Article Date: 2/1/2007

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Topography:
A Clinical Pearl

Create value by incorporating corneal topography into your practice.
PHOTO BY JAY MYRDAL/DIGITAL MANIPULATION BY MARK RYAN


Rita Mae Brown, a U.S. writer and playwright once said: "Language is the road map of a culture. It tells you where its people come from and where they are going." In the case of optometry, corneal topography is the road map to comfort and optimum vision for patients who have corneal irregularities. Its bright red, yellow and blue readings reveal where the patient is, in terms of condition, and what you should do, in terms of management. To provide these patients with the best care, however, you must first understand the nuances of the most commonly used corneal maps and be able to determine which of these maps to follow when dealing with certain corneal conditions.

Something else to consider: The new ICD-9 code 92025 now allows billing for the procedure of corneal topography. Providing proper documentation in the form of appropriate topography maps facilitates reimbursement (see "Diagnostic and Procedural Codes for Corneal Disorders," page 36).

 Mastering the maps

Most of the various topographers on the market have common map types to display corneal data. Each of these map types presents information differently and displays corneal data in a variety of ways. The following maps are most commonly used:

• "Axial" map. The axial map determines the radius of curvature of the cornea at each measured point. Any change in corneal curvature at a selected point correlates with the change in refractive power and therefore the change in refraction. This is most evident when clicking your cursor on the visual axis and comparing curvature changes before and after refractive surgery or orthokeratology. The change in curvature is directly related to the prescription change in dioptric power. You can also monitor the effects of contact lenses on refractive power by viewing the curvature chang-es to the corneal surface. The important note to make: You can draw a 1:1 relationship between the change in corneal curvature on the visual axis and the resultant refractive change.

The power of the cornea is revealed by clicking the cursor on any point of the map, making this feature very valuable when comparing the same cornea from one visit to the next. For example: if a patient has undergone refractive surgery, you may click the cursor on the visual axis to compare the pre- vs. post-curvature to determine the patient's change in refraction.

The axial map can also determine the type, shape and position of corneal astigmatism. Ax- ial interpretation reveals whether the patient has a normal "hour glass" astigmatism, an apical astigmatism and whether the astigmatism extends from limbus to limbus or is symmetrical or asymmetrical.

• Tangential or "instantaneous" map. This map effective-ly defines points of curvature change. It calculates each measured point of data at a 90° "tangent" to its surface, resulting in clearly defined, small or "instantaneous" curvature changes. This allows you to measure the size and shape of the cone in a keratoconus patient, for instance. Additionally, tangential maps define the position of the treatment or effect of corneal reshaping and refractive surgery. Specifically note the red ring of paracentral steepening outside the central area of flattening (blue). Comparing the red ring's position in relation to the pupil or visual axis clearly defines the position of effect. Consider using tangential maps when measuring a point of curvature or size of an area as opposed to relating to refractive power. This is because in dealing with a keratoconus patient, the ability to measure the size of the cone is very helpful in determining the ideal lens design and optic zone size.

• "Refractive" map. This map provides an interpretation of the quality of vision a patient may achieve from the corneal surface throughout the pupillary zone. The more consistent or uniform the refractive power within the margins of the pupil, the better able the anterior surface of the cornea is to refract light properly. Practitioners do not commonly employ the refractive map, as it does not provide information on curvature or size and shape of the cor-neal surface (for which the axial and tangential maps are more effective). However, this map can be very effective when used to interpret the quality of vision achievable from a patient's corneal surface.

For instance, when comparing pre- vs. post-corneal reshaping results, the refractive map illustrates the extent that corneal surface changes contribute to the patient's quality of vision and the position of the effect of treatment in relation to the pupil. Thus, the refractive power map can aid you in determining how well the patient sees specifically due to the contribution of the corneal surface to visual acuity. Additionally, following corneal reshaping or refractive surgery, it can show you how well or how poorly the effect is positioned.

• Elevation map. This map aids in fitting contact lenses on unusual corneal shapes. By placing a "best fit sphere" over the cornea (radius of curvature that best matches the average curvature of the map), the elevation map can clearly define areas of elevation or depression. Red shading on the map indicates peaks or elevations in relation to the best-fit sphere, and likely bearing points if fitting a contact lens. Conversely, colder colors, such as blue, signify areas of depression in relation to the sphere and likely points of clearance between the lens and cornea.

The elevation map can predict areas of concern prior to the diagnostic or custom fit, which can be helpful when optimizing lens parameters for the ideal patient outcome. By determining areas of clearance and bearing, you can design the appropriate lens for the patient.

