New software able to detect early ectatic disease

Screening tool incorporates data from the posterior corneal surface and corneal thickness map.

The detection of ectatic disease is of vital importance to the refractive surgeon. With subclinical disease, anterior curvature alone may not provide enough information to detect an early corneal abnormality.

The goal of the Belin/Ambrósio Enhanced Ectasia Display, a software feature of the Oculus Pentacam, is to combine anterior and posterior elevation and pachymetric data for a complete overview of the corneal structure. The combination of the pachymetric graphs and indices and elevation maps, which utilize an enhanced reference sphere, enables an increased sensitivity and specificity in the screening of patients for ectatic disease.

Raw elevation data are typically viewed by comparing the data to some standard reference surface. By subtracting (comparing) a known shape to the corneal surface, subtle elevation differences become highlighted. For refractive surgery screening, a best-fit-sphere (BFS) reference surface gives the most useful qualitative map (ie, easiest to read and understand). The clinician typically assumes that the reference surface (the shape being subtracted) closely approximates a “normal” cornea. This is actually not the case for abnormal corneas in which the calculation of the reference shape (best-fit-sphere or any best-fit-shape) incorporates all normal and abnormal corneal data. In the case of keratoconus, the cone will have the effect of steepening the BFS. This steepened BFS will actually minimize the elevation difference between the apex of the cone and the BFS (Figure 1).

Enhanced reference surface

The Belin/Ambrósio Enhanced Ectasia Display features a reference surface that more closely approximates the individual’s normal cornea after excluding the conical or ectatic region, as this will further magnify any existing pathology. We identified a 3.5-mm optical zone centered on the thinnest portion of the cornea (exclusion zone) and excluded it from the reference shape calculation. We calculated the new “enhanced BFS” by utilizing all of the valid elevation data within the central 8-mm zone and outside the exclusion zone (Figure 2).

The resulting new reference surface (enhanced BFS) more closely approximates the more normal peripheral cornea, thereby exaggerating any conical protrusion (Figure 1).

With a conical cornea, excluding the 3.5-mm zone from the BFS calculation eliminates the cone or steep portion of the cornea from the BFS calculation and results in a significantly flatter BFS based more on the normal peripheral cornea. The resulting elevation maps show a significant difference as the conical portion of the cornea is now more pronounced (ie, easier to identify) (Figure 3).

Because normal eyes are only minimally prolate, excluding this zone has little effect on the elevation maps. The elevation maps using the standard BFS and the enhanced BFS will look remarkably similar (Figure 4).

The relative elevation changes were highly significant when normal eyes and known keratoconic eyes were compared using the standard BFS and the enhanced BFS. The change (the elevation difference between the standard BFS and the enhanced BFS) appears to have significant prognostic value, as all normal eyes tested showed minimal change, while eyes with keratoconus or ectasia showed a significant increase in elevation values.

Elevation maps

The Belin/Ambrósio Enhanced Ectasia Display combines the already established pachymetric graphs and indices — cornea-thickness spatial profile (CTSP), percentage-thickness increase (PTI), progression index (PI) and the apex to thinnest location distance (IT) — with the enhanced anterior and posterior elevation maps (Figure 5).

The first two (upper) elevation maps are the regular elevation maps relative to the standard BFS. This anterior map is displayed on the left and the posterior on the right. For the Belin/Ambrósio Enhanced Ectasia Display, the area used to compute the reference surface is fixed at 8 mm. The enhanced elevation maps utilizing the 3.5-mm exclusion zone are immediately below the standard anterior and posterior elevation maps. These maps display the same elevation data as the baseline maps, but the reference surface is now the enhanced BFS, rather than the standard BFS.

This “exclusion map” may be similar to the standard display in normal eyes (Figure 5) or significantly different from the baseline elevation map as in keratoconus (Figure 6), depending on the relative impact the 3.5-mm exclusion zone made to the BFS computation.

The bottom two maps are difference maps showing the relative change in elevation from the baseline elevation map to the exclusion map. The bottom maps contain only three colors, each one corresponding to the amount of elevation change that occurs when moving between the baseline elevation map and the enhanced exclusion map.

Green on the difference map represents a change in elevation (from the baseline to the exclusion map) of less than 4 µm on the front surface and 10 µm on the back surface of the cornea. These values are typically within the range seen in normal eyes, as in Figure 6. Red represents elevation differences greater than 8 µm anteriorly and 18 µm posteriorly and are typically seen in eyes with known keratoconus, as in Figure 6. Yellow areas represent a change between 4 µm to 8 µm for the front surface and 10 µm to 18 µm for the back surface. These eyes fall in the suspicious or suspect zone.

Corneal thickness

The CTSP graph displays the sequence of pachymetric values along concentric circles of increasing diameter, beginning at and centered on the corneal thinnest point (TP). The PTI graph is calculated using a simple formula: (CT@x - TP)/TP, in which x represents the diameter of the imaginary circle centered on the TP with increasing diameters as provided by the CTSP. These graphs are depicted on the right side of the Belin/Ambrósio Enhanced Ectasia Display.

The Belin/Ambrósio Enhanced Ectasia Display is the first comprehensive refractive surgical screening tool to be fully elevation-based and to incorporate data from the posterior corneal surface and corneal thickness map. It is hoped that this additional information will provide greater sensitivity for detecting early ectatic disease than relying solely on anterior curvature.

• Michael W. Belin, MD, FACS, and Stephen S. Khachikian, MD, can be reached at the Lions Eye Institute of Albany Medical College, 1220 New Scotland Rd., Slingerlands, NY 12159, U.S.A.; +1-518-475-1515; e-mails: mwbelin@aol.com, stevekmd1@gmail.com. Dr. Belin serves as a consultant to Oculus. Dr. Khachikian has no direct financial interest in the products discussed in this article, nor is he a paid consultant for any companies mentioned.