Comparison of corneal epithelial and stromal thickness distributions between eyes with keratoconus and healthy eyes with corneal astigmatism ≥ 2.0 D

Determining the Relationship Between Regional Epithelial Thickness, Corneal Toricity, and Corneal Power in Normal Corneas

PURPOSE

To determine the relationship between regional epithelial thickness and corneal toricity and estimate the epithelium's impact on corneal power in normal corneas.

METHODS

This was a retrospective case-control study evaluating 200 eyes from 200 patients categorized into one of three groups based on corneal toricity obtained with Scheimpflug imaging: (1) 80 non-toric (NT) eyes (< 0.50 diopters [D] in any meridian), (2) 80 with-the-rule (WTR) eyes (⩾ +1.50 D @90 ± 22.5°), and (3) 40 against-the-rule (ATR) eyes (⩾ +1.50 D @180 ± 22.5°). Epithelial thickness maps (9 mm) were generated using anterior segment optical coherence tomography (ASOCT) imaging (Avanti RTVue XR; Optovue, Inc). Mathematical modeling was used to evaluate the epithelium's impact on corneal power.

RESULTS

There were no differences in mean curvature between groups (range: 43.2 to 43.7 µm). Average epithelial thickness profile within the central 9-mm zone varied by approximately 2 µm or less laterally but was thinner superiorly than inferiorly in all three groups (52.45 to 53.36 vs 56.01 to 56.48 µm) with no significant differences between groups. There were no differences in any measured epithelial metric at any location nor any deviation in overall epithelial thickness pattern between groups. There were minimal average variations (< 4 µm) across the cornea between central and peripheral values in any meridian. The modeled impact on corneal optics was a net reduction in curvature by approximately 0.30 D with less than 0.10 D of possible variation resulting from differences in regional thickness.

CONCLUSIONS

Regional epithelial thickness was unrelated to underlying corneal toricity and had minimal impact on corneal power in normal eyes. There was no evidence that normal epithelium significantly masks underlying normal stromal toricity, nor does the epithelium significantly contribute to corneal power under normal conditions. [J Refract Surg. 2025;41(2):e91-e101.].

The association of pre-photorefractive keratectomy Schirmer-1 test value with postoperative corneal epithelial thickness, ocular surface discomfort, and visual acuity

PURPOSE

To investigate the association of pre-photorefractive keratectomy Schirmer-1 test value with post-photorefractive keratectomy central corneal epithelial thickness, ocular surface disease index score, and uncorrected distance visual acuity.

METHODS

Patients were categorized according to preoperative Schirmer-1 value: the normal Schirmer Group (n=54; Schirmer-1 test value, >10 mm) and the low Schirmer Group (n=52; Schirmer-1 test value, between 6 and 10 mm). We analyzed ablation depth, visual acuity, result of Schirmer-1 test (with anesthesia), tear film break-up time, ocular surface disease index score, central corneal epithelial thickness, and spherical equivalent refraction.

RESULTS

We found significant differences between the groups in Schirmer-1 test value, tear film break-up time, and ocular surface disease index score, both preoperatively and postoperatively (p<0.001). The preoperative central corneal epithelial thicknesses of the two groups were similar (p>0.05). After photorefractive keratectomy, the Schirmer-1 test value and spherical equivalent refraction decreased in both groups (p<0.05), and ocular surface disease index scores and central corneal epithelial thickness values increased in the low Schirmer Group (p<0.001) but not in the normal Schirmer Group (p>0.05). The postoperative central corneal epithelial thicknesses of the low Schirmer Group were significantly higher than those of the normal Schirmer Group (p<0.001). Postoperative uncorrected distance visual acuity did not differ significantly between the two groups (p>0.05).

CONCLUSIONS

In patients with low Schirmer-1 test values before photorefractive keratectomy, the corneal epithelium thickened and ocular surface complaints increased during the postoperative period. However, changes in the corneal epithelium did not affect the postoperative uncorrected distance visual acuity. To reduce postoperative problems on the ocular surface in these patients, we recommend that dry eye be treated before photorefractive keratectomy.

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.

Corneal Epithelial Thickness in Normal Corneas with Low and High Toricity

Purpose

To compare the corneal epithelial thickness along the flat and steep meridians of corneas with low and high toricity.

Methods

This was a prospective observational comparative study on healthy subjects with normal corneas seeking preoperative evaluation for refractive surgery at a university-based hospital. Subjects with up to 2 diopters (D) of corneal with-the-rule astigmatism were defined as low corneal toricity (CT), whereas cylinder > two-dimensional was considered as high CT. The anterior segment optical coherence tomography was conducted to measure the epithelial thickness along the principle meridians of CT over a diameter of 9 mm. At the eye level, outcome variables (corneal and epithelial thicknesses in low- and high-astigmatism groups) were assessed using generalized estimating equation models.

Results

Included were 98 eyes (49 subjects): 46 eyes (23 subjects) with low CT and 52 eyes (26 subjects) with high CT. Two groups were similar with respect to the age (P = 0.82), sex (P = 0.49), and spherical equivalent (P = 0.11). Although the corneal thickness at steep and flat meridians was not different between two groups, high-CT group had significantly thinner epithelium at inferior 2.5-3.5 and 3.5-4.5 mm zones (P = 0.01 and 0.04) as well as superior 2.5-3.5 mm zone (P = 0.03) along the steep meridian. Two groups were similar with respect to epithelial thickness of flat meridian (all P > 0.05).

