J. Bradley Randleman, MD; Marcony R. Santhiago, MD, PhD; and William J. Dupps, MD, PhD
Identification of ectatic corneal disorders, such as keratoconus, pellucid marginal corneal degeneration, and postoperative ectasia after refractive surgery is one of the primary uses for corneal imaging in clinical practice. While late stage cases are easy to identify with any technology, early stages of disease can be challenging to identify accurately even with multiple technologies.
In Chapters 3 and 4, we introduced the types of curvature, pachymetry, and elevation patterns that are typically associated with ectatic corneal disorders and demonstrated the types of epithelial remodeling that typically occur in corneal ectasias. In this chapter, we will go through the various presentations of ectatic corneas in a systematic way, showing examples that do and do not follow the typical rules for varying stages of disease severity, from suspects to highly asymmetric disease to advanced cases.
There are a variety of maps with multiple indices that are used, and useful to varying extents, in making the diagnosis of an ectatic corneal disorder. We will highlight multiple maps from different devices to show both the useful information to make the correct diagnosis and also show maps of the same eye that might mislead the observer or provide less clarity than hoped.
When considering machine-derived screening metrics, it is important to recognize both the potential benefits and real shortcomings of these; there is simply no substitute for expert human evaluation when identifying ectatic corneas. There are numerous examples in the following pages that will demonstrate this phenomenon, where the overall evaluation of available imaging leads to the clear diagnosis of corneal ectasia but where many machine-based metrics fail to identify asymmetry and/or an ectatic cornea, or where the overall image evaluation does not demonstrate an ectatic process but where metrics show abnormalities.
Keratoconus is by far the most common presentation among corneal ectasias. That said, there is great variability in the patterns that present through imaging in patients with keratoconus. There are numerous classification systems to label the severity of keratoconus. Most of these systems are of minimal clinical value. For this text, we have simply divided imaging presentations into somewhat arbitrary mild, moderate, and severe classifications. There are also varying degrees of between-eye asymmetry that present; as these cases are some of the more challenging eyes to evaluate, we have separated out highly asymmetric disease presentations for their own evaluation.
In severe keratoconus, there are dramatic findings in anterior curvature and corneal thinning. In the most severe cases, the cornea becomes significantly thinned and the epithelial remodeling process begins to shift from thinning over the steepest regions to thickening in the areas of stromal tissue loss or fibrosis-induced flattening. This phenomenon may impact the safety of corneal cross-linking (CXL), because an area of maximal total corneal thickness may be functionally even thinner than predicted due to epithelial hypertrophy. Corneal hydrops occurs when a there is a rapid influx of aqueous into the corneal stroma. This causes significant corneal edema and massive temporary alterations to corneal thickness and curvature. Hydrops resolves over time with scarring that is sometimes visually significant and that occasionally can lead to corneal flattening and improved acuity depending on scar location.
PROGRESSIVELY ADVANCED PRESENTATIONS OF CORNEAL ECTASIAS
Figure 5-A. Placido composite maps of focal central steepening in truncated bowtie patterns for a series of different corneas with progressively more advanced stages of keratoconus from parts A to E.
Figure 5-B. Dual Scheimpflug/Placido composite maps showing anterior curvature (upper) and posterior curvature (lower) for a series of different corneas with progressively more advanced stages of keratoconus from parts A to E.
SECTION 1: CORNEAL ECTASIA SUSPECTS
The nomenclature used to describe these patients with varying stages of keratoconus or other ectatic cornea disorders has evolved over the years. As such, there are many terms that are overlapping and/or used interchangeably when they should have separate distinct meanings. Nowhere is this confusing terminology more acutely present than in describing the patient with suspicious topographic features but without clear evidence of ectatic disease. In the interest of simplicity, we are calling these individuals suspects, but many terms exist to describe similar eyes, and none are fully recognized as correct. None of these patients were offered laser vision correction.
23-year-old patient who presents for refractive surgery evaluation. The patient was a low myope (-3 D) with corrected distance visual acuity (CDVA) of 20/20 OU and no visual quality complaints.
Figure 5-1-1. Scanning slit imaging of the left cornea. Note the inferior steepening (lower left) with otherwise good corneal thickness and only mild posterior float/elevation (upper right). The other eye had similar findings (not shown).
