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Laser surface ablation offers an alternative to LASIK in eyes with corneal disease or that have undergone previous corneal surgery such as a corneal transplant. In such cases, where the cornea is already abnormal, a flap may introduce another element of unpredictability with respect to healing and postoperative outcome. In post-LASIK cases in which an enhancement is necessary, many surgeons prefer to perform a surface ablation procedure 18 to 24 months postoperatively to reduce the risk of epithelial ingrowth. Long-term follow-up data on eyes that underwent surface ablation before the advent of LASIK have demonstrated excellent stability over time.

Corneal collagen crosslinking (CXL) has revolutionized the treatment of keratoconus. Combining surface ablation with CXL in patients who are contact-lens intolerant affords modern corneal and refractive surgeons the potential to delay or reduce the need for a corneal transplant. Expertise not only in corneal surgery but also in laser refractive surgery is essential to appropriately manage these patients.

In the first of a two-part series on surface ablation, David OíBrart, MD, FRCS, FRCOphth, compared surface ablation with LASIK in normal corneas and described the use of mitomycin C and epithelial removal techniques (see Update on Surface Ablation Procedures: Part 1; February 2013; pp 58-60). In this second part, Dr. OíBrart summarizes the latest research on surface excimer ablation in keratoconus and eyes that have had previous corneal surgery. He also discusses the long-term safety and stability of these procedures. I hope you enjoy this installment of Peer Review, and I encourage you to seek out and review the articles in their entirety at your convenience.
-Allon Barsam, MD, MA, FRCOphth

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COMBINED PRK AND CXL FOR KERATOCONUS

Although ectasia has been reported after PRK,^1,2 it occurs much less frequently after PRK than LASIK.³ This is not surprising, given the greater degradation of corneal biomechanics that the LASIK flap induces.⁴ Several case series have reported successful and stable outcomes of topography-guided PRK in eyes with mild keratoconus (Amsler-Krumeich grade 1 to 2).^5,6

CXL with riboflavin and ultraviolet-A (UV-A) light therapy is the first treatment modality that appears to halt the progression of keratoconus and, to a limited extent, improve both refractive and topographic parameters. Multiple clinical studies, including a recently published randomized, bilateral study by OíBrart et al⁶ demonstrated the efficacy and safety of CXL with riboflavin and UV-A light therapy. As an isolated treatment, CXL has been used in combination with topography-guided PRK to optimize visual outcomes in patients with keratoconic and post-LASIK ectasia.^7-9 Combined CXL and limited topographyguided PRK with ablation depths generally less than 50 µm in selected eyes with moderate keratoconus and post-LASIK ectasia has been shown to be effective, with marked improvements in visual, refractive, and topographic parameters and stabilization of the ectatic process in most eyes.^7-9 In a series of 32 eyes with post- LASIK ectasia treated with transepithelial topographyguided PRK and sequential CXL, Kanellopoulos and Binder⁷ reported visual and/or topographic improvement in all but one case 18 months postoperatively. Kymionis et al⁸ reported significant improvements in refractive, visual, and topographic outcomes in 26 patients (31 eyes) with progressive keratoconus with a mean follow-up of more than 19 months.

Similarly, Tuwairqi and Sinjab⁹ evaluated 1-year visual and topographic outcomes and the safety and efficacy of CXL combined with topography-guided PRK to achieve near emmetropia in eyes with low-grade keratoconus. Fifteen patients (22 eyes) with low-grade keratoconus with evidence of progression, no scarring, a corneal thickness greater than 440 µm, and a maximum keratometry reading less than 51.00 D were included. After 1 year, statistically significant improvement was noted in all measured visual, refractive, and topographic parameters, with no evidence of progression in any of the eyes.

Such results for often visually devastating conditions are encouraging. Labiris et al¹⁰ demonstrated that combined CXL treatments are associated with significant improvements in quality-of-life scores. Follow-up for these studies, however, is limited to 1 to 3 years; long-term biomechanical stability has not been fully elucidated.