Conclusion

The epithelium of steep meridian was thinner in the high-CT group compared to the low-CT group.

Corneal layer thickness in keratoconus using optical coherence tomography

CLINICAL RELEVANCE

Accurate thickness measurement of corneal layers using anterior segment OCT can be used to improve visual outcomes. Understanding its applications is essential for optometric practices to enhance eye care procedures.

BACKGROUND

To evaluate the thicknesses of different corneal layers for identifying keratoconus (KCN) and subclinical keratoconus (SKCN) using spectral-domain optical coherence tomography (SD-OCT).

METHODS

This prospective study analyzed 60 eyes with KCN, 48 eyes with SKCN, and 53 normal eyes. The central corneal thickness (CCT) and thicknesses of the epithelium, Bowman, stroma, and Descemet-endothelium layers were measured using SD-OCT. One way analysis of variance and the area under the curve (AUC) were used to evaluate the parameters. The Delong method was used to compare AUCs.

RESULTS

In KCN, CCT and thicknesses of epithelium, Bowman, stroma, and Descemet-endothelium layers were 495.5 ± 41.7, 52.6 ± 6.4,11.5 ± 1.4, 415.5 ± 38.9, and 12.3 ± 1.7 µm, respectively. These thickness values were respectively 524.5 ± 33.3, 56.8 ± 6.8, 11.5 ± 1.6, 439.8 ± 30.6, and 12.4 ± 1.7 µm in SKCN and 563.8 ± 37.9, 57.7 ± 6.9, 12.2 ± 1.6, 469.5 ± 33.7, and 12.8 ± 2.1µm in normal group. Total cornea and stroma in KCN and SKCN, and epithelium in KCN were significantly thinner compared to the normal group (P < 0.001). The highest AUC values were observed for CCT in KCN (AUC 0.90) and SKCN (AUC 0.782). The diagnostic accuracy was significantly higher for stromal thickness in KCN (sensitivity 81.7%, specificity 73.6%, AUC 0.871) and SKCN (sensitivity 80.0%, specificity 56.6%, AUC 0.751) than other individual corneal layers (Delong, P < 0.001)                                    .

CONCLUSION

CCT can accurately distinguish keratoconus from normal eyes. However, central corneal stromal thinning was the most sensitive diagnostic index for early detection of SKCN. Developing standardized stromal maps may be helpful for detecting SKCN.

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.].

The Impact of Epithelial Remodeling on Surgical Techniques Used in Topography-guided Surface Ablation in Irregular Corneas

PURPOSE

To analyze the optical consequences of epithelial remodeling in irregular corneas and their impact on the choice of different surface ablation techniques.

METHODS

Anterior corneal and stromal surface topographies and epithelial thickness maps were analyzed in 24 eyes with irregular corneal optics. On two of the eyes, four different surface ablation techniques were simulated: (1) conventional anterior topography-guided photorefractive keratectomy (PRK), (2) transepithelial phototherapeutic keratectomy (PTK), (3) transepithelial anterior topography-guided PRK, and (4) stromal topography-guided PRK.

RESULTS

Stromal surface topographies showed higher keratometric values, astigmatism, asphericity, and corneal higher order aberrations compared to topographies of anterior corneas covered by epithelium. Transepithelial anterior topography-guided PRK and stromal topography-guided PRK both resulted in regularized stromal surface, transepithelial PTK achieved partial regularization corresponding to the smoothing effect of the epithelial remodeling, and conventional anterior topography-guided PRK delivered after epithelial removal resulted in residual stromal surface irregularities.

CONCLUSIONS

The difference in optical landscapes between the stromal and anterior surfaces in irregular corneas will represent a source of error when anterior topography-guided treatments are delivered on the deepithelialized stroma, as in conventional PRK. In contrast, anterior topography-guided ablations performed as transepithelial PRK and stromal topography-guided PRK delivered after epithelial removal address the full stromal irregularity, whereas transepithelial PTK alone may be used when topography-guided treatments are not possible. The authors conclude topography-guided PRK of irregular corneas should lead to significantly improved regularization only if it includes the effect of epithelial remodeling. [J Refract Surg. 2022;38(8):529-537.].

Heidelberg Anterion Swept-Source OCT Corneal Epithelial Thickness Mapping: Repeatability and Agreement With Optovue Avanti

PURPOSE

To assess the repeatability of corneal epithelial thickness mapping in virgin, post-laser refractive surgery (PLRS), and keratoconic eyes using a novel swept-source optical coherence tomographer (SS-OCT), and to determine the agreement of the measurements with a validated spectral-domain (SD) OCT.

METHODS

Analysis of 90 virgin, 46 PLRS, and 122 keratoconic eyes was performed. Three consecutive measurements of each eye were acquired with the Anterion SS-OCT and Avanti SD-OCT devices, and averages of the epithelial thickness mapping were calculated in the central 2-mm zone and in the 2- to 5-mm and 5- to 7-mm diameter rings. The repeatability was analyzed using pooled within-subject standard deviation (Sw). The agreement was assessed by Bland-Altman analysis and paired t tests.

