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Material properties of cornea, not central thickness, represent future of IOP measurement

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NEW YORK — Efforts to improve the accuracy of IOP measurements should focus on the material properties of the cornea, not just central corneal thickness, according to an ophthalmologist speaking here.

Adjusting Goldmann tonometry measurements by using patients' central corneal thickness (CCT) is a "fallacious attempt to add precision to a very noisy measurement," said James D. Brandt, MD, during his glaucoma keynote lecture at the OSN New York Symposium.

According to Dr. Brandt, several engineering models of the cornea show that the impact of CCT on IOP measurement is smaller than commonly believed. Material properties of the cornea, including corneal stiffness and viscoelastic properties, "can dwarf the effect of CCT" on tonometry measurements, he said.

The Ocular Hypertension Treatment Study and other trials have indicated that CCT has an effect on the measurement of IOP, but Dr. Brandt said that effect can vary depending on other properties of the cornea. Therefore, attempts to adjust tonometry readings with nomograms that take CCT into account do not improve the precision of IOP measurements, Dr. Brandt said.

"Recognizing your patient's CCT as being 'thin,' 'average' or 'thick,' and incorporating that information into your assessment of the overall picture will help you take better care of your patient," he said. "Trying to be more precise than this is not supported by the data and may be harmful to patient care."

New technologies, such as the Ocular Response Analyzer (Reichert Inc.), which is designed to measure hysteresis, or corneal stiffness, and the Pascal Dynamic Contour Tonometer (Zeimer Ophthalmic Systems AG), which takes tonometry measurements that are less affected by CCT, represent the next generation of glaucoma testing, Dr. Brandt noted.