Corneal topography devices are becoming increasingly more sophisticated with additional features and measurement capabilities such as wavefront aberrometry, OCT, Scheimpflug imaging, as well as improved software programs for detecting corneal abnormalities. The pearls presented here are aimed at interpretation of the more common curvature and elevation maps.
- Lack of standardization: Measurement and data presentation differ among different devices, therefore, intermachine comparison of maps is difficult. It is important to be aware of what device produced the map you are interpreting.
- Scale matters: The dioptric range, step size and number of colors influence the sensitivity of the image.
- Absolute scale: these factors are constant which allows for easier comparison to other exams.
- Relative/normalized scale: these factors adapt to the range of powers on the corneal surface and vary for each cornea. The same map plotted with a different scale or a different step size looks very different. Relative scales or large step sizes mask characteristic patterns of irregular astigmatism (i.e. keratoconus), while small step sizes tend to exaggerate normal patterns to appear like abnormal ones.
The Klyce/Wilson scale (ranging from 28.0 D to 65.5 D in 1.5 D steps) or the Universal Standard Scale (30.0 D to 67.5 D in 1.5 D steps) provides the widest range of powers without sacrificing the sensitivity to detect clinically significant features.
- Type of map influences appearance: The most common map is the sagittal or axial map, which gives an average picture (i.e. smoother appearance) of the anterior corneal curvature and is mainly used for screening. The tangential or instantaneous map shows true radius of curvature data at each point but appears more noisy/irregular. Refractive power maps are less commonly used but are most helpful for understanding the imaging power of the cornea and analyzing surgical effects.
- Characteristic patterns of irregular astigmatism include:
- Keratoconus (KC) and form fruste KC: Central or inferior steepening, asymmetric skewed bow-tie. Most devices have KC detection software, and three specific parameters can also be used to aid in the diagnosis:
- Central corneal power > 47.2 D
- Difference in corneal power between fellow eyes > 0.92 D
- I-S value (difference between average inferior and superior corneal powers 3 mm from the center of the cornea) >1.4 D
- Pellucid Marginal Degeneration (PMD): Irregular, against-the-rule astigmatism with an inferior annular pattern of steepening (“inferior claw” or “lazy C” pattern).
- Contact lens-induced corneal warpage: Irregular astigmatism that usually mimics KC (i.e., pseudokeratoconus) but may show superior steepening. Depends upon contact lens fit.
- Recognize topographic artifact: This can be produced by misalignment, improper focus, pressure on the globe, poor tear film, and lubricating eyedrops. If suspected, repeat the exam.
- Posterior elevation maps: Commonly seen from the Pentacam or Orbscan, have a completely different appearance and show focal elevations and depressions from a reference surface (i.e., best-fit sphere). The posterior float numerical data on this type of map is more sensitive for detecting corneal ectasias than the color pattern on curvature maps. A hot spot on a posterior float map (>20 microns on Pentacam or >40 microns on Orbscan is abnormal) is suspicious for ectasia. Posterior float can be difficult to interpret if the patient has had previous laser vision correction.
- Pachymetry maps: These are helpful for detecting thin corneas and correlating points of abnormal thinning to those of steep curvature and/or abnormal corneal float.