Repeatability of Epithelium Thickness Measured by an AS-OCT in Different Grades of Keratoconus and Compared to AS-OCT/Placido Topography

PURPOSE

To compare agreement of corneal epithelium thickness (ET) between AS-OCT system (RTVue, Optovue) and AS-OCT/Placido topographer (MS-39, CSO) in eyes with different stages of keratoconus (KC), and to assess the repeatability of RTVue AS-OCT.

DESIGN

Prospective reliability analysis.

METHODS

KC eyes were classified into forme fruste KC (FFKC), mild, moderate, and severe KC. Agreement was evaluated with Bland-Altman plots and 95% limits of agreement (LoA). The repeatability of RTVue was assessed via within-subject standard deviation (Sw), test-retest variability (TRT), coefficient of variation (CoV), and intraclass correlation coefficient (ICC).

RESULTS

A total of 119 KC eyes were enrolled, with 21 being FFKC, 26 mild, 39 moderate, and 34 severe. The 95% LoA ranged between -5.9 and 4.8 µm for center epithelium thickness (CET), between -5.7 and 8.2 µm for thinnest epithelium thickness (TET). At 1-mm measuring points, the 95% LoA of superior, inferior, nasal, and temporal were -4.2 to 4.7 µm, -5.2 to 6.0 µm, -7.9 to 10.2 µm, and -11.2 to 6.0 µm. At 3-mm measuring points, the corresponding values were -2.8 to 9.3 µm, -2.0 to 13.0 µm, -4.6 to 9.6 µm, and -6.3 to 9.7 µm, indicating that the 2 instruments were not interchangeable without adjustment. Despite that the repeatability of RTVue measurements in KC patients were acceptable, repeatability decreased gradually with the peripheralization of the measurement points.

CONCLUSIONS

The 2 OCT-based devices, RTVue and MS-39, do not provide interchangeable measurements of epithelium thickness in KC patients. Repeatability decreases in cases of more severe KC, emphasizing the importance of grading before clinical examination to avoid diagnostic errors.

Corneal Epithelial Thickness Mapping: A Major Review

The corneal epithelium (CE) is the outermost layer of the cornea with constant turnover, relative stability, remarkable plasticity, and compensatory properties to mask alterations in the underlying stroma. The advent of quantitative imaging modalities capable of producing epithelial thickness mapping (ETM) has made it possible to characterize better the different patterns of epithelial remodeling. In this comprehensive synthesis, we reviewed all available data on ETM with different methods, including very high-frequency ultrasound (VHF-US) and spectral-domain optical coherence tomography (SD-OCT) in normal individuals, corneal or systemic diseases, and corneal surgical scenarios. We excluded OCT studies that manually measured the corneal epithelial thickness (CET) (e.g., by digital calipers) or the CE (e.g., by confocal scanning or handheld pachymeters). A comparison of different CET measuring technologies and devices capable of producing thickness maps is provided. Normative data on CET and the possible effects of gender, aging, diurnal changes, refraction, and intraocular pressure are discussed. We also reviewed ETM data in several corneal disorders, including keratoconus, corneal dystrophies, recurrent epithelial erosion, herpes keratitis, keratoplasty, bullous keratopathy, carcinoma in situ, pterygium, and limbal stem cell deficiency. The available data on the potential role of ETM in indicating refractive surgeries, planning the procedure, and assessing postoperative changes are reviewed. Alterations in ETM in systemic and ocular conditions such as eyelid abnormalities and dry eye disease and the effects of contact lenses, topical medications, and cataract surgery on the ETM profile are discussed.

Assessing progression limits in different grades of keratoconus from a novel perspective: precision of measurements of the corneal epithelium

BACKGROUND

To assess repeatability and reproducibility of corneal epithelium thickness (ET) measured by a spectral-domain optical coherence tomographer (SD-OCT)/Placido topographer (MS-39, CSO, Florence, Italy) in keratoconus (KC) population at different stages, as well as to determine the progression limits for evaluating KC progression.

METHODS

A total of 149 eyes were enrolled in this study, with 29 eyes in the forme fruste keratoconus (FFKC) group, 34 eyes in the mild KC group, 40 eyes in the moderate KC group, and 46 eyes in the severe KC group. Employing the within-subject standard deviation (Sw), test-retest variability (TRT), coefficient of variation (CoV), and intraclass correlation coefficient (ICC) to evaluate intraoperator repeatability and interoperator reproducibility.

RESULTS

The repeatability and reproducibility of MS-39 in patients with KC were acceptable, according to ICC values ranging from 0.732 to 0.954. However, patients with more severe KC and progressive peripheralization of the measurement points had higher TRTs but a thinning trend. The current study tended to set the cut-off values of mild KC, moderate KC, and severe KC to 4.9 µm, 5.2 µm, and 7.4 µm for thinnest epithelium thickness (TET). When differences between follow-ups are higher than those values, progression of the disease is possible. As for center epithelium thickness (CET), cut-off values for mild KC, moderate KC, and severe KC should be 2.8 µm, 4.4 µm, and 5.3 µm. This might be useful in the follow-up and diagnosis of keratoconus.

