Photorefractive keratectomy results in myopic patients with thin cornea eyes

Abstract

Background:

To evaluate the results of visual acuity and quality, and corneal integrity in myopic patients with a thin cornea who received photorefractive keratectomy (PRK).

Materials and Methods:

In this before-after interventional study, 30 myopic eyes with a myopia −3.76 ± 1.72 (−6.50 to −1.25) D and a corneal thickness of 486.03 ± 11.93 (452-499) μm at the thinnest point received PRK. In myopia was more than 4D, mitomycin C was used with PRK. The surgery was performed with an excimer laser (VISX STAR, Abbott Medical Optics, Abbott Park, US).

Results:

The safety and the efficacy index of the surgery was 1.01 ± 0.05 and 1.00 ± 0.05 in these patients, respectively. All the patients were within ±0.5D of emmetropia 1 year after the surgery. Mesopic contrast sensitivity (CS) had a significant increase in two spatial frequencies of six (P = 0.003) and 12 (P = 0.003). Total coma (P < 0.001), spherical aberration (P < 0.001), and total higher-order aberrations (HOA) (P < 0.001) also showed a significant increase. Corneal hysteresis (P < 0.001) and corneal resistance factor (P < 0.001) showed a significant decrease after 1 year.

Conclusion:

PRK is a safe, effective, and predictable procedure with desirable effects on mesopic CS in patients with corneal thickness <500 μm, which increases HOAs and decreases corneal integrity proportionate to its value before the procedure.

Introduction

Treatment of low to moderate myopia with excimer laser refractive procedures like laser-assisted in situ keratomileusis (LASIK),[1] laser epithelial keratomileusis (LASEK),[2] and photorefractive keratectomy (PRK)[3] has desirable safety and efficacy. Studies have shown that LASIK in patients with corneal thickness <500 μm is a risk factor for the development of ectasia.[4] For this reason, PRK, despite complications like postoperative pain,[5] prolonged visual rehabilitation,[6] and corneal haze[3] is the technique of choice for the correction of refractive errors in these patients. Nowadays, with advances in the surgical techniques, these complications have noticeably decreased. For example, the use of mitomycin-C has substantially reduced post PRK haze.[7] However, since a number of cases with post PRK ectasia have been reported,[8] there are still hesitations regarding this procedure. We previously showed the safety and efficacy of LASEK in the treatment of myopic-thin cornea patients.[9] We performed this study to present the results of refraction, mesopic contrast sensitivity (CS), aberrations, and corneal integrity in patients with a thin cornea who received PRK.

Materials and Methods

In this before-after study, PRK was performed on 30 eyes of 15 myopic patients with a thin cornea. The patients had no sign of keratoconus in clinical examinations. It was confirmed by Pentacam scheimpflug imaging. The procedures were performed by two surgeons. The mean age of the participants was 29.47 ± 6.10 years old (range: 20-41 years) and 73.3% of them were female. The mean severity of myopia was -3.76 ±1.72 (−6.50 to − 1.25) D and the mean corneal thickness at the thinnest point was 486.03 ± 11.93 (452-499) μm. Based on the inclusion criteria, the refractive error had to be stable in the last 18 months before the procedure. Patients with any type of ocular pathology or a history of ocular surgery were excluded from the study. Patients with contact lenses stopped using them 4 weeks before the surgery.

The Institutional Review Board of Noor Ophthalmology Research Center reviewed and approved the study protocol. The details of the study were presented to the patients before the surgery and written informed consent was obtained from them.

According to the clinic policy, mitomycin-C was used in PRK surgery in patients with myopia more than 4D. Before the surgery, uncorrected visual acuity (UCVA) and best corrected visual acuity (BCVA) with a Snellen chart, manifest refraction spherical equivalent (MRSE) using an auto kerato-refractometer (Topcon 8800, Japan), CS using CVS1000 grating charts (VectorVision, Inc., Greenville, OH), aberrometry using the Allegretto WaveLight analyzer (WaveLigth Laser Technologie AG, Germany) and biomechanical properties of the cornea using an ocular response analyzer (ORA; Reichert Ophthalmic Instruments, Buffalo, NY, USA) were evaluated. CS was measured under mesopic condition (4 lux), with best distance correction, and without dilation. The data of aberrometry were obtained from the 6 mm setting of the device. For each patient, three measurements were performed and the best one was selected. Among aberrometry indices, C6 (trefoil), C7 (vertical coma), C8 (horizontal coma), root-mean-square (RMS) coma_total, C9 (trefoil), C12 (spherical aberration or SA), and RMS higher order aberration_total (RMS HOA_T) were reported. The following equation was used to calculate RMS coma_total : √(C7² + C8²). The indices of corneal hysteresis (CH) and corneal resistance factor (CRF) were reported from ORA. The patients received complete ophthalmic examination before the surgery and 1, 3, 6, and 12 months after it. Vision and refraction examinations were repeated in every follow-up visit, but the assessment of CS, aberrations, and corneal biomechanics was performed in the last follow-up. To evaluate the trend of the changes of the indices, repeated measures analysis of variance (ANOVA) and post-hoc test, and paired t-test were used with a significance level of 0.05.

