Ординатура / Офтальмология / Английские материалы / Elevation Based Corneal Tomography 2nd_Belin, Khachikian, Ambrósio_2011

Precise image registration is a prerequisite for accurate tomographic data. Because the distance travelled during rotation is less in the center, the point density is greatest in the central cornea (typically the area of concern) and decreases in the periphery. Additionally, the edge detection software on the Pentacam appears less susceptible to corneal haze than the Orbscan and the system does not suffer from the inaccuracy in locating the posterior corneal surface common to the Orbscan in the acute postoperative cornea.4

While initially designed primarily for refractive surgery, the Pentacam also provides cross-sectional visualization of the cornea and the anterior chamber, as well as an objective lens densitometry assessment. It calculates the pachymetry of the cornea from limbus to limbus with an accuracy of +/-5µm. True elevation measurements allow for computations of anterior and posterior sagittal (axial) and tangential (local or instantaneous) curvature, anterior and posterior elevation maps, anterior chamber depth measurement, angle approximation, corneal and anterior chamber volumes and anterior surface derived Zernike polynomials.

The authors’ goal is for this book to serve as both a primer and reference for the corneal refractive and anterior segment surgeon. Subsequent chapters will deal with the use of elevation based tomography in the assessment of screening patients for potential refractive surgery as well as others uses in the anterior segment (e.g. cataract grading).

CLINICAL APPLICATIONS

Refractive surgical screening is the most common clinical application of topography/ tomography. Screening should be rapid, technician friendly and accurate. The vast majority of patients can be quickly and safely screened with the 4 map composite (refractive) display or with the Belin/Ambrosio enhanced ectasia display. The recommended maps in the composite display include anterior and posterior corneal elevation using a best-fit-sphere, corneal thickness map (pachymetric distribution) and anterior sagittal curvature. Screening requires consistent scales, colors and settings to allow the practitioner to rapidly evaluate the pre-operative map. Each of these will be discussed in greater detail later, but consistent colors/scales/maps are key to rapid and safe screening of patients. Our recommended settings for refractive surgery screening are shown in TABLE 1. These settings are chosen to maximize sensitivity and allow for a quick visual screening based on color and pattern recognition. It should be understood that when screening for refractive surgery the goal is to identify patients that are not “normal.” The recommended settings below are designed for that purpose. (In a pathology oriented practice the scales have to be widened to encompass the more diverse range and shapes seen in abnormal corneas).

It is important to realize that what we call an “elevation” map does not really display the actual raw elevation data, but shows the elevation data against some reference surface. The most commonly used (and recommended for refractive screening) reference surface is a Best-Fit-Sphere (BFS). While other shapes can be utilized, they are all based on the same raw elevation data and so their differences are more qualitative than quantitative. The shape, scales and colors should be chosen to simplify screening. Different scales, shapes and color bars may be appropriate for other uses.

TABLE 1 - Recommended Refractive Surgery Screening Settings

How the BFS is computed is critical to a thorough understanding and will be discussed in greater detail in a separate chapter. The Pentacam has both an AUTO (Automatic) and MAN (Manual) setting. The understanding of how these differ is vital for proper assessment of the maps. The MAN setting utilizes the data within the user specified optical zone (e.g. MAN, Diameter = 8.0 mm uses all the data with the 8.0 mm optical zone centered on the apex). It uses all the data whether the data is accurate or extrapolated. Varying optical zone sizes will significantly affect the maps both qualitatively and quantitatively as the BFS (reference surface) will change. Because the normal eye is a prolate asphere, a larger optical zone results in a flatter BFS and a smaller optical zone a steeper BFS. As long as you have a good quality scan, without extrapolated data within the area used to compute the BFS, the MAN setting allows for comparisons over time and allows for the computation of normal values for a defined BFS diameter.

The AUTO setting utilizes all the data in an optical zone that is 90% the size of the largest optical zone obtainable without extrapolated data. In other words, the system draws the largest possible circle centered on the apex that does not include any extrapolated data and sets the optical zone used for the BFS computation to 90% of this value. The value of the AUTO setting is that it only uses “valid” data points, however the limitation is that the area used to define the BFS will be different between patients and different between exams of the same patient. This is a significant difference that limits the clinical usefulness of the AUTO setting. The MAN utilizes all data, including extrapolated data at a defined zone, while the AUTO uses only validated data but can have a varying zone. A varying optical zone in the AUTO setting makes establishing normal values and screening parameters problematic.

Our current recommendations are to use the MAN setting with the reference surface set to SPHERE and the DIAMETER set to 8.0 mm. It is important to insure that there is

minimal or no extrapolated data within the 8.0 mm zone. Extrapolated data appears as either white areas or black dots (user selectable, authors recommend “white area”). To assist the inspection of the maps, we recommend that the display be limited to show only the central 9.0 mm (this is an option in the drop down menu “max diameter 9.0 mm”). There are two numbers here that often lead to confusion. The “max diameter 9.0 mm” from the drop down menu refers only to the area of the cornea displayed on the maps. Often there is more of the cornea covered, but this setting will only show the central 9.0 mm, which is easier to read and adequate for refractive screening. The “max diameter 9.0 mm” has nothing to do with the BFS computation and only relates to how much of the cornea is being shown.

