Corneal Topography

How it works

A cone of illuminated concentric rings, called a Placido disc, is reflected off the cornea's tear film and photographed. Where the cornea is steep the rings appear closer together; where it is flat, farther apart; where it is irregular, distorted. Software converts the ring positions into curvature at each point and draws the map. Some instruments add a slit-scanning or rotating camera to measure the back surface and thickness as well, which is tomography.

Reading the map

A normal cornea is a smooth bow-tie of symmetric astigmatism or a uniform sphere. Keratoconus shows as an asymmetric island of steepening, usually below center, that grows over time. A post-LASIK cornea shows a flat central zone. A scar shows localized irregularity. Numeric indices summarize the steepest curvature, the astigmatism and the asymmetry, and are compared from visit to visit.

What it is used for

• LASIK, PRK and SMILE screening: to exclude keratoconus and irregularity, and to plan the treatment
• Keratoconus diagnosis and monitoring, alongside tomography
• Toric lens planning for cataract and lens surgery: the amount and axis of astigmatism
• Specialty contact lens design, from which rigid, hybrid and scleral lenses are built
• Assessment of corneal scars, transplants and post-surgical corneas
• Investigating vision that glasses cannot correct, to look for irregular astigmatism

Preparing for the scan

Contact lenses mold the cornea and must be out beforehand: soft lenses for a few days, rigid and scleral lenses for two to three weeks, so the map shows the cornea's true shape. A dry or unstable tear film blurs the rings and is treated or lubricated before scanning.