20-year-old patient who presents for refractive surgery evaluation. Patient has high astigmatism (4 D in both eyes) with CDVA of 20/20 in both eyes. There is scissoring on retinoscopy and prominent corneal nerves in both eyes on slit lamp examination.
Figure 5-1-2. (A) Placido imaging of the right and left eyes. The right eye exhibits a truncated mildly asymmetric bowtie pattern, while the left eye has an asymmetric truncated bowtie pattern that has greater asymmetry. The focal nature of steepening in better appreciated on tangential maps. (B) Time domain optical coherence tomography image showing corneal thickness values for the same right and left eyes. Central pachymetry is 460 to 480 µm in both eyes, and the thinnest points (450 to 460 µm) are displaced temporally in both eyes.
Figure 5-1-2. Placido imaging of the (C) right and (D) left eyes with difference maps show no progression over 14 months. 15-year-old patient who presents for an evaluation for keratoconus. The patient has not experienced any recent changes in vision. Patient was diagnosed with amblyopia in the right eye and was patched as a child. Current corrected visual acuity is 20/60 OD and 20/25 OS. There is scissoring on retinoscopy in both eyes. Figure 5-1-3. (A) Scheimpflug comparative images of the right and left eyes. There is a mildly truncated, symmetric bowtie pattern with no skewing of the radial axis in the right eye, with more than 5 D of corneal astigmatism. There is between-eye asymmetry, with a mildly truncated bowtie pattern with skewing of the radial axis up to 30 degrees but less than 3 D of corneal astigmatism. Both corneas are thin centrally but without significant displacement of the thinnest point and are relatively symmetric in overall thickness and pattern. Scheimpflug topometric displays of the (B) right and (C) left eyes. Only one asymmetry index is reported as abnormal in each eye. Scheimpflug ectasia screening display for the (D) right and (E) left eyes. There is a mildly elevated D score on the right but otherwise no demonstrable abnormalities reported on this map for either eye. Figure 5-1-3. Scheimpflug imaging showing exam stability over the course of 10 months for the (F) right and (G) left eyes. 44-year-old patient presents with a 20-year history of bilateral monocular diplopia, right eye worse than left eye. Patient reports ghosting around images in the right eye. CDVA is 20/50 OD, 20/20 OS. The patient has scissoring on retinoscopy reflex in both eyes. Figure 5-1-4. Scheimpflug imaging of the (A) right and (B) left eyes, showing inferior steepening of more than 2 D on axial curvature (upper left) with pronounced focal steepening on tangential curvature (lower left) in both eyes. Corneal thickness is normal and there is no displacement of the thinnest point (middle images) in either eye. There are no demonstrable abnormalities on anterior or posterior elevation (right images) in either eye. Figure 5-1-4. Scheimpflug topometric displays of the (C) right and (D) left eyes. There are multiple asymmetry indices identified as abnormal due to the anterior curvature asymmetry. Figure 5-1-4. Scheimpflug ectasia screening displays for the (E) right and (F) left eyes. There is a mildly elevated front elevation difference map (far left) on the (E) right eye but otherwise no demonstrable abnormalities reported on this display for either eye. (G) Spectral domain optical coherence tomography (SD-OCT) imaging of the right and left eyes. Total corneal thickness findings (upper) appear normal and are analogous to the values obtained with Scheimpflug imaging. There are no focal irregularities in epithelial thickness mapping in either eye (lower images). 27-year-old patient who presented for refractive surgery evaluation. Patient has low myopia with CDVA of 20/20 in both eyes. Patient has no visual quality complaints in either eye. There is scissoring on retinoscopy that is more prominent in the left eye. Figure 5-1-5. (A) Scheimpflug imaging of the right eye showing a mild asymmetric bowtie pattern with skewed radial axis (AB-SRAX) on axial curvature (lower left). Thinnest pachymetric point is slightly displaced temporally and there is an irregular corneal thickness spatial profile (CTSP). There are 2 asymmetry indices that are reported as abnormal (middle). (B) Scheimpflug imaging of the left eye showing a more pronounced AB-SRAX on axial curvature (lower left). Thinnest pachymetric point is displaced temporally to a greater extent than the right eye, and there is an abnormal CTSP. There are multiple asymmetry indices that are reported as abnormal (middle). Figure 5-1-5. Scheimpflug ectasia screening displays showing elevated D scores in the (C) right eye and multiple metrics identified as abnormal in the (D) left eye. (E) SD-OCT imaging of the right and left eyes showing analogous total corneal thickness findings to Scheimpflug imaging (upper), with a normal epithelial profile in the right eye and mild focal epithelial thinning with early surrounding hypertrophy in the left eye (lower right). 