A 9.0 mm map without extrapolated data can be considered valid. These “valid” maps will have reproducible BFS computations. All discussions that follow and suggestions for screening parameters are based on an MAN setting, BFS set to SPHERE, DIAMETER = 8.0, and Float and no extrapolated data within the central 8.0 mm zone.

The composite screening display outlined above has significant advantages for identifying ectatic change. Keratoconus, for example, is a progressive disorder, the hallmarks of which are stromal thinning, conical protrusion, corneal scarring, decreased spectacle corrected visual acuity, Fleisher’s ring, and in advanced cases Vogt’s striae.5 The identification of patients with keratoconus is essential when screening refractive surgery candidates. LASIK and most refractive procedures are contraindicated in keratoconus patients due to the potential for the disease to progress rapidly after surgery. “Forme fruste keratoconus” or early “subclinical keratoconus” may also be considered a contraindication to LASIK due to the likelihood of progression into clinically significant disease. Knowledge of the posterior corneal surface and full pachymetric maps are obviously of great clinical benefit.

The original Placido based keratoconus screening programs relied only on data from the anterior corneal surface. Earlier topographic systems either did not provide information on the posterior corneal surface or did so inaccurately, leading many to conclude that this information was not clinically important. It is now understood that changes on the posterior corneal surface are critical and often the earliest indicators of ectatic disease, in spite of normal anterior curvature maps and most likely predate other indicators (e.g. epithelial thinning) limited to the anterior surface.

FIGURE 4 is a 4-view composite map (4 Maps Refractive) of an asymptomatic patient presenting for refractive surgical evaluation. The map demonstrates the importance of both the posterior corneal surface and the pachymetric distribution map. The patient had 20/20 best spectacle corrected vision, a normal Placido based topography and central ultrasound pachymetry (520 microns) and was considered normal. The Pentacam composite display reveals the limitations of relying on anterior curvature and central corneal thickness measurements alone. The anterior elevation (upper right) and anterior sagittal curvature (upper left) are all normal, both maps displaying a small amount of astigmatism. The pachymetric map (lower left) shows a normal central reading of 517 microns. The pachymetric distribution is significantly displaced, however, where the thinnest region (502 microns) is infero/temporal to the central cornea. The difference between the thinnest reading and the apical reading

Figure 4. A Pentacam generated 4-view refractive composite map of an asymptomatic patient presenting for refractive surgical evaluation. Anterior and posterior elevation, sagittal curvature and pachymetry maps are shown. In spite of a normal anterior surface (both elevation and curvature) the posterior elevation and pachymetric maps demonstrate an early ectatic change.

is 15 microns. The posterior elevation (lower right) is clearly abnormal. The posterior map shows a well-demarcated, paracentral island of positive elevation (+30 microns) off the best- fit-sphere representing an ectatic change. The thinnest point displacement corresponds to the area of posterior elevation further increasing its significance.

Similarly, the patient depicted in FIGURE 5 would have a completely normal exam if one relied solely on ultrasound pachymetry and Placido based topography. The sagittal curvature (upper right), the map most commonly displayed on a Placido system, is completely normal. The curvature map is symmetric and shows minimal astigmatism.

The simulated keratometry readings are in the 39-40 D range. Central ultrasound pachymetry readings were 511 microns. All of these values would be considered well within the normal range (TABLE 2). This is, however, an incomplete picture. The pachymetric distribution is distinctly abnormal with a marked inferior displacement of the thinnest region (lower left). The thinnest area of the cornea is 482 microns, 29 microns thinner than the apical reading.

Figure 5. A Pentacam generated 4-view refractive composite map of an asymptomatic patient presenting for refractive surgical consultation. Evaluation of the images reveals that the area of posterior ectasia and the thinnest corneal region coincide.

The posterior elevation map (lower right) shows a well circumscribed, clearly demarcated, island of positive elevation (> 30 microns off the best fit sphere) representing the area of ectatic change. The area of posterior ectasia and the thinnest corneal region coincide. This patient has sub-clinical keratoconus in spite of normal Placido topography and normal ultrasound pachymetry (sub-clinical meaning retains good spectacle vision in spite of apparent anatomical changes). The Pentacam clearly reveals the shortcomings of the original incomplete corneal analysis.

The literature is replete with numerous articles of iatrogenic post LASIK ectasia in patients reportedly with normal preoperative exams.6,7 How many of these “so-called normal” exams are truly “normal” and how many had undiagnosed changes on their posterior corneal surface is unknown. Additionally, reported residual bed computations based on central ultrasound readings may significantly over-estimate the actual residual bed. Like flap thickness unpredictability, pachymetric distribution inconsistency may represent a previously unrecognized confounding variable.8