topography topics

The True Power of Refractive Maps

BY KENNETH A. LEBOW, OD, FAAO
DECEMBER 1999

Refractive maps are the only maps that describe the true refractive power of the cornea. They show the power of the eye in diopters and are calculated by modeling the cornea as a single refractive surface using Snell's Law. Refractive maps use arc-step reconstruction techniques to measure the refraction of light passing through the corneal surface. They take spherical aberration into account, and are adjusted for an aspheric surface, such as the human cornea, rather than for a simple spherical surface. These maps are also referred to as "Snell's Law Maps."

Remember, axial and tangential maps are actually a measure of the corneal curvature, not of true corneal power. Refractive maps show the power of the eye in its refractive state measured in diopters of power. Certainly, the most important portion of the refractive map is the area which appears over the pupil.

If aberrations of power due to surface irregularities appear over the pupil, visual performance will be affected.

Post-PRK Patients

Refractive maps are especially helpful for evaluating visual performance in post-operative refractive surgery patients. Figure 1 represents four views of a post-operative PRK patient with a rather large central island. The white plus sign is located over the area of highest elevation (2.2 �m) of the central island on the elevation map (lower left). That point on the corresponding refractive map (lower right) is associated with a power of 45.40D. When that refractive power is related to the surrounding powers falling within the pupil, the central island is seen to represent from 1.20D to 2.30D of difference. This power difference across the pupil created a significant visual problem for the patient for approximately one year and has prevented surgery on his fellow eye.

When this patient's refractive maps were viewed over a period of approximately one and a half years, it became apparent that the severity of the central island reduced significantly and the steepest power of the central island decreased from 45.90D to 45.20D. Over time, this regression improved the patient's central vision and eliminated most of the ghost images he was experiencing (Fig. 2).

Remember, understanding what information each map type represents enables you to select the proper view for any problem that you may encounter.

Dr. Lebow is a member of the AOA and a Fellow of the AAO. He is in private practice in Virginia Beach, Virg.