RESULTS

The repeatability ranges of the Anterion and Avanti epithelial thickness mapping measurements were Sw: 0.60 to 1.36 µm and Sw: 0.75 to 1.96 µm, respectively. The 95% limits of agreement of the Anterion and Avanti were 0.826 to 8.297. All values of the thickness measurements with the Anterion were lower than those of the Avanti, with the mean differences being 4.06 ± 1.81, 3.26 ± 2.52, and 3.68 ± 2.51 µm in virgin, PLRS, and keratoconic eyes, respectively (P < .001 for all).

CONCLUSIONS

The repeatability of the Anterion's epithelial thickness mapping was higher than that of the Avanti. In terms of the agreement between the Anterion and Avanti, the epithelium measured by the Anterion was always thinner than that of the Avanti, making their interchangeable use unsuitable without corrections. [J Refract Surg. 2022;38(6):356-363.].

Differentiating Between Contact Lens Warpage and Keratoconus Using OCT Maps of Corneal Mean Curvature and Epithelial Thickness

PURPOSE

To formulate an Epithelial Modulation index to differentiate between eyes with contact lens warpage and keratoconus.

METHODS

Normal eyes and eyes with either contact lens warpage or keratoconus were scanned by a Fourier-domain optical coherence tomography (OCT) system. Maps of epithelial thickness and anterior surface mean curvature were generated and converted to deviation maps by subtracting the average maps from a healthy population. The Epithelial Modulation index was defined as the covariance between the two types of deviation maps. A logistic regression model was used to classify eyes as non-keratoconus (normal or warp-age) or keratoconus (manifest, subclinical, or forme fruste).

RESULTS

The average Epithelial Modulation index value for normal eyes was -0.6 ± 1.0 µm/m. Eyes with keratoconus were characterized by coincident high anterior surface mean curvature and low epithelial thickness, resulting in a high Epithelial Modulation index (manifest: 103.0 ± 82.9 µm/m, subclinical: 37.0 ± 23.0 µm/m, forme fruste: 7.3 ± 13.2 µm/m). The Epithelial Modulation index was closer to normal for eyes with warpage (-1.9 ± 4.0 µm/m). The classification accuracy of the Epithelial Modulation index during five-fold cross-validation of the logistic regression model was 100 ± 0% for normal eyes and 99.0 ± 2.0% for eyes with warpage. The accuracy was 100 ± 0%, 100 ± 0%, and 53.1 ± 1.5% for the manifest, subclinical, and forme fruste keratoconus groups, respectively.

CONCLUSIONS

The Epithelial Modulation index is useful in distinguishing eyes with secondary epithelial modulation (keratoconus) from those with primary epithelial deformation (contact lens-related warpage). [J Refract Surg. 2022;38(2):112-119.].

Keratoconus: An updated review

Keratoconus is a bilateral and asymmetric disease which results in progressive thinning and steeping of the cornea leading to irregular astigmatism and decreased visual acuity. Traditionally, the condition has been described as a noninflammatory disease; however, more recently it has been associated with ocular inflammation. Keratoconus normally develops in the second and third decades of life and progresses until the fourth decade. The condition affects all ethnicities and both sexes. The prevalence and incidence rates of keratoconus have been estimated to be between 0.2 and 4,790 per 100,000 persons and 1.5 and 25 cases per 100,000 persons/year, respectively, with highest rates typically occurring in 20- to 30-year-olds and Middle Eastern and Asian ethnicities. Progressive stromal thinning, rupture of the anterior limiting membrane, and subsequent ectasia of the central/paracentral cornea are the most commonly observed histopathological findings. A family history of keratoconus, eye rubbing, eczema, asthma, and allergy are risk factors for developing keratoconus. Detecting keratoconus in its earliest stages remains a challenge. Corneal topography is the primary diagnostic tool for keratoconus detection. In incipient cases, however, the use of a single parameter to diagnose keratoconus is insufficient, and in addition to corneal topography, corneal pachymetry and higher order aberration data are now commonly used. Keratoconus severity and progression may be classified based on morphological features and disease evolution, ocular signs, and index-based systems. Keratoconus treatment varies depending on disease severity and progression. Mild cases are typically treated with spectacles, moderate cases with contact lenses, while severe cases that cannot be managed with scleral contact lenses may require corneal surgery. Mild to moderate cases of progressive keratoconus may also be treated surgically, most commonly with corneal cross-linking. This article provides an updated review on the definition, epidemiology, histopathology, aetiology and pathogenesis, clinical features, detection, classification, and management and treatment strategies for keratoconus.

Topical drug delivery to the retina: obstacles and routes to success

INTRODUCTION

Retinal diseases are one of the main reasons for vision loss where all available drug treatments are based on invasive drug administration such as intravitreal injections. Despite huge efforts and some promising results in animal models, almost all delivery technologies tested have failed in human trials. There are however examples of clinically effective topical delivery systems such as fast dissolving aqueous eye drop suspensions.