Surgical technique

The corneal epithelium was mechanically scraped with a blunt spatula under local anesthesia with proparacaine hydrochloride 0.5% drops without the use of alcohol. The VISX STAR S4 excimer laser (Abbott Medical Optics, Abbott Park, US) with software version 5:30 was used for ablation (target fluence: 160 mJ/cm², pulse rate: 10.0 Hz). The selected ablation zone in all patients was 6 mm with a blend zone.

In myopic eyes more than 4D, after performing laser, a sponge soaked in mitomycin C (MMC) 0.02% was applied on the ablated stroma for 10 s for each 1-diopter correction. After irrigation with 30cc of balanced salt solution, a bandage contact lens (Air Optix, Ciba vision) was used. After the surgery, betamethasone 0.1% eye drops 4 times/day, levofloxacin 5 mg/ml eye drops 4 times/day, and preservative free artificial tears (hypromellose) were prescribed for patients. To reduce inflammation, diclofenac sodium was used every 6 h until 24 h after the procedure. Daily examinations continued until complete epithelial healing was observed. On reepithelialization, the lens was removed and levofloxacin was discontinued but betamethasone and artificial tears were continued for 2 more weeks. After that, fluorometholone 0.1% was administered for 2 months in a tapering fashion. Then, as mentioned previously, follow-up visits were performed 1, 3, 6, and 12 months post operation.

Results

The safety index of the surgery was 1.01 ± 0.05 and the efficacy index was 1.00 ± 0.05. Regarding predictability, 100% of the participants were within ±0.5D of emmetropia after 1 year. UCVA showed a significant improvement in all follow-ups when compared to before the surgery (all P < 0.001). Preoperative BCVA was 0.001 ± 0.01 LogMAR which showed no significant difference after the surgery. MRSE showed a significant decrease in all follow-ups when compared to before the surgery (all P < 0.001). The results of the changes of UCVA, BCVA, and MRSE are presented in Table 1.

Table 1.

The changes in vision and refraction following PRK in thin cornea patients

After 1 year, mesopic CS showed a significant increase in two spatial frequencies of 6 (P = 0.003) and 12 (P = 0.003). The increase in vertical coma (P = 0.049), total coma (P < 0.001), SA (P < 0.001), and total HOA (P < 0.001) was also significant. CH (P < 0.001) and CRF (P < 0.001) showed a significant decrease after 1 year. Figures 1–3 show the results of CS, aberrations, and biomechanical properties of the cornea, respectively.

Figure 1.

One-year post-photorefractive keratectomy results of mesopic contrast sensitivity in patients with thin cornea

Figure 3.

One-year post-photorefractive keratectomy results of corneal biomechanical indices in patients with thin cornea

Figure 2.

One-year post-photorefractive keratectomy results of higher order aberrations in patients with thin cornea

No complications were noted after the surgery. Corneal haze Grade 1 was observed in three eyes (10%) 1 month after the surgery, which disappeared in two eyes after 3 months and in one eye after 6 months. No case of corneal haze was detected 1 year after the surgery.

