Biomechanical and Vascular Metrics Between Eyes of Patients With Asymmetric Glaucoma and Symmetric Glaucoma.

PRCIS: Corneal hysteresis (CH) and pulsatile ocular blood volume (POBV) were significantly lower in the eye with greater damage in asymmetric glaucoma, without a difference in intraocular pressure (IOP) or central corneal thickness (CCT), and no difference in elastic parameters. OBJECTIVE: To compare biomechanical and vascular metrics between the eyes of patients with asymmetric glaucoma (ASYMM) and those with symmetric glaucoma (SYMM). PATIENTS AND METHODS: Forty-five patients were prospectively recruited and divided into ASYMM, defined as cup-to-disc (C/D) ratio difference >0.1 between eyes and SYMM, with C/D difference ≤0.1. For ASYMM, the smaller C/D was defined as the best eye ("best") and the fellow eye was defined as the worst eye ("worse"). All metrics were subtracted as "worse" minus "best," including the viscoelastic parameter CH, and elastic parameters from the Corvis ST, including stiffness parameter at first applanation, stiffness parameter at highest concavity, integrated inverse radius, deformation amplitude ratio, IOP, CCT, mean deviation (MD), ganglion cell complex (GCC), and POBV were included. Paired t tests were performed between eyes in both groups. Statistical analyses were performed with SAS using a significance threshold of P <0.05. RESULTS: For ASYMM (16 patients), "worse" showed significantly lower CH (-0.76 ± 1.22), POBV (-0.38 ± 0.305), MD (-3.66 ± 6.55), and GCC (-7.9 ± 12.2) compared with "best." No other parameters were significantly different. For SYMM (29 patients), there were no significantly different metrics between eyes. CONCLUSIONS: Lower CH, POBV, GCC, and worse MD were associated with greater glaucomatous damage in asymmetric glaucoma without a difference in IOP or CCT. Lower CH and GCC are consistent with previous studies. POBV, a new clinical parameter that may indicate reduced blood flow, is also associated with greater damage.

A Review of Corneal Biomechanics and Scleral Stiffness in Topical Prostaglandin Analog Therapy for Glaucoma.

PURPOSE: The mechanism of action underlying prostaglandin analog (PGA) therapy involves changes in the expression of different metalloproteases to increase permeability of the sclera and allow increased aqueous humor outflow through this alternative drainage pathway. This alteration of structure impacts cornea/scleral biomechanics and may introduce artifact into the measurement of intraocular pressure (IOP) in the clinical setting. METHODS: A literature search reviewing the impact of PGA therapy on corneal and scleral biomechanics was conducted including basic studies, clinical studies with treatment naïve patients, and a clinical study examining the cessation of PGA therapy. Additional literature including engineering texts was added for greater clarity of the concepts underlying ocular biomechanics. RESULTS: One study with an animal model reported significant corneal stiffening with PGA treatment. Most longitudinal clinical studies examining the effects of initiation of PGA therapy in PGA naïve subjects failed to report biomechanical parameters associated with stiffness using the Corvis ST and only included those parameters strongly influenced by IOP. One study reported a significant reduction in scleral stiffness with IOP as a co-variate, highlighting the need to account for the effects of IOP lowering when assessing clinical biomechanics. The report of cessation of PGA therapy on corneal biomechanics showed no change in corneal compensated IOP after 6 weeks, raising the question of reversibility of the PGA-induced structural alteration. CONCLUSIONS: Given that the findings in several clinical studies may merely reflect a reduction in IOP, further studies are warranted using Corvis ST parameters associated with corneal and scleral stiffness. The gold standard for IOP measurement in the clinical setting is Goldmann applanation tonometry, a technique previously shown to be affected by corneal stiffness. Since PGA therapy has been reported to alter not only scleral biomechanics, but also corneal biomechanics, it is essential to consider alternative tonometry technologies in the clinic.

Asymmetric glaucoma in pseudoplateau iris syndrome.

A 39-year-old Caucasian man with bilateral narrow angles, a plateau-like iris configuration on gonioscopy and elevated intraocular pressure (IOP) presented with significant asymmetric glaucoma, left eye affected more than right. Initial management with topical medical therapy, laser iridoplasty and peripheral iridotomy in the left eye was ineffective in lowering the IOP or opening the anterior chamber angle. Ultrasound biomicroscopy demonstrated bilateral ciliary body cysts. The patient ultimately required surgical management, consisting of cataract extraction and endoscopic cyclophotocoagulation of ciliary body cysts in the left eye and trabeculectomy in the right eye, for persistent IOP control to prevent further optic nerve damage and subsequent visual field loss.

Corneal biomechanical properties affect Goldmann applanation tonometry in primary open-angle glaucoma.

PURPOSE: To study differences in corneal biomechanical properties between primary open-angle glaucoma (POAG) and central corneal thickness (CCT)-matched control subjects and their effect on intraocular pressure (IOP) measurement. METHODS: Thirteen eyes of 13 POAG subjects and 15 eyes of 15 normal subjects underwent corneal topography; IOP using Goldmann applanation tonometry (GAT), dynamic contour tonometry (DCT), and corneal compensated IOP (IOPcc) using the Reichert ocular response analyzer (ORA); corneal hysteresis; and CCT. Results from POAG and control eyes were then compared using t tests. RESULTS: Ages in the POAG group were slightly greater than that in the control group. CCT was closely matched between groups. Significant differences were found between GAT versus DCT and GAT versus IOPcc within both groups: Mean GAT IOP was not significantly different between POAG and controls, whereas mean DCT IOP did show a significant difference between groups as did mean IOPcc. The delta differences, GATΔDCT and GATΔIOPcc, were of greater magnitude in POAG subjects when compared with controls. Corneal hysteresis was significantly lower in POAG subjects. CONCLUSIONS: The delta differences between GAT and newer measures of IOP are greater in magnitude in patients with POAG than in the normal controls, independent of CCT. This is likely due to differences in the corneal biomechanical properties with POAG corneas being softer than healthy corneas, which causes greater underestimation of IOP by GAT in POAG than controls. Underestimation of IOP could affect treatment decisions and outcomes in POAG.