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Intrastromal cross-linking offers minimally invasive alternative for presbyopia correction
Current protocols use an epi-on approach and supplemental oxygen.
Photorefractive intrastromal cross-linking, or PiXL, holds great promise as a safe, easy and minimally invasive strategy to correct presbyopia.
“It is the least invasive procedure available today, particularly with the current protocols using an epi-on approach and supplemental oxygen. It is incomparable to any other presbyopic solution available currently,” A. John Kanellopoulos, MD, said in an interview with Ocular Surgery News.
Presbyopic PiXL is performed monocularly as a single treatment using the Avedro Mosaic system. Corneal cross-linking is induced in an annular section of the midperiphery of the cornea. CXL differentials modify the refractive properties of the cornea, producing a central steepening for near vision. The customized pattern, basically a doughnut-shaped beam profile between the 6 mm and 9 mm diameter, is delivered by the Mosaic device centered on the pupil while actively tracking throughout, he said.
Kanellopoulos has worked on the feasibility studies of PiXL since 2013. The technique has been used since then for myopic, hyperopic and astigmatic corrections, for the treatment of ectasia and as a monocular treatment to help with presbyopia. The outcomes have been within the range of 2 D.
“It has been relatively accurate as far as its delivery on the cornea, meaning centration. There has been, however, some variability in the response of individual patients. Thus, the titration of the dose-related effect is still something that is being investigated since the first clinical data that we have presented and reported in the literature,” Kanellopoulos said.
Minimally invasive, repeatable
Presbyopic PiXL has intrinsic interest because the actual cross-linking treatment preserves the center of the cornea. It is performed in a diameter of 6 mm to 9 mm from the center, thus causing no issues with quality of vision. A corneal steepening between 0.5 D and 2 D can be obtained, generating monocular monovision.
“Emmetropic presbyopic patients between the age of 45 and 60 can attain a reasonably good presbyopic correction,” Kanellopoulos said. “The alternatives, which include excimer ablation, an inlay or a premature removal of the crystalline lens and replacement with a trifocal multifocal lens, entail much more invasive procedures and a larger list of potential complications. Being a minimally invasive, safe option, PiXL is gaining a lot of interest from clinicians and patients alike.”
If patients do not achieve the fully targeted correction with one treatment, the procedure can be easily repeated, titrated and enhanced.
“A word of caution in our experience: If the patient is keratoconus suspect, the response to the PiXL procedure may be more than expected; thus, a titrated approach to achieve the optimal effect may be advised,” Kanellopoulos said.
The studies
Lim Wee Kiak, MD, has been using PiXL for low myopia, hyperopia and early presbyopia over the past 3 years. He was an investigator in the pilot trial 3 years ago, using PiXL to enhance vision in hyperopia and presbyopia, and is now part of the current multicenter trial, with 30 patients of 75 patients being recruited in Singapore. The new study is assessing the efficacy of PiXL vision enhancement in patients with presbyopia, using three treatment profiles varying the size of the treatment rings.
“We started recruitment and treatment in July 2019 and hope to complete all treatment by October 2019,” Lim said. “Our pilot study has demonstrated stability over a 12-month period, and we were encouraged to do more work to further refine this modality for visual enhancement. We need to expand the treatment amplitude and understand which patients will best benefit from PiXL.”
With the presbyopic treatment, a 0.75 D myopic change is induced in the treated eye, resulting in better near visual acuity. All of the patients he treated have improved near visual acuity with minor reduction in distance visual acuity.
“Some of these patients were post-cataract surgery and some were post-LASIK or other refractive procedures,” he said.
Advantages, limitations
Photorefractive CXL is a well-established procedure that has been studied, used and refined for many years. In 2013, Avedro obtained the CE mark for the KXL II device, which then evolved to become the Mosaic. The FDA approved the KXL system in 2016.
Current protocols entail the use of ParaCel, a riboflavin drop specifically formulated for the epi-on CXL procedure, applied for 4 minutes, followed by VibeX Xtra riboflavin isotonic saline solution for 6 minutes. In the oxygen-enhanced procedure, oxygen is provided in the last 2 minutes.
“Compared with current surgical or laser techniques, PiXL has a huge advantage as it is the most minimally invasive among all. There is no laser or surgical cut on the surface of the eye, the treatment duration is only 26 minutes, and no discomfort is experienced during the treatment process. All those who are afraid of surgery will find in PiXL an ideal approach,” Lim said.
The main limitation is the small range of refractive correction that can be achieved currently, between 0.5 D and 0.75 D. It is also not as precise and predictable as an excimer laser correction. Lim corroborated what Kanellopoulos said: The response depends on the patient’s own cornea collagen reaction.
PiXL is a work in progress, and research is ongoing to understand how to optimize the treatment for higher predictability and extended efficacy.
“We are doing the study on patients who did not have any refractive procedures or cataract surgery previously. I am hopeful that PiXL can also be used to help patients who had refractive procedures or cataract surgery done previously,” Lim said. – by Michela Cimberle
Editor’s note: PiXL for presbyopia is currently under clinical development using Avedro’s Mosaic system and is not available in the U.S.
- For more information:
- A. John Kanellopoulos, MD, can be reached at LaserVision Clinical and Research Eye Institute, 17 Tsocha Street, 115 21 Athens, Greece; email: ajkmd@mac.com.
- Lim Wee Kiak, MD, can be reached at Eagle Eye Center, 820 Thomson Road, Mount Alvernia Hospital, Singapore 574623; email: limweekiak@gmail.com.
Disclosures: Kanellopoulos and Lim report they are consultants for Avedro.