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ORIGINAL ARTICLE
Year : 2020  |  Volume : 33  |  Issue : 4  |  Page : 1160-1166

Assessment of epithelial changes by anterior segment optical coherent topography after photorefractive keratectomy


1 Department of Ophthalmology, Faculty of Medicine, Menoufia University, Menoufia, Egypt
2 Department of Ophthalmology, Tanta Ophthalmology Hospital, Tanta, Gharbia, Egypt

Date of Submission11-Mar-2020
Date of Decision14-Apr-2020
Date of Acceptance25-Apr-2020
Date of Web Publication24-Dec-2020

Correspondence Address:
Amina F Radwan
Shobraelnamla Country in Tanta
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/mmj.mmj_56_20

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  Abstract 


Objective
The aim of the study was to evaluate corneal epithelial changes after photorefractive keratectomy (PRK) by anterior segment optical coherence tomography (AS-OCT) and correlate these with ablation depth (AD) and refractive outcome.
Background
Evaluation of corneal epithelial changes profile after PRK is essential for understanding postoperative wound healing and clinical outcomes.
Patients and methods
This a prospective nonrandomized clinical study in 100 eyes of 50 patients treated with PRK for myopia, astigmatism, or compound myopic astigmatism. Epithelial thickness maps were obtained by AS-OCT preoperatively and at 2 and 3 months postoperatively. Correlation between epithelial changes and the amount of AD and spherical equivalent were analyzed.
Results
Compared with the preoperative value the central 1 mm and paracentral (superior, inferior, temporal, nasal) 1: 3 mm zone epithelium was 5.71 ± 2.221 and (6.23 ± 2.176, 6.31 ± 2.553, 8.35 ± 2.436, 6.06 ± 2.000 μm) thicker, respectively, at 3 months postoperatively (P < 0.01). Epithelial thickness reached approximately the preoperative thickness 2 months later with the central 1 mm and paracentral (Superior, inferior, temporal, nasal) epithelium being 0.06 ± 0.180 and (0.35 ± 1.021, 0.19 ± 0.790, 0.63 ± 1.496, 0.38 ± 1.362) μm, respectively. The spherical equivalent changed from −6.25 to −0.625 D preoperatively to −1.5–0.0 D at 2 months postoperatively and remained stable at 3 months. There was a significant correlation between epithelial thickness and AD.
Conclusion
The epithelial thickness was assessed by AS-OCT. Epithelial thickness reached approximately the preoperative thickness at 2 months after PRK but there was statistically significant increase up to 3 months. There was statistically significant positive correlation between AD and epithelial thickening. These changes do not affect refraction.

Keywords: anterior segment optical coherence tomography, corneal epithelial changes, photorefractive keratectomy


How to cite this article:
El-Sebaey AR, Ibrahim AM, Radwan AF. Assessment of epithelial changes by anterior segment optical coherent topography after photorefractive keratectomy. Menoufia Med J 2020;33:1160-6

How to cite this URL:
El-Sebaey AR, Ibrahim AM, Radwan AF. Assessment of epithelial changes by anterior segment optical coherent topography after photorefractive keratectomy. Menoufia Med J [serial online] 2020 [cited 2021 Apr 19];33:1160-6. Available from: http://www.mmj.eg.net/text.asp?2020/33/4/1160/304510




  Introduction Top


Photorefractive keratectomy was the first kind of corrective eye surgery to use a laser rather than a blade to remove the corneal tissue[1]. photorefractive keratectomy (PRK) avoids the use of the microkeratome or laser to make the LASIK flap. This leaves a greater portion of the cornea untouched by the surgery, which is important in patients who have thin corneas[2]. In addition, there appears to be more rapid recovery of the function of the corneal nerves, which minimizes the amount of dryness that can be present following the procedure. PRK may also provide an extra margin of safety in patients whose corneas have an unusual shape; this advantage is again due to leaving more of the cornea untouched by the surgery[2].

Optical coherence tomography (OCT) is a high-resolution cross-sectional imaging modality initially developed for retinal imaging. Anterior segment optical coherence tomography (AS-OCT) imaging was first described in 1994 by Izatt et al.[2], using the same wavelength of light as retinal OCT, namely 830 nm. This wavelength is suboptimal for imaging the angle due to limited penetration through the scattering tissue such as the sclera.

OCT imaging of the anterior segment with a longer wavelength of 1310 nm was developed later on and had the advantages of better penetration through the sclera as well as real-time imaging at eight frames per second[2].

