Studies redefine biomechanics of glaucoma

September 1, 2008

Investigators using a non-contact applanation tonometer (Ocular Response Analyzer [ORA], Reichert Inc.) have made finds pertaining to the importance of corneal hysteresis, the role of ocular biomechanical properties in the pathophysiology of glaucoma, and differences in these properties between African Americans and Caucasians.

Key Points

Fort Lauderdale, FL-New data from several research teams support a potentially significant relationship between ocular biomechanics and the glaucoma disease process. Investigators using a non-contact applanation tonometer (Ocular Response Analyzer [ORA], Reichert Inc.) have made finds pertaining to the importance of corneal hysteresis, the role of ocular biomechanical properties in the pathophysiology of glaucoma, and differences in these properties between African Americans and Caucasians.

At the annual meeting of the Association for Research in Vision and Ophthalmology (ARVO), several researchers provided updates on their research in this field, including Mitsugu Shimmyo, MD, an assistant clinical professor of ophthalmology at New York Medical College, Valhalla; Jing Wang, MD, University of Montreal, QC; and Charlotte Gore, a medical student at the University of California, San Diego (UCSD).

The importance of corneal biomechanics in the evaluation and management of glaucoma garnered widespread attention 6 years ago when investigators from the Ocular Hypertension Treatment Study (OHTS) reported that central corneal thickness was an important risk factor for the development of glaucoma. At the time, it was surmised that the relevance of corneal thickness pertained to its artifactual effects on the measurement of IOP by applanation tonometry. These biomechanical artifacts were recognized by Goldmann when the tonometer bearing his name was introduced more than 50 years ago. He concluded that IOP was likely underestimated in eyes with thin corneas and overestimated in eyes with thick corneas.

As a surrogate, CCT has offered up a mixed performance. While it is well established that CCT is a risk factor for glaucoma, the role of CCT as a prognostic factor in established glaucoma is less understood. Even less clear is the manner in which CCT-or the broader collection of ocular biomechanics-affects the glaucoma disease state. Researchers now speculate that corneal biomechanics could provide insight into the biomechanical properties of structures in the optic nerve head, where the damage of glaucoma is done.

Three perspectives

Dr. Shimmyo explained the results of utilizing the non-contact applanation tonometer to evaluate differences in corneal biomechanics among three groups: normal subjects, high-tension glaucoma, and normal-tension glaucoma. All the subjects with glaucoma were newly diagnosed and untreated at the time of testing.

"Goldmann applanation tonometry is a one-dimensional view of the universe of IOP, based on a single formula," Dr. Shimmyo said. "The [non-contact applanation tonometer] measures a biomechanical property of the cornea termed corneal hysteresis. It also provides an estimate of Goldmann IOP and an IOP estimate that is corrected for the cornea's biomechanical properties."

Corneal hysteresis is a measure of the viscoelastic damping of the cornea. An infrared electro-optical system monitors the conformation of the cornea as a metered puff of air deforms the cornea, continuing to collect data as the cornea rebounds to its natural configuration when the air puff ends. The cornea passes through applanation twice, once during deformation and once during rebound. Hysteresis is the calculated pressure difference of these two applanation events.

Results of the tests conducted by Dr. Shimmyo's team suggested that CCT might not be the optimal surrogate for corneal biomechanical properties. "We found that mean central corneal thickness was similar in normal eyes, in eyes with high-tension glaucoma, and in eyes with normal-tension glaucoma," he said. "In contrast, normal-tension eyes had significantly lower mean corneal hysteresis values than normal eyes or eyes with high-tension glaucoma."

Interestingly, lower corneal hysteresis values may have resulted in underestimates of IOP in the eyes with low-tension glaucoma. "In the normal eyes and the eyes with high-tension glaucoma, the values of Goldmann tonometry and the [non-contact applanation tonometer's] cornea-corrected IOP were similar," said Dr. Shimmyo. "But in the eyes with normal-tension glaucoma, the cornea-corrected IOP was higher than the Goldmann measurement by a mean of 5.4 mm Hg."

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