Corneal Imaging in Refractive Surgery Evaluations
J. Bradley Randleman, MD; Marcony R. Santhiago, MD, PhD; and William J. Dupps, MD, PhD
As an extension of Chapter 5, one of the primary clinical uses for corneal imaging is for the evaluation of refractive surgery candidates, specifically to identify patients who are suboptimal candidates for corneal refractive surgical procedures. Whereas Chapter 5 displayed corneal imaging from patients with corneal ectasias of varying severity, this chapter will focus on the range of patient presentations ranging from suitable candidates for refractive surgery to abnormal imaging that should exclude patients from corneal refractive surgery candidacy. In between these extremes, multiple iterations of subtle patterns that can be best amorphously categorized as suspicious mapping will be demonstrated.
There is no subject in corneal imaging more controversial than specific evaluation processes used to assess refractive surgery candidacy. The following image groupings are meant to be illustrative classifications rather than definitive, and the suspicious categories are by definition arbitrary in their groupings. The multitude of mapping options available for different imaging devices introduced in Chapter 2 are shown when possible in this chapter; thus, many images exist as a group from the same patient to show the range of maps and images obtainable.
There is some variability in patterns and imaging that are still typically considered suitable for corneal refractive surgery. Beyond simple, traditional, normal patterns, there are a variety of findings that may appear suspicious on first review but that are overall deemed to be normal variants in patients who can ultimately be deemed suitable refractive surgery candidates. These findings highlight the benefit of imaging multiple aspects of the cornea, including anterior curvature, regional and relational thickness, elevation evaluations, and epithelial thickness maps.
NOTE ON SCREENING RECOMMENDATIONS
The goal of this text is not to provide specific screening strategy recommendations, nor is it to recommend any particular technology or device over another. Rather, we are highlighting the capabilities of multiple technologies in isolation and in conjunction with other technologies to provide as comprehensive a view of each eye as possible. In most circumstances we are specifically not including patient age, gender, refraction, contact lens wear history, ocular history, or the patient’s goals and desires. These elements are all critical in determining refractive surgery candidacy but are beyond the scope of this imaging atlas.
SECTION 1: SUITABLE REFRACTIVE SURGERY CANDIDATES: NORMAL IMAGING AND VARIANTS
Amorphous Nonastigmatic Pattern
Figure 6-1-1. Scheimpflug refractive display for the (A) right and (B) left eyes from a patient presenting for refractive surgery evaluation. Anterior curvature maps (upper left) show symmetric with-the-rule regular astigmatism in both eyes, corneal thickness (lower left) is normal centrally and shows normal pachymetric progression to the periphery, and there are no focal abnormalities in elevation maps (right). Note the excellent between-eye symmetry in all maps.
Low Symmetric Astigmatic Pattern
Figure 6-1-2. Scheimpflug refractive display of the (A) right and (B) left eyes from a patient presenting for refractive surgery evaluation. Anterior axial curvature maps show symmetric with-the-rule astigmatism patterns in both eyes with relatively low astigmatism, corneal thickness is normal with minimal displacement of the thinnest point in either eye, and there are no significant focal elevations on the front or back surfaces.
Focal Pseudo-Steepening From Epithelial Hypertrophy
Figure 6-1-3. Scheimpflug refractive displays for the (A) right and (B) left eyes from a patient presenting for refractive surgery evaluation. Anterior curvature maps (upper left) show mild central steepening (≈0.5 D) in both eyes. Corneal thickness (lower left) is normal centrally with normal pachymetric progression to the periphery, and there are no focal abnormalities in elevation maps (right images).
Figure 6-1-3. Scheimpflug ectasia screening displays for the (C) right and (D) left eyes from the same patient. There are no focal anterior or posterior elevations demonstrated, and none of the indices are identified as suspicious. (E) SD-OCT imaging of the same patient showing normal total corneal thickness (upper) in both eyes. There is epithelial hypertrophy centrally and inferiorly in both eyes. The absence of central epithelial thinning relative to the surrounding periphery suggests that the epithelium is not masking an underlying stromal steepening.
Pseudo-Truncated Bowtie Pattern
Figure 6-1-4. Scheimpflug refractive display for the (A) right and (B) left eyes from a patient presenting for refractive surgery evaluation. Anterior curvature maps (upper left) show a flatter cornea overall than pervious examples, with a relatively normal pattern in both eyes, with subtle central steepening (≈0.5 D) in both eyes, more notable in the right. Corneal thickness (lower left) is thinner centrally than previous examples but still within normal range, with no displacement of the thinnest point in either eye and normal pachymetric progression to the periphery, and there are no focal abnormalities in anterior or posterior elevation maps.
Figure 6-1-4. Scheimpflug ectasia screening displays for the (C) right and (D) left eyes from the same patient. There are no focal anterior or posterior elevations demonstrated, and none of the indices are identified as suspicious. (E) SD-OCT imaging of the same patient showing normal total corneal thickness (upper) and normal epithelial maps (lower) for both eyes, with little variation in epithelial thickness across the central 6 mm in either eye. Case note: The findings in this patient are subtly different than previous cases, and on first review the focal central steepening could be considered suspicious. The degree of focal steepening, however, is mild (< 1 diopter [D] within the same meridian) and no other findings are suspicious. There are, further, no significant anterior or posterior surface elevations or focal epithelial thinning that would be considered suspicious.
