Understanding Corneal Topography
Within the past 10 years, corneal topography has grown from an elaborate and costly device used only for clinical research in large institutions to a critical in-office tool that many optometrists now use daily. Along with advances in computerization and software development, topographers have become smaller, more compact, more affordable and more precise. This primer describes the mechanics, methods of interpreting the data, and indications for performing corneal topography.
The Mechanics Behind Topography
Corneal topography--also known as videokeratography or corneal mapping--represents a significant advance in the measurement of corneal curvature over keratometry. Most corneal topographers evaluate 8,000 to 10,000 specific points across the entire corneal surface. By contrast, keratometers measure only four data points within the cornea's central 3-4mm; the small size of this area can lead to errors in determining precise toricity.
Topography provides both a qualitative and quantitative evaluation of corneal curvature. It does so by utilizing concentric rings, which project onto the cornea to create a virtual image. The device compares this image to the target size, and the computer then calculates the corneal curvature. Although many different systems are available, all share some unifying measurement characteristics. The computer ized topographer can generate various graphical representations. When performing corneal mapping for diagnosis of conditions and/or contact lens fitting, the two most common maps that practitioners use are:
Axial map. Also called the "power" or "sagittal" map, this output is the simplest of all the topographical displays. It shows variations in corneal curvature as projections and uses colors to represent dioptric values. Warm colors such as red and orange show steeper areas; cool colors such as blue and green denote the flatter areas.
Tangential map. Sometimes referred to as the instantaneous, local, or "true" map, it also displays the cornea as a topographical illustration, using colors to represent changes in dioptric value. However, the tangential strategy bases its calculations on a different mathematical approach that can more accurately determine the peripheral corneal configuration. It does not assume the eye is spherical, and does not have as many presumptions as the axial map regarding corneal shape. In fact it is the map that more closely represents the actual curvature of the cornea over the axial map. The tangential map recognizes sharp power transitions more easily than the axial map, and eliminates the "smoothing" appearance that appears on the axial map. This is not universally true for all topographers.
Compared with axial maps, tangential maps yield smaller patterns with details that are more centrally located. Tangential maps also offer a better visualization of the precise location of corneal defects. This display is most useful in following trends in the postsurgical or pathologic eye.
There are two other types of corneal topography printouts clinicians use:
Elevation map. This utilizes yet another algorithm to give additional information about the cornea. An elevation map shows the measured height from which the corneal curvature varies (above or below) from a computer-generated reference surface. Warm colors depict points that are higher than the reference surface; cool colors designate lower points.
Refractive map. This utilizes the measured dioptric power and applies Snell's law to describe the cornea's actual refractive power. A refractive map compensates for spherical aberrations as well as the aspheric contour of the cornea. The central portion of the refractive map is most important. This area overlies the pupil, so aberrations here almost invariably impact visual performance.
Clinicians use refractive maps to evaluate visual performance in post-refractive surgery patients. This view identifies central islands in patients who have undergone PRK or LASIK.
Scaling is another important consideration with corneal topography. In most topographers, the user can utilize the auto-size or normalized scale (relative scale). This strategy essentially subdivides the cornea into dioptric intervals based on its actual curvature range (usually a 6.00D range). Recognize that the actual colors are not specific to a dioptric value when using the normalized scale, but rather are relative to that particular patient's eye.
By contrast, the absolute or standard scale assigns a specific color to each dioptric value and constrains the data to fit within that range. This strategy allows you to directly compare images from different eyes or from significant curvature changes in one eye (e.g., pre- vs. postoperative refractive surgery status). The downside to using the standard scale is that the dioptric range is greatly expanded; hence, clinically significant irregularities may become somewhat obscured when comparing eyes with very different curvature readings. Clinically, it is probably best to use normalized (relative) maps when evaluating one particular eye, and use standard maps when comparing two different eyes or comparing the same eye over time.
Topography may be indicated in many clinical situations. Conditions such as keratoconus and pellucid marginal degeneration may exhibit corneal steepening before any biomicroscopic signs are evident. In keratoconus, the color maps provide information of the location, size and curvature of the cone's apex, and can help you follow the progression of the disease.
Topography also is invaluable when evaluating pre- and postsurgical patients, particularly those who have had penetrating keratoplasty, radial keratotomy or LASIK. Preoperatively, corneal maps give insight into potential obstacles, such as scarring or irregular astigmatism. Postoperatively, topography can help follow the healing phase and assist with contact lens fitting.
The most profitable and practical use of corneal topography in clinical practice is seen in the fitting of gas permeable lenses. Corneal topography is also useful for annually evaluating the topographical impact of corneal changes in soft contact lens patients. It is also virtually mandatory in corneal reshaping (Corneal Refractive Therapy or orthokeratology) to monitor the corneal changes occurring as well as the lens positioning from overnight wear.
Most corneal topographers provide software that can design an appropriate contact lens based upon the topography. They recommend lens material, size, design, and even simulate a fluorescein pattern. You can manually alter diameter; base curve and edge design and observe the impact on the simulated pattern. There are even bitoric fitting programs designed for highly astigmatic patients. This software can greatly simplify the gas permeable fitting process, reduce chair time and increase patient satisfaction.
When considering contact lens correction for these corneal conditions, corneal topography is practically mandatory:
Post-penetrating keratoplasty, which often results in high or irregular corneal astigmatism;
Post-RK or LASIK, in which the central cornea flattens relative to the periphery, potentially resulting in residual refractive error and irregular astigmatism;
Advanced keratoconus, in which the central and peripheral curves of the lens are critical to accommodate the protruding cone.
The information corneal topography provides can greatly enhance your ability to manage complex contact lens fits and increase your overall success rate. While many doctors were discouraged in years past by the high cost of this equipment, the prices have decreased dramatically. Also, many manufacturers now provide leasing packages that offset the initial investment costs.
Corneal mapping devices and contact lens fitting programs will continue to improve and expand their capabilities in the future. Indeed, today it is arguably the standard of care for our profession.
The authors have no direct financial interest in any company that manufactures corneal topographers.
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