Identifying early ectatic disease accurately

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.

A closer look at elevation maps

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