Use of AS-OCT for the evaluation of corneal epithelial changes after PRK using the AS-OCT.

The aim of this study was to evaluate corneal epithelial changes after PRK by AS-OCT and to correlate these with ablation depth (AD) and refractive outcome

Evaluation of these changes is essential for understanding the postoperative wound healing and to know if these changes affect refraction or not.


  Patients and Methods Top


This was a prospective nonrandomized clinical study conducted at Menoufia University Hospital, Ophthalmology Department and Tiba Eye Center, Menoufia during the period from May 2019 to January 2020 on patients with myopia, Astigmatism, or myopic mixed astigmatism. In all 100 eyes of 50 patients with myopia, astigmatism, or myopic mixed astigmatism were included, 15 men and 35 women, for which PRK was done by Eximer laser photoablation. Inclusion criteria was male or female in the age range of 20–40 years, patients without history of diabetes, with no previous history of refractive surgery, astigmatism (0.75–3.0 D), myopia (−1.00 to − 6.00), no visual dysfunctions other than myopia, astigmatism or myopic mixed astigmatism and contact lens wear was discontinued 3 weeks prior to the examination. Exclusion criteria were male or female less than 20 years old for PRK, with a refractive error less than − 1.00 or greater than − 6.00, patients with history of diabetes, autoimmune, or immunodeficiency diseases, pregnancy or breastfeeding, keratoconus, medications, such as accutane (isotretinoin) or cordarone (amiodarone hydrochloride), and a history of keloid formation and patients with severe ocular and systemic pathologies (e.g., history of herpes keratitis, glaucoma, cataract, diabetic retinopathy, and age-related macular degeneration).

All patients were subjected to the following: first, full history taking: personal history (age, sex), chronic systemic diseases with special emphasis on diabetes mellitus as regards age of onset, duration and type of treatment, eye diseases such as glaucoma, past history of previous ocular inflammation, surgery or trauma. Second, Best corrected visual acuity (BCVA) by decimal chart. Third, Slit-lamp biomicroscopy (Topcon, Tokyo, Japan) of the anterior segment for examination of corneal clarity, lens status. Fourth, intraocular pressure measurement with Goldmann applanation tonometer (Topcon). Fifth, Fundus examination by indirect ophthalmoscopy (Heine Omega, Loni, Ghaziabad, Turkey) and slit-lamp biomicroscopy with auxiliary lens (78 D lens) for evaluation of the optic nerve head and retina.

Special investigations (corneal topography), corneal topography was performed in all cases using the Pentacam System (Wave Light Allegretto Oculyzer; Wave Light, Chicago, Illinois, USA).

AS-OCTzeiss cirrus –5000 HD-OCT) (Carl-Zeiss, Oberkochen, Germany) was done in all cases to evaluate epithelial thickness.

The surgical technique: one surgeon performed the cases (Abdel Rahman El-Sebaey)., All patients underwent PRK using wave light Allegro EX-500 Excimer laser (ALCON, Texas, 6201 South Fwy, Fort Worth, TX 76134, United States).

PRK is an outpatient surgery and takes ∼5–15 min per eye to complete.

Postoperative: after PRK is completed one drop of topical Prednisolone acetate (1%) and one drop of moxifloxacin (0.5%) were instilled and then we inserted a bandage contact lens to protect the cornea as the epithelial layer grows back over the next 3–4 days. At 2 and 3 months after surgery, manifest refraction, uncorrected visual acuity, best corrected distance visual acuity BCVA; AS-OCT was assessed.

Follow-up: on the seventh postoperative day, contact lens was removed.

Regular follow-up was conducted on the first week, 1 month, 3 months postoperatively. In each visit careful examination of the patients was done with special attention to: first, BCVA; second, slit-lamp examination (cornea: corneal clarity); third, intraocular pressure measurement; and fourth, any postoperative complications.

Anterior segment OCT: it was done postoperatively to evaluate actual epithelial thickness changes at 2 and 3 months after PRK.

Administrative and ethical design: a written informed consent was obtained from the parents of all patients of the study. The study had been approved by the Local Ethics Committee on research involving human patients of the Faculty of Medicine, (Menoufia University Ophthalmology Department).

Statistical analysis

A total of 50 patients were admitted to this study. All data were fed to the computer and analyzed using IBM SPSS software package, version 20.0. (IBM CorpRHF; Armonk, New York, USA) Qualitative data were described using number and percentage. The Kolmogorov–Smirnov test was used to verify the normality of distribution. Quantitative data were described using range (minimum and maximum), mean, and SD. Significance of the obtained results was judged at the 5% level.

