Stability of epithelial thickness during 5 minutes immersion in 33°C 0.9% saline using very high-frequency digital ultrasound

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.

Epithelial thickness mapping for corneal refractive surgery

PURPOSE OF REVIEW

As more devices become available that offer corneal epithelial thickness mapping, this is becoming more widely used for numerous applications in corneal refractive surgery.

RECENT FINDINGS

The epithelial thickness profile is nonuniform in the normal eye, being thinner superiorly than inferiorly and thinner temporally than nasally. Changes in the epithelial thickness profile are highly predictable, responding to compensate for changes in the stromal curvature gradient, using the eyelid as an outer template. This leads to characteristic changes that can be used for early screening in keratoconus, postoperative monitoring for early signs of corneal ectasia, and for determining whether further steepening can be performed without the risk of apical syndrome following primary hyperopic treatment. Compensatory epithelial thickness changes are also a critical part of diagnosis in irregular astigmatism as these partially mask the stromal surface irregularities. The epithelial thickness map can then be used to plan a trans-epithelial PRK treatment for cases of irregularly irregular astigmatism. Other factors can also affect the epithelial thickness profile, including dry eye, anterior basement membrane dystrophy and eyelid ptosis.

SUMMARY

Epithelial thickness mapping is becoming a crucial tool for refractive surgery, in particular for keratoconus screening, ectasia monitoring, hyperopic treatment planning, and therapeutic diagnosis and treatment.

Comparison of very-high-frequency ultrasound and spectral-domain optical coherence tomography corneal and epithelial thickness maps

PURPOSE

To compare corneal thickness and epithelial thickness measurements in maps obtained using the RTVue spectral domain optical coherence tomography (SD-OCT) system and the Artemis 2 immersion arc-scanning very-high-frequency ultrasound (VHF-US) system.

SETTING

Department of Ophthalmology, Columbia University Medical Center, New York, New York, USA.

DESIGN

Comparative study.

METHODS

Eyes of normal volunteers were scanned with the SD-OCT system followed by the VHF-US system and then again by the SD-OCT system. On each map, the minimum corneal thickness and mean values of corneal thickness and epithelial thickness in the 3.0 mm radius zone and in 0.5 mm wide concentric annuli of up to a 3.0 mm radius around the corneal vertex were determined.

RESULTS

Both eyes of 12 normal volunteers were scanned. The corneal thickness values from both devices were highly correlated (R > 0.96); in the 3.0 mm radius zone, they were not statistically significantly different. There was no statistically significant change in epithelial thickness or corneal thickness in SD-OCT measurements taken before versus after immersion US. Although highly correlated (R > 0.76), the SD-OCT epithelial thickness values were systematically thinner (1.7 ± 2.1 μm) than the VHF-US measurements in the 3.0 mm radius zone (P < .01).

CONCLUSIONS

The corneal thickness measurements in the 3.0 mm radius zone in normal eyes were equivalent between the 2 systems. Although correlated, the VHF-US epithelial thickness measurements were systemically thicker than the SD-OCT values.

FINANCIAL DISCLOSURES

Drs. Silverman and Reinstein have a commercial interest in Arcscan, Inc. Dr. Reinstein is a consultant to Carl Zeiss Meditec. None of the other authors has a financial or proprietary interest in any material or method presented.

Comparison of Corneal Epithelial Thickness Measurement Between Fourier-Domain OCT and Very High-Frequency Digital Ultrasound

PURPOSE

To compare measurements of corneal epithelial thickness using optical coherence tomography (OCT) and very high-frequency digital ultrasound (VHFDU).

METHODS

Retrospective analysis of 189 virgin corneas and 175 post-laser refractive surgery (LRS) corneas that had corneal epithelial thickness measurement with RTVue Fourier-domain OCT (Optovue, Inc., Fremont, CA) (tear film included) and Artemis VHFDU (ArcScan Inc., Morrison, CO) (tear film excluded). Averages were calculated for the central 2-mm diameter zone and for two further concentric annuli of 1.5- and 0.5-mm width, each divided into eight sectors. Agreement was analyzed by mean difference (OCT - VHFDU), 95% limits of agreement (LoA) (1.96 standard deviation of the difference), and Bland-Altman analysis.

RESULTS

In virgin epithelium, mean central thickness was 53.4 ± 3.20 µm (range: 46 to 62 µm) with OCT and 54.1 ± 2.96 µm (range: 48 to 61 µm) with VHFDU; OCT measured thinnest in 70% with a mean difference of -0.71 µm (95% LoA of ± 3.94 µm, P < .001). In post-LRS epithelium, mean central thickness was 57.9 ± 6.08 µm (range: 42 to 77 µm) with OCT and 60.5 ± 6.47 µm (range: 42 to 79 µm) with VHFDU; OCT measured thinnest in 88%, with a mean difference of -2.48 µm (95% LoA of ± 5.33 µm, P < .001). A larger difference between methods was more common with thicker epithelium.

CONCLUSIONS

Corneal epithelial thickness measurements using OCT were found to be slightly thinner than for VHFDU. In contrast to VHFDU, OCT measurement includes the tear film, so the true difference is probably approximately 4 µm more than reported. The difference was greatest inferiorly and higher for post-LRS eyes and in thicker epithelium.