Discussion

The present study showed that PRK is a safe, effective, and predictable procedure for correcting low to moderate myopia in patients with corneal thickness < 500 μm; it improves CS but declines HOAs and corneal biomechanics. Since patients with thin cornea are not candidates for LASIK for the possibility of post-LASIK ectasia[4,10,11] and surface ablation method is preferred in corneal thickness <500 μm,[12] PRK can be regarded as treatment option for the correction of myopia in these patients. Long-term studies have confirmed the improvement of the vision and the refraction in myopic patients with normal corneal thickness after PRK.[13,14] In the patients with central corneal thickness (CCT) <500 μ, a retrospective long term study showed the safety and efficacy of excimer laser surface ablation procedures (PRK and LASEK) in these patients.[15] The 1-year results of the improvement of vision and refraction were better in our study; the 1-year safety and efficacy of the procedure were 0.97 ± 0.94 and 0.80 ± 1.2 in that study[15] versus 1.01 ± 0.05 and 1.00 ± 0.05 in our study, respectively. One reason for this difference can be the use of MMC in myopia more than 4D in our study. Slit lamp examination was used to evaluate corneal haze in both studies. It has been shown that MMC prevents the development of haze[7] and in contrast to our study in which no participant had haze after 1 year, the mean haze was 0.13 (0-2) in that study in the end of the 1^st year.[15] Moreover, we only used PRK and no participant received LASEK. The difference in the results of our study and that study[15] could be more obvious if the cases of retreatment were not included in that study.

In a retrospective study by Djodeyre et al.,[12] the 5-year results of surface ablation methods (PRK and LASEK) and LASIK in eye with CCT <470 μm were compared. The mean SE in the participants of this study was −3.51 ± 1.84D and − 0.23 ± 0.52D before and after the procedure respectively which may be due to the lower thickness of the cornea in this study when compared to our study.[16]

Following laser surgery, because the corneal shape becomes oblate, HOAs increase and CS decreases.[17,18] In our study, HOAs increased while mesopic CS not only did not decrease but also improved. This contradiction could result from the difference in pupil diameter between the two devices at the time of measurement. We already know that visual quality and image clarity decreases with pupil dilation. On the other hand, CS is related to neural processes in addition to optical factors while HOA exclusively shows optical aberration.

Hashemi et al.[19] performed a study to evaluate conventional PRK in the treatment of low to moderate myopia with normal corneal thickness and reported that although mesopic CS had a significant decrease 1 month after the procedure, it increased until the 6^th months after the operation and finally returned to preoperative values. In other words, there was no significant difference in mesopic CS between before and 6 months after the operation in this study.[20] Considering the trend of the changes of contrast up to 6 months, it seems that contrast improved similar to our study if the study continued for 1 year. It could be stated that there is a difference in mesopic CS changes between thin cornea patients and patients with normal corneal thickness who underwent PRK.

The increase in HOAs in our study was more than 1-year results of a study by Serrao et al.[20] which was conducted low myopic patients with normal corneal thickness. The reason for the difference could be difference in the corneal thickness between the two studies. Since the increase in spherical aberration after ablation depends on both the amount of refractive correction and the ablation profile, for example lower wavefront optimized ablations results in an increase in spherical aberration, not using this profile may explain the reason for the noticeable increase in spherical aberration. In addition, it could be assumed that more decrease in the corneal viscoelasticity in thin corneas may result in more changes in the biomechanical properties of the cornea and its pripheral protrusion which may cause more corneal ablateness and increase spherical aberration.

Different studies have evaluated the changes of the biomechanical properties of the cornea following PRK in patients with normal corneal thickness.[21,22,23] In our study, the decrease in CRF was less than the study performed by Qazi et al.[21] and more than the studies conducted by Kamiya[22] and Zare.[23] It is possible that the condition of the cornea was not stable and the results were not accurate due to the shorter follow-up of these studies, which should be evaluated in a comparative study between individuals with normal corneal thickness and patients with corneal thickness <500 micron.

The decrease of CH in our study was less than the three studies.[21,22,23] Comparison of results of the studies performed by Kamiya et al.[22] and Qazi[21] with our study suggests CH may decrease more with the increase in myopic correction. However, in the study which was conducted by Zare et al.[23] on cases with a lower SE than our study (−3.25D), the decrease in CH was more than our study, which could be due to the difference in the duration of follow-up between the two studies. The follow-up duration was 3 months in the study by Zare et al.[23] and 1 year in our study. Hjortdal et al.[24] reported partial recovery indices in a period of 12 months after the surgery, although these indices never return to preoperative values. The fact that a limited number of the cases of post PRK ectasia have been reported in the literature and studies have contributed it to primary ectasia and abnormal preoperative topography rather than an iatrogenic incident[25,26] can confirm this finding.

In general, it can be said that based on the 1-year results, PRK is a safe, effective, and predictable procedure with desirable effects on mesopic CS in patients with corneal thickness <500 μm, but it increases HOAs. Considering the slight biomechanical changes of the cornea and the effectiveness of the refractive results of PRK, it can be considered a treatment option in myopic patients with corneal thickness less 500 μ.