CONCLUSIONS

This study demonstrated that the precision of MS-39 was reduced in measuring more severe KC patients and more peripheral corneal points. In determining disease progression, values should be differentiated between disease-related real changes and measurement inaccuracies. Due to the large difference in ET measured by MS-39 between various stages of disease progression, it is necessary to accurately grade KC patients to avoid errors in KC clinical decision-making.

Short-term Changes in Epithelial and Optical Redistribution Induced by Different Orthokeratology Designs

OBJECTIVES

This study aimed to investigate corneal epithelial and topographic changes caused by two commercial myopia orthokeratology (ortho-k) designs.

METHODS

Twenty-six subjects fitted with vision shape treatment (VST) lenses and 30 subjects fitted with corneal reshaping therapy (CRT) lenses were reviewed 1 day, 1 week, and 1 month after lens initiation. A spectral-domain optical coherence tomography system was used to create epithelial maps that were in turn used to determine the average epithelial thickness of each zone and the diameter of treatment zone. By measuring the topographic tangential differential map, the treatment zone diameter and the power and width of the high convex zone (HCZ) were obtained. All epithelial thicknesses and topographic corneal variations recorded were analyzed.

RESULTS

At the central zone, the epithelial thickness changes (△ET) decreased significantly after 1 day of ortho-k in two groups. At 2- to 9-mm peripheral zone, ortho-k increased △ET until 1 week in the VST group, whereas it kept increasing in the CRT group after 1 week. At 1 month, the central △ET is -9.51±2.38 mm in the VST group, which was comparable to -8.72±3.43 mm in the CRT group. The nasal HCZ power and the △ET of nasal and inferior nasal were significantly larger in the CRT group. A positive correlation was found between the HCZ power and △ET generated by VST-type lenses inferiorly and temporally. For the CRT group, a positive correlation was found between inferior HCZ power and △ET.

CONCLUSIONS

At the early stage of ortho-k, epithelial thickness and topography change quickly and simultaneously. Epithelial changes were in line with corneal topography reshaping. Epithelial and optical remodelling were affected by different lens types.

New keratoconus staging system based on OCT

PURPOSE

To establish a numerical spectral-domain optical coherence tomography (SD-OCT)-based keratoconus (KC) staging system and compare it with existing KC staging systems.

SETTING

Eye Hospital of Wenzhou Medical University, Wenzhou, China.

DESIGNS

Retrospective case-control study.

METHODS

Scheimpflug tomography, air-puff tonometry, and SD-OCT were performed on 236 normal and 331 KC eyes. All SD-OCT-derived parameters of the corneal epithelium and stroma were evaluated based on their receiver operating characteristic (ROC) curves, area under the curve (AUC), sensitivity, and specificity to discriminate between normal and KC eyes. The best performing parameters were subsequently used to create an OCT-based staging system, which was compared with existing tomographic and biomechanical staging systems.

RESULTS

236 eyes from 236 normal patients and 331 eyes from 331 KC patients of different stages were included. The highest ranked AUC ROC SD-OCT parameters, derived from stroma and epithelium, were stroma overall minimum thickness (ST: AUC 0.836, sensitivity 90%, specificity 67%) and epithelium overall SD (EP: AUC 0.835, sensitivity 75%, specificity 78%). A numerical SD-OCT staging system called STEP including 2 parameters-"ST" and "EP"-with 5 stages was proposed.

CONCLUSIONS

The new SD-OCT-based KC staging system is the first to take the epithelium with its sublayer stroma information into account, showing a strong agreement to the existing staging systems. This system could be incorporated into daily practice, potentially leading to an overall improvement in KC treatment and follow-up management.

Epithelial Thickness Mapping in Keratoconic Corneas: Repeatability and Agreement Between CSO MS-39, Heidelberg Anterion, and Optovue Avanti OCT Devices

PURPOSE

To assess repeatability and agreement of corneal epithelial thickness mapping in eyes with keratoconus using three optical coherence tomography (OCT) devices featuring different technologies: spectral-domain (SD) OCT combined with Placido disk corneal topography (MS-39), swept-source OCT (Anterion), and SD-OCT (Avanti).

METHODS

Three consecutive measurements were acquired with the three devices in 60 eyes with keratoconus. The mean epithelial thickness was calculated in the central 2-mm zone and in 2- to 5-mm and 5- to 7-mm diameter rings. The repeatability was calculated using pooled within-subject standard deviation (Sw). The agreement was assessed by paired t tests and Bland-Altman plots.

RESULTS

The repeatability (Sw) of the epithelial thickness for the central 2-mm zone was 0.91, 0.71, and 0.93 μm for the MS-39, Anterion, and Avanti, respectively. All thicknesses with the MS-39 were greater than those of the Anterion and Avanti, with mean differences of 4.11 ± 1.34 μm (P < .001) and 0.52 ± 1.30 μm (P = .003), respectively. The 95% limits of agreement were 1.484 to 6.736 μm for the MS-39 and Anterion, -3.068 to 2.028 μm for the Avanti and MS-39, and 1.258 to 5.922 μm for for the Avanti and Anterion.

CONCLUSIONS

Epithelial thickness mapping results were most repeatable with the Anterion, followed by the MS-39 and Avanti. The MS-39 gave the thickest values, followed by the Avanti and Anterion. The differences were significant, making the devices not interchangeable for epithelial thickness mapping in eyes with keratoconus. [J Refract Surg. 2023;39(7):474-480.].

