corneal assessment
Corneal
Topography Basics and Clinical Applications
BY
MARJORIE J. RAH, OD, PHD
Advances
in technology in the cornea and contact lens field have made corneal topography
a key tool in many practices. The following is meant as a refresher of
topography basics and to provide
examples of various clinical applications of the technology.
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Figure 1. Refractive power difference map displaying
the difference between pre- and post-PRK refractive surgery.
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Axial and Tangential Maps
An axial map shows an assessment of corneal curvature in diopters.
Also called a sagittal or power map, it's commonly used to describe overall corneal
shape.
A tangential map is a more sensitive map that you can use to detect
more precise locations of a specific corneal defect. For example, when evaluating
a patient who has keratoconus, the location of the cone's apex is more apparent
when viewing the tangential map. When used with the difference display, a tangential
map can also depict the change in corneal shape following treatment with orthokeratology
or following a refractive surgical procedure.
Refractive Power Map
Axial and tangential maps measure corneal curvature, but not power.
A refractive power map describes the refractive power of the cornea in diopters.
This type of map is useful when evaluating the cornea following a refractive surgical
procedure. Figure 1 shows a difference display of refractive power maps for a patient
who underwent PRK surgery. The change in refractive power of the cornea is –4.50D,
which corresponds to the manifest refraction change of the eye.
Elevation Map
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Figure 2. Elevation map and simulated fluorescein
patterns of two lens designs for a patient who has keratoconus. |
An elevation map displays the difference in height of the corneal
surface when compared to a best-fit spherical surface. You can use the information
gleaned from this analysis to predict the fluorescein pattern when fitting GP contact
lenses. Figure 2 shows the elevation map for a keratoconic cornea along with the
simulated fluorescein patterns for two possible contact lens designs. The software
allows you to make adjustments in the lens designs and to then view the resulting
simulated fluorescein patterns.
Scratching the Surface
Each corneal topographer is different, but they all have extensive
clinical applications. What I've discussed here are but a few brief examples of
some of the basic applications of corneal topography. Many other features and benefits
exist, but are too broad for the scope of this column.
Dr. Rah is an assistant
professor at the New England College of Optometry where she works primarily in the
Cornea and Contact Lens Service in patient care, teaching and research.
Contact Lens Spectrum, Issue: April 2006