AREAS COVERED

Six obstacles to topical drug delivery to the eye have been identified and discussed in some details. These obstacles consist of static membrane barriers to drug permeation into the eye, dynamic barriers such as the lacrimal drainage and physiochemical barriers such as low thermodynamic activity. It is explained how and why these obstacles hamper drug permeation and how different technologies, both those that are applied in marketed drug products and those that are under investigation, have addressed these obstacles.

EXPERT OPINION

The reason that most topical drug delivery systems have failed to deliver therapeutic drug concentrations to the retina is that they do not address physiochemical barriers such as the thermodynamic activity of the permeating drug molecules. Topical drug delivery to the retina has only been successful when the static, dynamic, and physiochemical barriers are addressed simultaneously.

Long-term efficacy and safety results after iris-fixated foldable phakic intraocular lens for myopia and astigmatism: 6-year follow-up

PURPOSE

To evaluate the long-term efficacy and safety of iris-fixated foldable phakic intraocular lenses (pIOLs) for the management of myopia and astigmatism after 6-year follow-up.

SETTING

Ophthalmology Department, Hospital Braga, Portugal.

DESIGN

Retrospective cohort study.

METHODS

Patients included underwent iris-fixated foldable Artiflex pIOL implantation between January 2010 and December 2013. Follow-up evaluations were performed 6 months postoperatively and every 12 months until 72 months. Preoperative and follow-up data on uncorrected and corrected distance visual acuity (CDVA), manifest refraction, endothelial cell density (ECD), and anterior chamber depth (ACD) were analyzed.

RESULTS

A total of 177 eyes of 98 patients with a mean age of 32.33 ± 7.13 years were analyzed. There was a statistically significant improvement in CDVA from 0.09 ± 0.02 logMAR preoperatively to 0.04 ± 0.02 logMAR (P < .0001) at 6 years postoperatively. The spherical equivalent demonstrated a statistically significant improvement from -9.50 ± 2.93 diopters (D) to -0.41 ± 0.45 D at 6 years. At 72 months, the efficacy and safety indexes were 0.94 and 1.15, respectively. A mean loss of 31.77 cells/mm2 (1.19%) was observed each year until the sixth year. The overall mean ECD loss after correction for the physiological loss was 3.02% after 6 years. No correlations were observed between the mean ECD loss and the preoperative ACD.

CONCLUSIONS

The Artiflex pIOL was a safe, effective, and a stable option to correct myopia. A mean ECD loss of 3.02% was observed over 6 years. No pIOL was explanted due to corneal decompensation.

Topical drug delivery to the posterior segment of the eye: Thermodynamic considerations

Almost all studies on non-invasive topical drug delivery to the eye have emphasized the importance of biological barriers, static membrane barriers such as the cornea and the conjunctiva/sclera and dynamic barriers such as the lacrimal drainage. Hardly any have discussed the importance of the thermodynamic activity of the permeating drug molecules. Most drugs permeate from the eye surface into the eye by passive diffusion where, according to Fick's first law, the drug concentration gradient over the various permeation barriers (e.g., the tear fluid and the lipophilic membrane barriers) is the driving force. At the barrier interphases the dissolved drug molecules must partition from one barrier to another. For example, at the tear-cornea interphase the drug molecules must partition from the aqueous exterior into the lipophilic membrane. The drug partition coefficient between two phases is commonly defined as the equilibrium concentration ratio. However, these are only approximations. The actual driving force in Fick's first law is the gradient of the chemical potential and the equilibrium between two phases is attained when the chemical potential of the drug in one phase is equal to that in the other phase. Here the importance of thermodynamic considerations in topical drug delivery to the eye is reviewed.

Anterior-segment spectral domain optical coherence tomography in epidermolysis bullosa

PURPOSE

Assess epidermolysis bullosa (EB)-related corneal pathology using anterior segment optical coherence tomography (AS-OCT) and correlate imaging with clinical metrics in EB patients versus age-matched controls.

METHODS

EB patients and controls were recruited during an EB conference (July 2018) and at Tufts Medical Center (June-August 2019). Subjects completed a questionnaire, had best corrected visual acuity (BCVA) tested, and underwent AS-OCT scanning. Stromal and epithelial thickness were measured. Depth, length, and type of the three largest lesions were assessed by a masked examiner using a novel pathology grading system. Multivariate analysis of AS-OCT findings and clinical metrics was performed.

RESULTS

62 EB patients and 60 age-matched controls were enrolled. Mean BCVA was 1.8 lines worse in patients (p < 0.001). Vision loss was associated with increased stromal thickness. Discrete lesions were seen in 60.2% of patient eyes, averaging 1.71 ± 1.75 lesions in patients and 0.14 ± 0.42 in controls (p < 0.001). Mean primary lesion depth was 151.88 ± 97.49 μm in patients. Patients showed significant stromal thickening versus controls and lesions were most common in the periphery and inferiorly. Differences in frequency and duration of abrasions and severity of pain were all statistically and clinically significant in patients versus controls (p < 0.001).

CONCLUSIONS

AS-OCT can visualize and quantify differences in the corneas of EB patients compared with age-matched controls. Novel findings include quantification of average depth, length, and severity of discrete lesions, and sparing of the superior quadrant from stromal thickening in EB patients. These results support use of AS-OCT and a questionnaire in clinical trials for new EB therapies.

