Journal of Refractive Surgery

Original Article Supplemental Data

Predictors of Visual Acuity Improvement and Supernormal Vision After Refined Single-Step Transepithelial Photorefractive Keratectomy

Soheil Adib-Moghaddam, MD; Ali A. Haydar, MD; Marjan Razi-Khosroshahi, MD; Saeed Soleyman-Jahi, MD, MPH; Ghazale Tefagh, MD; Michael A. Grentzelos, MD; Samuel Arba-Mosquera, MSc, PhD; George D. Kymionis, MD, PhD

Abstract

PURPOSE:

To investigate predicting factors of visual acuity improvement and achieving uncorrected (UDVA) or corrected (CDVA) distance visual acuity supernormal vision (⩾ 15/10; Snellen 20/13) 1 year after refined transepithelial photorefractive keratectomy (TransPRK).

METHODS:

In this retrospective case series, a total of 155 eyes with myopia (mean ± standard deviation = −3.53 ± 1.81 diopters) with and without astigmatism (range = 0.25 to 4.50 diopters) were recruited and observed for 1 year. Patients underwent refined single-step TransPRK using the SCHWIND AMARIS 500 laser (SCHWIND eye-tech-solutions GmbH, Kleinostheim, Germany). The main predicting factors of postoperative supernormal vision and visual acuity improvements were sought by statistical modeling.

RESULTS:

Logistic models showed mesopic contrast sensitivity, high laser fluence, and coma higher-order aberration (HOA) as predicting factors for both UDVA and CDVA super-normal vision. Myopia, astigmatism, dynamic cyclotorsion correction, optical zone, and transitional zone showed associations only in simple models and were not retained in multivariable models. According to the linear regression models, CDVA, mesopic contrast sensitivity, coma HOA, and keratometry were common predictors of both postoperative UDVA and CDVA improvement compared to preoperative UDVA. Astigmatism, optical and transitional zones, and high laser fluence were common predictors only in simple regression models.

CONCLUSIONS:

Through statistical modeling, preoperative CDVA, mesopic contrast sensitivity, coma HOA, and simulated keratometry were found to be the main factors that predicted improvement of both postoperative CDVA and UDVA compared to preoperative CDVA. Furthermore, mesopic contrast sensitivity, coma HOA, and high laser fluence were the common predictors of achieving both CDVA and UDVA supernormal vision 1 year after refined single-step TransPRK.

[J Refract Surg. 2019;35(12):771–780.]

Abstract

PURPOSE:

To investigate predicting factors of visual acuity improvement and achieving uncorrected (UDVA) or corrected (CDVA) distance visual acuity supernormal vision (⩾ 15/10; Snellen 20/13) 1 year after refined transepithelial photorefractive keratectomy (TransPRK).

METHODS:

In this retrospective case series, a total of 155 eyes with myopia (mean ± standard deviation = −3.53 ± 1.81 diopters) with and without astigmatism (range = 0.25 to 4.50 diopters) were recruited and observed for 1 year. Patients underwent refined single-step TransPRK using the SCHWIND AMARIS 500 laser (SCHWIND eye-tech-solutions GmbH, Kleinostheim, Germany). The main predicting factors of postoperative supernormal vision and visual acuity improvements were sought by statistical modeling.

RESULTS:

Logistic models showed mesopic contrast sensitivity, high laser fluence, and coma higher-order aberration (HOA) as predicting factors for both UDVA and CDVA super-normal vision. Myopia, astigmatism, dynamic cyclotorsion correction, optical zone, and transitional zone showed associations only in simple models and were not retained in multivariable models. According to the linear regression models, CDVA, mesopic contrast sensitivity, coma HOA, and keratometry were common predictors of both postoperative UDVA and CDVA improvement compared to preoperative UDVA. Astigmatism, optical and transitional zones, and high laser fluence were common predictors only in simple regression models.

CONCLUSIONS:

Through statistical modeling, preoperative CDVA, mesopic contrast sensitivity, coma HOA, and simulated keratometry were found to be the main factors that predicted improvement of both postoperative CDVA and UDVA compared to preoperative CDVA. Furthermore, mesopic contrast sensitivity, coma HOA, and high laser fluence were the common predictors of achieving both CDVA and UDVA supernormal vision 1 year after refined single-step TransPRK.

[J Refract Surg. 2019;35(12):771–780.]

Supernormal vision is significantly enhanced visual acuity superior to that achieved with conventional methods of correction (glasses, contact lenses, and traditional refractive surgeries).1 It is an important feature for best performance in occupations such as target discrimination, firing weapons, aviation, night vision, and medical and surgical fields. The rapidly expanding technologies suggest that current visual performance might not meet the needs of the future. The first attempts to achieve supernormal vision began as early as the 1990s by investigating its possible determinant factors.1–6

Achieving postoperative supernormal vision is challenged by ocular factors, uncontrolled optical changes during the wound healing process, limitations of surgical equipment, and intractable surgeon variables.3 Studies suggested that preoperative corrected distance visual acuity (CDVA), total spherical aberration induction, pre-operative mixed astigmatism, higher myopia, and high relative humidity correlate with visual acuity after laser in situ keratomileusis (LASIK).7–9 Age and attempted correction were proposed to independently associate with a reduced likelihood of achieving 20/40 UDVA and decreased predictability in photorefractive keratectomy (PRK).10 A negative impact of high relative humidity on LASIK in terms of undercorrection has been reported.8,9

