Diagnostics

Understanding the uses of Topography measurements

Primary knowledge of corneal mapping can help you achieve the best outcomes for both your surgeon and patients.

Sue Stuhr, COT, OCS, Eugene, Ore.

Because the cornea contributes the majority of the eye’s refractive power, corneal aberrations have a significant impact on vision and, by extension, a patient’s quality of life. Corneal irregularities can be an unfortunate result of corneal disease, trauma or surgery. Analysis of corneal measurements can be one of the most valuable tools to understanding symptoms, making diagnoses, creating a treatment plan and designing surgeries.

Understanding the types of maps available with topography and uses for topography measurements is necessary to achieving optimal results.

Types of maps

Sagittal map - This map provides a view of the curvature across the entire cornea. Generally speaking, the axial map does not show minor variations in corneal curvature; however, it does use colors to represent the curvature and dioptric values at different spots across the cornea. The hotter colors (reds and oranges) represent steeper areas and the cooler colors (blues and greens) represent flatter areas.

Tangential map (Figure 1) - Corneal irregularities are often located on the anterior surface of the cornea, with two common causes being tear film disturbances and contact lens warpage. Colors are used to represent the curvature and dioptric values across the cornea and this curvature map resembles the true corneal shape.

Figure 1: This example of a tangential map was taken over a soft multifocal contact lens with a center-distance, peripheral add design. The colors represent the curvature and dioptric values across the cornea.

This map can also pinpoint the position of corneal defects such as cone location in keratoconus. On corneal topography, an eye with regular astigmatism will be represented by a bowtie pattern with two symmetric segments. If the symmetric bowtie is vertical, the eye has with-the-rule astigmatism; if it is horizontal, the astigmatism is against-the-rule; and if it is diagonal, oblique astigmatism is present. Additionally, the curvature map picks up corneal irregularities that have abnormal keratometry readings.

Elevation map - This map depicts the height at which corneal elevations and depressions deviate from a computer-generated reference surface. Shades of red show elevations and shades of green or blue show depressions.

Common uses for topography

Refractive surgery - Topography maps are used to screen refractive surgery candidates for normal corneal shape or to rule out suspicious patterns or risk factors of ectasia.

The detection of keratoconus is of particular importance in patients who plan to undergo refractive surgery. Postoperatively, topography can help to assess the dioptric change created, rule out decentred or incomplete ablation, post excimer ectasia or other changes.

Keratoconus - Early screening of keratoconus suspects is one of the most useful roles of topography. Early keratoconus suspects look normal on slit lamp examination, and the central keratometry gives only a limited assessment. That’s why topography has become the gold standard in screening keratoconus suspects. In cases with established keratoconus, topography is paramount for monitoring progression and doing a timely collagen cross linking and in contact lens fitting.

Post-surgical astigmatism - Post cataract surgery and post keratoplasty corneal astigmatism can be studied with the topographer and selective suture removal, or other interventions can be planned.

Surgical planning in cases with astigmatism - Limbal relaxing incisions and other methods of topography guided incision placement are used by surgeons to reduce post-operative astigmatism.

Effect of corneal and ocular surface disorders - Disorders such as pterygium, limbal dermoid, localized corneal scars can cause a change in the corneal topography and thus the monitoring is very useful.

Getting the best results

One of the simplest ways to ensure accuracy is proper patient education prior to the testing. Explaining the reason for the test and how it obtains results can be reassuring to the patient. This can ensure better patient cooperation. Anticipation of the testing may be especially stressful for a patient with a new medical diagnosis.

Another item to anticipate prior to corneal topography is to make sure all contact lens patients are out of soft lenses for at least a week for spherical correction, and two weeks or longer for toric or RGP lenses.

Make sure there is accurate surgical eye history from all patients to be tested. Patients forget to mention prior PRK or LASIK surgeries unless specifically asked about it. Additionally, make sure the surface of the cornea is lubricated appropriately. If surface irregularities are noted, use artificial tears to ensure better measurements.

Get familiar with the specific machine you’ll be using. Reading the instruction manual can provide additional pearls for successful testing. Some machines work better with lower ambient lighting.

Understanding the capabilities of topography, the uses for the measurements and how to get the best measurements, you can provide better care. OP

Sue Stuhr, COT, OCS has been working in eye care for 24 years. She has been at the practice of Drs. Fine, Hoffman and Sims in Eugene, Ore. for 14 years, where she is employed as technician, lead scribe, and clinical research coordinator.