Thickness profiles of the corneal epithelium along the steep and flat meridians of astigmatic corneas after orthokeratology

Background

The aim of this study was to investigate the changes in corneal epithelial thickness along the principle meridians of astigmatic corneas after six months of overnight spherical myopic orthokeratology (OK) lens wear.

Methods

This is a prospective study. Fifty-seven subjects with up to 1.50 diopters (D) of corneal toricity wore spherical OK lenses for 6 months. Evaluations of OK lens fit, visual acuity, refractions and corneal toricity (CT) were performed. Fourier-domain optical coherence tomography (FD-OCT) was conducted to measure the corneal epithelial thickness (ET) along the principle meridians of corneal toricity over a diameter of 6 mm. The means of △ET of the same diameter at individual meridians (△ETSm and △ETFm) were calculated and compared.

Results

Visual acuity and refraction improved significantly after OK lens wear. △ETFm thinned more than △ETSm (P = 0.027) at 1.5 mm in radius. △ETSm thickened more than △ETFm at 2.5 mm (P = 0.019) and 3.0 mm (P = 0.036).∣△ETSm - △ETFm∣ were significantly correlated with the baseline central CT at 2.0 mm, 2.5 mm and 3.0 mm. ∣△ETSm - △ETFm∣was significantly correlated with the baseline peripheral CT at 2.5 mm.

Conclusions

Overnight wear of spherical OK lenses resulted in differential changes in the thickness profiles of the corneal epithelium between the steep and flat meridians in eyes with corneal toricity.

Wide Stromal Mapping Using an Anterior Segment Optical Coherence Tomography

Purpose

To quantify and assess the reproducibility of the corneal stromal thickness profiles captured by the SD-OCT. Secondly, we correlated the zonal thicknesses to the age, gender and axial length.

Methods

We included 227 normal eyes of 227 patients with a maximum hypermetropia of +5 and myopia of –6 diopters (D). Subjects with an intraocular pressure exceeding 22 mm Hg, evidence of cataract formation, history of ophthalmic surgery or disease were excluded. Lastly, reproducibility was evaluated in a subset of 50 participants by means of an identical scan protocol repeated by 2 different OCT operators.

Results

Stromal values were consistently thicker in the peripheral cornea (p<0.001). Age was negatively correlated with approximately every sector of the stroma with notable exceptions of the center (r=0.117, p=0.088) and the superior inner (r=0.057, 0.409), middle (r=0.086, p=0.209) and outer locations (r=0.120, p=0.079). There was no statistical significance in most sectors when looking at the axial length, gender and K1/K2. This method was highly reproducible in terms of both the ICC and COV.

Conclusion

Corneal stromal mapping is highly reproducible and shows a negative correlation to age. Additionally, the periphery of the stroma is consistently thicker to the center. Other variables like gender and axial length show no relationship to the corneal stroma.

Ultrahigh-resolution anterior segment optical coherence tomography for analysis of corneal microarchitecture during wound healing

PURPOSE

To employ ultrahigh-resolution (UHR) optical coherence tomography (OCT) for investigation of the early wound healing process in corneal epithelium.

METHODS

A custom-built UHR-OCT system assessed epithelial healing in human keratoconic cornea after epi-off crosslinking (CXL) procedure and a wound healing model in rabbits with iatrogenic corneal injury. 3D OCT data sets enhanced obtaining epithelial thickness maps and evaluation of reepithelization stage. Accompanying changes in deeper corneal microarchitecture were analysed.

RESULTS

The mean central corneal thickness in 40 eyes with keratoconus at baseline was 482.7 ± 38.2 μm, while mean central epithelial thickness (CET) was 43.8 ± 6.4 μm. At the final visit 20 ± 5 days post-CXL procedure, CET was 35.0 ± 5.8 μm, significantly thinner after reepithelization (p < 0.001). Surgical success was assessed at the final visit through the demarcation line (DL), identified at 43.7 ± 13.5% stromal depth. In rabbits, the mean CET in 20 eyes at baseline was 35.9 ± 2.6 μm. In rabbits that revealed complete wound closure (10/20 eyes) at the last study day at 72 hr, CET was significantly thinner compared to baseline (30.4 ± 2.8 μm versus 35.4 ± 2.9 μm, p = 0.005). An intra-stromal landmark indicating early keratocyte apoptosis was measured at 30.0 ± 5.1% stromal depth. Epithelial thickness maps showed the time-course of corneal healing.

CONCLUSION

Ultrahigh-resolution (UHR)-OCT provided precise assessment of epithelial wound and its healing by 3D-mapping. In addition, microarchitectural changes in the cornea in early phases of epithelial healing were revealed.

Comparison of Corneal Epithelial and Stromal Thickness between Keratoconic and Normal Eyes in an Asian Population

PURPOSE

To assess corneal epithelial and stromal thickness in keratoconic and normal eyes by spectral domain optical coherence tomography (SDOCT) in an Asian population.