Combinations of Scheimpflug tomography, ocular coherence tomography and air-puff tonometry improve the detection of keratoconus

PURPOSE

To determine whether combinations of devices with different measuring principles, supported by artificial intelligence (AI), can improve the diagnosis of keratoconus (KC).

METHODS

Scheimpflug tomography, spectral-domain optical coherence tomography (SD-OCT), and air-puff tonometry were performed in all eyes. The most relevant machine-derived parameters to diagnose KC were determined using feature selection. The normal and forme fruste KC (FFKC) eyes were divided into training and validation datasets. The selected features from a single device or different combinations of devices were used to develop models based on random forest (RF) or neural networks (NN) trained to distinguish FFKC from normal eyes. The accuracy was determined using receiver operating characteristic (ROC) curves, area under the curve (AUC), sensitivity, and specificity.

RESULTS

271 normal eyes, 84 FFKC eyes, 85 early KC eyes, and 159 advanced KC eyes were included. A total of 14 models were built. Air-puff tonometry had the highest AUC for detecting FFKC using a single device (AUC = 0.801). Among all two-device combinations, the highest AUC was accomplished using RF applied to selected features from SD-OCT and air-puff tonometry (AUC = 0.902), followed by the three-device combination with RF (AUC = 0.871) with the best accuracy.

CONCLUSION

Existing parameters can precisely diagnose early and advanced KC, but their diagnostic ability for FFKC could be optimized. Applying an AI algorithm to a combination of air-puff tonometry with Scheimpflug tomography or SD-OCT could improve FFKC diagnostic ability. The improvement in diagnostic ability by combining three devices is modest.

The Impact of Corneal Epithelial Thickening and Inhomogeneity on Corneal Aberrations After Small Incision Lenticule Extraction

PURPOSE

To investigate the corneal epithelial remodeling profile after small incision lenticule extraction (SMILE), the correlated explanatory variables, and its potential impact on corneal higher order aberrations (HOAs).

METHODS

This single-center study prospectively evaluated 75 right eyes of 75 patients scheduled for SMILE. An anterior segment optical coherence tomography device was used to automatically obtain central 6-mm corneal epithelial thickness (ET), total corneal HOAs, and individual Zernike components before and after surgery. The ET inhomogeneity over the central 3- and 6-mm cornea was quantified with coefficient of variance (CV).

RESULTS

Both ET and CV significantly increased 1 month postoperatively (all P < .05). The stepwise multiple regression analysis showed that ET and CV were significantly correlated with preoperative ET and CV, respectively (all P < .01). The corrected spherical equivalent also significantly influenced ET and CV (all P < .01). Over the central 6-mm zone, the alterations of total corneal HOAs and individual Zernike components such as vertical coma (Z7) and spherical aberration (Z12, Z24) were significantly correlated with ET and CV (all P < .05).

CONCLUSIONS

The SMILE-induced epithelial remodeling involved both ET and ET inhomogeneity. The modulation was associated with preoperative and treatment parameters, and exerted a significant impact on corneal HOA alterations especially over the central 6-mm cornea. Together with the amount of correction and corneal curvature gradient change, preoperative assessment of ET and ET inhomogeneity might help predict postoperative epithelial remodeling. [J Refract Surg. 2023;39(1):23-32.].

Role of Corneal Epithelial Measurements in Differentiating Eyes with Stable Keratoconus from Eyes that Are Progressing

Purpose

To evaluate measures of corneal epithelium in eyes that showed documented signs of keratoconus (KC) progression and compare with stable eyes and healthy controls. Also, to determine the correlation of these epithelial parameters with maximum keratometry (K max) and pachymetry.

Design

Prospective, observational, comparative study.

Participants

One-hundred and fifty eyes from 150 patients. The study included 50 eyes from patients with documented KC progression, 50 eyes with stable KC, and 50 clinically normal eyes to serve as controls.

Methods

A spectral-domain (SD)-OCT imaging was obtained in all eyes, and mean values were compared between the groups. The correlation of epithelial parameters with K max and thinnest pachymetry was also investigated.

Main Outcome Measures

For the purposes of this study, the epithelial measures maximum, minimum, superior, and inferior values as well as the difference between the minimum and maximum (min-max) and epithelial standard deviation were considered, obtained from SD-OCT and compared between groups. Measurements of the thinnest point and min-max in pachymetry were also recorded.

Results

The only epithelial parameter that presented a statistically significant difference between stable and progressive KC was epithelium min-max. Although stable KC presented epithelium min-max mean values of -18.2 ± 6.6, progressive KC eyes presented mean values of -23.4 ± 10.3 (P < 0.0001). Epithelial maximum (P = 0.16), minimum (P = 0.25), superior (P = 0.28), inferior (P = 0.23), and standard deviation (P = 0.25) values were not significantly different between stable and progressive eyes. Difference min-max pachymetry points in stable (-108.3 ± 33.5) and progressive KC (-115.2 ± 56.0) were not significantly different (P = 0.723). There was no significant correlation between epithelium min-max with corneal thinning (P = 0.39) or K max (P = 0.09) regardless of disease progression.

