topographer

Maps You'll Treasure

How to use the capabilities of your corneal topographer to get the information you need.

DIANNE M. ANDERSON, O.D., F.A.A.O., Glen Ellyn, Ill.

Modern corneal topography systems are equipped with sophisticated computer programs and powerful software. They're loaded with features and capabilities of which most users are unaware.

The latest map-interpretation features, in particular, can be very helpful, whether you're fitting or designing contact lenses, diagnosing corneal conditions, such as keratoconus, or comanaging corneal refractive-surgery patients. These new features allow for a thorough evaluation of corneal irregularities as well as the effects of these irregularities on patients' quality of vision.

This article illustrates both the common and innovative features of various corneal topography systems and explains their clinical applications.

The basics

Corneal topographers can be small-cone or large-cone Placido disc or slit scanning devices. Small-cone Placido disc topographers have a shorter working distance and project a greater number of rings onto the cornea than large-cone topographers. The small-cone models available today include the Medmont E300 (Medmont); Scout and Keratron (EyeQuip); and Magellan Mapper (Nidek). (See figure 1.) Large-cone Placido disc topographers use a longer working distance and project fewer rings onto the cornea than small-cone topographers. Large-cone models available today include the Atlas Model 9000 (Carl Zeiss Meditec); and ReSeeVit (Veatch Ophthalmic Instruments). (See figure 2.)

Figure 1: The Medmont E300, Scout and Magellan Mapper are examples of small-cone placido disc topography systems.

Slit scanning devices, such as the Orbscan (Bausch & Lomb) and the Pentacam (Oculus), require more time for acquiring images, as they measure the elevation of both the anterior and posterior corneal surfaces.

Common map types

The various types of topographers can graphically represent the data they acquire in many different ways:

• The axial, or sagittal, map. This map supplies a general view of the anterior curvature of the cornea, providing a basic idea of the overall corneal shape. It also defines astigmatism as regular or irregular as well as oblique, apical or limbus-to-limbus. Further, the axial map reveals the presence of keratoconus or keratoectasia.

Figure 2: The ATLAS Model 9000 and ReSeeVit are examples of large-cone placido disc topography systems.

• The tangential map. This map provides precise interpretation of the size, shape and position of anterior corneal anomalies, such as irregular astigmatism and keratoconus. It also defines the position of the treatment zone in refractive surgery and orthokeratology patients. The red ring defines the treatment zone. A decentered treatment zone may induce lower-order aberrations, such as astigmatism, as well as higher-order aberrations, which cause halos and glare.

• The mean curvature map. This map shows the local average curvature at each point and suppresses such nonpathological features as corneal astigmatism and preserves pathological features, such as keratoconus, to detect subtle corneal-shape abnormalities.

• Irregularity maps. These maps display corneal-surface irregularities as wavefront errors, which helps to quantify areas of distortion. (See figure 3.)

Figure 3: Overview of axial, tangential, mean curvature and irregularity maps for a keratoconic eye. The steep apex of the cone is well defined by the tangential map and not suppressed by the mean curvature or irregularity maps.

• The refractive map. This map displays the dioptric effects of refractive surgery and orthokeratology on the cornea. It helps you determine the achieved magnitude of correction.

• Elevation maps. These maps measure the height of the cornea at a specific point in microns relative to a best-fit sphere or ellipsoid. In addition, they're helpful for predicting the sodium fluorescein (NaFl) pattern of spherical GP lenses on the eye. Blue shading indicates areas of NaFl pooling and red shading indicates areas of touch or bearing.

Placido disc topography systems convert curvature data into elevation values via a series of detailed algorithms. In contrast, slit scanning topographers measure elevation directly and reconstruct the data into curvature values.

Specific display options

You can display the aforementioned map types in many different ways. Most topographers allow you to view four different maps simultaneously. This list explains display options and their use:

• Single view
► Definition: displays one map type of one eye.
► Common usage: initial assessment of the corneal features.

• OD/OS comparison
► Definition: side-by-side display of the same map type for the right and left eye.
► Common usage: compare contour differences between the eyes.

Difference or subtractive
► Definition: quantifies the difference between two of the same map types of the same eye on two separate occasions (See figure 4.)
► Common usage: to view changes in curvature or refractive power in pre- and postoperative refractive-surgery cases and to monitor orthokeratology and corneal warpage cases.

Figure 4: A difference map display shows the -3.5D change through time in this orthokeratology patient.