METHODS

Forty-three keratoconic and 24 normal eyes were studied and examined using SDOCT. Keratoconic eyes with corneal scarring were excluded. Epithelial and stromal thickness was assessed at 25 points, 0.5 mm apart, across the central 6 mm of the pupil centre in the horizontal and vertical meridians. The correlation between epithelial and stromal thickness in both keratoconic and normal eyes (at the corneal centre) was also assessed.

RESULTS

The corneal epithelium at the pupil centre was significantly thinner in keratoconic eyes (p < 0.05) than in controls. Epithelial thickness varied widely in keratoconic eyes compared to controls (p < 0.05). The epithelium and stroma were significantly thinner inferiorly and temporally in keratoconic eyes (p < 0.05). There was a significant correlation between epithelial and stromal thickness (at the pupil centre) in the keratoconus group (rs = 0.348, p < 0.001) but not the normal group (rs = 0.036, p = 0.376).

CONCLUSIONS

Corneal epithelial thickness was markedly thinner and varied in keratoconic eyes compared to controls. Epithelial thinning occurred secondary to the abnormal elevation of the stroma. These findings are useful in detecting early keratoconus and in the evaluation of refractive surgery candidates.

Role of corneal epithelial thickness mapping in the evaluation of keratoconus

PURPOSE

To investigate the corneal epithelial thickness profiles in patients with a confirmed diagnosis of stable and progressive keratoconus.

SETTING

Studio Italiano di Oftalmologia, Rome, Italy.

DESIGN

Observational study.

METHODS

86 patients with either stable (n = 52) or progressive (n = 34) keratoconus and 182 healthy controls were enrolled in the study. Disease progression was confirmed by repeated corneal topographies over 1 year follow-up before inclusion in the study. All subjects had full corneal and epithelial thickness mapping taken by spectral domain optical coherence tomography (SD-OCT). The full corneal mapping was investigated by evaluating the central corneal thickness, the thinnest point, the superonasal-inferotemporal thickness difference and the minimum-median thickness difference. The epithelial mapping was investigated by assessing the 2 mm central thickness, the inferior paracentral (2-5 mm) thickness, and the minimum-maximum thickness difference.

RESULTS

No significant differences in full corneal mapping were found between stable and progressive keratoconic eyes. Of note, the inferior paracentral region of the corneal epithelium was significantly thinner in progressive (50 ± 3 μm) than stable (53 ± 4 μm) keratoconus (P < 0.001).

CONCLUSIONS

The SD-OCT corneal epithelial mapping was valuable for detecting local thickness changes in eyes with keratoconus. Monitoring the corneal epithelial changes across the inferior area in patients with keratoconus could be worthy for assessing disease progression.

Decreased epithelial to corneal thickness ratio in healthy fellow eyes of patients with unilateral bullous keratopathy

PURPOSE

To study the relationship between epithelial thickness (ET) and corneal thickness (CT) in healthy fellow eyes of patients with unilateral bullous keratopathy (BKP) and healthy subjects.

METHODS

Seventeen BKP patients (nine males, eight females, 73.2±10.4 years) and 40 healthy individuals (20 males, 20 females, 69.5±9.8 years) were included. All participants received anterior segment optical coherence tomography and specular microscopy with calculation of endothelial cell density. ET, CT, stromal thickness and ET-to-CT ratio were defined automatically (within 2 mm central area). Central epithelial thickness (CET) and central corneal thickness (CCT) were measured manually at the corneal centre and stromal thickness and CET-to-CCT ratio were calculated.

RESULTS

In healthy fellow eyes of BKP patients compared with eyes of healthy individuals ET was statistically significantly lower (p<0.001) while CT was statistically significantly higher (by 28.9 and 30.9 µm in 2 mm zone and corneal centre, respectively). Both 2 mm ET-to-CT ratio (0.091±0.01 and 0.10±0.004, respectively [p<0.001]) and CET-to-CCT ratio (0.083±0.006 and 0.97±0.005, respectively [p<0.0001]) were statistically significantly lower in healthy fellow eyes of BKP patients compared with eyes of healthy individuals. To identify healthy fellow eyes of BKP patients, area under curve for CET-to-CCT ratio and CET was 0.94 and 0.80, respectively (p=0.01), and for 2 mm ET-to-CT ratio and 2 mm ET was 0.91 and 0.80, respectively (p=0.03).

CONCLUSION

Decreased СET-to-СCT ratio resulting from simultaneous epithelial thinning and stromal thickening without significant corneal thickening indicates subclinical dysfunction of corneal endothelium in healthy fellow eyes in unilateral BKP patients.

A Review of Structural and Biomechanical Changes in the Cornea in Aging, Disease, and Photochemical Crosslinking

The study of corneal biomechanics is motivated by the tight relationship between biomechanical properties and visual function within the ocular system. For instance, variation in collagen fibril alignment and non-enzymatic crosslinks rank high among structural factors which give rise to the cornea's particular shape and ability to properly focus light. Gradation in these and other factors engender biomechanical changes which can be quantified by a wide variety of techniques. This review summarizes what is known about both the changes in corneal structure and associated changes in corneal biomechanical properties in aging, keratoconic, and photochemically crosslinked corneas. In addition, methods for measuring corneal biomechanics are discussed and the topics are related to both clinical studies and biomechanical modeling simulations.

