Ординатура / Офтальмология / Английские материалы / Elevation Based Corneal Tomography 2nd_Belin, Khachikian, Ambrósio_2011

(FIGURE 17) - The image below depicts the topographic changes seen in moderate to advanced keratoconus. The anterior elevation map (lower left) shows a well defined slightly inferior cone with a maximum elevation of greater than 31 microns. The posterior elevation map (lower right) shows a corresponding area of marked elevation (>54 microns). The pachymetry map (upper left) shows the thinnest point of the cornea overlying the cone (419 microns). The curvature map (upper left) depicts a very steep cornea, however, it does not properly localize the cone, nor does it convey its morphology.

Figure 17

(FIGURE 18) - In the anterior elevation map (lower left), there is a somewhat diffuse area of elevation inferotemporally that exceeds 37 microns in height off the BFS. The posterior surface shows a much more defined island (lower right) showing elevation of greater than 80 microns at its peak. As is often the case, the posterior changes exceed those seen on the anterior surface. These two areas of elevation enable the clinician to easily identify the location and boundaries of the cone. The pachymetry map (upper left) shows an inferotemporal area of thinning (414 microns). This eccentric thinning corresponds to the areas of elevation and confirms the location of the cone in this patient. The curvature map (upper right) shows a so-called “crab claw configuration” which neither accurately describes the cone nor properly localizes the cone. Without the elevation maps, this curvature pattern may erroneously lead the clinician to think that this is a case of Pellucid Marginal Degeneration. The pachymetry maps shows a central area of thinning and not the inferior band that would be present with Pellucid.

Figure 18

(FIGURE 19) - The image below depicts a patient with early keratoconus. The anterior elevation (lower left) shows an astigmatic cornea with an inferotemporal area of mild elevation to a maximum height of about 15 microns. There is a corresponding area of elevation on the posterior surface (lower right) with a maximum height greater than 45 microns. While the anterior elevation alone may be viewed as borderline, when viewed together, the anterior and posterior elevation maps clearly depict a cone. The location of the cone corresponds to the thinnest point of the cornea (556 microns) seen in the pachymetry map (upper left). Although the pachymetry falls within the normal range, the shift in the thinnest portion of the cornea towards the areas of elevation raises suspicion. The curvature map (upper right) shows a normal central curvature (K’s 42.7D and 42.4D) with steepening peripherally. In this case, the diagnosis of early keratoconus could be overlooked if relying on central curvature or pachymetric data alone.

Figure 19

(FIGURE 20) –This map shows the classic elevation and pachymetry findings seen in advanced keratoconus. The anterior elevation (lower left) shows a well defined paracentral area of marked elevation greater than 45 microns. The posterior elevation map (lower right) has a similar appearance showing a large paracentral cone with a maximum elevation of greater than 71 microns. These areas of elevation correspond to the location of the thinnest point on the cornea (496 microns) seen on the upper left pachymetry map. The curvature map shows marked astigmatism and places the steepest portion of the cornea over the cone.

Figure 20

(FIGURES 21A and 21B) – The most important indicator for diagnosing post LASIK (refractive surgery) ectasia is the comparison between the pre and post-operative posterior elevation. Refractive surgery induces planned changes in both the pachymetry and the anterior elevation (also anterior curvature). Routine, uneventful LASIK, however, should not cause changes to the posterior corneal surface. While it was earlier believed that routine changes did occur on the posterior surface after LASIK, it has now been shown that this is not the case and that this false assumption was based on the limitations in earlier topography systems to accurately measure the post-operative cornea.1,2 The two maps below show a normal pre-operative map on the left and an ectatic post-operative picture on the right. The map on the right shows the expected central flattening on the curvature map (upper left) and the expected depression on the anterior elevation map (upper right) corresponding to the high myopic ablation. The pachymetry map (lower left) reveal approximately 110 microns of corneal thinning in a well centered myopic ablation pattern. The posterior elevation map (lower right), however, shows a marked post-operative change with an over 30 micron difference. The post-operative posterior elevation map shows a central ectatic change. This type of picture is typical for post-LASIK ectasia.

(FIGURE 22) – Pellucid Marginal Degeneration (PMD) is a frequently misdiagnosed condition. Classic PMD has been described by a band of inferior thinning 1-2 mm from the inferior limbus and associated with significant against-the-rule astigmatism above the band and a rapid change in topography (steepening) at the band. The difficulty in diagnosing PMD with standard Placido based systems is that these reflective systems cannot image the area of the true pathology. Placido based systems are limited to imaging, at best, the central 9.0 mm of the cornea and typically this misses the area of maximal corneal thinning. Additionally, and as discussed in earlier chapters, sagittal curvature is a poor indicator of shape and cone location. The map below highlights these curvature limitations. The sagittal curvature map (upper right) incorrectly identifies the cone with a marked inferior displacement. Many practitioners would describe this as PMD. The anterior and posterior elevation maps (lower left and lower right) and the pachymetric distribution (upper left) more accurately reflects both the shape and location of the cone. In this case the conical shape is clearly evident and the pachymetry map reveals paracentral thinning and no evidence of a band of thinning. This is a classic keratoconus with an inferior cone.

Figure 22