Figure 5: A custom display of post-radial keratotomy regression. Axial maps are used to show the difference in corneal curvature with and without a hybrid contact lens on the eye. The corresponding image simulation shows the improvement in quality of vision with the lens on the eye.

• Overview
► Definition: different map types of the same eye on the same occasion.
► Common usage: to obtain a detailed description of corneal irregularities with different maps.

• Trend
► Definition: view of up to four different exams of the same map type of the same eye on different occasions.
► Common usage: to show healing trends in post-surgical and corneal-warpage cases.

• Custom
► Definition: four different views of any combination of exams for a selected patient. (See figure 5.)
► Common usage: to clinically illustrate a diagnosis for medical or educational purposes.

Wavefront aberrometry

Many of today's corneal topographers also incorporate wavefront aberrometry capabilities. These are useful because corneal irregularities have a major impact on both visual acuity and the quality of vision. It's important to keep in mind that when a device measures ocular wavefront, it's measuring the eye's total aberrations. Total ocular wavefront measurements are dependent on pupil size and accommodation. They provide objective information about refractive error. Most topography systems only measure corneal wavefront. As such, they measure only the aberrations contributed by the cornea, which are independent of pupil size and accommodation and don't provide refractiveerror information. However, both ocular and corneal wavefront data help illustrate the effects of higher-order aberrations on the patient's quality of vision. (See figure 6.)

If your topography system has wavefront capability, you can display corneal wavefront information in Zernike table format, image simulation format (a representation of what the patient sees) or point-spread function (a representation of how a point of light is degraded by the cornea's optics).

Figure 6: Post-radial keratotomy corneal wavefront display. Corneal wavefront data help to illustrate the effects of higher-order aberrations on quality of vision.

Wavefront aberrometry can also provide information about modulation transfer function, which is a measure of contrast sensitivity in cycles per degree. An increase in cycles per degree signifies an increase in contrast sensitivity. Patients with poor contrast sensitivity will show a decrease in cycles per degree. Keep in mind that in order to accurately view the effects of higher-order corneal aberrations on quality of vision, you must mask the lower-order aberrations of defocus and astigmatism. You can usually do this simply by checking off these coefficients on your topographer's Zernike table or pyramid.

Viewing data

You can evaluate and customize the displays you use based on your level of experience and what diagnostic or treatment information you're trying to obtain. For example, you can view your data in the following ways:

• Color scale settings. Expand or contract displays to produce general or detailed maps.
• Shape factor and eccentricity. This is a measure of corneal asphericity. Shape factor is equal to eccentricity squared and can have either a positive or negative value. Positive values indicate a prolate shape while negative values indicate an oblate shape.
• Horizontal visible iris diameter (HVID), or white-to-white. You can obtain this measurement either automatically or manually by drawing a chord from limbus to limbus. This information is helpful for determining the diameter and base curve of a contact lens.
• 3-D maps and cross sections. These allow you to obtain a view of the extent and location of corneal asymmetry. Keratoconic corneas generally show greater amounts of asmymetry in the vertical meridian and more symmetry in the horizontal meridian.
• Photopic and scotopic pupil measurements. You can obtain these measurements, but most topographers aren't true pupillometers, which measure with a standard universal light source.
• The inferior/superior index (I-S). Developed by Rabinowitz, this is a measure of the difference between the average inferior power and average superior power of the cornea. The ratio difference between these two measurements is the I-S value. Positive values signify that the cornea is steeper inferiorly. According to Rabinowitz, an IS value higher than 1.2 is characteristic of keratoconus.
• Surface asymmetry index (SAI). This is a measurement of corneal asymmetry in four perpendicular meridians. High readings — an SAI of 10.07, for example — are indicative of corneal pathology.
• Surface regularity index (SRI). This measures central corneal regularity. A large difference between the SAI and SRI values indicates the patient has corneal irregularity.

Exporting and networking

Many topographers allow you to export compressed files, enabling you to send these files to labs for contact-lens design.

If the lab has the compatible software, it can evaluate any map display of the selected patient. If it doesn't, it can still receive your data, but you must save a single image as a JPEG or bitmap.

Because compatibility can be an issue, check with your corneal topography vendor to determine whether review software or integration with other instruments, such as anterior segment photography, retinal photography and visual fields, are available.

The beauty of the various types of topography views and displays is how you can customize them to give you unique and comprehensive compilations of diagnostic and treatment data. The result of the use of these devices is comprehensive care for your patients. OM

Dr. Anderson practices in suburban Chicago, specializing in orthokeratology, keratoconus and post-surgical lens fits and anterior segment disease. E-mail her at dianne.ander son@comcast.net.