Relationship Between Central Epithelial Thickness and Central Corneal Thickness in Healthy Eyes and Eyes After Laser In Situ Keratomileusis

PURPOSE

To study the relationship between central epithelial thickness (CET) and central corneal thickness (CCT) in healthy individuals and to evaluate their changes after laser in situ keratomileusis (LASIK).

METHODS

In this study, we included 150 patients (150 eyes) who underwent LASIK and 83 healthy individuals (83 eyes). In all patients, CET and CCT were measured using anterior segment optical coherence tomography with Copernicus REVO on B-scans. Statistical analysis was performed using the Spearman rank test, t test, and receiver operating characteristic curve analysis.

RESULTS

In healthy individuals, CET positively correlated with CCT (r = 0.5, P < 0.001), the CET-to-CCT ratio being 0.100 ± 0.006. The CET-to-CCT ratio increased to 0.111 ± 0.009 (P < 0.001) and to 0.132 ± 0.021 (P < 0.001) at 1 day and 1 month postoperatively, respectively. The area under the curve for the CET-to-CCT ratio as a diagnostic criterion for previously performed myopic LASIK was 0.97 (sensitivity 93.5% and specificity 91.6%).

CONCLUSIONS

In healthy individuals, the corneal epithelium contributes a mean of 10.0% of corneal thickness. The CET-to-CCT ratio has a highly predictive value for indication of previously performed myopic LASIK. It seems that clinical application of epithelial thickness measurement can be improved by adjusting CET for CCT.

Drug delivery to the lens for the management of cataracts

Cataracts are one of the most prevalent diseases of the lens, affecting its transparency and are the leading cause of reversible blindness in the world. The clarity of the lens is essential for its normal physiological function of refracting light onto the retina. Currently there is no pharmaceutical treatment for prevention or cure of cataracts and surgery to replace the affected lens remains the gold standard in the management of cataracts. Pharmacological treatment for prevention of cataracts is hindered by many physiological barriers that must be overcome by a therapeutic agent to reach the avascular lens. Various therapeutic agents and formulation strategies are currently being investigated to prevent cataract formation as access to surgery is limited. This review provides a summary of recent research in the field of drug delivery to the lens for the management of cataracts including models used to study cataract treatments and discusses the future perspectives in the field.

Obtaining Thickness Maps of Corneal Layers Using the Optimal Algorithm for Intracorneal Layer Segmentation

Optical Coherence Tomography (OCT) is one of the most informative methodologies in ophthalmology and provides cross sectional images from anterior and posterior segments of the eye. Corneal diseases can be diagnosed by these images and corneal thickness maps can also assist in the treatment and diagnosis. The need for automatic segmentation of cross sectional images is inevitable since manual segmentation is time consuming and imprecise. In this paper, segmentation methods such as Gaussian Mixture Model (GMM), Graph Cut, and Level Set are used for automatic segmentation of three clinically important corneal layer boundaries on OCT images. Using the segmentation of the boundaries in three-dimensional corneal data, we obtained thickness maps of the layers which are created by these borders. Mean and standard deviation of the thickness values for normal subjects in epithelial, stromal, and whole cornea are calculated in central, superior, inferior, nasal, and temporal zones (centered on the center of pupil). To evaluate our approach, the automatic boundary results are compared with the boundaries segmented manually by two corneal specialists. The quantitative results show that GMM method segments the desired boundaries with the best accuracy.

Corneal Epithelium Thickness Profile in 614 Normal Chinese Children Aged 7-15 Years Old

The purpose of the study is to describe the values and distribution of corneal epithelium thickness (CET) in normal Chinese school-aged children, and to explore associated factors with CET. CET maps were measured by Fourier-domain optical coherence tomography (FD-OCT) in normal Chinese children aged 7 to 15 years old from two randomly selected schools in Shanghai, China. Children with normal intraocular pressure were further examined for cycloplegic autorefraction, corneal curvature radius (CCR) and axial length. Central (2-mm diameter area), para-central (2- to 5-mm diameter area), and peripheral (5- to 6-mm diameter area) CET in the superior, superotemporal, temporal, inferotemporal, inferior, inferonasal, nasal, superonasal cornea; minimum, maximum, range, and standard deviation of CET within the 5-mm diameter area were recorded. The CET was thinner in the superior than in the inferior and was thinner in the temporal than in the nasal. The maximum CET was located in the inferior zone, and the minimum CET was in the superior zone. A thicker central CET was associated with male gender (p = 0.009) and older age (p = 0.037) but not with CCR (p = 0.061), axial length (p = 0.253), or refraction (p = 0.351) in the multiple regression analyses. CCR, age, and gender were correlated with para-central and peripheral CET.

In pursuit of objective dry eye screening clinical techniques

Dry eye is a multifactorial, progressive, and chronic disease of the tears and ocular surface. The disease is multi-factorial and has intermittent symptoms. Discomfort, visual disturbance, tear film instability with potential damage to the ocular surface, and increased tear film osmolarity are known associates.

Dry eye is a common clinical problem for eye-care providers worldwide and there is a large number of clinical investigative techniques for the evaluation of dry eye. Despite this, however, there is no globally accepted guideline for dry eye diagnosis and none of the available tests may hold the title of the ‘gold standard’. The majority of the techniques involved in the diagnosis of the disease, particularly for its early stages, has a large degree of subjectivity.

