Induced IOP Increase Prompts Corneal Biomechanical Changes

An important part of the discussion regarding glaucoma management is the weight given to corneal mechanics in relation to intraocular pressure (IOP). Data published in Optometry and Vision Science indicate that corneal biomechanical metrics may be best interpreted in relation to IOP, according to a prospective, longitudinal study. Biomechanical elements explored are corneal hysteresis (CH) — the cornea’s “ability to dissipate energy,” and corneal resistance factor (CRF) — reflecting its rigidity.

The research was conducted at an outpatient clinic of Albuquerque Veterans Affairs Medical Center in New Mexico. Of 248 participants (248 right eyes), 4 groups included 88 participants with ocular hypertension, 27 suspected for glaucoma, 103 diagnosed with primary open-angle glaucoma (POAG), and 30 normal control individuals. A noncontact tonometer determined Goldmann-correlated IOP (IOPg), corneal-compensated IOP, CH, and CRF. Participation was required for at least 3 years after baseline ophthalmodynamometry in 2011 to 2012, and the median follow-up period after initial visit was close to 8 years.

An acute ophthalmodynamometry-induced IOP increase from 15.1 mm Hg to 29.9 mm Hg prompted a reduction in CH from a median of 8.4 mm Hg to 6.6 mm Hg, and an increase in CRF from a median of 8.8 mm Hg to 11.6 mm Hg. Results demonstrated “the degree of corneal resistance factor change was directly related to degree of IOPg elevation,” and CH was inversely related to IOPg change.

Although the induced CH reduction did not substantially co-vary between the 4 groups, a few unexpected differences occurred in one group. In the ocular hypertensive set, baseline CRF was highest, and induced IOP change and CRF increases were significantly lower than the other three cohorts. Researchers speculate that eyes with ocular hypertension may exhibit a naturally “stiffer ocular coat (corneal and sclera),” giving them a protective benefit.

A number of previous studies have shown that CH and CRF are not significantly altered if IOP stays within a typical range, while others find IOP is involved in changes for each of these elements. The current investigation affirms CH and CRF are “dynamic variables that co-vary with IOP.” Further, the amount of biomechanical change was demonstrated not to be significantly associated with future visual field loss; and acute, temporary changes in these factors were not predictive in the longer term.

In this study, requirements included age of at least 40 years, and having no other ocular disease that would impact IOP. Limitations included a somewhat narrow demographic profile — 95% of participants were men, and there was only 1 session of ophthalmodynamometry. However, strengths were a large sample size, and four varying diagnostic study groups. 

Baseline CH in the POAG cohort was lower than in the other groups. “Whether the lower baseline corneal hysteresis in the glaucoma group indicates a lesser reserve (and thus less room for corneal hysteresis to decrease further when challenged) or whether the findings suggest that all eyes faced with increased IOPs respond similarly with regard to corneal hysteresis remains unclear at this time and requires further study,” the analysis concludes.

Reference

Katiyar S, Tong J, Pensyl D, and Sullivan-Mee M. Corneal biomechanical changes caused by acute elevation of iop in eyes with and without glaucoma. Optom Vis Sci. April 2021;98(4):367-373. doi:10.1097/OPX.0000000000001668