Topography
Corneal topography is based on the Placido disk principle of reflecting images of multiple concentric rings onto the cornea; in corneal topography, the projected images are digitally captured and analyzed by computer software. Generally, in steeper parts of the cornea, the reflected mires appear closer together and thinner and the axis of the central mire is shorter. Conversely, along the flat axis, the mires are farther apart and thicker and the central mire is longer. Thousands of points are measured, and, using complex algorithms, the software generates color-coded maps of the topographic curvature of the cornea. These topographers assess only the anterior corneal surface and tear film.
There are 3 types of maps. The first is the axial curvature map, which closely approximates the power of the central 1–2 mm of the cornea but fails to describe the true shape and power of the peripheral cornea. The second is a tangential map, which typically shows better sensitivity to peripheral changes, with less “smoothing” of the curvature than the axial map (Fig 2-14). (In these maps, diopters are relative units of curvature and do not equate to diopters of corneal power.) A third map, the mean curvature map, uses an infinite number of spheres to fit the curvature. The algorithm determines a minimum- and maximum-size best-fit sphere and, from the radius of each, determines an average curvature (arithmetic mean of the principal curvatures) known as the mean curvature. These powers are then mapped using standard colors to represent diopter changes, allowing for more sensitivity to peripheral changes of curvature (Fig 2-15).
Before any topographic maps are interpreted, the color scale needs to be evaluated, because it can grossly influence the interpretation of the map. Arranging the color bar in 0.50-D increments may improve accuracy in interpretation of the map. Smaller increments greatly exaggerate minor or normal changes and increase sensitivity. Larger increments decrease sensitivity and can mask significant changes. The absolute scale is constant for all examinations and is useful for comparisons over time and between patients. The normalized or relative scale adjusts to the range of powers on the corneal surface and differs for each cornea. Thus, the power range and step size may be narrow or broad, thereby magnifying or minifying significant changes. Besides the limitations of the algorithms and the variations in terminology used by manufacturers, various other problems may affect the accuracy of corneal topography:
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misalignment
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limited stability (test-to-test variation)
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sensitivity to focus errors
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tear film effects
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distortions
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limited area of coverage (central and limbal)
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no standardized data maps
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colors that may be absolute or varied (normalized)
Despite these limitations, corneal topography maps allow clinicians to detect forme fruste and frank keratoconus in refractive surgery screenings and identify and predict corneal ectasia following refractive surgery. A normal astigmatic profile has a symmetric bow-tie pattern (Fig 2-16). Keratoconus can be represented by a central cone or inferotemporal steepening (Fig 2-17) (see Chapter 7 for a discussion of keratoconus). Skew deviations and asymmetric bow ties associated with focal steepening inferiorly are also suggestive of keratoconus. The ratio between the inferior and superior keratometry values can be indicative of keratoconus. The inferior–superior value (I–S value) is derived by calculating the difference between inferior and superior corneal curvature measurements at a defined set of 5 points above and below the horizontal meridian. I–S values greater than 1.4 and central corneal powers greater than 47.2 D are all suggestive of corneal ectatic disorders, but there is some overlap between normal and abnormal eyes (Table 2-1). Pellucid marginal degeneration has a characteristic “crab-claw” configuration with against-the-rule astigmatism (Fig 2-18). However, this pattern is not pathognomonic, as it can also be seen in keratoconus. Corneal topography can aid in accurately monitoring progression of these ectatic corneal diseases and in planning procedures such as corneal crosslinking. Prior refractive surgery leads to central flattening in myopic corrections and steepening in hyperopic corrections.
Table 2-1 Corneal Topography Signs Suggestive of Keratoconus or Corneal Ectasia
Corneal mapping is useful in managing congenital and postoperative astigmatism, particularly following penetrating keratoplasty. Complex peripheral patterns may result in a refractive axis of astigmatism that is not aligned with a topographic axis.
In addition to displaying power maps, computerized topography systems may display other data: pupil size and location, indices estimating degrees of regular and irregular astigmatism, estimates of the probability that the patient has keratoconus, simulated corneal curvature measurements, wavefront analysis, results of dry eye screening, meibomian gland imagery, and chord μ (angle kappa).
Angle kappa, the angle formed between the visual axis and the optical (or pupillary) axis, is particularly useful in the selection of basic or premium IOLs for cataract surgery and in detailed planning of astigmatic correction. Generally, an angle kappa less than 0.4 mm is desired. If angle kappa is greater than 0.4 mm, the clinician may choose to avoid multifocal IOLs. The ideal amount of spherical aberrations varies by individual, and appropriate IOL selection might offset these aberrations.
Corneal mapping can also be useful in the selection of lens implants for patients with previous refractive surgery who are considering cataract surgery. Hyperopic ablation induces negative spherical aberrations because the central cornea is steeper, and myopic ablation induces positive spherical aberrations because the central cornea is flattened. To avoid halos in eyes with positive aberrations, the clinician may consider an IOL with negative or zero spherical aberrations.
Tomography
Placido disk–based topography describes only the surface corneal curvature (power), whereas corneal tomography computes a 3-dimensional image of the cornea, providing details such as the anterior and posterior corneal curvature, corneal thickness, and anterior chamber depth, as well as information on the iris and lens. Various technologies, including scanning-slit and Scheimpflug imaging, are utilized to obtain these images.
Scanning-slit technology has been combined with Placido disk–based topography; this combination of technologies derives its posterior elevation map mathematically. This map may overestimate the posterior corneal curvature, however, especially in patients who have undergone LASIK procedures.
Other systems are based on the Scheimpflug principle, which is a geometric rule that describes the orientation of the plane of focus of an optical system, such as a camera, when the lens plane is not parallel to the image plane. The principle is named after Austrian army captain Theodor Scheimpflug, who used it to devise an apparatus for correcting perspective distortion in aerial photography. In Scheimpflug-based corneal tomography, a thin layer of the cornea is illuminated with the slit. Because they are not entirely transparent, the cells scatter the light, creating a sectional image. This image is photographed according to the Scheimpflug principle, resulting in an image of the illuminated plane in complete focus from the anterior surface of the cornea to the posterior surface of the crystalline lens. A number of devices have employed the Scheimpflug principle with rotational scanning, dual rotational scanning, and Placido disk–based topography.
Scheimpflug-based systems present considerable information, including anterior curvature, corneal thickness, anterior chamber depth, anterior and posterior elevation, and pupil diameter. As a result, the thickness and topography of the entire anterior and posterior surface of the cornea can be displayed (Fig 2-19). In addition, a densitometry function measures the amount of corneal or lens opacification.
Corneal tomography systems also employ keratoconus detection and classification algorithms. In keratoconus suspects, corneal tomography may provide additional useful information, as it may reveal subtle changes in the posterior corneal curvature that can precede anterior steepening. Measurements of the posterior corneal curvature and thickness provide important confirmation of thinning and steepening (Fig 2-20).
See Table 2-2 for an overview of methods and instruments used for corneal topography and tomography.
Table 2-2 Methods and Instruments for Corneal Topography and Tomography
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Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.