25-year-old patient with high myopia presents for refractive surgery evaluation. CDVA is 20/20 OU with -9 D correction OU. Patient has no visual quality complaints in either eye. Figure 5-1-6. Scheimpflug refractive maps of the (A) right and (B) left eyes from the patient show inferior steepening in both eyes, more pronounced in the left eye, with above-average corneal thickness in both eyes with no displacement of the thinnest point in either eye. There is a mild focal anterior surface elevation coincident with the steepest point in both eyes and a mild focal posterior surface elevation coincident with the steepest point and anterior elevation in the right eye but no clear pattern in the left eye. Figure 5-1-6. Scheimpflug topometric maps of the (C) right and (D) left eyes from the same patient. Multiple asymmetry indices are identified as abnormal in both eyes, particularly index of height decentration (IHD) in both eyes. Scheimpflug ectasia screening displays of the (E) right and (F) left eyes from the same patient. There is suspicious anterior surface elevation noted in both eyes and the Df metrics are elevated in both eyes. There are no additional metrics identified as abnormal in either eye. Figure 5-1-6. (G) SD-OCT imaging of the right and left eyes from the same patient. Total corneal thickness (upper) is thicker than average and analogous to Scheimpflug findings. Epithelial mapping appears normal in both eyes, with no focal thinning or hypertrophic reaction in either eye within the central 6 mm. Figure 5-1-6. Scheimpflug difference maps of the (H) right and (I) left eyes showing change in anterior curvature over the course of 1 year. In H, there is continued progression of steepening centrally/temporally over the course of the year. In I, there are fluctuations but no clear pattern of change or steepening. Case note: This case highlights the complexity of screening even when using a variety of imaging modalities. The patient’s anterior curvature appears highly suspicious in both eyes, while corneal thickness does not. The steepening does not appear to be attributable to epithelial hypertrophy, as there is no focal thickening to contribute to steepening; however, only the central 6 mm is available for review. It is possible that epithelial mapping beyond 6 mm would be useful in this case.
The term unilateral keratoconus has been applied to patients with definitive disease in one eye but without findings in the other, seemingly unaffected, eye. The identification of these unilateral eyes has evolved over time as corneal imaging has advanced; nevertheless, the choice of terms is unfortunate, as all current evidence suggests that all naturally occurring corneal ectasias (ie, not postoperative or traumatic) are a bilateral disease process, albeit with asymmetric presentations. We feel a more appropriate term to define this group is highly asymmetric keratoconus, with one seemingly clinically unaffected eye.
The line between highly asymmetric and asymmetric keratoconus is arbitrary, and the distinction is not meant to imply some distinct differences in disease process. Rather, the distinction lies in the findings of the less affected eye. In this grouping, the less affected eye is easier to identify using a combination of imaging technologies, but significant differences remain in the disease presentation between eyes.
Highly Asymmetric (Clinically Unilateral) Keratoconus
Figure 5-2-1. (A) Placido map of the right and left eyes of a patient with highly asymmetric keratoconus. The right (clinically affected) eye has a horizontally oriented asymmetric bowtie with significantly skewed radial axis with pronounced central steepening (upper left). The focal nature of steepening is best identified in the tangential curvature map (middle left). The patient has significant coma and trefoil (lower left). The left (clinically unaffected) eye has faint evidence of central focal steepening with a focal, skewed axis, but less than 1 D of steepening (upper right). There is no focal abnormality present in tangential imaging (middle right) and there is less coma than the right eye but significant trefoil (lower right).
