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M. Haustein, S. Hadjiraftis, L. E. Pillunat, E. Spoerl; The Impact of Glaucoma Stage on the Ocular Biomechanics. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4885.
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It is known that the corneal hysteresis (CH) and the corneal resistance factor (CRF) are reduced in glaucoma. Aim of the study was to evaluate the impact of these biomechanical parameters in glaucoma patients of different glaucoma stages.
In this prospective study 437 patients and 79 healthy volunteers were examined. Different groups were compared with each other: healthy eyes, abnormal discs, ocular hypertensives (OHT), normal tension glaucomas (NTG) and primary open angle glaucomas (POAG). The glaucomas were graded by cup to disc ratio (CDR) and visual field defects into different stages: 1. pre-perimetric, 2. moderate, and 3. advanced.The Ocular Response Analyzer (ORA) was used to obtain in vivo the parameters of the corneal biomechanics (CH, CRF) in the different groups. Those were correlated with further parameters [eye length (l), central corneal thickness (cCT), IOP, CDR, disc area (A), and corneal diameter].
The CH is statistically significant different between OHT, healthy and glaucoma eyes 11.7±1.91, 10.9±1.22, and 9.4±1.71 mmHg, respectively (p=0.001). Furthermore there were found differences between the different stages of glaucoma ( 9.7±1.23, 9.3±2.15, and 9.0±1.19 mmHg). Comparable results were found in CRF.CRF is statistically significant different between the groups (p=0.0021): normal 11.4±1.16, OHT 12.5±1.46, and glaucoma 9.6±1.64 mmHg. The glaucoma stages (p=0.032): 10.0±1.28, 9.5±1.34, and 9.3±1.46 mmHg, respectively. There was no statistical difference between NTG and POAG (p=0.37). CDR and cCT correlated with the biomechanical parameters. The bigger CDR or the thinner the cCT, the smaller CH (R=-0.30 and R=0.509, respectively) or CRF (R=-0,297 and R=0.542, respectively). Student t-test and Pearson correlation coefficient (SPSS® 15.0) were used to calculate the statistical significances.
Corneal biomechanics are different between healthy, OHT and glaucomatous eyes. The elasticity of ocular tissue decreases by increasing glaucoma stage. If this observation is also valid for other ocular tissues, i.e lamina cibrosa, IOP peaks might cause more mechanical stress. The ORA examination probably allows more insights into patients with reduced ocular rigidity and elasticity to treat them earlier and more intensive.
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