KEY CONCEPTS
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Corneal topography is an important tool for detecting corneal morphology, enabling both correct classification of keratoconus (KC) and detection of suspicious or initial patterns of the disease.
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Different color combinations can be used to help interpretation of corneal topographic maps and can increase or decrease the sensitivity for detecting deviations from normality.
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Qualitative information generated by the axial and tangential curvature maps of the anterior surface of the cornea is very useful in the screening of patients with suspected KC, especially for the experienced clinician.
Introduction
Keratoconus (KC) is defined as a progressive, non-inflammatory, bilateral, and asymmetric disease that leads to bulging and thinning of the cornea generating irregular astigmatism and visual impairment. Early recognition and correct diagnosis of the disease are essential and allow prevention of complications. Alternatives to halt progression of the disease such as corneal collagen cross-linking potentially avoid corneal transplantation in many young patients. ,
Anamnesis and a careful physical examination are sometimes insufficient for detecting KC, especially because in the early stages of the disease there may be no symptoms or loss of best-corrected visual acuity. Corneal topography is an important tool for detecting corneal morphology, enabling both correct classification of KC and detection of suspicious or initial patterns of the disease. ,
Corneal Topography
Videokeratoscopy, based on Placido disk topography, is based on the principle that the cornea behaves like a convex mirror, and the reflection of known white placid disks interspersed with black color disks directly on the surface of the cornea can be measured and analyzed.
The distance in millimeters between the disks is converted to diopters using a formula that uses the theoretical refractive index of the cornea. This information allows evaluation of thousands of data points both in the center and at the periphery of the cornea. Greater distances between the disks indicate flatter corneas and are represented by cooler (blue, violet, purple) colors, whereas smaller distances between disks reveal more curved or prolate corneas and are represented by warmer colors (yellow, orange, red). The shades of green, in general, represent measurements more compatible with the aspheric pattern of a normal cornea. ,
To extract the maximum amount of information with adequate quality, direct observation of the Placido disk map also allows the examiner to observe if there is a lack of information or any inaccuracy. Adequate exposure, centralization of the image, and avoidance of shadowing by the nose and lashes are important to preserve the complete image and linearity of the Placido disk. The quality and the integrity of the tear film and corneal surface may also interfere with the clarity and linearity of the image. Severe dry eye with debris and mucus in the tear film may create an artificially altered surface and generate a secondary irregular curvature ( Fig. 14.1A,B ). Treatment of the dry eye, allergies, and other ocular surface disease is important, and repeating the examination after treatment may be necessary in more severe cases. Modern capture software has quality assessment tools with standard scores to assist the clinician in cases in which the capture is not of sufficient quality for an adequate interpretation of the examination.
In general, corneas with KC, when analyzed through corneal topography, exhibit greater complexity in the interpretation of measurements. This is primarily because keratoconic corneas are more prolate with higher keratometry values and are more irregular in shape. Together with the limitations of technology, this leads to low reproducibility between measurements and adds complexity in the evaluation, especially when comparing different topography and keratometry devices ( Fig. 14.2A,B ).
SCALES
Different color combinations can be used to help interpretation of corneal topographic maps and can increase or decrease the sensitivity for detecting deviations from normality. Absolute scales display fixed color values for each curvature regardless of the map analyzed. Although such scales may not be useful in cases of subtle curvature changes, they allow an appropriate comparison between the two eyes of the same patient, detecting asymmetries between them. ,
Relative scales display the range of curvature calculated based on the specific map. A specific combination of colors can be used and different steps between colors (1.5 diopters [D], 1.0 D, 0.5 D, or 0.25 D) ( Fig. 14.3A–C ). Screening using small steps increases sensitivity and allows detection of subtle and initial KC patterns. These small steps are less interesting for moderate and advanced cases of KC, because those corneas usually have steeper keratometry and a wide variation of value from the apex to the periphery. This combination (small steps plus relative scales) provides quantitatively inaccurate values and generates noisy patterns with red color spreading over large areas of the axial map. Steps of 1.5 D can better locate the apex and the variation from the center to the periphery of the cone. ,
It is important that the examiner chooses a scale that is sensitive enough to detect suspicious patterns but which is not noisy enough to lead to a misclassification of normal corneas as suspicious corneas or KC.
Axial Versus Tangential Map
The more important maps extracted from the Placido disk data are the anterior axial curvature map, the tangential map, the anterior elevation map, and the aberration map (high-order aberration) ( Fig. 14.4A–D ).
The anterior axial curvature map is the reconstruction of the corneal surface by an algorithm that measures the radius of curvature of the cornea centered on the optical axis. This strategy attempts to illustrate the aspherical geometry of the corneal surface. The transition from the center to the periphery on the axial curvature map is smoother, and there is a gradual transition of colors from the center to the periphery of a normal cornea.
In the tangential curvature map , the measurement of the radius of curvature is performed locally at each point of the cornea, generating more precise information about the real radius of curvature at the each analyzed point. This highlights possible outliers, generating a rougher map and showing the peripheral region with greater precision.
In KC, the tangential map highlights the focal region of steepening and detects with greater precision the apex of the cone and the real region of steepening in the cornea ( Fig. 14.5A,B ).