After refractive surgery, aberrations, diffraction effects, scattered light, and errors of focus due to any surface irregularity may impair visual acuity.2,11 Correction of higher-order aberrations (HOAs) could lead to supernormal vision after laser-assisted surgeries.1,3 Correcting monochromatic wavefront aberrations was reported to proportionately improve visual acuity of normal eyes.12 Yet, not every surgery addressing HOAs has resulted in supernormal vision. Moreover, there are many unaddressed potential confounding and modifying factors that may substantially distort currently proposed interrelations. No study has conducted a consolidated approach to investigate a comprehensive list of candidates' contributing factors.

Since the 1990s, transepithelial photorefractive keratectomy (TransPRK) has been introduced as a potential alternative to other laser-assisted refractive techniques that involve mechanical manipulation of the epithelium.13 It also may prevent LASIK's flap-related complications and ectasia.14 In this technique, a laser is used to ablate both the epithelium and stroma without any mechanical or chemical insult. Both epithelial and stromal ablation occur in a single continuous session in the more recent single-step TransPRK platform, explaining its promising results compared with the two-step platform.15 This was more pronounced in results of refined single-step TransPRK for different types of refractive error.16–18 The refined method includes modification in the laser nomogram based on each patient's demographic and visual parameters and makes use of the Iran regimen.15 No previous studies have investigated the main predictors of supernormal vision after any modality of TransPRK.

In this study, we conducted statistical modeling of a comprehensive list of patients' personal, surgical, and optical parameters to unravel the independent predictors of achieving supernormal vision, defined as UDVA of 15/10 or worse (Snellen 20/13) after refined single-step TransPRK.

Patients and Methods

This study was conducted as a chart review retrospective case series. Eyes with myopia and astigmatism were recruited from Bina Eye Hospital from 2012 to 2015. A written informed consent from the patients and institutional review board approval from Shahid Beheshti University of Medical Sciences were obtained. The inclusion criterion was mild to high myopia with or without astigmatism that had been stable for at least 12 months. Any patient with concomitant eye disease, severe dry eye, systemic disease with ophthalmic involvement, history of corneal or ocular surgery, retinal disorders, and keratoconus was excluded from the study. All patients were asked to stop wearing hard or soft contact lenses for 4 weeks prior to surgery.

The following preoperative optical parameters were determined for all of the patients: spherical equivalent, astigmatism, UDVA, CDVA, photopic and mesopic contrast sensitivities (M&S Smart System 20/20; M&S Technologies Inc., Niles, IL), keratometry and topography with Scout (Optikon 2000 SPA, Rome, Italy) and Orbscan (Bausch & Lomb, Rochester, NY), pupil diameter in a photopic condition of 270 lux (Keratron Scout Corneal Analyser; Optikon, Rome, Italy), ocular wavefront (OWF) HOA (ORK Wavefront Analyzer; SCHWIND eye-tech-solutions GmbH, Kleinostheim, Germany), and anterior corneal wavefront (CWF) HOAs at 4 and 6 mm pupil diamters (Keratron Scout Corneal Analyser; Optikon, Rome, Italy). Total root mean square (RMS) of 3rd and 4th order OWF and CWF aberrations were also considered. In contrast sensitivity assessment, the testable contrast ranges from 0.0 to 2.5 log units, with each level corresponding to a change of 0.1 log units. Finally, one number is reported as the contrast sensitivity of the examined eye. One-year postoperative visual parameter data were also collected.

To assess UDVA and CDVA, the patients were asked to remove their glasses and sit 13 feet (4 meters) from the Smart System M&S LCD chart. A trial frame was gently placed and fixed on the patient's face. Enough illumination was provided on the chart and in the examination room to make sure that the patients could see the letters well and the pupil diameter was in the normal range. After occluding the non-test eye with a plain occluder, the test eye was assessed. We asked the patients to stand at 6 meters from the chart and not to make any efforts to focus. The optotypes in full line basis were read from the largest to the smallest (continuing up to 20/10) and the smallest line fully read was considered to be the visual acuity. In case of CDVA, the patients were provided with the best optical correction for each eye based on subjective cyclorefractions and objective refractions.

Surgical Procedure

All eyes underwent refined single-step TransPRK by the same surgeon (SA-Moghaddam) as previously described.15,17–19 The procedures were done using the SCHWIND AMARIS 500 laser (SCHWIND eye-tech-solutions GmbH). Each eye was irrigated with 60 cc of chilled balanced salt solution. Epithelial and stromal ablations elapsed in a single continuous session. In the refined technique, some modifications are applied in surgical parameters and in the postoperative medical regimen and eye care. Determination of target ablation and optical zone was individualized for each eye based on personal and optical characteristics. Mitomycin C 0.02% was only applied up to 14 seconds in eyes with myopia of greater than 6.00 diopters (D).19 The Iran regimen was prescribed for all patients postoperatively.15 In this regimen, topical nonsteroidal anti-inflammatory drugs and steroids are not used until the third postoperative day, low-dose topical steroids are prescribed for a longer duration thereafter, and patients are also prescribed vitamin C (ascorbic acid). These modifications are considered to accelerate corneal defect recovery, prevent corneal haze induction, and reduce pain after ablation.