PRK reduces the biomechanical strength of the cornea,³ and progression after PRK with CXL has been reported,⁷ along with the rare occurrence of significant corneal haze and scarring.¹¹ Despite these limitations, in carefully selected contact-lensñintolerant patients with low-grade ectasia, combined CXL and topographic PRK treatments may be considered with adequate counseling and fully informed consent as to the risk of ectasia progression.

SURFACE ABLATION IN EYES WITH PREVIOUS CORNEAL SURGERY

Excimer laser surface ablations to treat postkeratoplasty ametropia and iatrogenic astigmatism were first reported 20 years ago.¹² Initial results with PRK, although encouraging for that time, demonstrated limited efficacy and safety, with reduction in cylinder limited to only a few diopters, frequent occurrences of significant iatrogenic haze, and a loss of more than 2 lines of BCVA.^12-14 The development of LASIK in the mid- 1990s afforded surgeons better outcomes for treating patients with postkeratoplasty astigmatism by avoiding epithelial-stromal wound healing interactions.¹⁵ Results were generally better than standard PRK, with no haze and fewer lines lost of BCVA.¹⁵

With the development of modern laser platforms and the use of adjunctive medications such as mitomycin C, surface ablation has once again become a viable option for managing complex cases. Leccisotti¹⁶ conducted a prospective study of 10 eyes with compound myopic astigmatism treated for keratoconus with PRK and adjunctive mitomycin C 0.02% for 45 seconds after deep anterior lamellar keratoplasty. Postoperatively, the safety index was 1.15, and the efficacy index was 0.81. No lines of BCVA were lost. Trace haze was observed in two eyes, and endothelial counts were unchanged. Sixty percent of eyes were within ±1.00 D of defocus equivalent. Leccisotti also reported a significant improvement in BCVA.

In a prospective study, Forseto Ados et al¹⁷ assessed the safety and efficacy of PRK with mitomycin C in 36 eyes to correct refractive errors after corneal transplantation. Sixteen months postoperatively, more than 60% of eyes were within ±1.00 D of emmetropia, and more than 50% had a UCVA of 20/40 or better. Eleven months postoperatively, endothelial cell decompensation was observed in one eye, and haze developed in 8.3% of eyes.

Hodge et al¹⁸ reported a consecutive series of 41 patients (49 eyes) with refractive cylinders greater than or less than 6.00 D who underwent PRK with mitomycin C after penetrating keratoplasty for keratoconus. Similar to other published series of postkeratoplasty eyes treated with LASIK, no severe iatrogenic haze occurred.

Ward et al¹⁹ retrospectively evaluated the efficacy, refractive predictability and stability, and complications of PRK with adjunctive mitomycin C in patients with postkeratoplasty anisometropia. Of 20 cases, 100% reported that they were satisfied, with 90% having a BCVA of greater than 20/40 and 50% gaining 1 to 2 lines of BCVA. The mean reduction in astigmatism was 2.90 D. A 2-line loss of BCVA was reported in one eye.

Astigmatism after corneal surgery in these problematic cases is typically irregular. Topography-guided and corneal wavefront-guided ablations have proved invaluable for the correction of irregular refractive errors because they address both higher- and lower-order aberrations.²⁰ Because the epithelium will typically try to smooth out underlying stromal irregularities, and because the vast majority of higher-order aberrations are located at the anterior corneal surface, a surface approach rather than treatment under a flap seems logical.

Rajan et al²⁰ assessed topography-assisted corneal wavefront surface ablation for the correction of ametropia and irregular astigmatism after keratoplasty. Fifteen patients (16 eyes) who were intolerant of spectacle and contact lens correction due to astigmatic anisometropia after keratoplasty (15 penetrating and one lamellar) underwent topography-assisted customized treatments. The investigators reported significant improvements in UCVA and BCVA and significant reductions in both lower- and higher-order aberrations. They also documented less haze in the eyes treated with mitomycin C.

