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    Identifying early ectatic disease accurately

    New screening software can detect the subtle abnormalities

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    Sreening for early ectatic disease is essential in the evaluation of the refractive surgical candidate. There have, however, been many cases of severe post-LASIK ectasia without evidence of preoperative risk factors. Improving preoperative screening with better detection of subtle corneal topographic abnormalities may therefore help eliminate these rare but devastating cases.

    Elevation topography is rapidly emerging as the new standard for topographic screening. Elevation topographers can provide both anterior and posterior corneal elevation measurements, a complete pachymetric distribution map along with the more traditional axial and sagittal curvature. One limitation that is faced, however, is difficulty with interpretation of the elevation map. While obvious cases of ectatic disease are readily identified, more subtle elevation changes may only be apparent to the most experienced refractive surgeons.

    Moreover, careful attention to the posterior corneal surface is necessary to detect some of these subtle changes. Often, evaluation of the posterior surface is overlooked as in the past the posterior corneal surface was viewed as less important for refractive surgical evaluation. This may no longer be true, and the posterior surface may actually serve as an early indicator of ectatic disease.


    Figure 1: Topographic example of raw elevation data as measured by an elevation topographer. Without an appropriate reference surface it is impossible to qualitatively compare the elevation data between normal corneas and those with mild or advanced disease.
    Because detecting early ectatic change can be difficult, we have developed an elevation topography screening tool to aid in identifying early changes. The tool uses an enhanced reference surface to highlight corneal elevation abnormalities and allows the physician to more easily identify suspicious but subtle changes.

    A closer look at elevation maps


    Figure 2: Elevation topography map of a normal cornea showing minimal astigmatism and minimal change in elevation throughout.
    To properly examine elevation maps of the cornea, an appropriate reference surface is required. Without an appropriate reference surface, the changes in the raw elevation seen with abnormal corneas are too small for qualitative evaluation (Figure 1). When looking at a corneal elevation map with a best-fit sphere reference surface, as seen in Figure 2, it is important to understand that the numbers displayed are not absolute elevation values, but rather elevation values above or below an imaginary sphere fit to the patients' cornea. This sphere is known as the "best-fit sphere" and it is used to help qualitatively compare different elevation points on the corneal surface. Figure 3 shows an astigmatic cornea with elevation values labeled as compared with a best-fit sphere.

    Typically, ectatic change is seen as an abnormal "island" of elevation above the reference sphere (Figure 4). This needs to be compared with elevation changes seen in an astigmatic cornea. The key difference is the pattern recognition. Often, particularly in advanced cases, this island of elevation will be associated with localized corneal thinning and a change in curvature. In preclinical ectatic conditions, however, a small isolated change in elevation, without associated curvature or pachymetric changes, may be all that is seen. The elevation is still abnormal, but the change is so subtle it is difficult to detect, despite using an appropriate reference surface. This is because, when these elevation abnormalities exist, the reference sphere is calculated using both the normal portions of the cornea and the abnormal areas. Even subtle changes in elevation can significantly alter the best-fit sphere calculation. Using this abnormal data in the best-fit sphere calculation no longer helps identify corneal abnormalities, it hides them. Although the reference sphere is technically still "best-fit" we need an "ideal fit" in these cases, so that we can easily locate the early ectatic changes (Figure 5).

    Identifying early ectasia in corneas that may have been considered healthy


    Figure 3: Elevation topography map of a normal astigmatic cornea showing large changes in elevation, but no evidence of a central island of elevation.
    Working with the Pentacam Eye Scanner (Oculus), we have developed a screening tool that selectively eliminates the abnormal corneal elevation data and recreates the best-fit sphere using only the more normal elevation data. This makes even small areas of abnormal corneal elevation more pronounced while leaving areas of normal elevation unchanged.


    Figure 4: Elevation topography map of the posterior cornea in a patient with keratoconus. Note the island of elevation in the inferior cornea.
    We first tested this screening tool on 35 normal eyes and 30 eyes with documented keratoconus to see what the change in elevation would be using the best-fit sphere and our modified ideal sphere. Our hypothesis was that for normal corneas without evidence of ectatic disease, there would be little to no change in elevation between the two reference shapes. We theorized that for corneas with ectatic disease, however, there would be a significant change in corneal elevation as measured with the traditional best-fit sphere and our modified ideal sphere, which is based on the more normal portion of the cornea (Figure 6). Our results confirmed this.

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    Stephen S. Khachikian, MD
    Stephen S. Khachikian, MD is a Cornea and Refractive Surgery Fellow at the Cornea Consultants of Albany and Albany Medical Center, ...
    Michael W. Belin, MD
    Michael W. Belin, MD is Professor of Ophthalmology and Vision Science at the University of Arizona Department of Ophthalmology and ...

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