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Topography:
A Clinical Pearl
Create value by incorporating corneal
topography into your practice.
BY DIANNE ANDERSON,
O.D., AURORA, ILL., AND RANDY KOJIMA
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