Basic Topographic Patterns and Tomographic Correlates
J. Bradley Randleman, MD; Marcony R. Santhiago, MD, PhD; and William J. Dupps, MD, PhD
Topography/tomography pattern recognition remains the mainstay of patient evaluation, as expert pattern recognition provides a more detailed corneal analysis than any automated screening techniques or strategies. Recognizing the most common normal, suspicious, and abnormal patterns is the first step of corneal imaging interpretation. The hallmark of corneal ectatic disorders is asymmetry. This asymmetry manifests in irregular curvature patterns, focal asymmetric corneal thinning, focal changes in elevations, induced lower- and higher-order aberrations, and interocular (between eye) asymmetry. As corneal ectasias worsen, asymmetry increases.
Topographic and tomographic patterns are easy to recognize in their extreme manifestations; subtler presentations, however, are frequently overlooked or misinterpreted. Any pattern displaying asymmetry warrants detailed evaluation.
Normal patterns include nonastigmatic patterns (ie, round, oval, or nondescript patterns) and symmetric astigmatic patterns. Truly symmetric astigmatic patterns can be oriented in any meridian, typically categorized as vertical (with the rule), horizontal (against the rule), or oblique orientations. As astigmatism increases, topographic patterns become more distinct and identifiable.
Figure 3-A. Composite image showing pattern manifestations in eyes with low amounts of astigmatism with overall unremarkable topographic patterns.
Asymmetric astigmatic patterns represent the most challenging to interpret clinically, as they range from normal variants to highly suspicious/abnormal cases.
The patterns shown in the abnormal category are representations of corneal ectatic processes in various stages. While there may be discrepancy from source to source regarding terminology, we have used the following terminology and pattern classifications: against-the-rule astigmatism, inferior steepening, focally steep, skewed radial axes, asymmetric bowtie with skewed radial axes (AB-SRAX), truncated bowtie, vertical D, drooping D, and pellucid marginal degeneration (PMD)–like (also called crab claw) patterns.
Figure 3-B. Composite image showing progressively asymmetric anterior curvature, ranging from clearly normal to clearly abnormal. (A) Placido image showing a regular anterior curvature pattern with mild, symmetric astigmatism. (B) Placido image showing a mildly truncated bowtie anterior curvature pattern. This focal steepening centrally is concerning for an early ectatic corneal disorder (suspicious). (C) Placido image showing moderate anterior curvature asymmetry with inferior steepening. This pattern is concerning for an early ectatic corneal disorder (suspicious). (D) Placido image showing focal anterior curvature steepening in the inferior periphery and a “crab claw”–type appearance. This pattern is indicative of an ectatic corneal disorder (abnormal). (E) Placido image showing more pronounced focal anterior curvature steepening in the inferior periphery and a crab claw–type appearance. This pattern is indicative of an ectatic corneal disorder (abnormal). (F) Placido image showing more pronounced focal anterior curvature steepening in the inferior periphery. This pattern is indicative of an ectatic corneal disorder (abnormal).
The patterns demonstrated in this chapter arise from multiple imaging technologies, the majority of which are Placido, Scheimpflug, and dual Scheimpflug/Placido imaging. Thus, while some images do not have tomographic correlates, most do. Multiple maps are shown from many cases to provide the clearest picture possible about the nature of the pattern and the hallmarks for its identification.
Of note, images shown in this chapter arose from optically clear corneas free of any scarring that influenced topographic pattern unless indicated. Additional irregular patterns resulting from scars and opacities are shown in Chapter 8 (“Corneal and Refractive Surgery Complications”) and Chapter 9 (“Clinical/Topographic Correlations”).
Finally, as the classifications used are obviously artificially segregated, there are significant overlaps in pattern groupings, particularly for suspicious and abnormal pattern groupings. The categorization of images is done to facilitate recognition or particular pattern elements.
SECTION 1: SYMMETRIC NONASTIGMATIC PATTERNS (NORMAL PATTERNS AND VARIANTS)
The round, oval, minimally astigmatic pattern is commonly found in normal corneas. The normal unoperated cornea is slightly steeper in the center, but the gradation from periphery to center is typically small. In corneas with minimal astigmatism, there will be a rather unremarkable, amorphous central pattern. In eyes with low amounts of astigmatism, there will be a slight bowtie pattern that becomes more easily discernible as the amount of astigmatism increases. These patterns are notable for within- and between-eye symmetry.
In eyes with round or oval anterior curvature patterns, there are typically unremarkable tomographic findings. Thickness maps tend to be well centered with normal thickness progressions, and elevations are low and well centered.
