An in-depth discussion of corneal topography is beyond the scope of this handbook. However, a working knowledge of topography is necessary to perform LASIK safely. Topography is used as a screening tool during the preoperative evaluation to be certain the contour of the cornea is regular (see Index for topographyrelated cases). The surgeon must not only rule out the presence of ectatic diseases such as keratoconus and pellucid marginal degeneration (PMD) in an overt form, but must also be able to detect these conditions in a subclinical or subtle form. Occasionally, an adult may present with mild keratoconus in an arrested form, known as forme fruste keratoconus, which does not progress. The patient may be completely asymptomatic and might never develop the clinical disease unless the cornea is thinned and weakened during LASIK surgery. The age of the patient is an important consideration. When the subclinical form is present in a young adult or teenager, one cannot be certain that the condition will not progress into a manifest ectatic disease over time. It is therefore advisable to avoid LASIK surgery in this situation.

Serial corneal topography can also be used to track the corneal contour in a contact lenswearing patient who is avoiding lens wear in preparation for final LASIK measurements. Topography, keratometry, and refraction are all used during repeated examinations as indicators that the corneal contour has become stable.

Corneal topography is also an essential part of the postoperative management of the LASIK patient. Retreatment should not be performed until the corneal contour has become stable, and topography can help with this determination. Topography can help evaluate a patient with postoperative vision that is less than expected. This can be related to the presence of central or paracentral islands, that is, small, steep, and/or elevated areas detected by topography. Postoperative corneal ectasia following keratorefractive surgery can occur even when no apparent preoperative pathology was detected. The alterations in corneal contour related to flap complications such as epithelial ingrowth, a flap buttonhole, or striae can be evaluated and monitored using this technology. A decentered excimer ablation can also be identified with corneal topography. Comparative maps can help assess the effect of corrective surgical treatment. For all of these reasons, the topography unit is indispensable to the refractive surgeon.

The results from the more common topography units can be confounded in several
important ways. For example, if the patient’s fixation is off, the map will not be meaningful. If the surface of the cornea is irregular as a result of a dry eye or exposure keratopathy, the data collection will be limited. Excessive tearing is associated with an enlarged tear meniscus that can distort the map inferiorly. Finally, some topography units will extrapolate data to fill areas for which data could not be obtained. This extrapolation can be misleading. It may be more reliable to use a unit that shows areas of data dropout as blank areas. The computer analysis used to derive color maps is based on certain physical and mathematical assumptions. The data generated are only as accurate as the information captured and the accuracy of the assumptions made. Therefore, careful interpretation of the output from this equipment in the context of the particular patient is always advised.

Modern topography units may generate maps with the potential to demonstrate corneal power, elevation, and thickness. Before interpreting a map, it is important to look at the scale on which the map is based. Maps generated on different days or on different machines should have the same scale in order to be meaningfully compared. The scale for an axial or power map is given in diopters. Steeper areas are red or orange, and flatter areas are blue or purple. The scale for an elevation maps is given in micrometers. This refers to the micrometer above or below a calculated best-fit sphere. Cornea below the sphere is blue and above the sphere is red or orange. In some cases, the elevation maps are mathematically derived rather than the result of a direct measurement. Pachymetry maps are expressed in micrometers.

The difference between power and elevation can be a source of confusion in interpreting maps. How can a cornea be flat and elevated at the same time? The appearance of a mesa or a flattened mountaintop is a geological structure that is flat and elevated. A similar contour abnormality on the cornea would appear blue (indicating flattening) on a power map and orange or red on an elevation map. Likewise, a spire at the bottom of a valley would appear steep on a power map, but depressed on an elevation map. Understanding the differences between power and elevation gives the surgeon a deeper understanding of the corneal contour when interpreting these maps.