Conclusions

Epithelial measures are useful to identify KC eyes that are progressing; the parameters that measure the difference between min-max epithelium points were significantly different between stable and progressive groups, unlike this difference in pachymetry. Finally, this epithelial parameter seems to be independent of corneal thinning and K max.

Financial Disclosures

Proprietary or commercial disclosure may be found after the references.

Combining Spectral-Domain OCT and Air-Puff Tonometry Analysis to Diagnose Keratoconus

PURPOSE

To investigate the diagnostic capacity of spectral-domain optical coherence tomography (SD-OCT) combined with air-puff tonometry using artificial intelligence (AI) in differentiating between normal and keratoconic eyes.

METHODS

Patients who had either undergone uneventful laser vision correction with at least 3 years of stable follow-up or those who had forme fruste keratoconus (FFKC), early keratoconus (EKC), or advanced keratoconus (AKC) were included. SD-OCT and biomechanical information from air-puff tonometry was divided into training and validation sets. AI models based on random forest or neural networks were trained to distinguish eyes with FFKC from normal eyes. Model accuracy was independently tested in eyes with FFKC and normal eyes. Receiver operating characteristic (ROC) curves were generated to determine area under the curve (AUC), sensitivity, and specificity values.

RESULTS

A total of 223 normal eyes from 223 patients, 69 FFKC eyes from 69 patients, 72 EKC eyes from 72 patients, and 258 AKC eyes from 258 patients were included. The top AUC ROC values (normal eyes compared with AKC and EKC) were Pentacam Random Forest Index (AUC = 0.985 and 0.958), Tomographic and Biomechanical Index (AUC = 0.983 and 0.925), and Belin-Ambrósio Enhanced Ectasia Total Deviation Index (AUC = 0.981 and 0.922). When SD-OCT and air-puff tonometry data were combined, the random forest AI model provided the highest accuracy with 99% AUC for FFKC (75% sensitivity; 94.74% specificity).

CONCLUSIONS

Currently, AI parameters accurately diagnose AKC and EKC, but have a limited ability to diagnose FFKC. AI-assisted diagnostic technology that uses both SD-OCT and air-puff tonometry may overcome this limitation, leading to improved treatment of patients with keratoconus. [J Refract Surg. 2022;38(6):374-380.].

Agreement Between Predicted and Actual Measured Ablation Depth After FS-LASIK Using Different Rotating Scheimpflug Cameras and OCT

Purpose

To compare the predicted ablation depth (AD) with the postoperatively measured corneal ablation depth (postop-AD) at central, paracentral, and midperipheral locations using two rotating Scheimpflug analyzers and a Fourier-domain optical coherence tomographer in eyes that underwent femtosecond laser-assisted LASIK (FS-LASIK).

Methods

The values of corneal thickness were measured preoperatively and postoperatively at one and three months. The difference between preoperative and postoperative was defined as postop-AD. Measurements were performed at the corneal vertex and mid-peripheral area. The mid-peripheral corneal thickness was measured at the superior, inferior, nasal, and temporal locations at a distance of 1.0 or 2.5 mm from the corneal vertex. The predicted AD was calculated by ORK-CAM software (Schwind eye tech-solutions GmbH, Kleinostheim, Germany), and the difference between the predicted AD and postop-AD was defined as Δ-AD. Paired t-test analysis was employed to evaluate the differences, agreement was assessed by the Bland-Altman method.

Results

Forty-two eyes of 42 patients were investigated. At one month, the predicted AD in the central and paracentral areas was underestimated by the Pentacam HR (Oculus, Wetzlar, Germany), Sirius (Costruzione Strumenti Oftalmici, Florence, Italy) and RTVue OCT (Optovue Inc., Freemont, CA, United States), whereas Δ-AD was negative as established by all devices and predominantly statistically significant. The Δ-AD values approximated zero at three months. The mean difference of Δ-AD at three months at the corneal vertex was 0.67 ± 9.39 mm, −7.92 ± 9.05 mm and −1.36 ± 8.31 mm, respectively. The mid-peripheral measurements had positive values at one month and even more highly positive at three months (with statistically significant differences in most of the cases). The agreement between the predicted and postop-AD was moderate with all devices, but slightly better with RTVue.

Conclusion

The predicted AD seems to be underestimated in the central and paracentral corneal area and overestimated in the mid-periphery.

Translational Relevance

The study could help to partly explain and prevent the refractive errors after FS-LASIK.

Evaluation of Wide Corneal Epithelial Remodeling after Small Incision Lenticule Extraction (SMILE) with Wide-Field Optical Coherence Tomography

Purpose

This study aims to assess the corneal epithelial remodeling within a 9 mm diameter zone induced by small incision lenticule extraction (SMILE) and evaluate its relationship with the refractive outcomes.

Methods

A total of 64 eyes of 64 patients were included. Wide-field optical coherence tomography (OCT) was used to measure the epithelial thickness (ET) across a 9 mm diameter area, preoperatively, and after one day, one week, one month, three months, and six months postoperatively. The ET changes were compared among the different time points and analyzed zones.

