Biomechanics of the corneoscleral shell and hemodynamics of the glaucomatous eye: is there a connection?

Purpose. To find the connection between the parameters of the corneoscleral shell and lamina cribrosa (LC) and hemodynamic parameters of the eyes with primary open angle glaucoma (POAG).Materials and methods. The study involved 111 eyes of 63 patients, including 41 eyes without ophthalmic pathology (control group, mean age M ± SD 66.9 ± 18.2 yrs), 22 eyes with the initial glaucoma stage (group 1, mean age 65.9 ± 11.3 yrs), 31 eyes with the developed glaucoma stage (group 2, mean age 69.2 ± 17.3 yrs), and 17 eyes with the advanced glaucoma stage (group 3, mean age 69,7 ± 7.3 yrs). LC parameters and the density of surface (SVL) and deep (DVL) vascular layers were measured using Spectralis OCT2 with an angiography module (Heidelberg Engineering, Germany) in the enhanced deep imaging (EDI) mode using AngioTool software. Corneal hysteresis (CH) and corneal resistance factor (CRF) were determined using ORA (Ocular Response Analyzer, Reichert, USA). The rigidity coefficient (E) of the corneoscleral shell was measured by a GlauTest-60 tonograph (Russia) operation in the differential tonometry mode. The elasticity coefficient (s) was determined by a modified differential tonometry technique. The rheographic index (RI) and pulse blood volume (PBV) were measured using transpalpebral rheoophthalmography.Results. A statistically significant correlation was revealed between CH and SVL (р = 0.005, r = 0.288), LC thickness and SVL and DVL (р = 0.001, r = 0.374 and р = 0.003, r = 0.397, respectively). A negative statistically significant correlation was found between E and RI (р = 0.000, r = -0.538) as well as between E and PBV (р = 0.001, r = -0.376). A similar correlation was revealed between s and theses parameters (RI and PBV; р = 0.027, r = -0.404 and р = 0.024, r = -0.410, respectively). E increase is accompanied by a decrease in the SVL density (р = 0.000, r = -0.376); besides, PBV is negatively correlated with the LC depth (р = 0.022, r = -0.257).Conclusion. In POAG, deteriorated blood supply of inner ocular shells (decreased RI, PBV, SVL and DVL density) correlates with (a) increased corneoscleral rigidity (manifested in the increase of rigidity and elasticity coefficients, and the decrease of CH), and (b) decreased LC thickness and its increased depth. Increased rigidity of the corneoscleral shell contributes to an LC posterior displacement. It must be concluded that LC thickness is a more sensitive biomechanical parameter that changes even in the initial stages of POAG and has a high diagnostic value.

Non-Invasive Ocular Rigidity Measurement: A Differential Tonometry Approach

Purpose: Taking into account the fact that Goldmann applanation tonometry (GAT) geometrically deforms the corneal apex and displaces volume from the anterior segment whereas Dynamic Contour Tonometry (DCT) does not, we aimed at developing an algorithm for the calculation of ocular rigidity (OR) based on the differences in pressure and volume between deformed and non-deformed status according to the general Friedenwald principle of differential tonometry. Methods: To avoid deviations of GAT IOP from true IOP in eyes with corneas different from the “calibration cornea” we applied the previously described Orssengo-Pye algorithm to calculate an error coefficient “C/B”. To test the feasibility of the proposed model, we calculated the OR coefficient (r) in 17 cataract surgery candidates (9 males and 8 females). Results: The calculated r according to our model (mean ± SD, range) was 0.0174 ± 0.010 (0.0123–0.022) mmHg/μL. A negative statistically significant correlation between axial length and r was detected whereas correlations between r and other biometric parameters examined were statistically not significant. Conclusions: The proposed method may prove a valid non-invasive tool for the measurement method of OR, which could help in introducing OR in the decision-making of the routine clinical practice.