Koch et al²¹ investigated the outcomes of wavefrontguided PRK with mitomycin C in 32 eyes that had previously undergone radial keratotomy (RK). According to the study authors, 100% of eyes that underwent myopic corrections and 74% of eyes that underwent hyperopic corrections were within ±1.00 D of intended correction. None of the eyes lost more than 2 lines of BCVA, and there were no cases of postoperative haze greater than grade 1. Similarly, Camellin and Arba Mosquera,²² in a study of 26 patients (35 eyes) with previous keratoplasty and RK, performed transepithelial corneal wavefrontguided PRK. Sixty percent of eyes achieved a UCVA of 20/40 or better, 71% were within ±1.00 D of the attempted correction, and none of the eyes lost lines of BCVA. One eye developed trace haze.

These results are encouraging and compare with LASIK but without the flap-associated complications.

LONG-TERM FOLLOW-UP OF PRK

Although the short- and medium-term outcomes of surface ablation procedures are excellent, for any keratorefractive procedure to be viable, the induced corrections must be stable for many decades without the occurrence of long-term complications that may degrade either the transparency or biomechanics of the cornea. Many previous keratorefractive procedures such as RK were unsuccessful, as continued biomechanical instability led to progressive peripheral ectasia and hyperopic shift.²³ Recently, several long-term follow-up studies have demonstrated reasonable refractive and excellent biomechanical stability after PRK. In a retrospective study of 29 patients (42 eyes) 10 years postoperatively, Koshimizu et al²⁴ demonstrated mild myopic regression of approximately -0.50 D, with minimal haze scores and no more than 1 line loss of BCVA. Interestingly, the investigators noted a slightly higher rate of endothelial cell loss than would be expected with patient age.

Guerin et al²⁵ reported 16-year follow-up data for 23 patients (39 eyes) after PRK with 5.00-mm optical zones and corrections up to -7.00 D. They found slight regression of correction over the follow-up period, but none of the eyes lost more than 1 line of BCVA, and all patients reported being satisfied. In a randomized study of myopic PRK and LASIK with 7-year follow-up, Ivarsen and Hjortdal26 reported stabilization of corneal power from 1 to 7 years in PRK-treated eyes. In contrast, corneal power continued to increase from 1 to 7 years after LASIK.

In a study of 44 patients (44 eyes) treated 17 to 20 years earlier with PRK with a 6.00-mm optical zone for corrections between -2.00 and -7.00 D, OíBrart et al found an increase in myopic spherical equivalent of only -0.30 D over follow-up (personal data). The safety index was 1.01. None of the eyes lost more than 1 line of BCVA and there was no evidence of ectasia on Scheimpflug topographic examination. Two eyes had trace haze. The results of these studies suggest acceptable refractive stability over 2 decades for surface ablation procedures, with no long-term sight-threatening complications regarding corneal transparency and biomechanical stability.

CONCLUSION

The literature supports the use of advanced surface ablation combined with CXL in selected cases of keratoconus. There is also good evidence to support its use in eyes that have previously undergone corneal surgery. Longterm stability and safety appear to be satisfactory.

Allon Barsam, MD, MA, FRCOphth, is a Consultant Ophthalmic Surgeon at Luton & Dunstable University, The Western Eye Hospital, Imperial College Healthcare NHS Trust, and Focus Clinics in London. Dr. Barsam is a member of the CRST Europe Editorial Board. He may be reached at e-mail: eyes@allonbarsam.com.

David P. S. OíBrart, MD, FRCS, FRCOphth, is a Consultant Ophthalmologist in the Department of Ophthalmology at St. Thomasí Hospital, London. Dr. OíBrart may be reached at tel: +44 1702 586656; e-mail: davidobrart@aol.co.uk.

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