The following tests used:

  1. Student's t-test: for normal quantitative variables, to compare between two studied groups
  2. Mann–Whitney test: for abnormal quantitative variables, to compare between two studied groups
  3. Spearman's correlation test: to detect correlation between epithelial thickness and AD.


A P value equal to or less than 0.05 was considered statistically significant. P value more than 0.05 was considered not significant


  Results Top


Demographic data of the studied group. Age ranged from 20 to 40 with a mean value of 30.79 ± 6.241. Male cases were 15 (30%), while female cases were 35 (70%).

Preoperative refraction data of the studied sample. Sphere diopter ranged from to 5.75–1.5 with a mean value of −3.244 ± 1.659 and cylinder diopter ranged from −2.75 to 1.5 with a mean value of −0.880 ± 1.040 while AXIS ranged from 0 to 180 with a mean value of 74.60 ± 68.306.

AD data of the studied sample ranged from 20 to 109 with a mean value of 62.42 ± 20.911.

Spherical equivalent (SE) preoperative and postoperative data of the studied sample. At preoperative it ranged from −6.25 to − 0.625 with a mean value of −3.827 ± 1.523 and there were no differences between SE postoperatively after 2 months and 3 months with a mean value of − 0.495 ± 0.405 [Table 1].
Table 1: Distribution of the studied sample according to patient's demographic data and preoperative refraction

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Change in epithelial thickness data of the studied sample: Inferior thickness after 2 months ranged from −1 to 3 with a mean value of 0.19 ± 0.790 and after 3 months it was increased with a mean of 6.31 ± 2.553. Superior thickness after 2 months ranged from − 2 to 3 with a mean value of 0.35 ± 1.021 and after 3 months it was increased with a mean of 6.23 ± 2.176.Temporal thickness after 2 months ranged from − 2 to 4 with a mean value of 0.63 ± 1.496 and after 3 months it was increased with a mean of 8.35 ± 2.436. Nasal thickness after 2 months ranged from − 2 to 7 with a mean value of 0.38 ± 1.362 and after 3 months it was increased with a mean of 6.06 ± 2.00 and central thickness after 2 months ranged from − 2 to 3 with a mean value of 0.06 ± 0.810 and after 3 months it was increased with a mean of 5.71 ± 2.221. There were statistically significant differences between postoperative values after 2 months and after 3 months [Table 2] and [Figure 1].
Table 2: Comparison between preoperative and postoperative time according to patient's spherical equivalent

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Figure 1: Graphic representation show average preoperative epithelial thickness distribution (left) and its change at 3 months postoperatively (right).

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Inferior thickness at preoperative ranged from 47 to 60 with a mean value of 56.50 ± 3.603 and at postoperative time it was increased after 3 months with a mean of 60.81 ± 4.485. Superior thickness at preoperative ranged from 47 to 63 with a mean value of 53.29 ± 4.207 and at postoperative time it was increased after 3 months with a mean of 59.52 ± 4.868. Temporal thickness at preoperative ranged from 47 to 65 with a mean value of 55.56 ± 4.105 and at postoperative time it was increased after 3 months more than the other with a mean of 63.92 ± 4.685. Nasal thickness at preoperative ranged from 47 to 60 with a mean value of 53.29 ± 4.037 and at postoperative time it was increased after 3 months with a mean of 59.33 ± 4.764 and central thickness at preoperative ranged from 49 to 59 with a mean value 53.83 ± 3.097 and at postoperative time it was increased after 3 months with a mean of 59.54 ± 3.764. There were statistically significant differences between preoperative and postoperative values [Table 3]. There was more thickening in temporal and inferior regions [Table 4].
Table 3: Comparison between postoperative times according to patient's change in epithelial thickness

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Table 4: Correlation between epithelial thickening and ablation depth

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Correlation between epithelial thickening and AD and it shows that there was statistically significant positive correlation between epithelial thickening and AD [Table 5] and [Figure 2].
Table 5: Comparison between preoperative and postoperative time according to patient's epithelial thickness

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Figure 2: Graphic representation show correlation between epithelial thickness and ablation depth.

