Correlation between epithelial thickness in normal corneas, untreated ectatic corneas, and ectatic corneas previously treated with CXL; is overall epithelial thickness a very early ectasia prognostic factor?

Anastasios John Kanellopoulos, Ioannis M Aslanides, George Asimellis, Anastasios John Kanellopoulos, Ioannis M Aslanides, George Asimellis

Abstract

Purpose: To determine and correlate epithelial corneal thickness (pachymetric) measurements taken with a digital arc scanning very high frequency ultrasound biomicroscopy (HF UBM) imaging system (Artemis-II), and compare mean and central epithelial thickness among normal eyes, untreated keratoconic eyes, and keratoconic eyes previously treated with collagen crosslinking (CXL).

Methods: Epithelial pachymetry measurements (topographic mapping) were conducted on 100 subjects via HF UBM. Three groups of patients were included: patients with normal eyes (controls), patients with untreated keratoconic eyes, and patients with keratoconic eyes treated with CXL. Central, mean, and peripheral corneal epithelial thickness was examined for each group, and a statistical study was conducted.

Results: Mean, central, and peripheral corneal epithelial thickness was compared between the three groups of patients. Epithelium thickness varied substantially in the keratoconic group, and in some cases there was a difference of up to 20 μm between various points of the same eye, and often a thinner epithelium coincided with a thinner cornea. However, on average, data from the keratoconic group suggested an overall thickening of the epithelium, particularly over the pupil center of the order of +3 μm, while the mean epithelium thickness was on average +1.1 μm, compared to the control population (P = 0.005). This overall thickening was more pronounced in younger patients in the keratoconic group. Keratoconic eyes previously treated with CXL showed, on average, virtually the same average epithelium thickness (mean -0.7 μm, -0.2 μm over the pupil center, -0.9 μm over the peripheral zone) as the control group. This finding further reinforces our novel theory of the "reactive" component of epithelial thickening in corneas that are biomechanically unstable, becoming stable when biomechanical rigidity is accomplished despite persistence of cornea topographic irregularity.

Conclusion: A highly irregular epithelium may be suggestive of an ectatic cornea. Our results indicate that the epithelium is thinner over the keratoconic protrusion, but to a much lesser extent than anticipated, and on average epithelium is thicker in this group of patients. This difference appears to be clinically significant and may become a screening tool for eyes suspected for ectasia.

Keywords: cornea epithelial thickness; corneal pachymetry; ectasia; keratoconus screening.

Figures

Figure 1
Figure 1
B-scan reconstruction, as obtained from the HF UBM system showing epithelium and corneal thickness measurements via the caliper tool. We observe the anterior and posterior cornea, as well as the cornea–epithelium interface. (A) Full scan, scale 8.4 μm/pixel. (B) Detail.
Figure 2
Figure 2
Corneal report produced by the Zeus software showing total corneal, epithelial, and stromal thickness pachymetry maps over 8 mm diameter. Notes: The subject’s eye is normal. We observe the overall thicker epithelium over the pupil center.
Figure 3
Figure 3
Central epithelial thickness versus central corneal thickness of the same data points as produced by 540 different combinations of select meridional scans. Note: All data correspond to the same eye.
Figure 4
Figure 4
Central corneal (A) and central epithelial (B) thickness histogram plots of the same data points as produced by 540 different combinations of select meridional scans. Note: All data correspond to the same eye.
Figure 5
Figure 5
The four transitional zone points used to calculate peripheral epithelium thickness on each eye. Note: Epithelium thickness from these points was averaged, and is referred to as peripheral epithelium thickness.
Figure 6
Figure 6
Epithelial thickness maps of the same (control) patient, same eye, as produced by two consequtive acquisitions. Both maps demonstrate a thicker epithelium over the pupil center. Different acquisitions (A and B) of the same eye may produce epithelium maps which may vary by an estimated ± 4 μm.
Figure 7
Figure 7
Epithelium thickness across the three study groups, at the periphery, mean, and pupil center. Abbreviation: KCN, keratoconus.
Figure 8
Figure 8
Corneal and epithelial thickness maps of a KCN patient. Note: A significantly thicker epithelium over the pupil center is observed. Abbreviation: KCN, keratoconus.
Figure 9
Figure 9
Example of localized thin epithelium (lower segment), over an area of a significantly thinner stroma.
Figure 10
Figure 10
Examples of localized thinner epithelium over protruding corneas. (A and B) KCN patients, (C) a PMD patient. Abbreviations: KCN, keratoconus; PMD, pellucid marginal degeneration.
Figure 11
Figure 11
Examples of very smooth epithelium of a patient subjected to CXL. Abbreviation: CXL, crosslinking.
Figure 12
Figure 12
Examples of epithelium (patients subjected to CXL and partial topography guided PRK) demonstrating localized variations. Abbreviations: CXL, crosslinking; PRK, photorefractive keratectomy.

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