Statistical Analysis

We used the Hosmer and Lemeshow analysis approach to fit the final multivariable regression models and identify the independent factors that predict the chance of a patient gaining lines of postoperative UDVA or CDVA supernormal vision (≥ logMAR −0.176 equivalent to 20/13 or 15/10) or visual acuity improvement. This cut-off was based on mean ± standard deviation of postoperative visual acuities. We also fit linear regression models to determine predicting factors of postoperative UDVA or CDVA improvements, both compared to preoperative CDVA. First, simple logistic or linear models were used to screen all candidate parameters and spot the potential parameters associated with a chance of achieving supernormal vision. In this stage of the analysis, parameters with a screening P value of .20 or less were eligible to enter the final multivariable model. Next, we built the primary multivariable model consisting of the variables spotted in the screening stage. A backward elimination method was used to determine the final independent parameters. In this stage, only parameters that added to fitness of the model and/or those with P values of .05 or less were kept in the final method. We also checked the possible interaction terms among the finally retained parameters. Once the final multivariable models were fit, we focused only on common predictors of UDVA-and CDVA-based indices. This approach was considered because factors that predict individually UDVA-or CDVA-based indices will probably be of less clinical relevance.

We first used each parameter in its original continuous scale. If any parameter survived the model, we also tried other categorical scales of the same variable in the final model. If other scales of the variable significantly added to fitness of the final model, we retained the corresponding scale. Other scales were usually generated based on the 50th with/without the 25th and 75th percentiles of the variable.

Results

Initially, 163 eyes were enrolled in this study, but 155 eyes (95%) completed all follow-up visits until 1 year after the surgery and were included in the evaluation. The mean age of the patients was 28.4 ± 5.5 years (range: 19 to 48 years); 112 (72.3%) eyes belonged to females and the remaining 43 (27.7%) belonged to males. The mean preoperative spherical equivalent and astigmatism values were −3.97 ± 1.81 and −1.16 ± 1.02 D, respectively (Table 1 and Table A, available in the online version of this article).

Patient Data

Table 1:

Patient Data

Descriptive Data and Associations of Preoperative HOAs With Postoperative Visual Acuity Indices

Table A:

Descriptive Data and Associations of Preoperative HOAs With Postoperative Visual Acuity Indices

Figure 1 shows the visual outcomes. One year postoperatively, 57 eyes (37%) achieved UDVA of 20/13 or better (equivalent of 15/10 as supernormal vision). UDVA of 20/20 or better was detected in 145 (93.5%) eyes. No eye lost two or more preoperative CDVA decimal lines and 6 eyes (3.8%) lost one CDVA line. Mean efficacy and safety indices were 1.18 ± 0.28 and 1.25 ± 0.18, respectively. Mean differences of postoperative UDVA and CDVA with preoperative CDVA were 0.16 ± 0.29 and 0.24 ± 0.20 decimal lines, respectively. Furthermore, ±0.50 and ±1.00 D predictabilities were 98.7% and 99.3%, respectively.

Standard graphs showing refractive and visual outcomes. D = diopters; UDVA = uncorrected distance visual acuity; CDVA = corrected distance visual acuity

Figure 1.

Standard graphs showing refractive and visual outcomes. D = diopters; UDVA = uncorrected distance visual acuity; CDVA = corrected distance visual acuity

Multivariable logistic models revealed that eyes with mesopic contrast sensitivities of median or better have approximately five-fold higher chances of achieving UDVA (odds ratio [OR] = 4.83, P = .02) and CDVA (OR = 4.95, P = .002) supernormal vision (Tables AB, available in the online version of this article, Figure 2). Each 0.1 µm increment of preoperative OWF coma resulted in a three- to two-fold decrease in the chance of UDVA (OR = 0.32, P = .001) and CDVA (OR = 0.52, P = .02) supernormal vision. Using higher amounts of high laser fluence during surgery resulted in an average of 4% per mJ/cm2 decreased probability to achieve UDVA and CDVA (OR = 0.96, P = .002 for both) super-normal vision (Table B, Figure 2).

Common Predictors of UDVA- and CDVA-Based SNV Extracted From Simple and Multivariable Logistic Regression Models

Table B:

Common Predictors of UDVA- and CDVA-Based SNV Extracted From Simple and Multivariable Logistic Regression Models

Common predicting factors of two multivariable logistic regression models fit for postoperative uncorrected (UDVA) and corrected (CDVA) distance visual acuity ⩾ 20/13 (15/10 supernormal vision [SNV]) in eyes undergoing refined single-step transepithelial photorefractive keratectomy. Other factors of the multivariable models that could predict individually UDVA- or CDVA-based SNV, but not both, are excluded. MCS = mesopic contrast sensitivity (subgroup of eyes with MCS equal or better than median was compared with subgroup of eyes with MCS less the median); HOA = higher-order aberration (per 0.1 µm increments); HF = high fluence (mJ/cm2); OR = odds ratio

Figure 2.