Figure 3-1-1. Placido maps of the (A) right and (B) left eyes from a patient showing a regular anterior curvature pattern with good within-eye and between-eye symmetry, minimal central steepening, and an inferior-superior value (I-S value) less than 0.5 D in both eyes.
Figure 3-1-2. Placido maps showing a regular anterior curvature pattern bilaterally, with good within-eye and between-eye symmetry and no significant focal steepening evident in tangential images (lower images). Figure 3-1-3. Placido maps showing regular anterior curvature patterns bilaterally, with-the-rule astigmatism (symmetric bowtie pattern), good within-eye and between-eye symmetry, and no significant focal steepening evident in tangential images (lower images). Figure 3-1-4. Placido maps showing a regular anterior curvature pattern bilaterally, with-the-rule astigmatism (symmetric bowtie pattern), good within-eye and between-eye symmetry, and mild with-the-rule astigmatism (symmetric bowtie pattern) evident in tangential images (lower images). Figure 3-1-5. (A) Scheimpflug imaging of both eyes from a patient showing normal, symmetric anterior curvature (upper right and left images), normal central and regional corneal thickness (lower right and left images), and good between-eye symmetry for both curvature and thickness. (B) Scheimpflug refractive map of the right eye from the same patient showing normal, symmetric anterior curvature (upper left), normal central and regional corneal thickness (lower left), and unremarkable anterior and posterior elevation maps (right), with no focal elevations on either surface. (C) Scheimpflug topometric map from the same eye showing normal anterior and posterior axial curvature values and asymmetry values within a normal range for all indices. Figure 3-1-5. (D) Scheimpflug ectasia display showing normal front and back elevations, normal relational thickness profiles, and values within a normal range for all indices. (E) Scheimpflug Zernike analysis map showing normal higher-order aberration values for all measures including coma and spherical aberration. Figure 3-1-6. (A) Scheimpflug overview imaging of both eyes from a patient showing normal, symmetric anterior curvature (upper right and left images), thinner but still normal central and regional corneal thickness (lower right and left images), and good between-eye symmetry for both curvature and thickness. Scheimpflug refractive map of the (B) right and (C) left eyes from the same patient showing normal, symmetric anterior curvature (upper left), thinner but still normal central and regional corneal thickness (lower left), and unremarkable anterior and posterior elevation maps, with minimal but unremarkable focal elevations in both eyes. Figure 3-1-6. Scheimpflug ectasia display for the same (D) right and (E) left eyes showing normal front and back elevations, normal relational thickness profiles, and values within a normal range for all major indices. Figure 3-1-7. Dual Scheimpflug/Placido image of the left eye from a patient showing essentially normal anterior surface curvature, with mild central steepening not following any specific pattern, normal corneal thickness with no displacement of the thinnest point, and no abnormal focal elevation on the anterior or posterior surface. Note the simulated keratometry (SimK) astigmatism is less than 0.5 D, similar to the patterns shown in Figure 3-A.
SECTION 2: SYMMETRIC ASTIGMATIC PATTERNS (NORMAL VARIANTS)
As corneal astigmatism increases, the classic bowtie pattern becomes easily discernible. Normal astigmatic corneas have pronounced patterns but remain highly symmetric. Because a normal cornea with astigmatism has a shape akin to a straight cylinder along its entire steep meridian, the resulting bowtie pattern shape should extend through the majority of the meridian and remain orthogonal (without skew).
Figure 3-2-1. (A) Placido maps and ring images of the right and left eyes showing a regular anterior curvature pattern bilaterally with significant with-the-rule astigmatism (symmetric bowtie pattern) and good within-eye and between-eye symmetry. (B) The same Placido image as A with black arrows highlighting the regular astigmatic pattern. Note the bowtie pattern extends to the periphery and there is no skewing of either axis.
Figure 3-2-2. Dual Scheimpflug/Placido maps of the (A) right and (B) left eyes from a patient showing a regular anterior curvature pattern bilaterally, with significant with-the-rule astigmatism (symmetric bowtie pattern) in both eyes, good within-eye and between-eye symmetry, and normal central and peripheral corneal thickness in both eyes (upper right).