Results

The ET increases from one week to three months and stabilized from three months to six months. Compared to the preoperative values, the mean ET changes at six months in central (2 mm), paracentral (2-5 mm), mid-peripheral (5-7 mm), and peripheral (7-9 mm) zones were 4.37, 4.36, 1.61, and -1.59 μm, respectively. The correlation between the epithelial thickening and the amount of myopia correction was positive in central (P = 0.001) and paracentral zones (P < 0.001) and negative in peripheral zone (P = 0.006). The intended diameter of the optical zone was negatively related to epithelial hyperplasia in the central (P = 0.020) and paracentral zone (P = 0.006), and the correlation was positive in the mid-peripheral zone (P = 0.001). The epithelial thickening of central zone (P = 0.012) and the difference of mean ET between central and paracentral zone (P = 0.020) were negatively related to the spherical equivalent at six months.

Conclusion

An asymmetric lenticule-like pattern of epithelial remodeling occurred in 9 mm diameter cornea at six months after myopic SMILE. The epithelial remodeling may affect the refractive outcomes of SMILE.

Corneal and Epithelial Thickness Mapping: Comparison of Swept-Source- and Spectral-Domain-Optical Coherence Tomography

Objective

To compare the results and repeatability of the corneal thickness (CT) and epithelial thickness (ET) maps provided by Enhanced Spectral-Domain-Optical Coherence Tomography with those of Spectral-Domain-OCT in normal eyes.

Methods

30 normal eyes of 30 patients were assessed by 3 trained operators with ESD-OCT and SD-OCT.

Results

The central and minimum ET obtained with both devices were correlated: central ET, r = 0.86, p < 0.05; minimum ET, r = 0.72, p < 0.05. Compared with SD-OCT, ESD-OCT tended to underestimate these figures by 1.4 and 1.9 μm on average. The central and minimum CT obtained with both devices were strongly correlated: central CT, r = 0.994, p < 0.05; minimum CT, r = 0.995, p < 0.05. ESD-OCT tended to overestimate these figures by 11 and 14 μm on average. Repeatability was good for both devices with a mean coefficient of variation of measurements <6% for ET and <2% for CT. Interoperator variability (standard deviation and COV) was significantly higher for ESD-OCT than for SD-OCT for all local epithelial thicknesses and significantly lower for the central CT and several local corneal thicknesses, whereas no significant differences between both technologies were found for the central and minimum ET and the minimum CT.

Conclusion

ESD-OCT and SD-OCT provide reproducible measurements of CT and ET in normal corneas with a strong correlation between both technologies. However, both technologies are not interchangeable when the main thickness parameters (i.e., central and minimum CT and minimum ET) are used for diagnosing early keratoconus or calculating the expected residual stromal bed thickness before corneal refractive surgery or anterior lamellar keratoplasty.

Clinical outcomes of transepithelial PRK using SCHWIND AMARIS laser platform with actual versus default epithelial thickness values

PURPOSE

to investigate the clinical outcomes of transepithelial photorefractive keratectomy (tPRK) with actual epithelial thickness versus default software values.

METHODS

eighty-three patients with refractive spherical error of -1.50 to -7.00 diopters (D), and refractive astigmatism up to 4.00 D were consecutively enrolled and divided into two groups: group 1 undergone tPRK with actual central and peripheral epithelial thickness input in right eyes, group 2 undergone tPRK with actual central and 10 μm higher peripheral epithelial thickness in right eyes, left eyes underwent tPRK with default protocol in both groups. Outcome measures were induced refractive error, achieved optical zone (OZ), and wasted stromal tissue.

SETTING

Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran, IranDesign: prospective controlled study.

RESULTS

Mean ± SD of induced spherical equivalent (SE) was +0.15 ± 0.39 D and +0.01 ± 0.35 D in right and left eyes of group 1 (p=0.01), and +0.04 ± 0.22 D and +0.03 ± 0.23 D in right and left eyes of group 2 (p=0.75), respectively. There was no statistically significant difference between wasted tissue between right and left eyes in group 1 and group 2 (p=0.77 and p=0.49, respectively). OZ contraction was significantly higher in right compared to left eyes in group 1 (p=0.05), but not in group 2 (p=0.95).

CONCLUSION

In tPRK, refractive outcomes, wasted tissue, and OZ contraction depend little on pre-existing corneal epithelial thickness in corneas with normal range epithelial thickness. However, OZ contraction may be a concern in lower amount of ablations.

Reliability and agreement of the central and mid-peripheral corneal thickness measured by a new Scheimpflug based imaging

Background

To assess the intra-observer repeatability and inter-observer reproducibility of central corneal thickness (CCT) and mid-peripheral corneal thickness (MPCT) measurements using a new Scheimpflug imaging instrument (Scansys) and compare the agreement with the rotating Scheimpflug corneal tomographer (Pentacam HR).

Methods

The same well-trained operator performed the measuring using the two devices, after which Scansys measurements were repeated by another operator. Both instruments required three consecutive measurements per subject. Corneal thickness measurements were obtained by each instrument, including CCT, thinnest corneal thickness (TCT), pupil corneal thickness (PCT), and MPCT. Test-retest repeatability (TRT), within-subject coefficient of variation (CoV), and intra-class correlation coefficient (ICC) were used to evaluate repeatability and reproducibility. A paired t-test was used to compare the differences between Scansys and Pentacam, and the agreement was compared with Bland-Altman plots.