Common predicting factors of two multivariable logistic regression models fit for postoperative uncorrected (UDVA) and corrected (CDVA) distance visual acuity ⩾ 20/13 (15/10 supernormal vision [SNV]) in eyes undergoing refined single-step transepithelial photorefractive keratectomy. Other factors of the multivariable models that could predict individually UDVA- or CDVA-based SNV, but not both, are excluded. MCS = mesopic contrast sensitivity (subgroup of eyes with MCS equal or better than median was compared with subgroup of eyes with MCS less the median); HOA = higher-order aberration (per 0.1 µm increments); HF = high fluence (mJ/cm2); OR = odds ratio

In addition to these common predicting factors retained in multivariable models, age (OR < 1), degrees of myopia (OR < 1) and astigmatism (OR < 1), preoperative CDVA (OR > 1), RMS of CWF HOA at 4-mm pupil (OR < 1), ablation depth (OR < 1), degrees of ocular cyclotorsion during surgery (OR < 1), and optical (OR > 1) and transitional (OR < 1) zones of ablation were common predicting factors of achieving both UDVA and CDVA supernormal vision only in simple regression models of the screening stage. In each case, an OR of greater than or less than 1 indicated associations of corresponding parameter with higher and lower changes of achieving visual acuity supernormal vision, respectively. After adjusting for other contributing factors, these predictors were not retained in final multivariable models (Tables AB, Figure 3).

Common predicting factors spotted in simple (univariate) logistic regression models conducted for postoperative uncorrected (UDVA) and corrected (CDVA) distance visual acuity ⩾ 20/13 (15/10 supernormal vision [SNV]) in eyes undergoing refined single-step transepithelial photorefractive keratectomy. Other factors that could predict individually UDVA- or CDVA-based SNV, but not both, are not shown. OR = odds ratio; MCS = mesopic contrast sensitivity; HOA = higher order aberration; HF = high fluence; RMS = total root mean square of 3rd and 4th order corneal wavefront HOAs at 4-mm pupil size; AD = ablation depth; DC = dynamic cyclotorsion; OZ = optical zone; TZ = transitional zone

Figure 3.

Common predicting factors spotted in simple (univariate) logistic regression models conducted for postoperative uncorrected (UDVA) and corrected (CDVA) distance visual acuity ⩾ 20/13 (15/10 supernormal vision [SNV]) in eyes undergoing refined single-step transepithelial photorefractive keratectomy. Other factors that could predict individually UDVA- or CDVA-based SNV, but not both, are not shown. OR = odds ratio; MCS = mesopic contrast sensitivity; HOA = higher order aberration; HF = high fluence; RMS = total root mean square of 3rd and 4th order corneal wavefront HOAs at 4-mm pupil size; AD = ablation depth; DC = dynamic cyclotorsion; OZ = optical zone; TZ = transitional zone

After adjusting for preoperative CDVA, multivariable linear regression models showed that mesopic contrast sensitivity predicted further improvements of both postoperative UDVA (βs = 0.24, P < .001) and CDVA (βs = 0.20, P < .001) compared to preoperative CDVA. Preoperative CWF coma HOA at 4-mm pupil (βs = −0.11, P < .001) and OWF coma HOA (βs = −0.19, P = .03) revealed inverse correlations with CDVA and UDVA improvements, respectively. Furthermore, preoperative simulated keratometry values were in favor of both UDVA (βs = 0.06, P < .001) and CDVA (βs = 0.07, P < .001) improvements compared to preoperative CDVA (Figure 4, Table C, available in the online version of this article).

Common predicting factors of two multivariable linear regression models fit for improvements of postoperative uncorrected (UDVA) and corrected (CDVA) distance visual acuity compared to preoperative CDVA in eyes undergoing refined single-step transepithelial photorefractive keratectomy. Other factors of the multivariable models that could predict individually UDVA- or CDVA-based improvements, but not both, are excluded. VA = visual acuity; βs = standardized beta; MCS = mesopic contrast sensitivity (subgroup of eyes with MCS equal or better than median was compared with subgroup of eyes with MCS less the median); HOA = higher-order aberration (per 0.1 µm increments); Sim Kerat = simulated keratometry (diopters)

Figure 4.

