Common glaucoma drugs may affect IOP measurements

Temporary corneal changes resulting from long-term prostaglandin analogue (PGA) use may trick the current gold-standard test into underestimating pressure in patients with primary open-angle glaucoma (POAG), new research claims.

Published in the British Journal of Ophthalmology this May, the study found that topical PGAs led to reversible reductions in corneal hysteresis, corneal resistance factor and central corneal thickness that contributed to an underestimation of intraocular pressure (IOP) measured by Goldmann applanation tonometry.

Researchers studied pairs of eyes from 35 POAG patients taking PGA therapy for at least one year. Patients paused therapy in one eye for six weeks while concurrently continuing its use in the other eye, and after six and 12 weeks—after PGA therapy had been reinstated—the eyes were compared via corneal hysteresis.

At six weeks, the study eyes showed marked increases in corneal hysteresis, corneal resistance factor and central corneal thickness while IOP, measured via Goldmann tonometry, rose from 15.4 to 18.4 mm Hg, according to Medscape. Once PGA therapy had been reinitiated, the effects were reversed in the study eyes at week 12, as well as discrepancies in the IOP as measured by Goldmann tonometry versus corneal-compensated IOP.

"(These results) warrant caution when clinicians assess accuracy and adequacy of IOP control in patients under chronic PGA therapy," the study's authors conclude.

The 'sneak thief of sight'

Glaucoma, a group of eye disorders that lead to progressive damage of the optic nerve, is the second-leading cause of blindness in the U.S., occurring mostly in people over age 40. The most common form of glaucoma, POAG, results from fluid buildup inside the eye exerting progressive pressure on the optic nerve that transmits visual signals to the brain.

High IOP can damage the optic nerve, and vision lost to glaucoma cannot be restored. While glaucoma cannot be prevented, if it's diagnosed and treated early enough, it can usually be controlled. That's why such research into IOP accuracy is critical.

Leo Semes, O.D., professor at the University of Alabama-Birmingham School of Optometry and founding member of the Optometric Glaucoma Society, notes the study confirms that lower corneal hysteresis readings are associated with a glaucoma diagnosis. Additionally, Dr. Semes stresses that while severity of glaucoma damage was measured by the mean deviation index and retinal nerve fiber layer thickness, lower corneal hysteresis (and corneal resistance factor) data were consistent with greater degree of damage. These emerging data points could influence management of glaucoma.

"Future research will focus on the influence of these measurements in relationship to the diagnosis and progression of glaucoma," he says. "Biomechanics parameters may give us a better understanding of patients who continue to progress in the face of lowered IOP."

Reference the AOA's clinical practice guideline on POAG, and find AOA Marketplace resources for patient education and awareness of the disorder.