Results

This study enrolled 112 healthy subjects. The CoV were <0.91% and 0.55% for repeatability and reproducibility, respectively. The ICC was close to 1 in all measurements. For intra-observer repeatability in the CT_2mm region, TRT was <10.30 µm. Moreover, TRT was <15.26 µm within the CT_5mm region. The paired t-test showed significant differences in all corneal thickness measurements (P<0.001). The central region and CT_2mm agreement were high, but the largest range of 95% limits of agreement (LoA) appeared in the CT_nasal-5mm.

Conclusions

The new Scheimpflug imaging instrument showed excellent intra-observer repeatability and inter-observer reproducibility for corneal thickness measurements. The agreement analysis suggested that Scansys and Pentacam could be interchangeably used between the central region and CT_2mm, except CT_5mm.

Effect of intraoperative mitomycin-C application on epithelial regeneration after transepithelial photorefractive keratectomy

PURPOSE

To investigate the effect of mitomycin-C (MMC) intraoperative application on postoperative corneal epithelial remodeling, haze incidence, and refractive correction in transepithelial photorefractive keratectomy (transPRK).

SETTING

Eye Hospital of Wenzhou Medical University, Wenzhou, China.

DESIGN

Prospective randomized controlled study.

METHODS

Eyes that completed all follow-ups were included. Epithelial thickness (ET) map measurement was conducted preoperatively and at 1 week, and at 1 month, 3 months, and 6 months postoperatively, generating ET in central, paracentral, and midperipheral zones. The difference between postoperative and preoperative ET (ΔET) was computed for each zone. During follow-up, haze incidence and visual acuity were assessed and compared between groups.

RESULTS

A total of 100 eyes completed all follow-up, including 40 eyes treated with MMC in MMC group and 60 eyes in control group without MMC treatment. For ΔET, between-group difference was found in midperipheral (P = .011) zone at 1 week postoperatively and in central (P = .036) and paracentral zones (P = .039) at 1 month postoperatively. Haze incidence was lower in MMC group at 1 week and 1 month postoperatively (P = .035 and .018, respectively). Safety index (postoperative uncorrected distance visual acuity/preoperative corrected distance visual acuity [CDVA]) and efficacy index (postoperative CDVA/preoperative CDVA) were higher in MMC group (P = .012 and P = .036, respectively) at 1 month postoperatively. No difference was found after 3 months postoperatively.

CONCLUSIONS

With a temporary impact on corneal epithelial regeneration and refractive correction, intraoperative MMC application in transPRK decreased the haze incidence but had no effect on epithelial remodeling.

Repeatability of corneal and epithelial thickness measurements with anterior segment optical coherence tomography in keratoconus

PURPOSE

To investigate the repeatability in corneal thickness (CT) and epithelial thickness (ET) measurements using spectral domain anterior segment optical coherence tomography (AS-OCT, REVO NX, Optopol) in keratoconus, and examine the effect of corneal crosslinking (CXL) on repeatability.

METHODS

A cross-sectional study of 259 eyes of 212 patients with keratoconus attending the corneal disease clinic at a university hospital tertiary referral center were enrolled. Two groups were analysed: eyes with no prior history of CXL (Group A) and eyes with prior CXL (Group B). Repeatability of measurements was assessed using the intraclass correlation coefficient (ICC) and coefficient of variation (CV).

RESULTS

In Group A, central corneal thickness (CCT) was 472.18 ± 45.41μm, and the ET was found to be the thinnest in the inferior-temporal aspect at 51.79 ± 5.97μm and thickest at the superior-nasal aspect at 56.07 ± 5.70μm. In Group B, CCT was 465.11± 42.28μm, and the ET was the thinnest at the inferior-temporal aspect at 50.63 ± 5.52μm and thickest at the superior aspect at 56.80 ± 6.39μm. When evaluating CT measurements, ICC was above 0.86 and 0.83 for Group A and Group B respectively. When evaluating ET measurements, ICC was above 0.82 for both groups. CXL had no statistically significant impact on the repeatability of measurements.

CONCLUSIONS

AS-OCT provides repeatable CT and ET measurements in the central and peripheral cornea in patients with keratoconus. Repeatability is not affected by a history of CXL.

Remodeling of Cornea With Isotretinoin Treatment

OBJECTIVES

To evaluate the change of corneal epithelial thickness (ET) in subjects using isotretinoin with spectral-domain optical coherence tomography and further to explore reflection of changes on corneal topography.

METHODS

Forty eyes of 40 subjects with acne vulgaris scheduled for oral isotretinoin were included in this prospective study. Subjects were examined with RTVue-XR and Pentacam at baseline, 1th, 3rd, and 6th months of treatment, and 3rd month of isotretinoin cessation.

RESULTS

A statistically significant increase was detected in each sector of ET map except inferonasal 7 to 9 mm between baseline and following visits (P<0.05, for all visits). The increase in superior (2-7 mm), inferior (2-7 mm), and maximum values in epithelium statistics and the decrease in superior (2-7 mm), inferior (2-7 mm), minimum, and maximum values in stroma statistics at follow-up visits were significant (P<0.05, for all visits). Central corneal thickness, maximum Ambrosio-relational thickness, average pachymetric-progression index at 1th, 3rd, and 6th months, and thinnest pachymetry, index of surface variance (ISV) at 3rd, and 6th months differed significantly (P<0.05, for specified visits). The regression in parameters was observed at 3rd month of isotretinoin cessation.

