June 2017

 

COVER FEATURE

 

Rebirth of laser vision correction
Treating irregular corneas


by Michelle Stephenson EyeWorld Contributing Writer

   

Figure 1. Topography-guided PRK and corneal crosslinking: preoperative, postoperative, and difference map. The postop map shows significant reduction in the irregular
astigmatism; the difference map shows 7.4 D of flattening over the cone and 5.9 D
of steepening to the superior cornea; best corrected spectacle acuity improved from 20/80 to 20/25.


Figure 2. Topography-guided PRK and corneal crosslinking: preoperative, postoperative, and difference map. Postop map shows normalization of the corneal contour; best
corrected spectacle acuity improved from 20/50 to 20/25.
Source: Raymond Stein, MD



Several new technologies are allowing refractive surgeons to significantly improve these patients’ quality of vision

Keratoconus is the most common cause of irregular corneas. Other causes include ectasia, trauma, scarring, and previous procedures, such as radial keratotomy. These patients are often left with significant visual disabilities and require gas permeable contact lenses or penetrating keratoplasty to improve their quality of vision.
“One of the greatest advances in refractive surgery has been the ability to treat irregular corneas,” said Eric Donnenfeld, MD, Rockville Centre, New York. “Now, thanks to the advent of several new technologies, refractive surgeons are able to significantly improve many of these patients’ quality of vision. Many times, these patients will no longer need to wear gas permeable lenses, but can wear soft contact lenses or glasses. On occasion, these patients can go spectacle-free.”
For patients with mild corneal irregularities, Dr. Donnenfeld prefers performing wavefront aberrometry treatment, which treats the entire visual system including the anterior cornea/posterior cornea and lens. “This can be for very mild cases of forme fruste keratoconus and some other corneal irregularities,” he explained.
New technologies are allowing surgeons to treat more severe irregularities.

Topography-guided laser ablations

A new technology approved in the U.S. in 2016 is topography-guided laser ablations. This is unique in that it combines myopic and hyperopic ablations to create a more regular corneal surface. “With topographic ablations, we are able to treat more significantly irregular corneas, such as more advanced keratoconus,” Dr. Donnenfeld said. “We have found that we can capture images with the Topolyzer [Alcon, Fort Worth, Texas] in patients who have corneas as steep as 60 D. The key aspect of topographic ablations is that they can not only flatten the steep areas of the cornea, but they also steepen the flat areas of the cornea so that we can improve patients by up to 10 D of corneal irregularity on a fairly routine basis.”
Topography-guided PRK (rather than LASIK) is performed on these patients because they already have thinner corneas. “Mitomycin is used to prevent scarring, and patients are told that visual rehabilitation may take several months. They are also told that their vision will be improved, but that they will not be getting rid of their glasses. However, these patients have had significant improvements, and [at the recent ASCRS•ASOA Symposium & Congress] we presented a series of 68 patients who had a mean improvement in best corrected visual acuity of two lines and a mean improvement in uncorrected visual acuity of four lines,” Dr. Donnenfeld said.
According to Raymond Stein, MD, director, Bochner Eye Institute, Toronto, and associate professor of ophthalmology, University of Toronto, who has been performing topography-guided PRK for the past 8 years, the best candidates for the procedure are those with 20/30 or worse best corrected visual acuity with keratoconus, pellucid marginal degeneration, and ectasia after laser vision correction. “Preferred cases are corneas at least 450 µm thick with a dioptric difference across the cornea of less than 10 D. I would not advise topography-guided PRK if the area of prominent steepening is outside of the pupillary zone because this would result in excessive central steepening and a large myopic shift. Be cautious in patients with good uncorrected acuity as the treatment could decrease uncorrected visual acuity while improving best corrected visual acuity. Patients may not be happy if they need to wear corrective glasses or contact lenses after surgery if they didn’t prior to surgery,” Dr. Stein said.
Dr. Stein noted that this procedure can benefit patients of all ages, not just young ones with progressive disease. Older patients with stable corneas have the potential to discontinue rigid gas permeable lenses and return to soft contacts or glasses. 
A. John Kanellopoulos, MD, clinical professor of ophthalmology, New York University Medical School, and medical director, Laservision.gr Institute, Athens, Greece, said patients who have decentered ablations and irregular ablations resulting from refractive procedures with older laser technologies who complain of ghosting, halos, and difficulties with visual function are good candidates for treatment with topography-guided PRK. This procedure will enlarge optical zones and recenter optical zones on the cornea vertex, which is closer to the visual axis of the patient. “Topography-guided PRK does a great job of treating these patients. The pearl here is that topography-guided PRK addresses only the irregularity of the cornea in regard to the vertex, which we assume is the line of sight, and it may hide some refractive surprises postoperatively. Usually, enlarging optical zones or recentering optical zones will result in myopic shift as these treatments with topography resemble hyperopic treatments,” Dr. Kanellopoulos said.
Most of these issues can be preempted with a technique called topography neutralization, which may require two steps: (1) to optimize the cornea as a lenticular system and (2) to address potential myopic shift and more rarely a slight hyperopic shift, in a second, mainly spherical ablation.
According to Dr. Stein, the amount of improvement in best corrected visual acuity after topography-guided PRK is dependent on the preoperative level of best corrected spectacle visual acuity. The greater the preoperative loss of acuity, the higher the potential for lines gained. “In general, the improvement is one to six lines of gain,” he explained.
There is a learning curve with this procedure. “Fortunately, this procedure has been performed outside the U.S. since 2003, and our group in particular has worked on topography-guided treatments extensively, with more than 50 peer-reviewed publications and hundreds of presentations in meetings over the past 15 years,” Dr. Kanellopoulos said. “Our experience, along with that from other investigators around the world, can serve as a great introduction for clinicians getting involved with topography-guided treatments.”