Common predicting factors of two multivariable linear regression models fit for improvements of postoperative uncorrected (UDVA) and corrected (CDVA) distance visual acuity compared to preoperative CDVA in eyes undergoing refined single-step transepithelial photorefractive keratectomy. Other factors of the multivariable models that could predict individually UDVA- or CDVA-based improvements, but not both, are excluded. VA = visual acuity; βs = standardized beta; MCS = mesopic contrast sensitivity (subgroup of eyes with MCS equal or better than median was compared with subgroup of eyes with MCS less the median); HOA = higher-order aberration (per 0.1 µm increments); Sim Kerat = simulated keratometry (diopters)

Common Predictors of Postoperative UDVA and CDVA Improvement Compared to Preoperative CDVA, Extracted From Simple and Multivariable Linear Regression Models

Table C:

Common Predictors of Postoperative UDVA and CDVA Improvement Compared to Preoperative CDVA, Extracted From Simple and Multivariable Linear Regression Models

In addition to these factors, preoperative UDVA (βs < 0), astigmatism (βs < 0), RMS of CWF HOA at 4-mm pupil (βs < 0), CWF secondary astigmatism HOA at 6-mm pupil (βs < 0), high fluence of laser ablation (βs < 0), and optical (βs > 1) and transitional (βs < 0) zones of ablation were identified as common predicting factors of both UDVA and CDVA improvements in the screening stage; however, they were not retained in the final models (Table A, Table C, Figure 5). In each case, βs values of greater than or less than 0 indicated associations of corresponding parameter with higher and lower improvements of postoperative visual acuity, respectively. Subanalysis based on the intraoperative mitomycin C application did not reveal different results in any of the models discussed above.

Common predicting factors spotted in simple (univariate) linear regression models conducted for improvements of postoperative uncorrected (UDVA) and corrected (CDVA) distance visual acuity compared to preoperative CDVA in eyes undergoing refined single-step transepithelial photorefractive keratectomy. Other factors that could predict individually UDVA- or CDVA-based improvement, but not both, were not shown. VA = visual acuity; βs = standardized beta; MCS = mesopic contrast sensitivity; HOA = higher-order aberration; Sim Kerat = simulated keratometry; RMS = total root mean square of 3rd and 4th order corneal wavefront HOAs at 4mm pupil size; Sec Ast HOA = secondary astigmatism corneal wavefront HOA at 6-mm pupil size; OZ = optical zone; TZ = transitional zone; HF = high fluence

Figure 5.

Common predicting factors spotted in simple (univariate) linear regression models conducted for improvements of postoperative uncorrected (UDVA) and corrected (CDVA) distance visual acuity compared to preoperative CDVA in eyes undergoing refined single-step transepithelial photorefractive keratectomy. Other factors that could predict individually UDVA- or CDVA-based improvement, but not both, were not shown. VA = visual acuity; βs = standardized beta; MCS = mesopic contrast sensitivity; HOA = higher-order aberration; Sim Kerat = simulated keratometry; RMS = total root mean square of 3rd and 4th order corneal wavefront HOAs at 4mm pupil size; Sec Ast HOA = secondary astigmatism corneal wavefront HOA at 6-mm pupil size; OZ = optical zone; TZ = transitional zone; HF = high fluence

Discussion

Exploiting a comprehensive and consolidated statistical modeling, we found that mesopic contrast sensitivity, coma HOA, and high laser fluence could predict the likelihood of achieving both UDVA and CDVA supernormal vision 1 year after refined single-step TransPRK. Linear regression models revealed that mesopic contrast sensitivity, coma HOA, simulated keratometry, and CDVA could predict improvements of both postoperative UDVA and CDVA compared to preoperative CDVA. In addition to these predictors, simple regression models of the screening stage introduced other common predictors for UDVA- and CDVA-based indices. However, they were not retained in final multivariable models (Tables BC).

This is the first study evaluating predictors of achieving supernormal vision after TransPRK. Although previous investigations of other modalities of refractive surgery have mainly focused on the effects of HOAs,20 contrast sensitivity,21 and pupil size22 on postoperative visual acuity, this is the first study investigating wide-ranging personal, surgical, and optical parameters. In the study by Lipshitz in 2002,3 several key factors mainly attributed to inherent physiological differences, changes in optical biomechanical properties, uncontrolled corneal reepithelialization, technological limitations, and surgeon-dependent uncontrollable variables were presented as challenges in achieving supernormal vision after an excimer laser corneal surgery. The authors reported 15% of patients achieved UDVA of 20/15 or better (13/10) after standard spherical and astigmatic correction. In our study, 37% of patients achieved a UDVA of 15/10 or better.

We will discuss the predictors of visual acuity outcomes in two categories: patient-related predictors that could help surgeons advise their patients about probable outcomes of surgery and procedure-related predictors that could be considered and modified (in case of some factors) by the surgeons to improve surgery outcomes.

Patient-Related Predictors

Distance and chromatic aberrations are the principal factors that attenuate retinal image quality. Theoretically, our biological receptors are naturally limited between 20/8 and 20/10.23 Therefore, correcting HOAs may potentially improve visual performance. Several studies have investigated the impact of monochromatic aberrations on refractive status. Some suggested that more myopic eyes would have higher amounts of aberrations.24 Other studies reported no significant difference in amounts of aberrations between eyes with myopia, hyperopia, and emmetropia.6,25 This dilemma could be explained by differences in the age, population, and methodologies of these studies.