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There was statistically significant positive correlation between epithelial thickness and changes in epithelial thickness at 3 months after PRK and no significant correlation at 2 months [Table 6].
Table 6: Correlation between epithelial thickness and change in epithelial thickness at 2 and 3 months after photorefractive keratectomy

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  Discussion Top


The corneal epithelium accounts for an average of 1.03 D of the power of the eye at the central 2.0 mm zone. This power was 0.85 D at the 3.6 mm zone, suggesting that the corneal epithelium makes an individual contribution to the prolate; the spherical nature of the cornea typically attributed to the corneal stromal surface alone[3].

In our study, we evaluated postoperative changes in epithelial thickness after PRK by using high-resolution anterior segment optical coherence tomography (HD-As-OCT).

This study was a prospective nonrandomized clinical study which included 50 patients: 35 (70%) were women and 15 (30%) were men. The age of the studied patients ranged from 20 to 40 years with a mean value of 30.79 ± 6.241 years.

The study included 50 candidates for PRK with no previous history of refractive surgery, diabetes mellitus or autoimmune diseases. Candidates were either myopes (−1.00 to − 6.00), astigmatism (0.75–3.0 D), or had myopic mixed astigmatism.

Corneal epithelial thicknesses, central and paracentral (1.0: 3.0 mm from the center), (inferiorly, superiorly, nasally, and temporally), were measured using AS-OCT preoperatively and after 2 and 3 months postoperatively.

The preoperative refractive data of the studied sample were sphere diopter ranged from − 5.75 to 1.5 with a mean value of −3.244 ± 1.659 and cylinder diopter ranged from −2.75 to 1.5 with a mean value of − 0.880 ± 1.040 while axis ranged from 0 to 180 with a mean value of 74.60 ± 68.306.

There were no differences between SEs of the studied sample postoperatively after 2 and 3 months. Preoperatively, it ranged from −6.25 to −0.625 D with a mean value of −3.827 ± 1.523 D and after 2 and 3 months postoperatively; the mean value was −0.495 ± 0.405 D so there is no correlation between changes of epithelial thickness postoperatively and SE (refraction).

There were statistically significant differences between preoperative and postoperative epithelial thickness with more thickening temporally and inferiorly. Temporal thickening was the highest.

The epithelial thickness at 2 months postoperatively returned approximately to its normal thickness as preoperative thickness.

There were statistically significant differences between postoperative epithelial thickness after 2 months and after 3 months.

The AD of the studied sample ranged from 20 to 109 μ with a mean value of 62.42 ± 20.911 μ.

There was statistically significant positive correlation between epithelial thickness and AD.

The nonuniform epithelial thickening might be explained by the aspheric ablation with increased peripheral tissue ablation compared with the center of the cornea. Therefore, the corneal epithelium may compensate for this transition zone by having a reduced thickness centrally. This was confirmed by the statistically significant positive correlation between AD and epithelial thickening after PRK in our study.

Hou et al.[4] showed that the epithelial debridement in the surface ablation caused an initial edema and nonuniform thickening in the central corneal epithelium 1 week postoperatively, and then epithelial thickness was reduced after 1 month, followed by a gradual epithelial thickening over the following 6 months. The mechanism underlying the thinning of the corneal epithelium at 1 month postoperatively may be attributed to transaction of sensory nerves and reduction of trophic modulator secretion after laser ablation.

In agreement with our results, Hou et al.[4] also reported that the epithelium thickening was characterized by a thicker epithelium inferiorly than superiorly and temporally than nasally after PRK with a maximum amount of epithelial thickening observed temporally, in accordance with the findings of Chen et al.[5].

Moreover, Chen et al.[5] showed that the epithelial thickness in all of the measured areas continued to increase between 1 and 3 months after the surgery, whereas the refractive stability was achieved by 1 month. No correlation was found between epithelial thickening and postoperative refraction change.

Ivarsen et al.[6] reported that PRK induced increase in epithelial thickness of ∼ 15–20% that persisted after surgery. Preoperative epithelial debridement caused an initial decrease in epithelial thickness, followed by a gradual epithelial thickening over the next 12 months. There was no correlation between the change in epithelial thickness and the change in refraction after PRK.

Hamberg-Nyström et al.[7] showed that the corneal epithelium was significantly thicker in eyes treated with PRK and small zone diameters (from 4.1 to 5.0 mm). The most important variables accounting for greater epithelial hyperplasia were small ablation zones, higher attempted corrections, and deeper ablations resulting in large changes in power from the center to the edge of the ablation.

Kanellopoulos and Asimellis[8] reported that the preoperative epithelium in male eyes is thicker than in female eyes. The nonuniform preoperative epithelial thickness profile is characterized by a thinner epithelium superiorly than inferiorly and temporally than nasally is also in accordance with Reinstein et al.[9] and Li et al.[10].