CONCLUSIONS

Isotretinoin treatment induces epithelial thickening and stromal thinning. Remodeling of corneal layers causes statistical differences in ISV and pachymetry-related parameters of Pentacam. The pachymetry changes in cornea return to baseline at the 3rd month of discontinuation of treatment.

Corneal Epithelial Thickness Changes Following SMILE for Myopia With High Astigmatism

PURPOSE

To evaluate the corneal epithelial thickness (CET) profile changes after small incision lenticule extraction (SMILE) surgery for myopic astigmatism correction of greater than 2.00 diopters (D).

METHODS

This prospective observational study included 40 eyes (23 patients) treated with SMILE for myopia with cylinders of -2.25 to -4.50 D. Along with standard ophthalmic examinations, CET maps with a diameter of 9 mm were measured by high-resolution spectral-domain optical coherence tomography preoperatively and postoperatively. Correlations between the degree of residual astigmatism and the difference in CET values between preoperative flat and steep meridians were analyzed.

RESULTS

The CET showed significant changes in the central (2 mm), paracentral (2 to 5 mm), midperipheral (5 to 7 mm), and peripheral (7 to 9 mm) zones 6 months after SMILE (P < .001). Among the regions, the CET in the paracentral zones displayed the largest increase (9.75%) with the highest average thickness (57.29 µm). Moreover, symmetrical regional epithelial thickening at the preoperative astigmatism axis was observed in the midperipheral zones. The difference in CET between preoperative flat and steep meridians in the mid-peripheral zones continuously increased from postoperative 1 day to 6 months. This difference was positively correlated with the residual cylinder errors at 6 months postoperatively (r = -0.334, P = .035).

CONCLUSIONS

The 9-mm diameter CET in eyes with high astigmatism significantly increased 6 months after SMILE. Additionally, the difference in CET between preoperative flat and steep meridians in midperipheral zones may be related to astigmatic undercorrection in SMILE. [J Refract Surg. 2021;37(4):224-230.].

Epithelial and stromal remodelling following femtosecond laser-assisted stromal lenticule addition keratoplasty (SLAK) for keratoconus

The purpose of this study was to evaluate corneal epithelium and stromal remodelling with anterior segment optical coherence tomography in patients who have undergone stromal lenticule addition keratoplasty (SLAK) for advanced keratoconus. This was a prospective non-comparative observational study. Fifteen eyes of 15 patients with advanced keratoconus underwent implantation with a cadaveric, donor negative meniscus-shaped intrastromal lenticule, produced with a femtosecond laser, into a stromal pocket dissected in the recipient cornea at a depth of 120 μm. Simulated keratometry, central corneal thickness (CTT), corneal thinnest point (CTP), central epithelial thickness (CET), central and peripheral lenticule thickness, anterior and posterior stromal thickness were measured. Regional central corneal epithelial thickness (CET) and variations in the inner annular area (IAT) and outer annular area (OAT) were also analysed. All parameters were measured preoperatively and 1, 3, and 6 months postoperatively. The average anterior Sim-k decreased from 59.63 ± 7.58 preoperatively to 57.19 ± 6.33 D 6 months postoperatively. CCT, CTP, CET, and OAT increased and IAT decreased significantly after 1 month. All parameters appeared unchanged at 6-months except that of OAT that further increased. Lenticule thickness was stable. In conclusion we observed that SLAK reshapes the cornea by central flattening with stromal thickening and epithelial thickness restoration.

Repeatability and agreement of total corneal and sublayer pachymetry with 2 different algorithms of Fourier-domain optical coherence tomography in myopic and postphotorefractive keratectomy eyes

PURPOSE

To investigate repeatability and agreement of total corneal and sublayer pachymetry with 2 different algorithms of Fourier-domain optical coherence tomography (OCT) in myopic and postphotorefractive keratectomy (PRK) eyes.

SETTING

Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran, Iran.

DESIGN

Prospective observational study.

METHODS

Total corneal, epithelial, and stromal thicknesses were measured using RTVue-XR OCT with Pachymetry + Cpwr (6.0 mm algorithm) and PachymetryWide (9.0 mm algorithm) scan patterns. The repeatability of 25 zones of 9.0 mm map and 17 zones of 6.0 mm map and the agreement between measurements of these 2 algorithms were calculated.

RESULTS

Ninety-five myopic and 117 post-PRK patients were evaluated. By the 9.0 mm algorithm, coefficient of variation (CoV) for total cornea was 2.33% or lesser and 2.49% or lesser and for epithelium was 5.14% or lesser and 5.18% or lesser; and by the 6.0 mm algorithm, CoV for total cornea was 1.80% or lesser and 2.59% or lesser and for epithelial thickness was 3.08% or lesser and 4.80% or lesser in myopic and post-PRK eyes, respectively. Bland-Altman mean difference for epithelial thickness was 0.69 or lesser and 1.16 or lesser and 95% limits of agreement for epithelial thickness was 6.81 or lesser and 8.56 or lesser in myopic and post-PRK eyes, respectively.

CONCLUSIONS

Good repeatability was seen in measurements of total corneal, stromal, and epithelial thicknesses by both algorithms. Agreement of the 2 algorithms in central zone was also good. However, large range of variation in paracentral thickness measurements did not allow us to consider these algorithms as interchangeable.