The impact of HOAs on visual performance is not clear. Seiler et al.26 reported an increase in total wavefront aberrations in myopic eyes undergoing PRK, especially for eyes with a pupil diameter of greater than 5 mm. In addition, the increase observed in wavefront error was significantly correlated with the loss in CDVA, low-contrast visual acuity, and glare vision. Another study on LASIK proposed that greater amounts of myopia correction lead to increased ocular HOAs, compromising the postoperative contrast sensitivity ability.21

Accordingly, customized corneal ablation, such as wavefront-guided refractive surgery, was anticipated to correct or minimize HOAs in a step toward super-normal vision.4,5 However, Levy et al.27 reported similar amounts of ocular HOAs in eyes with natural supernormal vision (UDVA > 20/16 or 12/10) and those undergoing refractive surgery. Another study20 reported no statistically significant difference in spherical and coma aberrations between eyes that gained super-normal vision after wavefront-guided laser epithelial keratomileusis and those with natural supernormal vision (UDVA > 20/16). The mean RMS results for all orders were higher in LASEK eyes only at a pupil diameter of greater than 4 mm. They agreed that laser refractive surgery does induce HOAs, but this increase does not impact the performance. In our study, coma HOA reduced the likelihood of achieving UDVA and CDVA supernormal vision and predicted less improvement of visual acuity. The total RMS of CWF HOAs showed a negative impact on visual acuity outcomes in simple regression models. However, the latter association was not retained in multivariable models. Considering the possible role of minimizing HOAs using customized corneal ablation on the chance of achieving supernormal vision following refractive surgery could help surgeons decide about the modality of choice for their patients.

Refractive surgeons need to know the effects of preoperative visual indices on postoperative visual acuity when consulting their patients. A study on customized LASIK reported that patients with lower than average preoperative visual acuity will continue to gain acuity, whereas patients with better than average preoperative visual acuities may remain the same or even lose acuity.28 Furthermore, improvements in visual acuity after corneal cross-linking increase with lower preoperative CDVA values.29 Similarly, we found that eyes with higher preoperative CDVA show lower net improvements in postoperative CDVA or UDVA. In addition, regression to the mean artifact combined with removal of the minimization effect of the correcting spectacles could explain our findings concerning the role of preoperative CDVA.

Refractive surgeons prefer a postoperative keratometry value of greater than 35.00 D (and less than 50.00 D) to avoid any deteriorations in spherical aberration, contrast sensitivity, and night vision.3,30 Clinically, the change in keratometry needed to reach a diopter of myopic correction is inversely proportional to the amount of refractive correction. In our study, higher preoperative keratometry value was associated with greater postoperative CDVA and UDVA improvement compared to preoperative CDVA.

According to the current literature,31 a spherical error of ±0.50 D or a cylindrical error of ±1.00 D will reduce visual acuity by roughly half the maximum Snellen fraction. Studies have shown myopic correction and the amount of it to have deteriorative effects on postoperative subjective and objective visual functions and to induce HOAs.21,32 Although patients with higher preoperative refractive errors will show higher net postoperative improvements, those with lower refractive errors will gain a better visual acuity.7 We showed less chance of achieving postoperative supernormal vision in patients with higher preoperative myopia and astigmatism. Furthermore, higher astigmatism had a negative impact on visual acuity improvement. These predictive roles observed for myopia and astigmatism in simple regression models were not retained in the final multivariable models. The predictive role of astigmatism could be argued by considering the probable confounding effect of ocular residual astigmatism (ORA). Although cylindrical error does not necessarily correlate with ORA, higher ORA has been linked to unpredictable postoperative refraction correction.33

Procedure-Related Predictors

Based on the dual fluence concept,34 approximately 80% of corneal ablation is made with a high fluence level that speeds up the correction. Low fluence level is implemented for fine correction, hence maximizing resolution. Therefore, the timing of the laser procedure will be significantly shorter without adverse effects on safety and precision, which is favorable in high myopic corrections. However, in this study on mild to moderately myopic eyes, we found that high laser fluence has deteriorative effects on postoperative visual acuity.

In theory, high fluence induces a wide zone of burn damage and increases fragmentation of tissue in the corneal stroma.35 High fluence photoablation is linked to thermal vaporization, shock wave effects, and surface plasma that might justify any mechanical damage to deeper tissue. Yet, stromal undulations or corneal endothelium damage have not always been noticed. It is argued that using fluence above a threshold of 600 to 1,200 mJ/cm2 per pulse may not induce greater amounts of tissue ablation; yet, thermal and mechanical damages will ensue. Fluence of the ablative laser beam is not directly determined by the surgeon. Instead, the laser platform is programmed to deliver an optimum amount of energy to the surface unit of the ablation area in different configurations of the ablation plan. Therefore, considering the pros and cons of high fluence ablation could be helpful in programming laser platforms and choosing the best ablation protocol for each eye based on its myopic degree. Other than ablation plan configurations, the fluence can be influenced by ambient factors such as the temperature and humidity of the operation room.

Many studies have shown a correlation between optical zone and refractive outcomes, reporting a 6-mm optical zone to provide better visual outcomes compared to a 5-mm optical zone.36,37 The microsurface irregularities are fairly uniform in ablations over a 6-mm optical zone with less impact on functional vision. Adjusting for other contributing factors diminished the initial common predictive role of the optical zone for visual acuity outcomes in our study. However, the optical zone retained its positive effect on postoperative CDVA.