Normal Astigmatic Pattern
Figure 6-1-5. Placido images for the (A) right and (B) left eyes from a patient presenting for refractive surgery evaluation. There is symmetric against-the-rule astigmatism in the right eye and an indistinct pattern of against-the-rule/oblique astigmatism in the left eye. There is low astigmatism in both eyes. Scheimpflug refractive display for the same (C) right and (D) left eyes showing asymmetric bowtie patterns orientated against the rule in the right eye and obliquely in the left eye, thicker than average pachymetry with minimal displacement of the thinnest point, and normal front and back elevation maps in both eyes. Dual Scheimpflug/Placido imaging for the same (E) right and (F) left eyes showing asymmetric bowtie patterns orientated against the rule in the right eye and obliquely in the left eye, thicker than average pachymetry with minimal displacement of the thinnest point, and unremarkable anterior and posterior elevation maps in both eyes.
Asymmetric Pseudo-Inferior Steepening Due to Epithelial Hypertrophy
Figure 6-1-6. (A) Scheimpflug comparative display from a patient with inferior steepening in both eyes (≈1 D in the right eye and ≈2 D in the left eye). Posterior elevation is shown in the lower images; there is no focal posterior elevation associated with the anterior steepening in either eye.
Figure 6-1-6. Scheimpflug refractive display of the (B) right and (C) left eyes from the same patient. Anterior curvature maps (upper left) show the same pattern as Figure 6-1-6A. Pachymetry maps (lower left) show normal central and peripheral thickness in both eyes, with good between-eye symmetry and minimal displacement of the thinnest point in either eye. There are no focal elevation changes in either eye in anterior (upper right) or posterior (lower right) elevation maps. Scheimpflug ectasia screening displays of the same (D) right and (E) left eyes. There are no abnormalities identified in either eye in any of the metrics reported in this map. Figure 6-1-6. (F) SD-OCT imaging of the same patient. Total corneal thickness (upper) appears normal in both eyes with findings analogous to those demonstrated by Scheimpflug imaging. Epithelial maps (lower) show epithelial asymmetry, with up to 10 μm of focal inferior epithelial hypertrophy in both eyes. This hypertrophy corresponds in location to the anterior curvature steepening seen on both eyes. Case note: This case initially appears suspicious due to inferior steepening on anterior curvature maps. There is, however, epithelial hypertrophy coincident in location with anterior curvature steepening in both eyes. Given this finding and lack of associated changes in total thickness or elevation findings, the patient was deemed a suitable candidate for refractive surgery. Figure 6-1-7. (A) Scheimpflug comparative display from a patient with inferior steepening in both eyes, more pronounced in the left eye. Posterior elevation is shown in the lower images; there is no focal posterior elevation colocalized with the anterior steepening in either eye. Figure 6-1-7. Scanning slit imaging of the (B) right and (C) left eyes from the same patient. Placido curvature maps (lower left) show the same focal inferior steepening in both eyes. Pachymetry maps (lower right) show thick corneas in both eyes with slight displacement of the thinnest points in different directions in the right (inferior) and left (temporal) eyes. There are no focal findings in anterior float maps (upper left) in either eye. There is some concordance with thinnest point and posterior float (upper right); the significance of this is unknown as the method of generating posterior float values is a known limitation for Orbscan technology. Scheimpflug refractive display of the same (D) right and (E) left eyes. Anterior curvature maps show the same pattern of inferior steepening. Pachymetry maps show normal central and peripheral thickness in both eyes, with good between-eye symmetry and minimal displacement of the thinnest point in either eye. There are no significant focal elevation changes in either eye in anterior or posterior elevation maps. Scheimpflug ectasia screening displays of the (F) right and (G) left eyes from the same patient. There are no metrics identified as abnormal in either eye. Figure 6-1-7. (H) SD-OCT composite maps of the same right (upper) and left (lower) eyes. Total corneal thickness appears normal in both eyes with findings analogous to those demonstrated by Scheimpflug imaging. Epithelial maps show epithelial asymmetry, with up to 9 μm of focal inferior epithelial hypertrophy in both eyes. This hypertrophy corresponds in location to the anterior curvature steepening seen on both eyes. Case note: Similar to the previous case, this case initially appears suspicious due to inferior steepening on anterior curvature maps. There is, however, epithelial hypertrophy coincident in location with anterior curvature steepening in both eyes. Given this finding and lack of associated changes in total thickness or elevation findings, the patient was deemed a suitable candidate for refractive surgery.