• Subtractive or "difference" map. This highly under-utilized map compares two cor-neal maps from one point in time to another, which can be very helpful when monitoring corneal changes from one exam to the next.

For example, the subtractive map can clearly define the absolute effect of contact lenses, refractive surgery and corneal reshaping on the corneal surface. This function subtracts each measured point from one map to the other. So, when selecting the subtractive map function, both selected maps will appear with a third "comparison" or "difference" plot displayed. Areas of the cornea that are now flatter are displayed in cooler colors (blue), and areas of the cornea that are steeper are represented in warmer colors (red), as with pre- and post-op, orthokeratology, contact lens-induced changes, keratoconus, etc. The result: You can precisely measure corneal changes.

This map is indispensable if you practice cor-neal reshaping. When comparing the overnight changes or monitoring the patient over time, the subtractive map indicates the absolute results enacted by the lens.

Used in combination, the following interpretations provide a clear picture of the effect: Employ the axial interpretation to determine treat- ment zone position and prescription changes. The tangential map determines the position of the corneal reshaping lens in the closed eye environment. Lastly, use the refractive power map to measure the treatment zone size. (See "Setting up your topographer," page 35.)

Applying the maps

The patient's clinical diagnosis dictates which topography map should be utilized. Several corneal conditions are best represented with specific map types:

• Keratoconus and pellucid marginal degeneration (PMD). The axial map can differentiate the diagnosis between these two very similar, non-inflammatory thinning disorders. The distance from the center of the cornea (apex) to the steepest part of the cornea is termed the "peak elevation index" or PEI. The average PEI for eyes with keratoconus is 1.95mm from the corneal apex, while the average PEI for PMD is 3.5mm from the apex.

The axial map also generates the shape factor. Shape factor (P) measures the asphericity of the cornea and is a derivative of eccentricity(e). These values can be converted by the equation P=e2. A positive, high-value shape factor (prolate shape) is evident in keratoconus, while a negative or low-value shape factor (oblate shape) is indicative of PMD (see figures 1a and 1b, page 36). The tangential map helps "pinpoint" the shape and location of the cone in keratoconus.

• Corneal edema and war-page. It's possible to define these conditions with a series of axial maps compared over time. The subtractive map shows the change in curvature, which helps you differentiate edema and war-page from keratoconus or irregular astigmatism.

• Post-radial keratotomy and post-LASIK. In some cases, post-refractive surgical patients experience regression or distorted vi- sion. The best way to determine the reason for these outcomes: View the refractive and tangential topography maps. The refractive map quantifies and qual- ifies the treatment zone, allowing you to tell whether the treat-ment zone is large or small and whether it's centered over the patient's pupil. The tangential map defines the exact location of the treatment zone relative to the patient's pupil. This is helpful in determining the quality of vision the patient experiences. Patients with decentered treatment zones may complain of aberrations, such as haloes and glare.

Contact lens applications include:

• Orthokeratology. It's best to use an axial map to determine baseline corneal data, such as apical radius, sagittal depth or eccentricity and horizontal visible iris diameter (HVID). Use these measurements to determine how well the treatment is working. Compare these maps over time to show curvature changes. Axial maps also indicate refractive changes that have occurred, enabling you to determine if the treatment is effective. The red ring of the tangential map best defines the location of the ortho-keratology treatment. This allows you to determine the lens changes necessary to achieve centration.

• Limbal-to-limbal astigmatism. Patients with high amounts of limbal-to-limbal astigmatism require precise fitting of soft-toric or bitoric gas permeable (GP) lenses. The axial map provides you with the best view of the location and amount of astigmatism. An elevation map illustrates the expected fluorescein pattern beneath the best-fit sphere or reference sphere on that cornea (see figures 2a and 2b, page 38). When the toricity stretches from limbus to limbus, a bitoric GP provides better stability by matching the different corneal elevations. You don't see the typical toric fluorescein pattern on patients who have apical astigmatism. These patients may be better fit with a spherical GP lens which aligns well with the spherical peripheral cornea.

• Keratoconus. Many patients with keratoconus have one eye that's more progressed than the other. The axial map can determine the degree of curvature and define the more progressed cor-nea. The elevation map can determine whether a keratoconic lens design is appropriate for one or both corneas. If a normal toric fluorescein pattern appears beneath the reference sphere on the elevation map, the patient may do well with a bitoric lens design on that eye.



Optometric Management, Issue: February 2007