The purpose of this article is to review existing dry eye investigative techniques and to present a new objective dry eye screening technique based on optical coherence tomography.

WNT10A exonic variant increases the risk of keratoconus by decreasing corneal thickness

Keratoconus is a degenerative eye condition which results from thinning of the cornea and causes vision distortion. Treatments such as ultraviolet (UV) cross-linking have proved effective for management of keratoconus when performed in early stages of the disease. The central corneal thickness (CCT) is a highly heritable endophenotype of keratoconus, and it is estimated that up to 95% of its phenotypic variance is due to genetics. Genome-wide association efforts of CCT have identified common variants (i.e. minor allele frequency (MAF) >5%). However, these studies typically ignore the large set of exonic variants whose MAF is usually low. In this study, we performed a CCT exome-wide association analysis in a sample of 1029 individuals from a population-based study in Western Australia. We identified a genome-wide significant exonic variant rs121908120 (P = 6.63 × 10(-10)) in WNT10A. This gene is 437 kb from a gene previously associated with CCT (USP37). We showed in a conditional analysis that the WNT10A variant completely accounts for the signal previously seen at USP37. We replicated our finding in independent samples from the Brisbane Adolescent Twin Study, Twin Eye Study in Tasmania and the Rotterdam Study. Further, we genotyped rs121908120 in 621 keratoconus cases and compared the frequency to a sample of 1680 unscreened controls from the Queensland Twin Registry. We found that rs121908120 increases the risk of keratoconus two times (odds ratio 2.03, P = 5.41 × 10(-5)).

OCT corneal epithelial topographic asymmetry as a sensitive diagnostic tool for early and advancing keratoconus

PURPOSE

To investigate epithelial thickness-distribution characteristics in a large group of keratoconic patients and their correlation to normal eyes employing anterior-segment optical coherence tomography (AS-OCT).

MATERIALS AND METHODS

The study group (n=160 eyes) consisted of clinically diagnosed keratoconus eyes; the control group (n=160) consisted of nonkeratoconic eyes. Three separate, three-dimensional epithelial thickness maps were obtained employing AS-OCT, enabling investigation of the pupil center, average, mid-peripheral, superior, inferior, maximum, minimum, and topographic epithelial thickness variability. Intraindividual repeatability of measurements was assessed. We introduced correlation of the epithelial data via newly defined indices. The epithelial thickness indices were then correlated with two Scheimpflug imaging-derived AS-irregularity indices: the index of height decentration, and the index of surface variance highly sensitive to early and advancing keratoconus diagnosis as validation.

RESULTS

Intraindividual repeatability of epithelial thickness measurement in the keratoconic group was on average 1.67 μm. For the control group, repeatability was on average 1.13 μm. In the keratoconic group, pupil-center epithelial thickness was 51.75±7.02 μm, while maximum and minimum epithelial thickness were 63.54±8.85 μm and 40.73±8.51 μm. In the control group, epithelial thickness at the center was 52.54±3.23 μm, with maximum 55.33±3.27 μm and minimum 48.50±3.98 μm epithelial thickness. Topographic variability was 6.07±3.55 μm in the keratoconic group, while for the control group it was 1.59±0.79 μm. In keratoconus, topographic epithelial thickness change from normal, correlated tightly with the topometric asymmetry indices of IHD and ISV derived from Scheimpflug imaging.

CONCLUSION

Simple, OCT-derived epithelial mapping, appears to have critical potential in early and advancing keratoconus diagnosis, confirmed with its correlation with established Scheimpflug-derived asymmetry topometric indices.

Evaluation of corneal epithelial and stromal thickness in keratoconus using spectral-domain optical coherence tomography

PURPOSE

We sought to assess the corneal thickness of the epithelium and stroma in keratoconic and normal eyes by spectral-domain optical coherence tomography (SD-OCT).

METHODS

Fifty-seven keratoconic and 20 normal eyes were studied. The eyes were examined by SD-OCT, and the keratoconic eyes were subdivided into 2 groups: those showing only smooth corneal thinning and corneal protrusion on the image (KC1 group) and those showing abnormalities in the Bowman layer or in the stroma, or in both (KC2 group). The thicknesses at the corneal vertex and at the superior, inferior, nasal, and temporal cornea 1.5 mm from the corneal vertex in the KC1 group were compared with those in the normal group. The OCT findings in the KC2 group were described.

RESULTS

The epithelial thickness at the corneal vertex and at the inferior and temporal cornea, and the stromal thickness at all points were significantly thinner in the KC1 group than in the normal group (p < 0.05). The epithelial and stromal thicknesses at the corneal vertex were significantly correlated in the KC1 group and the normal group (r (2)  = 0.427, p < 0.0001).The epithelial thickness in the KC2 group was not uniform owing to Bowman layer scarring, stromal scars, and secondary corneal amyloidosis.

CONCLUSIONS

Although epithelial thinning is associated with stromal thinning, when the cornea remains clear, the epithelial thickness may vary because of the irregularity of the stroma beneath the epithelium in patients with keratoconus.