Figure 3-2-2. (C) The same dual Scheimpflug/Placido map as in A showing the anterior curvature map with a black arrow to highlight the regular astigmatic pattern. Placido maps of the same (D) right and (E) left eyes showing pattern stability over 4 successive visits (10 months). Figure 3-2-3. Scheimpflug maps of the (A) right and (B) left eyes from a patient showing a regular anterior curvature pattern bilaterally (upper left) with significant with-the-rule astigmatism, good within-eye and between-eye symmetry, and normal central and peripheral corneal thickness (lower left) and elevations (right images) in both eyes. Figure 3-2-3. (C) The same Scheimpflug maps of the right eye in A, with black arrows highlighting the regular astigmatic pattern in anterior curvature (upper left) and both anterior (upper right) and posterior (lower right) elevations. Scheimpflug topometric map of the same (D) right and (E) left eyes, showing normal, regular anterior and posterior axial curvature values. The index of surface variance value falls outside the normal range in both eyes, but other asymmetry indices are within the normal range. Note extensive interpolation of missing data (black stippling) in peripheral regions highlighted by the wide 12-mm axis ranges in this view. Figure 3-2-3. Scheimpflug ectasia display of the same (F) left and (G) right eyes, showing normal front and back elevations, normal relational thickness profiles, and normal D score metrics. Figure 3-2-4. (A) Scheimpflug maps of the left eye showing a regular anterior curvature pattern bilaterally (upper left), with significant with-the-rule astigmatism, a thin cornea (lower left), and normal elevations (right images). (B) The same Scheimpflug map of the left eye in A with a black arrow highlighting the regular astigmatic pattern in anterior curvature. There is a small skewed axis (15 degrees), which is determined as the distance in degrees between the steep meridian (superior arrowhead) and solid black line passing through 90 degrees. Figure 3-2-4. (C) Scanning slit map of the same left eye showing analogous findings in Placido-based anterior curvature (lower left), corneal thickness (lower right), and elevations (upper images) as those obtained with Scheimpflug imaging. (D) The same scanning slit imaging of the left eye in C with a black arrow highlighting the regular astigmatic pattern in anterior curvature. In this imaging device, there is no skewed pattern, although this may be due to the truncated nature of the image due to data loss superiorly and inferiorly with the scanning slit beam Placido device in use. In this eye, despite the relatively normal curvature pattern, the low corneal thickness is cause for concern for refractive surgery screening. Figure 3-2-5. Scheimpflug refractive maps of the (A) right and (B) left eyes in a patient with regular, oblique astigmatism showing a regular anterior curvature pattern bilaterally (upper left), with significant oblique astigmatism, good within-eye and between-eye symmetry, thick central and peripheral corneal thickness (lower left), and normal anterior and posterior elevations (right images) in both eyes. Figure 3-2-6. Dual Scheimpflug/Placido maps of the (A) right and (B) left eyes in a patient with oblique astigmatism. Note the mild asymmetry in the left eye and mild between-eye asymmetry. Thickness values are normal and symmetric between eyes. There is no apparent coma in the right eye, while the left eye has more significant coma (lower right).
SECTION 3: ASYMMETRIC ASTIGMATIC PATTERNS (SUSPICIOUS PATTERNS)
Asymmetric patterns may still be variants of normal and/or minimally concerning in patient screening if they exhibit only mild asymmetry and/or occur with other findings that lessen their significance. Classic examples include an asymmetric bowtie pattern with less than 1 diopter (D) of asymmetry, or an asymmetric bowtie with steepening coincident with epithelial hypertrophy (see Chapter 4).
As asymmetry increases, concern grows for a corneal ectatic process. Increasing asymmetry can manifest either as an increased relative steepening in one meridian as compared to its opposite, as increasing deviation of the radial axis (called skewing), or both. Small skewing is typically not significant, while larger angles of skew show clinically significant irregularity.
As asymmetry progresses, it becomes easier to identify corneal pathology. Yet, even within the abnormal category, there are ranges of presentations. Three patterns that are challenging to recognize include vertically steep patterns (also called vertical D patterns), against-the-rule patterns with skewed axes, and centrally steep patterns that manifest as truncated bowtie patterns.
Figure 3-3-1. Placido map showing a classic asymmetric bowtie pattern, with up to 1 D of inferior steepening compared to superior, and no skewing of the radial axis. I-S ratio is 1.46 D.
