Different topographers' results vary for dry eye patients

Issue: April 1997

FOREST GROVE, Ore. — Different topography systems produce varying results when evaluating dryness, and many experienced topography users recommend grid and slit systems over placido disk systems for use in dry eye patients.

Most of the nine topographers on the market today use placido disk- or placido cone-based technology to generate their maps. However, the CTS (PAR Vision Systems) uses raster-stereography, a projected grid, and the Orbscan (Orbtek) uses projected slits. Many practitioners feel these projection systems yield superior results when evaluating dry eye patients.

Patrick J. Caroline, COT, assistant professor of optometry at Pacific University College of Optometry, here, said all topography systems map the surface of the tear film, not the surface of the cornea, so dry eye patients can present a challenge for all systems. "If there is a break-up in the tear film, the systems using reflected images are at some compromise," he said.

Perry Rosenthal, MD, senior scientific advisor for Euclid Systems Corp., which is developing a new topographer, said this compromise occurs because placido technology uses the front surface of the cornea as if it were a convex mirror, while projection technologies use the front of the cornea as a projection screen.

Placido machines start with an invalid assumption about the shape of the cornea, said Dr. Rosenthal. "Also, they don't measure the center of the cornea, because there's a ring there, a space," he said. "When there are abrupt changes, they can't record anything, even when something is out of focus, but you don't know they're not recording it because they extrapolate a result. Placido disks pretend they know but they make a lot of assumptions."

Ken Daniels, OD, an assistant clinical professor and director of the Contact Lens Research Program at the Pennsylvania College of Optometry, added that several of the placido-based systems "use an extrapolation mode where, if data are not acquired by the image, the mathematics of the system will use algorithms to fill in the blanks and extrapolate what it would presume to be the information for that area."

Slit, grid good for dry eye patients

Experienced with many topographers, Mr. Caroline said the slit technology of the Orbtek system may be advantageous for the dry eye patient. "The difference with the Orbtek system is that it takes multiple, parallel piped slits of the cornea to obtain its raw data, then pastes the slits together to create the topography," he said. "When you're taking a narrow slit of the cornea, the tear film break-up appears to be less of an issue."

Dr. Daniels, who also has used most topographers, uses the CTS for dry eye patients. He finds a big difference between "placido imaging and the rasterography of PAR, be cause the grid allows elevation-depression mapping, which won't miss any spots on the cornea."

However, he also said the placido cone-based TMS-2 Video-keratoscope (Tomey Technologies) is a high-resolution placido system that does not extrapolate data. "If there is a significant 'dry spot' or 'surface irregularity', differential mapping will better highlight the relativeness of the discrete area of defect," Dr. Daniels said.

Albert Morier, OD, instructor in clinical ophthalmology at Albany Medical College, Department of Ophthalmology, has also used most systems, and he, too, prefers the CTS for evaluating dry eye. "The PAR actually measures elevation, and you have 14-mm limbal to limbal coverage. You can also go beyond the limbus," he said.

Dr. Rosenthal said the new topographer by Euclid goes into production this month. "The Euclid does not extrapolate," he said. "It measures every coordinate, regardless of how quickly or abruptly the curvature changes. It shows tear break-up in dry eye. There may be irregularity, but at least it shows that and you know what you have."

Video, drops can help placido images

While he prefers the CTS for dry eye patients, Dr. Daniels said problems mapping a dry eye patient with any topographer can be avoided by evaluating the tear film with high-resolution video before taking the map. In a normal eye, the tear film is even. With dry eye and other problems, irregularities become evident: the video evaluation enables the operator to judge if the picture of the cornea will be reliable or if there are inconsistencies from make-up, a mucous strand or an inconsistent tear film.

Mr. Caroline said adding drops to the eye prior to taking the image and waiting a few minutes can increase the success of a placido topographer's evaluation of dry eye.

Drs. Daniels and Morier do not support the use of drops. Dr. Morier said adding drops or blinking "pollutes the sample," although he pointed out that the CTS requires a drop of fluorescein.

For Your Information:
Patrick J. Caroline, COT, may be contacted at the Oregon Health Sciences University, Casey Eye Institute, 3375 S.W. Terwilliger Blvd., Portland, OR 97201; (503) 597-9507; fax: (503) 494-4286. Mr. Caroline has no direct financial interest in the products mentioned in this article. He is a paid consultant for Humphrey Instruments.
Perry Rosenthal, MD, can be contacted at (617) 735-8810. Dr. Rosenthal has a financial interest in the Euclid topographer and is a paid consultant for Euclid Systems Corporation.
Ken Daniels, OD, can be reached at 1200 West Godfrey Ave., MOD 6, Philadelphia, PA 19141-3399; (215) 276-6104; fax: (215) 276-1329. Dr. Daniels has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
Albert Morier, OD, can be contacted at Albany Medical Center, Albany, NY 12203; (518) 262-4353; fax: (518) 262-6936. Dr. Morier has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
CTS is available from PAR Vision Systems, (315) 738-0600; fax: (315) 738-0562.
Orbscan is available from Orbtek, (801) 484-8777; fax: (801) 484-3982.
Euclid Systems Corp. can be contacted at (800) 477-9396; fax: (703) 471-7577.