Sedaghat et al.[11] stated that there was a marked decrease in epithelial thickening pattern at 1 month after PRK, with gradual thickening at 3 and 6 months. Changes in epithelial thickness and SE were significant only for the paracentral peripheral zone. Repeated thickness measures before and after PRK at different follow-up times showed a significant difference in thickness separately in various zones (P < 0.001). A significant decrease in thickness was seen 1 month after PRK in all zones. Afterward, epithelial thickening continued in all zones and reached the preoperative thickness in the midperipheral and peripheral zones 6 months later, whereas the thickness in the central 5-mm zone was significantly thicker than before surgery. There was also a significant correlation between changes in SE and epithelial thickness from before to 6 months postoperatively in the paracentral and peripheral zones.

In our study, we follow up the epithelial thickness for up to 3 months so a long-term study is needed with epithelial thickness to be measured at consecutive follow-up visits.


  Conclusion Top


In this study, we assessed the epithelial thickness by AS-OCT. The present study demonstrated that a significant epithelial thickness profile change occurred after PRK due to the increase in thickness up to 3 months postoperatively, and returned to its normal preoperative thickness up to 2 months postoperatively. There was also statistically significant positive correlation between AD and epithelial thickening. The postoperative epithelial thickening did not affect the refractive outcome (no correlation between epithelial thickening and SE). This thickening may be attributed to legalized postoperative corneal stromal shape. This study showed that epithelial thickening increased temporally and nasally rather than superiorly and inferiorly. The temporal thickening was the highest.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Huang D, Eas C, Lin JS, Schuman WG, Stinson WC, Hee MR, et al. Optical coherence tomography. Science 1991; 4:1178–1181.  Back to cited text no. 1
    
2.
Izatt JA, Hee MR, Swanson EA, Lin CP, Huang D, Schuman JS, et al. Micrometer-scale resolution imaging of the anterior eye in vivo with optical coherence tomography. Arch Ophthalmol 1994; 112:1584–1589.  Back to cited text no. 2
    
3.
Gatinel D, Racine L, Hoang-Xuan T. Contribution of the corneal epithelium to anterior corneal topography in patients having myopic photorefractive keratectomy. J Cataract Refract Surg 2007; 33:1860–1865.  Back to cited text no. 3
    
4.
Hou J, Wang Y, Lei Y, Zheng X, Zhang Y. Corneal epithelial remodeling and its effect on corneal asphericity after transepithelial photorefractive keratectomy for myopia. J Ophthalmol 2016; 2016:7.  Back to cited text no. 4
    
5.
Chen X, Stojanovic A, Liu Y, Chen Y, Zhou Y, Utheim TP. Postoperative changes in corneal epithelial and stromal thickness profiles after photorefractive keratectomy in treatment of myopia. J Refract Surg 2015; 31:446–453.  Back to cited text no. 5
    
6.
Ivarsen A, Fledelius W, Hjortdal JØ. Three-year changes in epithelial and stromal thickness after PRK or LASIK for high myopia. Investig Ophthalmol Vis Sci 2009; 50:2061–2066.  Back to cited text no. 6
    
7.
Hamberg-Nyström H, Fagerholm P, Tengroth B. A comparative study of epithelial hyperplasia after PRK: summit versus VISX in the same patient. Acta Ophthalmol Scand 2009; 74:228–231.  Back to cited text no. 7
    
8.
Kanellopoulos AJ, Asimellis G. Longitudinal postoperative LASIK epithelial thickness profile changes in correlation with degree of myopia correction. J Refract Surg 2014; 30:166–171.  Back to cited text no. 8
    
9.
Reinstein DZ, Archer TJ, Gobbe M. Change in epithelial thickness profile 24 hours and longitudinally for 1 year after myopic LASIK: three-dimensional display with Artemis very high-frequency digital ultrasound. J Refract Surg 2012; 28:195–201.  Back to cited text no. 9
    
10.
Li Y, Tan O, Brass R. Corneal epithelial thickness mapping by Fourier-domain optical coherence tomography in normal and keratoconic eyes. Ophthalmology 2012; 119:2425–2433.  Back to cited text no. 10
    
11.
Sedaghat MR, Momeni-Moghaddam H, Gazanchian M. Corneal epithelial thickness mapping after photorefractive keratectomy for myopia. J Refract Surg 2019; 35:632–641.  Back to cited text no. 11
    


    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]



 

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