In combination with crosslinking

Patients with thin corneas that are ectatic should undergo corneal crosslinking, which has recently been approved by the U.S. Food and Drug Administration and has been used in Europe for more than 15 years. “Some surgeons perform crosslinking at the same time as the refractive procedure. I prefer to do them as separate procedures, separated by approximately 3 months or more,” Dr. Donnenfeld said. “The reason for this is that there is improved epithelial healing and a more stable refractive error once the crosslinking has been stabilized.”
Dr. Stein prefers performing corneal crosslinking immediately after topography-guided PRK. “It is important to strengthen a cornea, especially if one removes tissue. Results are more predictable if the corneal crosslinking is done after and not before topography-guided PRK,” he said. 
Dr. Kanellopoulos found in a landmark study that the combination of topography-guided partial PRK and crosslinking appears to have a synergistic effect in the amount of corneal flattening and normalization.1 It also results in less scarring.
“Additionally, there is the fact that if a cornea has been crosslinked and a surface ablation is attempted after that, the ablation will remove the most biomechanically stable part of the stroma that has been reinforced with the crosslinking process. This may be counterintuitive in the long-term stability of those eyes, so we have since shifted our clinical and surgical paradigm into combining two procedures in what has been known as the Athens protocol. This technique has been adopted by hundreds of surgeons internationally, and recently in the United States. It entails the customized topography- guided or wavefront-guided normalization of the very irregular cornea combined with corneal crosslinking,” he explained.
Dr. Kanellopoulos said that the improvement in vision is dramatic, but the postoperative recovery can be lengthier than with standard PRK. “Sometimes, 2 weeks may be required for the cornea to re-epithelialize and the surface to normalize. Most of these eyes achieve at least 20/40 best corrected visual acuity, which compares favorably to penetrating keratoplasty. A larger percentage of patients enjoy uncorrected visual acuities in the 20/20 to 20/25 range. However, I think it would be unwise to view the Athens protocol as a refractive procedure aiming for emmetropia. Topography-guided PRK should aim to normalize the cornea and address potential significant anisometropia at a later time, either with a phakic intraocular lens or with a lens-based procedure, such as clear lens extraction or cataract surgery with a multifocal, toric, or extended depth of focus intraocular lens. Since this technique has been introduced, it has become one third of my clinical practice and has reduced my cornea transplantation rate by 90%,” he said.

Treat irregular corneas before cataract surgery

According to Dr. Kanellopoulos, many surgeons have previously used toric lenses in patients with irregular corneas and have achieved a relatively good visual result. “I, nevertheless, think that in these cases, the optimal approach would be to first normalize the cornea and give that eye the ability to become the best possible lenticular system for a cataract procedure to come at a later time, perhaps 2 or 3 months later. At that time, intraocular lens calculation would be far more accurate and the use of a premium intraocular lens, such as a multifocal, extended depth of focus, or a toric IOL, may address visual rehabilitation better, despite the fact that this would be two procedures. I think that doing the cataract procedure first and then trying to normalize the cornea if the patient was symptomatic would be far more difficult,” he said.

Other options

According to Dr. Donnenfeld, if patients are not candidates for topography-guided PRK or wavefront aberrometry, the cone can be flattened using phototherapeutic keratectomy with the excimer laser. “This flattens the cone and uses the epithelium to prevent ablation in the flatter areas, while the cones that have thinner epithelia over their surface ablate more rapidly. Very commonly, this can flatten the cone by up to 5 D. We have followed this procedure with a topographic ablation several months later,” he said.
Finally, for patients who are not candidates for excimer laser, corneal inlays are helpful in improving cones. “We find this particularly helpful in patients with thin inferiorly displaced cones, as with pellucid marginal degeneration. In summary, there is a whole new world of technologies that allow the corneal surgeon to improve the vision in patients with irregular corneas to dramatically improve their quality of vision as well as their quality of life,” Dr. Donnenfeld said.

Reference

1. Kanellopoulos AJ. Comparison of sequential vs same-day simultaneous collagen cross-linking and topography-guided PRK for treatment of keratoconus. J Refract Surg. 2009;25:S812–8.

Editors’ note: Drs. Donnenfeld and Kanellopoulos have financial interests with Alcon. Dr. Stein has no financial interests related to this article.

Contact information

Donnenfeld
: ericdonnenfeld@gmail.com
Kanellopoulos: ajkmd@mac.com
Stein: raymondmstein@gmail.com

Treating irregular corneas Treating irregular corneas
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