The simple regression models in our study showed that ocular cyclotorsion during surgery has deteriorative effects on postoperative visual indices. Although this effect was not retained after adjusting for other contributing factors (eg, HOAs) in multivariable models, it is supported by previous studies reporting detrimental effects of cyclotorsion through inducing postoperative residual astigmatism and HOAs.38 We have previously investigated the correlates (eg, age, visual function, and axis) of cyclotorsion in eyes undergoing single-step TransPRK.39 Considering these predisposing factors could help delineate patients at higher risk of cyclotorsion and its potential detrimental effects.

Due to the smaller laser spot size presented by the SCHWIND AMARIS platform and the dual fluence system, single-step TransPRK effectively minimizes the irregularities of the remaining stromal bed, which could effectively accelerate corneal reepithelialization. It is also equipped with a high-tech laser ablative system with integrated intelligent thermal effect control that ameliorates the detrimental effects of high laser fluence on supernormal vision.15 Moreover, in a study by Adib-Moghaddam et al.,40 intraoperative ambient temperature and humidity levels did not remarkably influence visual outcome in TransPRK. In the refined variant, we took advantage of an aberration-free mode that reduced corneal tissue loss that occurs in other aberration-guided modes.15

In this study, we have only evaluated an objective index of visual function. Assessment of subjective visual indices, such as information obtained from questionnaire-based data, could shed additional light on clinical and practical aspects of supernormal vision. Moreover, we have only included a single ethnic population. Future investigations on multi-ethnic populations could produce outcomes that are more generalizable and reliable. Finally, we primarily intended to study predicting factors of achieving supernormal vision after refined single-step TransPRK. Lack of comparison with other refractive platforms would not remarkably affect our main goal in this study.

We have investigated surgical and inherent optical parameters capable of predicting postoperative visual acuity index supernormal vision (visual acuity ≥ 15/10) and improvement of postoperative UDVA and CDVA compared to preoperative CDVA in eyes undergoing refined single-step TransPRK. Our findings could help surgeons predict postoperative outcomes more precisely and therefore improve case selection in refined single-step TransPRK. It also could provide clinical clues to further improve efficacy of surgical laser platform. Further studies on multi-ethnic populations, examining objective and subjective visual function indices, and integrating neuroscience approaches could provide more robust evidence.

References

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Patient Data

VariableMean ± SDRange
Inherent
  Age (y)28.4 ± 5.50(19 to 48)
  SimK max (D)44.94 ± 1.61(41 to 50)
  SimK min (D)43.67 ± 1.52(40.10 to 47.50)
  Central corneal thickness (µm)551.01 ± 35.89(471 to 633)
  Pupil diameter (mm)4.50 ± 0.73(3.01 to 6.60)
  Spherical equivalent (D)−3.97 ± 1.81(−9.62 to −0.87)
  Astigmatism (diopter)−1.16 ± 1.02(−4.50 to −0.25)
  UDVA (logMAR)0.52 ± 0.34(−0.08 to 1.00)
  CDVA (logMAR)−0.02 ± 0.08(−0.18 to 0.30)
  Kappa axis angle (degrees)5.22 ± 1.30(2.38 to 9.76)
  Kappa axis location (degrees)250.68 ± 85.94(1 to 360)
  Photopic CS (log unit)0.94 ± 0.29(0.2 to 2.5)
  Mesopic CS (log unit)0.96 ± 0.38(0.1 to 2.5)
Procedure-related
  Optical zone (mm)6.69 ± 0.36(6.1 to 7.80)
  Transitional zone (mm)1.41 ± 0.53(0.39 to 2.50)
  Absolute SCC (degrees)2.62 ± 2.33(0 to 10.1)
  DCC range (degrees)2.96 ± 1.91(0 to 9.61)
  Central ablation (µm)136.90 ± 31.25(49.33 to 202.39)
  Temperature (°C)23.93 ± 1.23(20.4 to 26.5)
  Humidity (%)27.82 ± 5.81(14 to 47)
  Low fluence (mJ/cm2)186 ± 36(155 to 274)
  High fluence (mJ/cm2)347 ± 51(272 to 540)

Descriptive Data and Associations of Preoperative HOAs With Postoperative Visual Acuity Indices

HOAAssociations With Postoperative Visual Acuity Indices

Mean ± SD (µm)UDVA SNVCDVA SNVPostop UDVA – Preop CDVAPostop CDVA – Preop CDVA
Ocular wavefront
  Coma0.14 ± 0.07OR = 0.61, P = .07aβs = −0.1, P = .08a
  Trefoil0.12 ± 0.09
  RMS0.28 ± 0.14OR = 0.64, P = .17a
Corneal wavefront in 4-mm pupil
  Coma0.13 ± 0.09OR = 0.64, P = .09aβs = −0.05, P = .17a
  Trefoil0.12 ± 0.08OR = 0.72, P = .17a
  Spherical0.12 ± 0.07
  Cylindrical0.03 ± 0.03
  Tetrafoil0.04 ± 0.06βs = −0.1, P = .15aβs = −0.1, P = .03a
  RMS0.22 ± 0.11OR = 0.46, P = .12aOR = 0.33, P = .04aβs = −0.1, P = .09aβs = −0.05, P = .10a
Corneal wavefront in 6-mm pupil
  Coma0.19 ± 0.09
  Trefoil0.14 ± 0.08
  Spherical0.23 ± 0.06
  Cylindrical0.05 ± 0.04OR = 0.49, P = .17aβs = −0.1, P = .18aβs = −0.1, P = .12a
  Tetrafoil0.04 ± 0.04
  RMS0.32 ± 0.08OR = 0.5, P = .16aβs = −0.12, P = .04a

