custom ablation, we need to develop technology that can analyze how optical information is processed in the visual system. This requires a device that involves subjective patient input.” Only one research group has made progress on this issue (see Enter InterWave below).

Nagging Questions About Low Myopes 
In addition to the issue of quality of vision, refractive surgeons are at odds over who will benefit most from wavefront technology. Dr. Wilson maintained that patients with a “+1 D or +2 D of hyperopia and anything over –4 D or –5 D for myopia” are good candidates. “The uncertainty is with the low myope,” Dr. Wilson said. “Is it really beneficial for a –2 D to undergo custom ablation compared to traditional LASIK? There is no good data demonstrating that low corrections can benefit.”

Dr. Durrie disagreed. “The early generation lasers corrected vision but induced halos and glares at night. Custom ablation reduces this and is especially beneficial to those with low myopia and lower higher-order aberrations. Indeed, these patients tend to be really bothered if you induce spherical aberration or residual coma because they haven’t experienced those distortions.”

Dr. Wilson countered that “the induced aberrations in these low myopia corrections are small enough that the difference between custom and traditional LASIK is not statistically significant.”

A Middle Path?
Dr. Cabler, who has practiced in the Houston area for 28 years, is an owner/ director of Laser Eye Institute of Houston, which has five different excimer lasers on the premises.

“This allows me the freedom to be more objective because I’m not married to one laser platform—and I can tell you that the custom laser industry has overpromised what it can deliver. In general, most patients will achieve a comparable result from a traditional treatment, which is simpler, less expensive and less time-consuming for both patient and surgeon,” said Dr. Cabler.

Dr. Cabler recently became a proponent of the “wavefront-optimized” Allegretto Wave, which uses a different proprietary technology to reduce aberrations. This is done by incorporating the keratometric readings into their algorithm. The smaller diameter of the laser treatment beam combined with high-speed scanning capability allows for the correction of spherocylindrical refractive errors and induces less spherical aberration by maintaining the prolate shape of the cornea, Dr. Cabler said. “You are getting a ‘modified custom’ LASIK treatment with the Allegretto Wave [and] can achieve excellent results in less time with less expense.”

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1 The Alcon LadarWave/LadarVision 4000 excimer laser.
2 Wilson, S. E. Am J Ophthalmol 2003;136(2): 340–342.

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Dr. Durrie is a consultant for Alcon and Bausch & Lomb. Drs. Cabler and Wilson have no related financial interests.

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Enter InterWave

Keith P. Thompson, MD, medical director of InView Vision Center in Atlanta, along with researchers at the General Electric Company Research Center at Harvard and the Massachusetts Institute of Technology, developed the InterWave scanner to directly address the concerns raised by Dr. Wilson.

“We were interested in trying to explain situations in which the patient achieved 20/20 after a refractive surgery but was unhappy with the quality of vision,” Dr. Thompson said. “We didn’t have a good way to characterize what was occurring in the visual system, so our idea was to develop a device that measured aberrations of the entire visual system, from the cornea to the visual cortex.”

While traditional autorefraction wavefront technology sends light waves through a single region of the cornea to the retina, where they are reflected off the retina and returned to the aberrometer, InterWave aberrometry involves patient feedback to account for any neural processing of aberrations by the intraretinal layers and the higher visual pathways.

The device, based on spatially resolved refractometry, uses patient feedback to measure multiple points for each eye. During evaluation, the patient uses a joystick to center a light spot onto a target and then clicks a button when he or she deems the alignment satisfactory. The device then moves the test spot to another location on the cornea, and the test sequence is repeated. The testing takes approximately three minutes per eye to measure the 50 to 75 testing points. These results can then help the refractive surgeon tailor the laser dosing to the variations in optical power across three discrete pupillary zones as measured by aberrometry.

Dr. Thompson and his colleagues published an article comparing aberrometry-guided laser treatment with standard LASIK treatment based upon the manifest refraction.1 They found that the InterWave LASIK achieved acuity and refractive results equivalent to those of standard LASIK treatment based upon refraction, but actually resulted in superior quality mesopic vision—decreasing halo and glare.

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1 Ophthalmology 2004;111:1368–1379.

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