Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard Mini Atlas Series CORNEALTOPOGRAPHY_Agarwal, Jacob_2009

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

values suggest oblate corneas, such as those with a history of myopic treatment.

The presentation of the now widely accepted “Quad map” includes the sagital curvature, anterior and posterior elevation, and pachymetry maps. When examining the Pentacam quad map to determine candidacy for elective vision correction, the relationship between the maps must be assessed. Since the posterior surface is the first to manifest ectatic changes, inspect this map first. Elevated areas noted on the posterior surface map may correspond to a displaced “thinnest area” on the pachymetry map, elevated area on the anterior elevation map, and irregular astigmatism on the curvature map. Examples are shown in Figures 2.8 to 2.10. Astigmatism often manifests as a “saddle” on the posterior surface as seen in Figure 2.11 and may not be pathological. Checking the elevation map using the toric ellipse is an excellent method to ruling out ectasia in these eyes.

Optical pachymetry is calculated from direct measurement of the anterior and posterior surface elevations. While ultrasound remains the standard for pachymetry measurement, optical pachymetry has been reported to be reproducible and reliable in the literature. One advantage of optical pachymetry is display of values

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 2.11: Astigmatism manifests as a “saddle” pattern on the posterior surface

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CHAPTER 2: PENTACAM

over the corneal surface (Fig. 1.12). It is important to inspect the pachymetry values relative to those around it as well as the thinnest value. One study found Pentacam provided measurements that were slightly but systematically lower than the measurements provided by ultrasonic pachymetry in normal corneas, and another found measurements obtained with the Orbscan II (CF) are thinner than those obtained with the Pentacam in normal eyes. Most essentially reported significant agreement with one another, but these methods may not be simply interchangeable.

Pachymetry after PRK has been shown to be problematic for slit-scanning devices. Corneal thickness was measured using Pentacam, Orbscan II, and ultrasonic pachymetry in unoperated eyes, and in those after myopic PRK. Authors reported following myopic PRK, the Pentacam measurement were comparable to ultrasonic pachymetry, while Orbscan measurements were thinner. Pachymetry after Lasik has been shown to be variable as well. Several studies have been published, with reports of good repeatability in pachymetry measurements and strong correlation. such that authors suggested Pentacam pachymetry may be substituted for ultrasound in the postLasik patient.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 2.12: A pachymetry map of a patient with keratoconus. Note the displacement of the thinnest point, and the overall reduction of corneal thickness

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CHAPTER 2: PENTACAM

As a tomographer, the Pentacam measures the posterior surface directly, as shown in (Fig. 2.13A). Calculations are based on the Gullstrand eye model. Values are negative, due to the negative refractive index of the cornea and aqueous humor.

Literature reports suggest good repeatability for the posterior Best Fit Sphere (BSF), with significant differences when compared to Orbscan (Fig. 2.13B). Hashemi et al studied LASIK patients using the Orbscan and Pentacam pre and postoperatively. They reported that, the Orbscan II yielded larger posterior elevation values before and after surgery, and significant postoperative changes in posterior corneal elevation and posterior maximum elevation compared to the Pentacam.

Ciolino found no statistically significant difference in posterior corneal displacement between the Lasik and PRK patients as measured by the Pentacam and suggested ectasia may not routinely occur after Lasik. Quisling et al found the Pentacam and Orbscan IIz measure similar thinnest points but differ in posterior elevations above the best-fit sphere, despite similar radii of curvature. They were not able to determine if the Pentacam underestimated posterior vault or if Orbscan overestimates the height.

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