A biomechanically compensated intraocular pressure (bIOP) measuring device initiates an even greater temporary indentation of the cornea than Goldmann applanation tomography (GAT), potentially leading to a prolonged pressure drop of up to 1 hour, according to an analysis posted in Ophthalmology Science. These devices are intended to be significantly less dependent on corneal stiffness than GAT.
The retrospective cohort study examined 20 subjects with healthy eyes and 20 who were previously diagnosed with bilateral primary open-angle glaucoma (POAG) who visited the Royal Liverpool University Hospital between April 2019 and September 2019. At baseline, IOP was measured three times in each eye with a rebound tonometer. Researchers selected 1 eye at random for bIOP tonometry, using the rebound tonometry-only eye as a control. Immediately after the bIOP screen, both eyes were measured with rebound tonometry at specified time intervals.
In the bIOP eye, the pressure changed significantly in healthy eyes (P <.01) and those with glaucoma (P <.01). Following bIOP tonometry, healthy subjects’ IOP was reduced from a mean of 13.75 mm Hg to 10.84 mm Hg at 4 minutes, and in those with glaucoma, from 13.28 mm Hg to 11.11 mm Hg at 8 minutes. Approximately 83% of normal eyes and 92% of eyes with POAG approached baseline IOP at 1 hour.
Investigators suggest it took longer for the healthy eyes to return to baseline IOP than glaucomatous eyes because they had a greater pressure drop. Further, the biggest change in IOP happened earlier and more prominently in the normal group. The researchers speculate that eyes affected by glaucoma display variations in scleral rigidity or increased resistance to aqueous outflow, causing a less sudden and distinct drop in IOP.
Previous studies have shown that applanation temporarily alters the chamber angle and shape of the anterior chamber, with the bIOP device indenting the cornea and “deforming” the anterior chamber more than Goldmann applanation tonometry, according to investigators. “It is likely that this deformation alters IOP and aqueous outflow and accounts for the significant and prolonged reduction in IOP after [bIOP],” the study explains, adding the outcome is potentially caused by a number of factors.
Mean age of participants was slightly more than 54 years. Patients with smaller cataracts were not excluded. Limitations of the study included difficulty in quickly changing the rebound device’s probe at the first measurement intervals. Also, researchers did not assess participants’ corneal curvature or thickness, although biomechanically-calibrated devices take into account corneal elasticity and viscosity. Theoretically, bIOP measurements may be less affected by corneal thickness and patient’s age than other methods of tonometry, the study explains.
Borroni D, Gadhvi K, Hristova R, et al. Influence of Corvis ST on intraocular pressure. Ophthalmol Sci. Published online January 13, 2021. doi:10.1016/j.xops.2021.100003