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

(FIGURE 3) - The map below illustrates a normal astigmatic cornea. The anterior elevation map (upper right) shows a purely with-the-rule astigmatic pattern with the steep axis (blue/purple) at about 90 degrees. The posterior elevation map (bottom right) shows a similar pattern but with a much more pronounced astigmatic pattern. Note that on both the anterior and posterior elevation maps, while there may be a significant amount of peripheral elevation (normal in an astigmatic cornea), there are no focal islands of elevation centrally. The corneal thickness map shows a normal distribution with a central thickness of about 559 and no displacement of the thinnest point of the cornea. The curvature map (upper left) shows a normal symmetric anterior curvature pattern.

Figure 3

(FIGURE 4) – Similar to Figure 3, the map below shows a normal astigmatic pattern. While there are no focal islands of elevation, the change in elevation is slightly asymmetric (compare the astigmatic pattern to Figure 3). There is a larger, but very gradual increase in elevation temporally due to the mild asymmetry that is a very common normal variant. The posterior elevation map also shows a normal astigmatic pattern. The central corneal thickness is temporally displaced and slightly thinner than average but still within the normal range. The curvature map shows a normal bowtie pattern consistent with the anterior astigmatism.

Figure 4

(FIGURE 5) – It is assumed that individuals look through the “center” of their corneas. This, however, is not always the case. A patient’s line of sight, pupillary center and corneal apex are not the same. Sagittal curvature maps are derived relative to a measurement axis. This axis is the vertex normal to the cornea (for a Placido system to work it needs to reflect its own image and so must be normal to the surface being measured). If a patient’s line of sight and corneal apex differ then the sagittal curvature maps do a poor job of reflecting shape. Elevation maps will also change depending on the line of sight, but their overall appearance (other than rotation) will be unchanged. This is like taking an American football and rotating it so that the laces are in a different location. If you rotate the laces up or down, it still looks like a football. This is similar to elevation. In curvature however, if you rotate the football the entire image changes and it often no longer is recognizable as a football.

(FIGURE 7) – This is another example of a sagittal curvature “false positive.” You will notice that the pachymetry map (lower left) is completely normal. Both the anterior (upper right) and posterior (lower right) elevations reveal a normal astigmatic pattern but the pattern appears as if the eye was rotated down and nasally. The sagittal curvature map (upper left), however, appears highly asymmetric with an area of “steepening” (false positive) in the area of the displaced apex.

Figure 7

(FIGURE 8) – Demonstrates a relatively rare curvature pattern of superior steepening (upper left) and the explanation for this pattern. Again, this is a “false positive” on the curvature map and is due to the displaced apex pattern seen on the anterior elevation (upper right). In this case the apex appears to have been rotated superiorly. Both anterior and posterior elevation patterns are completely normal as is the pachymetric distribution (lower left). The anterior elevation map enables the physician to appreciate that this is a normal astigmatic eye and that true ectatic change does not exist.

Figure 8

(FIGURE 9) – The image below shows a patient with normal astigmatism but a very thin cornea. The anterior elevation (lower left) shows a normal astigmatic pattern. The posterior elevation map (lower right) has a similar appearance revealing an astigmatic pattern with no focal areas of abnormal elevation. The curvature map (upper right) reveals a symmetric bowtie pattern of astigmatism on the anterior surface. The pachymetry map (upper left) shows a symmetric pachymetric distribution, however the central thickness of 456 microns suggests that this patient may not be an ideal candidate for lamellar refractive surgery.

Figure 9

(FIGURE 10) - This image shows a patient with normal astigmatism but a thin cornea. The anterior elevation (upper right) reveals an astigmatic pattern with no abnormal areas of elevation. The posterior elevation map (lower right) has a similar appearance showing only a strongly astigmatic pattern. The curvature map (upper left) shows a bowtie pattern of astigmatism on the anterior surface. The pachymetry map (lower left) has a fairly normal pachymetric distribution but the cornea thins below 500 microns at the thinnest region (499 microns).

Figure 10

(FIGURE 11) - The image below shows the importance of evaluating a complete pachymetric map. The anterior elevation (lower left) shows a normal symmetric astigmatic pattern without evidence of elevation abnormalities. The posterior elevation (lower right) also reveals normal astigmatism. The curvature map (upper right) shows a normal bowtie pattern. The pachymetry map (upper left) shows a central thickness of 495 microns. Careful examination of the map shows that the thinnest area of the cornea is 483 microns. This area is displaced inferotemporally and would be considered outside of the normal range. The thinnest region (483 microns) would likely be missed by stand ultrasonic pachymetry which typically looks just at the central cornea.

Figure 11

(FIGURE 12) – This is another example of where a single central ultrasonic pachymetry reading would be misleading. While the apical ultrasound reading would be on the order of 520 it would fail to reveal the significant inferior displacement of the thinnest region with a thickness of below 500 microns. It is important to examine not only the central thickness but the thinnest region and the overall distribution.

Figure 12