The most common long-term complications of radial keratotomy (RK) include concomitant hyperopia, astigmatism, and corneal irregularity, with visual impairment that cannot be corrected with spectacles. A hyperopic shift can occur anytime in approximately 50% of these patients.1,2
Management of this complication is challenging and controversial and the prognosis is not usually good. Different techniques including laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK), with or without customization, have been used to treat these patients; however, these procedures have had variable results and are associated with a high rate of complications such as incision opening, multiple buttonholes in the flap,3–5 epithelial downgrowth,6 and corneal haze formation, in particular in eyes that require reoperation.7 Little evidence exists with variable mixed results on PRK8,9 or LASIK.5,10–12 Topography-guided transepithelial PRK is 1 of the latest techniques used to correct the residual refractive errors after refractive surgery.13,14
In this study, we performed transepithelial PRK as a no-touch single-step procedure in a series of post-RK patients with extreme corneal irregularity and refractive error. Using this technique, the excimer laser smoothly removes the epithelium without manual intervention. Alternatively, corneal wavefront data are precisely registered on the cornea without modifying the corneal surface by manual intervention.
To our knowledge, the present study is the first prospective interventional study to evaluate the efficacy and safety of topography-guided transepithelial PRK as a new technique for treating irregular astigmatism in post-RK eyes.
PATIENTS AND METHODS
Study Design and Patients
A prospective interventional clinical study was performed at Feiz University Hospital, a tertiary ophthalmology referral center, and the Persian Eye Clinic between 2015 and 2016. This study was approved by the Institutional Review Board and Ethics Committee of Isfahan University of Medical Sciences, Isfahan, Iran, and followed the tenets of the Declaration of Helsinki. Written informed consent was obtained from all patients before they participated in the study.
Consecutive patients whose refractive and topographic data could be measured and had stable refractive errors for at least 1 year, were included in the study. Exclusion criteria were pregnancy, breastfeeding, systemic or ocular disease that could interfere with the wound-healing process (eg, dry-eye syndrome, uveitis, corneal dystrophy, and/or degenerative diseases); eyelid disease; autoimmune disease and collagen vascular disease; previous ocular pathologies (glaucoma, retinal detachment); previous corneal or intraocular surgery other than RK; previous ocular trauma; central corneal thickness less than 490 μm; and predicted residual stromal thickness less than 300 μm after surgery. Withdrawal criteria included absence during the follow-up visits and discontinuing the study protocol by the patient.
All patients had a complete preoperative eye examination and measurements including cycloplegic and subjective refraction, uncorrected (UDVA) and corrected (CDVA) distance visual acuities, manifest refraction spherical equivalent (MRSE), slitlamp evaluation including intraocular pressure (IOP) (Goldmann tonometer), and fundoscopy. Corneal topography and wavefront were measured with the Scheimpflug analyzer (Sirius, Costruzione Strumenti Oftalmici).
Three topography measurements were evaluated by an experienced technician and the best value meeting the criteria proposed by the company was selected and subjected to the excimer laser via a Universal Serial Bus port.
All eyes had transepithelial PRK with the excimer laser Schwind Amaris 1050RS (Schwind eye-tech-solutions GmbH & Co. KG). All surgeries were performed by the same surgeon (M.G.).
After scrubbing the eyelids and skin with povidone–iodine 5.0%, tetracaine 0.5% was instilled twice for anesthesia into each eye before the procedure. A blepharostat speculum was inserted to allow maximum exposure of the globe. Topographic and refractive data were loaded, evaluated, and modified if necessary, according to the manufacturer's guidelines. The eye tracker was initiated and ablation was centered on the corneal vertex.
The epithelium and stroma were ablated in a single step using the corneal wavefront-guided transepithelial PRK nomogram of the Amaris laser software (Optimized Refractive Keratectomy-Custom Ablation Manager software, version 5.0.3913.505, Schwind eye-tech-solutions GmbH & Co. KG).
The selected optical zone varied between 6.0 mm and 6.8 mm. The transition zone was calculated by the algorithm of the excimer laser machine; it varied between 0.86 mm and 2.91 mm. The total ablation zone varied between 7.36 mm and 9.3 mm. After the laser ablation, a single topical application of mitomycin-C 0.02% (MMC) was administered instantly to the stromal bed for 30 to 40 seconds; thereafter, the surface was irrigated with 40 cc of a balanced salt solution and a soft bandage contact lens (Acuvue, Johnson & Johnson Vision Care, Inc.) was inserted.
