Connecting Cosmopolitanism and Science Fiction: The Modern/Colonial Complex This sub-section begins by situating science fiction and cosmopolitanism in the context of

Baseline Elevation Maps - We can begin by looking at the left half of the Belin / Ambrosio ectasia display (FIGURE 9) where the elevation data of a normal eye is shown.

The first two (upper) elevation maps (placed side by side) are the regular elevation maps of the cornea relative to the standard best fit sphere. This map is displayed for the front

All red readings should be repeated as the coverage area is insufficient for valid interpretation. To the left of the elevation map is the number 9 mm. This is the size of the cornea being displayed. To the right of the map is the eye being examined, OD or OS.

Enhanced Maps - Immediately below the standard anterior and posterior elevation maps are the anterior and posterior enhanced elevation maps using the 4 mm exclusion software. These are “enhanced” elevation maps, which display the same elevation data as the

Figures 9. Belin/Ambrosio Enhanced Ectasia Display of a normal cornea (Left side of display). The baseline elevation maps (top) show normal corneal elevation map. The exclusion map (middle) has a similar appearance. The change in elevation from the baseline to the exclusion map (bottom) shows almost no change in elevation (Green) on both anterior and posterior surfaces.

Figure 10. (Belin/Ambrósio Enhanced Ectasia Display – Elevation side) A poor quality Pentacam image will contain less valid data. If there is extrapolated data within the 8.0 mm zone the “Diameter” label will turn YELLOW or RED to notify the clinician of the reduced diameter and to suggest that the patient be reimaged.

The green areas on the difference map represent a change in elevation (from the baseline to the exclusion map) of less than 5 microns on the front surface and 12 microns on the back surface of the cornea. These values are typically within the range seen in normal eyes, as in this example.^7,8 Red would represent areas where the elevation difference between the 2 maps is ≥ 7 microns anteriorly or ≥ 16 microns posteriorly and are the magnitude typically seen in eyes with known ectatic disease. Yellow areas represent a change between 5 and 12 microns for the front surface and 12 to 16 microns for the back surface. These eyes fall in the suspicious or suspect zone. In this example (FIGURE 9) the difference maps are both all green as this is an otherwise “normal” eye. The next example (FIGURE 11) shows an eye with

“sub-clinical” or form-fruste keratoconus. While the anterior surface is completely normal (both the standard elevation and the enhanced elevation show no islands and the difference map is all green), the posterior elevation maps shows a suspicious, paracentral, positive island of elevation on the standard display that is greatly exaggerated on the enhanced elevation display. The radius of curvature of the standard posterior BFS changes from 6.74 to a much flatter 6.91 mm when the 4 mm zone is excluded and the maximal elevation of the island increases over 25 microns. The difference map shows a corresponding prominent RED zone.

Figure 11. Belin/Ambrósio Enhanced Ectasia Display of a patient with early keratoconus. The baseline elevation maps (top) show the presence of a mild cone limited to the posterior cornea, while the exclusion map (middle) enhances the visibility of the abnormal cone. The change in elevation from the baseline to the exclusion map (bottom) shows a significant change on the posterior surface (Red) and yet no significant change (Green) on the anterior surface. (The pachymetric analysis (right side of map) is discussed in Chapter 7).

CONCLUSION

The Belin / Ambrósio Enhanced Ectasia Display is the first comprehensive refractive surgical screening tool to be fully elevation based. The goal of the software is to assist the

3. Belin MW, Khachikian SS. Corneal diagnosis and evaluation with the OCULUS Pentacam. Highlights of Ophthalmology. 2007; 35:5-8.

Keratoconus is defined as a bilateral, non-inflammatory condition, in which the cornea assumes a conical shape associated with progressive thinning. In fact, progressive corneal thinning is a hallmark of ectatic diseases including keratoconus (KC), pellucid marginal degeneration (PMD) and keratoglobus. The secondary steepening and protrusion that occur represent consequence of the altered and weakened cornea. These changes lead to a progressive increase in keratometry, astigmatism and higher order aberrations.^1,2

Classically, the diagnosis of keratoconus relies on the assessment of the anterior surface contour over the central and paracentral cornea. Central keratometry, however, is an insensitive indicator because the measurements may remain normal until more advance stages of the disease. Traditional Placido corneal topography enables identification of earlier forms of keratoconus than possible with just keratometry because it reveals mild levels of irregularity such as inferior steepening (IS) and asymmetric bow tie patterns (ABT). These abnormalities have been recognized as early signs of disease² and the term sub-clinical or forme-fruste keratoconus has been used to refer to such conditions where this topographic pattern is present while the patient retains good BCVA. Changes to the posterior surface and to the corneal thickness, both in magnitude and distribution, commonly predate any measurable changes on the anterior surface and may be abnormal in spite of normal Placido topography.

Because progressive corneal thinning is an essential component of ectatic conditions, elevation tomography based pachymetric mapping is believed to provide a more sensitive method for detecting early forms of disease than is possible with Placido topography. In addition, patients with keratoconus may present with anterior topographic levels of irregularity similar to normal corneas and therefore, complete corneal tomographic measurements including, anterior and posterior elevation maps, corneal surface curvature maps, and pachymetric maps, may increase specificity and confirm ectasia among otherwise questionable cases.³