J. Bradley Randleman, MD; Marcony R. Santhiago, MD, PhD; and William J. Dupps, MD, PhD
In Chapters 1 and 2, we introduced the anterior segment optical coherence tomography and very high–frequency digital ultrasound (VHFDU) devices and discussed their overall utility in evaluating corneal pathology in isolation and particularly in conjunction with corneal topography and tomography. In this chapter, we will specifically focus on the utility of epithelial mapping in evaluating corneal ectasias, in screening for refractive surgery candidacy, and in evaluating patient complaints after surgery.
EPITHELIAL THICKNESS AND REMODELING PATTERNS
In classical textbooks, epithelial thickness is typically reported as being 50 μm, consistent across the cornea and across populations. In reality, while epithelial thickness is relatively consistent and predictable in normal corneas, it nevertheless varies across the cornea and from patient to patient. When evaluating regional epithelial thickness, it is often the pattern rather than the absolute thickness that assists in the diagnosis. Epithelial mapping in normal eyes shares many similarities between eyes in terms of overall thickness and minimal change across the cornea (expressed as low thickness standard deviation [SD]). Increasing variation, especially in concordance with subtle topographic changes, may suggest early ectatic changes and are particularly relevant to identify in refractive surgery screening.
Epithelial Remodeling
There are basic tenants to epithelial remodeling when the cornea is altered in any way. The epithelium attempts to reduce curvature irregularities. Thus, it compensatorily thins over steep areas, thickens to compensate for tissue loss (troughs), and epithelial thickness will become maximally irregular to compensate for underlying stromal irregularities to reduce irregular curvature. Therefore, if irregular curvature remains, the epithelium has reached its compensatory limit. Epithelial remodeling is driven by the rate of curvature change, so more abrupt regions of curvature difference will stimulate greater epithelial remodeling.
Epithelial Remodeling in Ectatic Corneas
Epithelial thickness remodels early in the ectatic process, typically with thinning occurring over the steepest region. Thus, any focal epithelial thinning with coincident steepening is cause for concern. On the other hand, there are many nonpathologic situations where focal epithelial hypertrophy occurs and results in focal steepening on curvature maps. This phenomenon can occur with or without contact lens wear. Focal epithelial hypertrophy can lead to false concern on screening when it causes focal steepening on curvature maps, when in reality, these patients are typically not manifesting early signs of an ectatic corneal process and are often reasonable candidates for refractive surgery.
Epithelial Remodeling After Refractive Surgery
Epithelial maps can also be useful to identify possible sources of refractive error/regression after laser vision correction (LVC) and may be of particular use to monitor cases after high hyperopia, where maximal central epithelial thinning may contribute to corneal and refractive instability.
Epithelial Thickness: Clinical Measurements
Epithelial thickness has been demonstrable using VHFDU for decades, but this device has not been commercially available until recently. Epithelial measurements have also been obtainable clinically using anterior segment spectral domain optical coherence tomography (SD-OCT) imaging across the central 6 mm of the cornea. Recently, 2 additional devices have been approved by the US Food and Drug Administration for epithelial measurements across the central 9 mm of the cornea. Thus, in the near future, there will likely be increasing awareness and availability of epithelial measurements in clinical practice.
This chapter will highlight the appearance of epithelial maps in normal and ectatic eyes, the typical appearance before and after corneal refractive surgery, and basic concepts in utilizing epithelial maps for refractive surgery screening. Further detailed examples of the use of epithelial maps will be shown in subsequent chapters.
Progressively Abnormal Epithelial Thickness Profiles
Figure 4-A. Composite image showing different eyes with varying degrees of epithelial thickness patterns and remodeling responses, ranging from normal corneas to eyes with advanced keratoconus. (A) SD-OCT image of a normal cornea, showing a normal epithelial distribution pattern. (B) SD-OCT image of a cornea with mild keratoconus, showing a small point of focal thinning without any surrounding epithelial thickening. (C) SD-OCT image of a different cornea with mild keratoconus, showing a small but slightly more pronounced focal thinning with very mild surrounding epithelial thickening. (D) SD-OCT image of a patient with keratoconus, showing focal thinning of 5 μm or more (roughly 10% of total epithelial thickness) with mild surrounding epithelial thickening. (E) SD-OCT image of a patient with advanced keratoconus. There is pronounced focal epithelial thinning of more than 15 µm (roughly 30% of total epithelial thickness). (F) SD-OCT image of a patient with advanced keratoconus. There is pronounced focal epithelial thinning with surrounding epithelial hypertrophy. This is the hallmark finding of epithelial remodeling in keratoconus.
SECTION 1: GENERAL EPITHELIAL MAPPING IMAGES IN NORMAL EYES
Figure 4-1-1. SD-OCT imaging of a normal right eye showing the scanner view (upper left), which allows the reviewer to confirm appropriate centration at the time of image capture, raw optical coherence tomography output in cross section (upper right), and computed total (pachymetry map, lower left) and epithelial (epithelium map, lower right) thickness across the central 6 mm. Specific metrics are listed on the far left: visual examination reveals minimal deviation in epithelial thickness, and the SD is 0.8 μm (left).
Figure 4-1-2. Composite SD-OCT image of the right (upper) and left (lower) eyes from a patient with a normal corneal evaluation. There are nearly identical regional thicknesses between eyes both in total and epithelial thickness. There is slightly greater epithelial thickness variation across the central 6 mm in this patient (highlighted by the higher SD values) as compared to the previous figure, but this is still an overall normal evaluation. Figure 4-1-3. SD-OCT comparative image of the right and left eye from a patient with a normal corneal evaluation. The left cornea is slightly thinner than the right, but the overall patterns are similar between eyes for both total and epithelial thickness. Figure 4-1-4. SD-OCT image of a patient with a thin cornea. Central total thickness is 455 μm. Despite this, the epithelial thickness pattern is normal with minimal variation across the central 6 mm and a SD of 1.1 μm. These findings do not confirm a normal cornea, but there is no specific suggestion of an ectatic cornea based on epithelial thickness patterns. Figure 4-1-5. SD-OCT image of the right eye from a patient with a normal corneal evaluation showing the central 9 mm of the cornea. The scanner view (upper left) shows the image is relatively well centered. The raw OCT output in cross section (upper right) appears the same as with the 6 mm mapping software. Total thickness (pachymetry map) and epithelial thickness (epithelium map) show similar information to previous images but show values out to 9 mm. There is minimal epithelial thickness deviation across the cornea. Figure 4-1-6. SD-OCT image of the right eye from a patient with a normal corneal evaluation. The scanner view shows that the upper lid partially impacted the image (large white arrow), which can also be seen as the break on the right side of the cross-section scan (small white arrow). This lid position is seen as thinning in the upper portion of the epithelial map (black arrow). The speckling dot pattern shows that these data were interpolated.
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