Figure 3-3-2. (A) Dual Scheimpflug/Placido maps showing an asymmetric anterior curvature pattern, with-the-rule astigmatism (asymmetric bowtie pattern), thinner central corneal thickness (upper right), and significant coma (lower right). (B) Composite dual Scheimpflug/Placido anterior curvature map of the same eye, with a black arrow highlighting the asymmetric but regular astigmatic pattern in anterior curvature. Cone Location and Magnitude Index (machine-derived white circle) is 1.58 D, indicating a notable amount of asymmetry identified by the device. Figure 3-3-3. (A) Dual Scheimpflug/Placido map of the right eye from a different patient showing an asymmetric anterior curvature pattern, with mild skewing of the radial axis, normal central and peripheral corneal thickness (upper right), and no focal anterior or posterior surface elevation abnormalities (lower images). (B) Scheimpflug refractive map of the same eye showing a similar but less distinct asymmetric anterior curvature pattern (upper left) without obvious skewing of the radial axis, normal corneal thickness (lower left) with no displacement of the thinnest point, and no significant focal anterior or posterior surface elevations (right). Figure 3-3-3. (C) Scheimpflug topometric map of the same eye showing keratoconus index and index of height decentration (IHD) indices identified as suspicious/abnormal. (D) Scheimpflug ectasia display showing mild focal elevation on the anterior surface (lower left), with higher than typical D metrics. (E) Scheimpflug Zernike display showing elevated coma inferiorly (-0.438 μm). Figure 3-3-4. (A) Scheimpflug composite maps of the right and left eyes of a young patient with Marfan syndrome, showing an overview of anterior curvature, corneal thickness, and posterior elevation. The 2 eyes are similar in their overall appearance. Figure 3-3-4. (B) Scheimpflug refractive display of the same right eye showing significant astigmatism, truncated asymmetric anterior curvature pattern with mild skewing of the radial axes (< 20 degrees), thin central corneal thickness (lower left) with mild displacement of the thinnest point, and maximum anterior and posterior elevations that are shifted temporally (right images). These corneal images are suspicious for a weaker cornea due to their asymmetry combined with thin corneas. (C) Scheimpflug topometric map of the same right eye showing multiple asymmetry indices identified as abnormal. (D) Scheimpflug ectasia display of the same right eye showing mild focal elevation on anterior elevation with multiple indices identified as abnormal. Figure 3-3-5. Dual Scheimpflug/Placido maps of the (A) right and (B) left eyes in a patient with overall steep corneas (> 47 D), significant astigmatism (> 2 D) in both eyes, an asymmetric anterior curvature pattern with between 1 to 2 D of superior-inferior asymmetry, mild skewing of the radial axes of less than 20 degrees in both eyes, low normal central corneal thickness (upper right) that is well centered in both eyes, and no demonstrable findings on elevation maps. These corneal images are suspicious for a weaker cornea due to their anterior curvature asymmetry. (C) Close up of anterior curvature map from dual Scheimpflug/Placido imaging of the left eye showing the mild skewing of less than 20 degrees (difference between inferior arrow and line bisecting 270 degrees). Figure 3-3-5. (D) Scheimpflug maps of the right and left eyes showing similar asymmetric anterior curvature patterns with more pronounced inferior steepening as compared to dual Scheimpflug/Placido imaging, with thin/normal central corneal thickness and minimal thinnest point displacement in both eyes. (E) Scheimpflug topometric map for the same right eye showing the same asymmetric steepening on anterior and, to a lesser extent, posterior curvature, with an elevated IHD but otherwise without asymmetry indices identified as abnormal. (F) Scheimpflug ectasia display for the same right eye showing no significant findings on thickness or anterior or posterior elevation (left images), but with deviation of average pachymetric progression index (Dp) and the overall D score coded in the suspicious range.
SECTION 4: ABNORMAL ASYMMETRIC PATTERNS
Table 3-1
ABNORMAL PATTERNS
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*Against-the-rule steepening is not a definitively abnormal pattern, but in many instances, subtle asymmetries in this orientation are missed; therefore, all cases with this pattern should be thoroughly evaluated. |
Against-the-rule astigmatism, with the steepest corneal meridian around 180 degrees and resembling a proper bowtie on topography, can be a completely normal finding in a non-ectatic cornea, especially in older patients. Abnormal patterns oriented against the rule, however, are overlooked more frequently than more traditional inferior steepening patterns (discussed in the next section). Against-the-rule astigmatism is common in older patients, such as those presenting for cataract surgery, but is an uncommon and unusual pattern to find in younger patients, such as those presenting for corneal refractive surgery. All against-the-rule patterns, therefore, warrant added review to determine if there is any skewing of the axes and/or other manifestations of asymmetry.
Figure 3-4-1. Placido maps of a patient with low levels of against-the-rule astigmatism in both eyes (axis 175 right eye, 180 left eye). There does not appear to be any significant skewing of the radial axes, although a small degree of skew may be present in both eyes nasally. There is significant between-eye asymmetry in both pattern and amount of astigmatism, and the steepening pattern in the right eye is truncated. These findings make this overall imaging pattern highly suspicious at a minimum. Note the I-S value is negative in the right eye; in the case of against-the-rule patterns, the I-S value has no significance.
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