Pellucid marginal degeneration versus keratoconus: distinction with wide-field SD-OCT corneal sublayer pachymetry

PURPOSE

To evaluate the capability of wide-field spectral-domain optical coherence tomography (SD-OCT) corneal sublayer pachymetry to distinguish between keratoconus and pellucid marginal degeneration (PMD).

METHODS

This prospective cross-sectional study included 69 eyes (59 with keratoconus and 10 with PMD) of 69 patients. All patients were examined three subsequent times with the RTVue XR system. Pachymetry maps of the total cornea (CT), the epithelium (ET) and the stroma were generated using the onboard software. For analysis of reliability, the coefficients of variation and intraclass correlation coefficients (ICC) were calculated. Receiver operating characteristic analyses were performed to elaborate the most accurate pachymetric parameters for distinguishing between PMD and keratoconus.

RESULTS

Overall repeatability of (sublayer) pachymetry was comparably good in both keratoconus (ICC ranging between 0.827 and 0.986) and PMD (ICC ranging between 0.753 and 0.998). Measurement reliability in keratoconic eyes was a negative function of Kmax (p<0.05). As compared with keratoconus, PMD exhibited higher CT (526±31 µm vs 503±30 µm; p=0.02) and ET (56±7 µm vs 51±5 µm; p=0.02) in the inferotemporal 2-5 mm sector as well as lower ET in the inferior 7-9 mm sector (52±5 µm vs 57±5 µm; p<0.01). The calculated ratio between CT in the inferotemporal 2-5 mm and in the inferior 7-9 mm sector yielded the highest diagnostic accuracy for distinguishing between PMD and keratoconus with an area under the curve of 0.977 and an optimal cut-off value of 0.90.

CONCLUSION

Wide-field SD-OCT corneal sublayer pachymetry showed good reliability in PMD and keratoconus and may be useful to differentiate between the two ectatic diseases.

Longitudinal Epithelial Thickness Profile Changes 18 Months After Photorefractive Keratectomy

PURPOSE

Long-term evaluation of corneal epithelial thickness (ET) profile changes after photorefractive keratectomy (PRK) using Fourier-domain anterior segment optical coherence tomography.

METHODS

Three hundred twenty-six eyes of 163 patients were included in this prospective observational study. The corneal epithelial map was obtained across a 9-mm diameter area of the cornea before and up to 27 months after surgery. ET was assessed in 25 sectors and 4 annular zones (central 2 mm, paracentral 2-5 mm, midperipheral 5-7 mm, and peripheral 7-9 mm).

RESULTS

There was a significant reduction in mean ET in all zones 1 month after PRK. Subsequently, ET increased in all annular zones. The change in mean ET became stable in the midperipheral and peripheral zones at 3 to 6 months and in the central zone at 12 months, and it continued to increase in the paracentral zone even after 18 months after surgery. The ET was 3.40 μm and 4.05 μm in the central and paracentral zones at 6 months, respectively. Postoperative spherical equivalent changed significantly only from 1 to 3 months (P < 0.04). There was a significant correlation between postoperative spherical equivalent at month 1 and ET change in the paracentral and midperipheral zones (P < 0.027).

CONCLUSIONS

There is a significant reduction in ET 1 month after myopic PRK with a gradual thickening thereafter until it reaches stability at 12 months in the central zone. However, it continues to change even after 18 months in the paracentral zone. The greatest thickening is in the paracentral zone, followed by the central zone.

Anterior segment optical coherence tomography scanning protocols and corneal thickness repeatability

PURPOSE

To examine the influence of anterior segment optical coherence tomography imaging protocols on the intraobserver and intrasession repeatability of epithelial, stromal, and total corneal thickness measurements.

METHODS

Repeated anterior segment optical coherence tomography (AS-OCT) images (Spectralis, Heidelberg) were obtained from 15 adults using single 8.3 mm wide horizontal line scans with an average of 2, 10, 20, 30, 50 and 100 B-scans. Volumetric scans consisting of nine 8.3 mm horizontal line scans encompassing a 1.3 mm vertical region were also captured (with 20 B-scans per line scan). Single point thickness measures (at the normal to the tangent of the anterior corneal surface) were compared with thickness measures averaged over the central 6 mm. The impact of B-scan averaging and intraobserver variability were examined for single line scans. For volumetric scans, the impact of the number of line scans upon intraobserver and intrasession variability were calculated.

RESULTS

Intraobserver repeatability did not vary significantly as a function of the number of averaged B-scans per line scan, but was lowest for 20-30 averaged B scans. For volumetric scans, increasing the number of line scans did increase scan duration (p < 0.001), with minimal impact upon the average scan quality index (p = 0.06). Averaging more than 3 line scans did not significantly improve intraobserver or intrasession repeatability for either single point or average thickness measurements.

CONCLUSION

AS-OCT volumetric scans with 3 lines each consisting of 20 B-scans with measurements averaged over a central 6 mm of the cornea provide highly repeatable measures of epithelial, stromal and total corneal thickness (95 % LoA ≤ ±3.2 μm for intraobserver repeatability and ≤ ±3.7 μm for intrasession repeatability). This scanning protocol can provide reliable information when monitoring subtle changes in corneal thickness.