Postoperatively, a topical steroidal (betamethasone 0.1%) and a topical antibiotic (ciprofloxacin 0.3%) 4 times a day were prescribed. Preservative-free artificial tears were prescribed as necessary. The bandage contact lens was left in place for 7 days or until complete healing of the epithelial defect. The topical steroidal agent was gradually reduced over 6 to 8 weeks by monitoring the IOP.
The main outcomes of the study were MRSE, UDVA, and CDVA. The secondary outcome measurements included changes in the corneal topography and corneal wavefront aberration parameters including spherical aberration and coma. All patients were evaluated at day 1, daily until the epithelium healed, at 1 week, and at 1 and 6 months after the intervention.
The intraoperative and postoperative complications included unexplained visual disturbance, persistent epithelial defects, keratitis, corneal perforation, and corneal haziness recorded at 1 month and 6 months after surgery.
Vector Analysis for Evaluation of Astigmatic Treatment
Manifest refraction was converted to the corneal plane value before performing astigmatic vector analysis by the Alpins method.15 The target-induced astigmatism, surgically induced astigmatism, magnitude of error, angle of error, difference vector, correction index, index of success, flattening effect, flattening index, torque effect, spherical correction index, spherical difference, and index of success for spherical change were calculated.
Statistical analysis was performed using SPSS software (version 16.0, SPSS, Inc.). Efficacy was evaluated by the UDVA and safety was assessed by changes in the CDVA observed at 6 months. Predictability was determined by the mean MRSE and defocus equivalent at 6 months.3 The continuous variables were presented as means ± SD. The categorical variables were presented as frequency and percentage. The parametric t test (paired student t tests) or nonparametric test (Mann-Whitney) was used to the compare the mean values.
The study comprised 22 eyes (22 patients) and all patients completed the 6 months of postoperative follow-up. The interval between RK and transepithelial PRK ranged from 17 to 22 years. Table 1 shows the demographic characteristics and baseline refractions of the patients.
Postoperative Refraction and Visual Outcomes
Statistically significant differences were observed before, between, and after the intervention regarding mean UDVA logarithm of the minimum angle of resolution (logMAR) (P = .001), CDVA logMAR (P = .014), mean MRSE (P < .001), and defocus equivalent (P < .001) (Table 2).
At the end of the study, the mean absolute cylinder improved; however, this improvement was not statistically significant (P = .066). The keratometric astigmatism decreased postoperatively; however, this reduction was not significant (P = .084) (Table 2).
Table 2 shows the postoperative aberration parameters before and after intervention. The mean higher-order aberrations (HOAs), mean spherical aberration, and mean coma were significantly reduced at the 6-month visit (P < .001).
The UDVA also improved after the intervention (P < .001) (Table 2). Figure 1 shows a comparison between the preoperative CDVA and postoperative UDVA.
Figure 2 shows the percentage of eyes that gained lines of logMAR. At the end of the study, only 1 eye (4.5%) lost 2 lines of CDVA and 2 eyes (9.1%) lost 1 line of CDVA.
Figure 3 shows the scattergram of attempted versus achieved correction. Figure 4 shows the preoperative and postoperative refractive astigmatism values.
Vector Analysis of Astigmatism Changes (Alpins Method)
Table 3 shows the results of the vector analysis of astigmatic correction using the Alpins method after transepithelial PRK.
During transepithelial PRK surgery, all RK incisions remained intact with no separation or splitting, and no intraoperative complications were observed. No retreatment was performed. No adverse MMC effects were observed during the postoperative follow-up.
The results of our study indicate that transepithelial PRK can be used as an effective and safe method to treat post-RK patients who have been refractively stable for at least 1 year. No major complications occurred during surgery or in the postoperative period.
The post-RK refractive errors can be corrected by several means including phakic intraocular lenses (pIOLs), refractive lens exchange (RLE), and femtosecond-assisted LASIK.3,16
All these procedures have certain limitations. In pIOL cases, because most of the patients with RK are aged 45 years or older, preserved accommodation and decreasing anterior chamber depth are concerns. In RLE cases, there is a potential risk for vitreoretinal complications.17,18
In a few studies that evaluated the efficacy and safety of PRK and LASIK as refractive procedures to correct the refractive and aberration errors after RK,7,9,19–22 the results were variable.
