In addition to the limitations of the specific algorithms and the variations in terminology among manufacturers, the accuracy of corneal topography may be affected by other potential problems:
tear-film effects
misalignment (misaligned corneal topography may give a false impression of corneal apex decentration suggestive of keratoconus)
instability (test-to-test variation) insensitivity to focus errors
limited area of coverage (central and limbal)
decreased accuracy of corneal power simulation measurements (SIM K) after refractive surgical procedures
decreased accuracy of posterior surface elevation values in the presence of corneal opacities or, often, after refractive surgery (with scanning-slit technology)
Roberts C. Corneal topography: a review of terms and concepts. J Cataract Refract Surg. 1996; 22(5):624–629.
Corneal Tomography
Whereas surface corneal curvature (power) is best expressed by Placido imaging, overall corneal shape, including spatial thickness profiles, is best expressed by computed tomography. A variety of imaging systems are available that take multiple slit images and reconstruct them into a corneal-shape profile, including anterior and posterior corneal elevation data. These include scanning-slit technology and Scheimpflug-based imaging systems (Fig 1-12). To represent shape directly, color maps may be used to display a z-height from an arbitrary plane such as the iris plane; however, in order to be clinically useful, corneal surface maps are plotted to show differences from best-fit spheres or other objects that closely mimic the normal corneal shape (Fig 1-13). In general, each device calculates the best-fit sphere for each map individually. For this reason, comparing elevation maps is not exact because they frequently have different referenced best-fit sphere characteristics.
Figure 1-12 Different options for corneal imaging. All images are of the same patient taken at the same visit. A, Placido disk–based corneal curvature map showing axial and tangential curvature maps as well as the elevation map and the Placido rings image. Recall that this mapping technology analyzes only the surface characteristics of the cornea. B, Optical coherence tomography (OCT) image of the same cornea shown in A. Note that the corneal thickness profile (of the stroma as well as the epithelium) is well demonstrated, but the overall surface curvature is not. Had this patient previously undergone either LASIK or Descemet membrane–stripping keratoplasty (DSEK), which he has not, the demarcation line would have been well imaged with this technology. C, Corneal tomography image using dual Scheimpflug/Placido–based technology of the same patient and eye shown in A and B. The surface curvature, pachymetry, and anterior and posterior elevation mappings are demonstrated. Numerical values are shown along the right side. D, Wavescan image from a device like that illustrated in Fig 1-1A, taken of the fellow eye to that represented in A, B, and C. Note that this map does not show any corneal surface contours or features but rather provides information about the optics of the entire ocular system. As such, it can provide information on the refractive error and aberrations of the entire eye. (Images courtesy of M. Bowes Hamill, MD.)
Figure 1-13 Height maps (typically in µm). A, Height relative to plane surface; z1 is below the surface parallel to the corneal apex, and z2 is above the surface parallel to the corneal limbus. B, Height relative to reference sphere; z3 is below a flat sphere of radius r1, and z4 is above a steep sphere of radius r2. (Illustration by Christine Gralapp.)
Elevation-based tomography is especially helpful in refractive surgery for depicting the anterior and posterior surface shapes of the cornea and lens. With such information, alterations to the shape of the ocular structures can be determined with greater accuracy, especially postoperative changes.
Indications for Corneal Imaging in Refractive Surgery
Corneal topography is an essential part of the preoperative evaluation of refractive surgery candidates. About two-thirds of patients with normal corneas have a symmetric astigmatism pattern that is round, oval, or bow-tie shaped (see Fig 1-10). Asymmetric patterns include asymmetric bowtie patterns, inferior steepening, superior steepening, skewed radial axes, or other nonspecific irregularities.
Corneal topography detects irregular astigmatism, which may result from abnormal tear film,
contact lens warpage, keratoconus and other corneal ectatic disorders, corneal surgery, trauma, scarring, and postinflammatory or degenerative conditions. Repeat topographic examinations may be helpful when the underlying etiology is in question, especially in cases of suspicious steepening patterns in patients who wear contact lenses or who have an abnormal tear film. Contact lens wearers often benefit from extended periods without contact lens wear prior to preoperative planning for refractive surgery; this period allows the corneal map and refraction to stabilize. Patients with keratoconus or other ectatic disorders are not routinely considered for ablative keratorefractive surgery because the abnormal cornea has an unpredictable response and/or progressive ectasia. Forme fruste, or subclinical, keratoconus typically is considered a contraindication to ablative refractive surgery. Studies are under way to determine the suitability of some keratorefractive procedures in combination with corneal collagen crosslinking as alternative therapeutic modalities for these patients (see also Chapter 7).
Corneal topography and tomography can also be used to demonstrate the effects of keratorefractive procedures. Preoperative and postoperative maps may be compared to determine the refractive effect achieved (difference map; Fig 1-14). Corneal mapping can also help explain unexpected results, including undercorrection and overcorrection, induced astigmatism, and induced aberrations from small optical zones, decentered ablations, or central islands (Fig 1-15).
