Biomechanical criteria for estimating the risk of Primary Open-Angle Glaucoma Progression

The aim of the investigation was to identify new biomechanical parameters characterizing elastic properties of the ocular corneoscleral shell and to determine their significance for estimating the risk of primary open-angle glaucoma (POAG) progression. Materials and Methods. The study involved 41 patients (43 eyes) aged 55 to 72 (mean age 64.0 +/-4.0 years) with non-operated POAG, including 20 eyes with stage I POAG and 23 eyes with stage II, as well as 15 patients of the same age group with no eye pathology (except for age-related cataract) who served as control. The examination included Maklakov elastic tonometry with three different weights, differential Schiotz tonometry using a GlauTest 60 tomograph (Russia), optic nerve imaging using HRT3 (Germany), and static Humphrey (Germany) perimetry. All patients were re-examined 18 months after the initial examination. Results. The biomechanical parameters were calculated based on mathematical modeling using elastic tonometry and differential tonometry data according to the method proposed by the authors. The value of a new parameter, elastic rise coefficient.m, characterizing mainly the rigidity of the cornea and determined using Maklakov elastic tonometry data averaged 0.88+/-0.20 mm Hg/g in stage I POAG and 0.80+/-0.04 mm Hg/g in stage II, whereas the control group showed an average of 0.86+/-0.07 mm Hg/g. At the same time, we revealed increase in the other biomechanical parameter, elastic rise coefficient Y-S, characterizing mainly the rigidity of the sclera and determined according to Schiotz elastic tonometry data. Its value amounted to 1.65+/-0.25 mm Hg/g for stage I POAG and 1.88+/-0.13 mm Hg/g for stage II, with the mean of 1.47+/-0.10 mm Hg/g in the control group. It was found that with the value=Y-S/Y-M (an index showing the individual ratio of scleral and corneal rigidity) increasing above the threshold level clinical signs of glaucomatous process progression were observed. Conclusion. The results indicate growing imbalance between biomechanical parameters of the sclera (Y-S) and the cornea (Y-M) during POAG development. The ratio of these parameters (K=Y-S/Y-M) can serve as a measure of risk in progression of glaucomatous process related to the change in the state of the ocular corneoscleral shell.