Figure 5-2-1. (B) Placido imaging difference maps of the same clinically unaffected left eye showing no change in curvature over 14 months. Figure 5-2-2. (A) Scheimpflug comparative display showing the right and left eyes of a different patient with highly asymmetric keratoconus. The patient’s right eye has focal inferior steepening up to 8 D with a displaced thinnest point. The patient’s right eye has mild inferior steepening of less than 1 D with minimal displacement of the thinnest point. Figure 5-2-2. Dual Scheimpflug/Placido imaging of the affected (B) right and unaffected (C) left eyes. In the right eye, the upper left axial map shows analogous focal inferior steepening with focal thinning, while there are no focal abnormalities in the left eye. (D) Scheimpflug maps of the same left eye showing analogous curvature and thickness, but there is a difference of 13 μm in thinnest pachymetry between dual Scheimpflug/Placido and Scheimpflug devices. (E) Scheimpflug topometric display of the same left eye shows no asymmetry indices identified as abnormal. (F) Scheimpflug ectasia screening display of the same left eye shows an elevated D score and reduced Ambrósio relational thickness max value. Figure 5-2-2. (G) SD-OCT composite image of the right (upper) and left (lower) eyes from the same patient showing focal total cornea and epithelial thinning in the affected right eye. The temporal region of the left eye appears thinner than anticipated for total thickness (502 μm both centrally and inferotemporally), but there are no readily identifiable focal abnormalities in epithelial thickness (lower right). (H) Scheimpflug difference map showing no change in curvature over 2 years in the left eye. Figure 5-2-3. (A) Scheimpflug comparative display of a different patient with highly asymmetric keratoconus. The right (clinically affected) eye shows a pronounced truncated bowtie pattern with up to 10 D of focal steepening centrally with coincident focal posterior surface elevation. The clinically unaffected left eye shows focal inferior paracentral steepening of 2 D with a mildly increased focal posterior elevation. (B) Dual Scheimpflug/Placido maps of the same right eye showing the same truncated bowtie pattern on Placido mapping (upper left) with a well-centered thinnest pachymetry of 495 μm (upper right) and focal anterior and posterior elevations (lower right and left). (C) Dual Scheimpflug/Placido image of the same left eye showing analogous focal inferior paracentral steepening on Placido imaging (upper left) with thinnest pachymetry of 546 μm (upper right) and only mild anterior and posterior elevations (lower right and left). Figure 5-2-3. (D) Scheimpflug imaging of the same left eye showing analogous findings to the dual Scheimpflug/Placido imaging of Figure 5-2-3C. Thinnest pachymetry measures 535 μm as opposed to 546 μm with dual Scheimpflug/Placido imaging. (E) Scheimpflug topometric imaging of the same left eye. There are 2 asymmetry indices identified as abnormal and multiple indices that are higher than average values. (F) Scheimpflug ectasia screening display shows a mildly elevated D score, but no other variables are identified as suspicious. Figure 5-2-3. (G) SD-OCT composite imaging of the affected right (upper) and unaffected left (lower) eyes. There is focal total and epithelial thinning in the affected right eye. In the left eye, the inferior temporal region appears thinner than anticipated (540 μm both centrally and inferotemporally) and there is subtle focal epithelial thinning with surrounding thickening, which is the hallmark of early ectatic disease. (H) Scheimpflug imaging showing no change in curvature of the clinically unaffected left eye over 3 years. Figure 5-2-4. (A) Scheimpflug comparative display of a different patient with highly asymmetric keratoconus. The right (clinically affected) eye shows a pronounced AB-SRAX pattern, with more than 10 D of focal inferior steepening. The clinically unaffected right eye shows a mildly asymmetric bowtie pattern. Corneal thickness is slightly lower in the affected right eye, but there are otherwise no major differences in pachymetry between eyes. Figure 5-2-4. (B) Dual Scheimpflug/Placido imaging of the same affected right eye showing the same AB-SRAX pattern with 10 D of focal steepening. Thinnest pachymetry is 500 μm and is displaced inferotemporally. There is a clear focal posterior surface elevation. (C) Dual Scheimpflug/Placido imaging of the same clinically unaffected left eye showing a relatively symmetric anterior curvature pattern. Thinnest pachymetry is 535 μm with minimal displacement. There is no focal elevation in anterior or posterior surfaces. (D) Scheimpflug topometric display of the same left eye. There are no asymmetry indices identified as abnormal or suspicious. (E) Scheimpflug ectasia screening display of the same left eye shows no identified abnormalities in any metrics. Figure 5-2-4. (F) SD-OCT composite image of the affected right (upper) and unaffected left (lower) eyes shows focal total and epithelial thinning in the right eye. In the left eye, there is displacement of the thinnest point (lower left) but no focal thinning in epithelial mapping. Figure 5-2-4. Scheimpflug difference maps of the same left eye show no change in (G) anterior curvature or (H) corneal thickness over 15 months.
