Comparative analysis of biomechanical parameters of the fibrous membrane of the eye in clinical variants of the course of primary open-angle glaucoma

PURPOSE. To analyze the biomechanical parameters of the fibrous membrane of the eye in patients with normaltension glaucoma (NTG) in comparison with primary openangle glaucoma (POAG).

METHODS. The study compared 37 eyes with NTG and 78 eyes with POAG, which were comparable in stages (initial and developed NTG was in 78%, POAG — in 77% of cases), in age (NTG — 63.62±1.9 years, POAG — 59.85±1.1 years (p=0.088)). The control group included 19 healthy eyes. Corneal tomography and biomechanical parameters were measured using Pentacam (Oculus) and Corvus ST, respectively.

RESULTS. The following differences were revealed in NTG compared to POAG: axial length (24.8±2.3 mm, 23.97±2.3 mm, respectively, p<0.01), central corneal thickness (CCT; 513.97± 5.2 µm, 557.7±3.9 µm, respectively, p<0.01). Corneal stiffness in patients with NTG is reduced (DA ratio 4.7±0.07, R 8.9±0.15) in comparison to POAG (DA ratio 4.07±0.08, R 7.56±0.2) with p<0.01. SP-A1 is lower in NTG (101.65±3.8) than in POAG (128.49±2.6), p<0.01. Stress–strain index (SSI) is also reduced in NTG (1.14±0.04) compared to POAG (1.25±0.03), p=0.026. Biomechanical glaucoma factor (BGF) in NTG is 57±3.5; in POAG — 17.1±1.9 (p<0.01). In NTG corneal stiffness is lower than in a healthy eye (DA Ratio 4.02±0.08 and R 7.63±0.2), p<0.01, the differences in other parameters are not significant. POAG differs from a healthy eye only in the increased stiffness of the fibrous membrane SP-A1 (p=0.044). In NTG the P0 pressure did not differ from biomechanically corrected intraocular pressure (bIOP; 13.95±0.5 and 13.96±0.3, respectively) and was lower than in POAG in all stages of glaucoma. At the same time, in NTG the bIOP level did not differ, while in far-advanced stage of POAG it was elevated.

CONCLUSION. NTG differs from POAG in reduced rigidity of the entire fibrous membrane of the eye, and from healthy eyes — in cornea being more prone to displacement.

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