Raising IOP changes corneal hysteresis, resistance factor
Acutely raising IOP by ophthalmodynamometry increased corneal resistance factor and decreased corneal hysteresis regardless of glaucoma status, according to a study published in Optometry and Vision Science.
“One of the primary confounders of both corneal hysteresis and corneal resistance factor is IOP,” the researchers wrote. “Although some reports suggest that corneal hysteresis (CH) and corneal resistance factor (CRF) measurements change little when IOP remains within the statistically normal range, other studies convincingly show that IOP is independently linked to variation in both parameters.”
Suchitra Katiyar, MPH, OD, FAAO, of Eye Associates of New Mexico in Albuquerque, and colleagues conducted a prospective, longitudinal cohort study to determine the relationship between IOP, CH and CRF. They evaluated 248 right eyes for changes in IOP, CH and CRF at ophthalmodynamometry visits for at least 3 years starting in 2011 or 2012. Eyes were diagnosed with primary open-angle glaucoma, ocular hypertension, suspected glaucoma or as normal. Patients underwent at least five reliable visual field (VF) tests.
The study included participants aged at least 40 years with normal gonioscopy angles; no corneal or scleral conditions that might affect IOP; no secondary glaucoma; no VF loss from disorders aside from glaucoma; no previous keratorefractive, corneal or incisional glaucoma surgery; and refractive error under 5 D and astigmatism under 3 D.
On average, IOP was doubled from 15.1 to 29.9 mm Hg, causing a CRF increase of 34% (from 8.8 to 11.6 mm Hg) and a CH decrease of 21% (from 8.4 to 6.6 mm Hg).
CH reduction was not statistically different between the four diagnostic groups or between eyes with or without VF loss progression. CH and CRF levels and future VF progression were weakly connected, but Katiyar and colleagues wrote that it was not enough to use ophthalmodynamometry as a predictive measure in glaucoma treatment.
Limitations included demographic profile, evaluation of only one instance of acute corneal biomechanical parameter response to raising IOP and use of pressure on the eyelid over the inferotemporal sclera, which may have affected the results.
Perspective
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Corneal hysteresis (the cornea’s ability to absorb and dissipate energy) and corneal resistance factor (a measure of rigidity) are both biomechanical markers for glaucoma. Hysteresis has been shown to be closely correlated to glaucoma progression. In fact, some studies suggest hysteresis may have a stronger relationship to glaucoma than corneal thickness. Katiyar and colleagues found that the increased corneal stress and strain from IOP elevation changes both hysteresis and resistance factor, making clinical interpretation challenging. Additionally, they found that ocular hypertensives’ corneas may be more resistant to applied force, potentially offering protection from conversion to glaucoma.
While biomechanical properties clearly have a role to play in glaucoma management, the real question for the clinician is the cost vs. diagnostic utility of the machines. Artificially elevating IOP in office as a provocative test seems unnecessary at this time, as the results are difficult to interpret.
A measure of hysteresis and corneal resistance factor can be important and may add value in difficult to diagnose cases. As costs decrease and artificial intelligence-assisted strategies emerge (like the glaucoma risk calculators that properly put all these factors into proper context), these tests will become more commonplace in eye care.
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