Figure 5-2-5. (A) Scheimpflug comparative display of a patient with asymmetric keratoconus. The left eye shows a distinct focal inferior steepening pattern, while the right eye has a similar but subtler inferior steepening pattern present. Corneal thickness is lower in the left eye with similar patterns between eyes and both eyes having a slightly displaced thinnest point. Scheimpflug topometric display from the same patient showing coincident focal changes anterior and posterior curvature, (B) with 2 asymmetry indices identified as abnormal in the less affected right eye and similar but (C) more pronounced coincident focal changes anterior and posterior curvature with most asymmetry indices identified as abnormal in the more affected left eye.
Figure 5-2-5. Scheimpflug ectasia screening display of the same (D) right and (E) left eyes showing an increased total D score (2.26) but no other identified abnormalities in the less affected right eye, with multiple metrics identified as abnormal in the left eye. (F) SD-OCT image of the same right and left eyes, showing displacement of the total thinnest point inferotemporally in both eyes, more pronounced in the left eye, with mild focal epithelial thinning temporally in the right eye and more pronounced epithelial thinning with surrounding thickening in the left eye. Figure 5-2-6. (A) Scheimpflug comparative display of a different patient with asymmetric keratoconus, showing an asymmetric bowtie anterior curvature pattern in the right eye with mild inferior steepening, and focal inferior steepening up to 4 D in the left eye. (B) Placido-based anterior curvature map of the same left eye, showing similar asymmetric bowtie pattern with significant inferior steepening. (C) Scanning slit imaging of the same left eye showing analogous inferior steepening with an irregularly thin cornea inferiorly (537 μm centrally and 542 μm inferiorly), with increased anterior and posterior elevation (float) values. Figure 5-2-6. (D) Scheimpflug refractive display of the same right eye, showing a centrally steep symmetric bowtie pattern with mild inferior steepening. Central thickness measures 521 μm. (E) Scheimpflug refractive display of the same left eye with analogous anterior curvature pattern. Central thickness measures 510 μm as compared to 535 μm with scanning slit imaging. (F and G) Scheimpflug topometric displays show coincident focal curvature changes on anterior and posterior surfaces in both eyes, more pronounced in (G) the left eye. There are multiple asymmetry indices identified as abnormal in both eyes. Scheimpflug ectasia screening display of the (H) right and (I) left eyes. There are no identified abnormalities in either eye. Case note: This case highlights the occasional discrepancy in imaging, where there is disagreement between different evaluations. In this case, anterior curvature, asymmetry indices, and subjective pachymetry distribution all confirm the presence of keratoconus, while elevation and pachymetry metrics do not. Figure 5-2-7. (A) Scheimpflug comparative display of a different patient with asymmetric keratoconus, showing distinct focal steepening inferiorly up to 8 D in the right eye, with focal steepening of less than 2 D in the left eye. Despite the significant difference in anterior curvature, there are minimal differences in central or regional corneal thickness between eyes. Scheimpflug topometric displays of the (B) right and (C) left eyes, showing multiple asymmetry indices identified as abnormal in the right eye, but only IHD identified as abnormal in the left eye. Figure 5-2-7. Scheimpflug ectasia screening map showing multiple identified abnormal indices in (D) the right eye, with multiple indices identified as suspicious in (E) the left eye. (F) SD-OCT image of the right and left eyes, showing a displaced total thickness thinnest point in both eyes. There is focal epithelial hyperplasia in both right and left eyes, more pronounced in the right eye (lower images) with focal epithelial thinning noted inferiorly in both eyes within the central 6 mm. Figure 5-2-8. (A) Scheimpflug comparative display of a different patient with asymmetric keratoconus, showing focal inferior steepening in both eyes, more pronounced in the left eye, with up to 4 D in the right eye and more than 10 D in the left eye. There is focal corneal thinning in both eyes, also more pronounced in the left eye. (B) Dual Scheimpflug/Placido imaging of the same less affected right eye showing focal inferior steepening with an inferiorly displaced thinnest point (489 μm), with focal anterior and posterior elevations. (C) Dual Scheimpflug/Placido imaging of the same more affected left eye showing focal inferior steepening up to 10 D (upper left) with an inferotemporally displaced thinnest point (472 μm), with focal anterior and posterior elevations. Figure 5-2-8. (D) SD-OCT composite images of the same right (upper) and left (lower) eyes showing inferior displacement of the total thinnest point in both eyes, with subtle coincident focal epithelial thinning in the right eye and pronounced focal epithelial thinning with adjacent epithelial thickening in the left eye.
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