Asymmetric Pseudo–Crab Claw Pattern Due to Epithelial Hypertrophy
Figure 6-1-8. Dual Scheimpflug/Placido imaging of the (A) right and (B) left eyes from a patient who presented for refractive surgery evaluation. Anterior curvature maps (upper left) show irregular inferior steepening in both eyes in a modified/truncated crab claw–type pattern, with ≈ 2 D of focal steepening in both eyes. Pachymetry maps show normal central and peripheral thickness in both eyes, with good between-eye symmetry and no displacement of the thinnest point in either eye. There is a mild increase in anterior elevation in both eyes in the region corresponding to anterior curvature steepening. There are no focal elevation changes in the posterior elevation maps in either eye. Scheimpflug refractive displays of the (C) right and (D) left eyes from the same patient. Anterior curvature maps show the same irregular inferior steepening pattern in both eyes in a modified/truncated crab claw–type pattern, with slightly less focal steepening in both eyes as compared to dual Scheimpflug/Placido imaging. Pachymetry maps show normal central and peripheral thickness in both eyes, with good between-eye symmetry and no displacement of the thinnest point in either eye. There is a mild increase in anterior elevation in the left, but not right, eye in the region corresponding to anterior curvature steepening. There is also a mild inferior tilt pattern (lower superior elevations progressing to higher elevations in the inferior cornea) in the posterior elevation (lower right) of the right eye that is largely absent from dual Scheimpflug/Placido imaging and the left eye. Scheimpflug ectasia screening displays of the (E) right and (F) left eyes from the same patient. There are no metrics identified as abnormal in either eye.
Figure 6-1-8. (G) SD-OCT composite maps of the same right (upper) and left (lower) eyes. Total corneal thickness appears normal in both eyes with findings analogous to those demonstrated by Scheimpflug and dual Scheimpflug/Placido imaging. Epithelial maps show epithelial asymmetry, with up to 12 μm of focal inferior epithelial hypertrophy in both eyes. This hypertrophy corresponds in location to the anterior curvature steepening seen on both eyes. Case note: Similar to previous cases, there is epithelial hypertrophy coincident in location with anterior curvature steepening. Corneal thickness appeared normal in all imaging modalities, while mild subtle difference existed between technologies regarding anterior and posterior surface elevations. This case highlights the potential benefit of obtaining data from multiple imaging modalities to come to a consensus determination regarding patient suitability for refractive surgery.
SECTION 2: SUSPICIOUS IMAGING IN REFRACTIVE SURGERY EVALUATIONS
Thin Cornea
Figure 6-2-1. (A) Scheimpflug comparative display from a patient with essentially symmetric oblique astigmatism, with mild asymmetry in the left eye, and thin central corneas in both eyes. There is good between-eye symmetry in both curvature and thickness.
Figure 6-2-1. Placido imaging of the (B) right and (C) left eyes from the same patient. The symmetric oblique astigmatism patterns are analogous to those seen in Scheimpflug imaging. Scheimpflug refractive displays of the (D) right and (E) left eyes from the same patient. Anterior curvature maps show the same pattern as Figure 6-2-1A. Pachymetry maps show thin central pachymetry with adequate peripheral thickness in both eyes, with good between-eye symmetry and minimal displacement of the thinnest point in either eye. There are no significant focal elevation changes in either eye in anterior (upper right) or posterior (lower right) elevation maps. Figure 6-2-1. Scheimpflug topometric displays of the (F) right and (G) left eyes from the same patient. There are no asymmetry indices identified as abnormal in the right eye, while index of height decentration (IHD) is displayed as suspicious in the left eye. Scheimpflug ectasia screening displays of the (H) right and (I) left eyes from the same patient. The Belin-Ambrósio enhanced ectasia display total deviation (BAD-D) score is mildly elevated in both eyes, more so in the left, which is identified as suspicious. These scores are likely due to the thin central pachymetry values, as pachymetric progression appears normal in both eyes. Figure 6-2-1. (J) SD-OCT imaging of the right and left eyes from the same patient. Total corneal thickness values are analogous to those shown in Scheimpflug imaging. There is no clear pattern of epithelial thinning in either eye that corresponds to regions of anterior steepness, but there is a deviation of up to 8 μm within the central 6 mm in both eyes. Case note: The primary suspicious issue in this patient is thin central pachymetry, as curvature appears symmetric and elevations and epithelial thickness maps are unremarkable.
Thin Cornea With Asymmetric Steepening
Figure 6-2-2. Scheimpflug refractive display of the (A) right and (B) left eyes from a patient who presented for refractive surgery evaluation. Anterior curvature maps show an asymmetric bowtie with skewed radial axis (AB-SRAX; ≈30 degrees) but with low astigmatism in both eyes. Pachymetry maps show thin central pachymetry in both eyes, with good between-eye symmetry and minimal displacement of the thinnest point in either eye. There is a minimally increased anterior elevation in both eyes that is shifted slightly temporally. There are no focal elevation changes posterior elevation maps (lower right) in either eye.
Very Thin Cornea
Figure 6-2-3. Scanning slit imaging of the (A) right and (B) left eyes from a patient who presented for refractive surgery evaluation. Anterior curvature show symmetric with-the-rule bowtie patterns in both eyes. Pachymetry maps (lower right) show thin central thickness (≈460 µm). Anterior and posterior elevations (floats) are unremarkable in both eyes.
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