Photorefractive keratectomy showed limited success because of the potential irregularity of the corneal surface, followed by haze and regression. The correction of residual refractive errors after RK using PRK is less effective and less predictable than PRK for naturally occurring myopia and astigmatism without previous refractive surgery. In these cases, PRK is usually associated with greater corneal haze.23
The major disadvantages of LASIK are related to the creation of the lamellar flap. Complications include free, incomplete, irregular, thin, or buttonholed flaps.12,24 Moreover, the risks include uncontrolled shearing forces when the corneal flap is lifted and extension of RK wound dehiscence, which could lead to epithelial ingrowth and loss of visual function.25
Another limitation of LASIK is that this procedure should be avoided in eyes with more than 8 RK incisions because of an increased risk for intraoperative separation of the radial incisions5; furthermore, in LASIK cases, treatment of the astigmatic component was less predictable and major complications such as ectasia and flap necrosis were observed.6,26
Using transepithelial approaches during PRK, the epithelium acts as a natural mask in ablations for thinning over stromal protrusions and thickening over troughs; hence, maximum correspondence is observed between the corneal topography and the ablation in patients with irregular corneal astigmatism. The most notable finding of our study was a significant improvement in vision, both corrected and uncorrected.27,28
Except for the study by Camellin and Arba Mosquera,28 no combination therapy exists for treating the residual refractive errors after RK. Moreover, no direct comparison is observed among the outcomes of other refractive surgeries after RK.
Camellin and Arba Mosquera28 showed that the simultaneous use of corneal wavefront-guided transepithelial PRK and phototherapeutic keratectomy using an excimer laser was safe and effective in correcting the aberrations and refractive errors after RK; however, a limitation of that study was the short follow-up duration for evaluating the delayed regression.
Ghanem et al.29 showed that wavefront-guided PRK was effective, predictable, and safe for treating hyperopia and hyperopic astigmatism after RK. Significant improvement was observed in the UDVA, CDVA, and HOAs, with a low incidence of visually significant corneal haze.
In a study by Leccisotti and Fields,3 18 eyes of 10 patients were treated with LASIK for consecutive hyperopia after RK. Although the authors reported the procedure to be a safe and effective approach to treat post-RK hyperopia, a major opening of a single radial incision was noted in 3 eyes but never extended more than 3.0 mm.
In another study that used LASIK in the setting of previous RK, the UDVA significantly improved postoperatively and remained stable through the final follow-up interval at 9 to 12 months postoperatively with no complications.30 All our patients completed the 6-month follow-up and during this duration, no case of corneal haziness was observed, presumably because of the prophylactic effect of using MMC on corneal apoptosis and cellular proliferation.31
We observed a decrease in mean astigmatism in both keratometric and refractive values; however, statistical tests failed to show a significant change with regard to a predetermined cutoff for a significance level of the P value. A possible explanation might be the limited sample size of the study, high variability of the outcome, and possibly, small effect size (efficacy of the procedure for treatment of astigmatism). The higher variability and smaller effect size of the procedure in turn could be explained by the corneal irregularity in the patients enrolled in this study.
Although we have shown significant improvements in the UDVA, CDVA, and reduced corneal astigmatism after 6 months of follow-up, our study had certain limitations including the relatively short duration of follow-up, small sample size, lack of randomization, and absence of a control group. However, to our knowledge, this was the first evaluation that addressed the use of transepithelial PRK on post-RK as a relatively rare condition. A longer follow-up with larger cohorts is necessary to validate these findings and to show the stability of refraction in these patients.
In conclusion, considering our results, transepithelial PRK is an effective and safe procedure to treat the refractive instability in post-RK patients. Therefore, transepithelial PRK might prove to be a promising and effective procedure in treating residual aberration and refractive error. Large, prospective, and comparative studies are required to show the efficacy of transepithelial PRK in comparison to several refractive surgeries for post-RK patients.
WHAT WAS KNOWN
- The management of RK complications is controversial and prognosis is not usually good. Different techniques including LASIK and PRK, with or without customization, have been use in these patients; however, these procedures have had variable results and are associated with a high rate of complications.
WHAT THIS PAPER ADDS
- When using transepithelial approaches during PRK, the epithelium acted as a natural mask in ablations for thinning over stromal protrusions and thickening over troughs. Therefore, transepithelial PRK might prove to be a promising procedure in treating the residual aberration and refractive error after RK.
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Disclosures:None of the authors has a financial or proprietary interest in any material or method mentioned.