Common Predictors of UDVA- and CDVA-Based SNV Extracted From Simple and Multivariable Logistic Regression Models

Common PredictorsSimple Regression Model (P < .20)Multivariable Regression Modeld


UDVA SNVCDVA SNVUDVA SNVCDVA SNV




ORPORPORPORP
Mesopic CSa,b3.06.0041.95.044.83.024.95.002
Coma HOAa,c (per 0.1 µm increments)0.61.070.64.090.32.0010.52.02
High fluence (mJ/cm2)a0.98.020.98.0010.96.0020.96.002
Age (year)0.92.020.91.01
Myopia (absolute diopter)0.67.0010.74.003
Astigmatism (absolute diopter)0.47.010.40.001
CDVA (per 1/10 decimal increments)1.46.011.6< .001
RMS CWF HOA 4-mm (per 0.1 µm increments)0.46.120.33.04
Ablation depth (µm)0.98.0020.98.01
Dynamic cyclotorsion (degrees)0.89.170.83.09
Optical zone (mm)3.7.019.97< .001
Transitional zone (mm)0.24< .0010.32.001

Common Predictors of Postoperative UDVA and CDVA Improvement Compared to Preoperative CDVA, Extracted From Simple and Multivariable Linear Regression Models

Common PredictorsUnivariate Regression Analysis (P < .20)Multivariable Regression Analysisd


UDVA ImprovementCDVA ImprovementUDVA ImprovementCDVA Improvement




βsPβsPβsPβsP
CDVA (per 1/10 decimal increments)a−0.05< .001−0.04< .001−0.07< .001−0.07< .001
Mesopic CSa,b0.12.190.17.100.24< .0010.2< .001
Coma HOAa,c (per 0.1 µm increments)−0.16.08−0.12.17−0.19.03−0.11< .001
Simulated keratometry (D)a0.12.190.16.080.06< .0010.07< .001
UDVA (per 1/10 decimal increments)−0.02.03−0.01.19
Astigmatism (absolute diopter)−0.14.08−0.25.005
RMS CWF HOA 4-mm (per 0.1 µm increments)−0.10.09−0.05.10
CWF secondary astigmatism 6-mm (µm)−0.11.18−0.13.10
Optical zone (mm)0.15.060.32< .001
Transitional zone (mm)−0.18.04−0.16.05
High fluence (mJ/cm2)−0.14.09−0.21.01
Authors

From Bina Eye Hospital, Tehran, Iran (SA-Moghaddam, AAH, MR-K, GT); TransPRK Research Group, Tehran, Iran (SA-Moghaddam, AAH, MR-K, GT); Universal Council of Ophthalmology, Universal Scientific Education and Research Network, Tehran, Iran (SA-Moghaddam, AAH, MR-K, SS-J, GT, SArba-Mosquera); School of Medicine, Tehran University of Medical Sciences, Tehran, Iran (AAH, SS-J); Student Research Committee, School of Medicine, Iran University of Medical Sciences, Tehran, Iran (MR-K); Vardinoyiannion Eye Institute of Crete, Faculty of Medicine, University of Crete, Heraklion, Crete, Greece (MAG); SCHWIND eye-tech-solutions, Kleinostheim, Germany (SA-Mosquera); University of Valladolid, Valladolid, Spain (SA-Mosquera); the Department of Ophthalmology, Faculty of Medicine, University of Athens, Athens, Greece (GDK); and Jules-Gonin Eye Hospital, University of Lausanne, Lausanne, Switzerland (GDK).

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (SAdib-Moghaddam, SS-J, MAG, GDK); data collection (SAdib-Moghaddam, AAH, MR-K, GT, SArba-Mosquera); analysis and interpretation of data (SAdib-Moghaddam, AAH, MR-K, SS-J, GT); writing the manuscript (AAH, MR-K, SS-J, GT); critical revision of the manuscript (SAdib-Moghaddam, AAH, MR-K, SS-J, GT, MAG, SArba-Mosquera, GDK); statistical expertise (SS-J); administrative, technical, or material support (SAdib-Moghaddam, AAH, MR-K, SS-J, GT, MAG, SArba-Mosquera, GDK); supervision (SAdib-Moghaddam, SS-J, SArba-Mosquera)

Correspondence: Soheil Adib-Moghaddam, MD, Bina Eye Hospital, Resalat Highway, Tehran 1634764651, Iran. E-mail: Soheil.adibmoghaddam@gmail.com

Received: June 11, 2019
Accepted: October 25, 2019

10.3928/1081597X-20191025-01

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