KEY CONCEPTS
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Scheimpflug imaging provides a complete anterior segment analysis that is not possible with older Placido-based systems.
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Imaging of the posterior cornea allows identification of subclinical keratoconus prior to vision loss.
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The addition of a Placido disk to a Scheimpflug device offers no clinical advantage.
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The Belin/Ambrósio Enhanced Ectasia Display (BAD) is the most commonly used refractive screening tool.
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The Belin ABCD classification/staging eliminates the limitations of the older Amsler-Krumeich classification.
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The Belin ABCD Progression Display monitors ectatic patients over time and displays when significant progression occurs prior to permanent vision loss.
Background
There has been a dramatic increase in the need to recognize the earliest forms of keratoconus and other ectatic disorders and to eliminate the false-positives seen with some older technologies. As in other areas of medicine, imaging has played a large part in this change. The new information offered by anterior segment tomography not only allows for earlier identification of disease, but has altered our perception of what constitutes keratoconus. Tomographic imaging (Scheimpflug, ocular coherence tomography [OCT]) offers significant advantages over traditional Placido-based curvature analysis (topography) and ultrasonic pachymetry. Elevation-based tomographic imaging allows for the measurement of both the anterior and posterior corneal surfaces. The accurate measurement of both the anterior and posterior corneal surfaces and the anterior lens allows for the creation of a three-dimensional reconstruction of the anterior segment, which affords significantly more diagnostic information than was previously available. Posterior measurements and/or changes in pachymetric distribution are often the first indicators of ectatic disease, despite completely normal anterior curvature. Examination of the posterior corneal surface can often reveal pathology that would otherwise be missed if one were relying on anterior analysis alone , , ( Fig. 15.1 ). The remainder of this chapter will deal solely with Scheimpflug tomography. For purposes of ectatic disease diagnosis and refractive screening, Scheimpflug imaging has certain advantages over OCT in that it allows for a much greater area of coverage (up to limbal to limbal coverage) typically not possible with current/older OCT technology (newer and imaging OCT systems have improved coverage). Scheimpflug imaging also covers significantly more of the cornea than was possible with Placido-based devices, the coverage of which is limited to, at best, 40% of the cornea. This added coverage is critical in the correct diagnosis of peripheral diseases such as pellucid marginal degeneration (PMD) ( Fig. 15.2 ). Although multiple Scheimpflug devices exist, this chapter will deal solely with the OCULUS Pentacam (OCULUS GmbH, Wetzlar, Germany), as this is the most commonly used Scheimpflug device. Other devices have combined Scheimpflug imaging with older Placido imaging. Other than offering some familiarity with older technology, the addition of a Placido disk makes little, if any, clinical sense. There has been a long-standing misconception that Placido-based curvature is in some way inherently superior to elevation-based curvature or that elevation-based curvature is derived and not a direct measurement. Both Placido systems and elevation systems derive curvature. Placido systems measure slope and derive curvature and Scheimpflug systems measure elevation (points in space) and derive curvature. Other than the greater area of coverage afforded by Scheimpflug imaging and the lower susceptibility to tear abnormalities, the curvature maps are virtually identical ( Fig. 15.3 ). The importance of corneal coverage is not just limited to PMD. In addition to being able to measure and locate the true thinnest point, a full corneal thickness map allows the generation of pachymetric progression graphs that reflect the rate of change in corneal thickness. A single thickness reading is very limited in determining what is normal. Two corneas can have the same central corneal thickness but share dramatically different pachymetric progressions. Abnormal corneas (i.e., corneas showing ectatic change or tendency) have a more rapid thinning from the corneal periphery to the thinnest point. , This more rapid rate of pachymetric progression, when seen in a preoperative cornea, is highly suggestive of ectatic change or an eye at greater risk of postoperative ectatic change ( Fig. 15.4 ).
The biggest advantage of Scheimpflug imaging, however, is the measurement of the posterior corneal surface. Although the posterior surface contributes minimally to the overall refractive power of the eye (due to the minimal difference between the index of refraction of the cornea and aqueous) and was, in the past, considered less important, posterior surface changes are now recognized to serve as the earliest indicator of ectatic change and typically predate any changes on the anterior corneal surface , , , ( Fig. 15.5 ).
Elevation Maps
That the standard anterior and posterior corneal tomographic maps are referred to as elevation maps is somewhat of a misnomer. True elevation would require displaying the data against a planar (flat) surface. Maps generated against a flat surface are not clinically useful (intuitive), as the surface changes cannot be visually appreciated. The reason is that the raw elevation data from normal eyes and markedly ectatic corneas look remarkably similar ( Fig. 15.6 ). To make the maps clinically useful and to allow for a rapid visual inspection, the raw data are compared with some nonplanar reference surface. The purpose of the reference surface is to magnify or amplify the surface differences that would otherwise not be appreciated by the naked eye. The so-called “elevation” maps depict how the corneal surface differs from a defined reference shape. Although the appearance of the map will vary greatly depending on the reference surface used, all maps are generated using the same raw elevation data. Recognizing that the reference surface will alter the appearance of a map, but not its accuracy, is important. ,
The choice of the reference surface will often depend on the clinical situation, the population being evaluated, and the specific pathology. For most applications, the best-fit-sphere (BFS) is the most qualitatively intuitive (easiest to read and understand) surface and the most commonly used. A BFS allows for the visualization of astigmatism, as the flat meridian rises above the BFS while the steep meridian drops below the BFS. The normal astigmatic pattern generated against a BFS is easily recognizable ( Fig. 15.7 ). When screening for ectatic disease, one is trying to identify an abnormal conical protrusion. A focal protrusion will appear as an elevated area against the BFS (positive island of elevation) ( Fig. 15.8 ). As the cornea is normally aspherical, steeper in the center, and flatter toward the periphery, normal corneas will have some central positive elevation. The goal of screening is to allow for a rapid visual inspection to separate normal from abnormal. This task, however, is made more difficult by the fact that the normal cornea is aspherical and displays, to a smaller degree, a positive elevation (“positive island of elevation”), similar to what is seen with ectatic disease. The BFS and the resultant elevation map will vary depending on how much of the cornea is utilized to construct the reference surface. If the entire cornea is used to construct the BFS, then the normal asphericity of the cornea will be clearly demonstrated. As the area (optical zone) to compute the BFS is decreased, the BFS steepens as less of the flatter periphery is incorporated into the BFS. Taking the BFS from the central 8.0-mm optical zone steepens the BFS enough to mask effectively the normal corneal asphericity ( Fig. 15.9 ). Masking the normal asphericity makes screening for ectatic disease easier and allows for rapid visual inspection. The BFS taken from the central 8.0-mm optical zone has become somewhat standardized.
