Investigation of regulatory factors of the extracellular matrix and cornea-scleral biomechanical properties in physiological aging and primary open-angle glaucoma
PURPOSE: Comparative analysis of the biomechanical parameters of the fibrous tunic of the eyeball and extracellular matrix (ECM) characteristics in patients with suspected POAG, early-stage POAG and healthy elderly people, as well as control groups of various ages. METHODS: The study included 60 people aged 21 to 74 years. The first group consisted of 16 healthy young adults (n=16; mean age 26.3±5 years; “young" control group), the second group included 17 healthy elderly subjects (n=17; 69.2±4 years “age" control group) with no signs of ophthalmological pathology. The third group consisted of 17 people with suspected POAG (n=17; mean age 68.2±6 years; with IOP above 21 mm Hg for three consecutive measurements (average central corneal thickness) and absence of visual field defects on the threshold perimetry test). The fourth group consisted of 10 patients with early-stage POAG (n=10; mean age 67.6±5 years; lesion of one or both eyes). Therapy was discontinued 2 weeks prior to the study. Ophthalmic examination included both the standard (visometry, tonometry, biomicroscopy, gonioscopy, automatic static perimetry) and additional methods (laser scanning tomography). Investigation of the fibrous tunic biomechanical parameters was conducted using the Ocular Response Analyzer (ORA) («Reichert», USA). All subjects underwent lab tests. The content of the regulation of extracellular matrix factors, MMP-9 and TIMP-4, in the serum and tear fluid was determined by enzyme linked immunosorbent assay (ELISA; diagnostic test systems Bender MedSystems® («eBiosciences», Austria; «R & D Diagnostics Inc.», US). We investigated 60 tear fluid samples and 60 serum samples. Statistical processing was made in the SPSS program 17.0 (SPSS, Inc., Chicago, IL). Two independent groups were compared with the use of nonparametric U-test Mann-Whitney test. Correlation analysis was performed using the Spearman test (r). Differences were considered statistically significant at p <0.05. RESULTS: The results showed that elderly age, suspected POAG, and the early-stage of POAG were associated with a set of changes in the cornea-scleral capsule and more or less expressed changes in the ECM regulation system both on local and systemic levels. Thus, the data obtained from the bi-directional applanation tonometry showed a reduction of viscoelastic properties of fibrous tunic associated with age and the development of the early-stage of POAG: the correlation between the age and corneal hysteresis (CH) (significant decrease with age), CH and changes in the early-stage of POAG (similar dynamics). In the study of MMP-9 and TIMP 4 content a significant increase in TIMP-4 level in tear fluid was detected in the 2-nd group compared to the 1-st (p=0.0001). This tendency to a progressive increase of tear fluid TIMP 4 was observed in patients with suspected of POAG and early-stage of POAG. Pair correlations analysis revealed a statistically significant strong inverse relationship between CH and TIMP-4 level in the tear fluid (r=-0,510; p=0,01). It was found that physiological aging was associated with a significant rise of TIMP-4 on local level (p=0.0001), whereas patients suspected of POAG had an increased MMP-9 production on systemic level. CH and CH/corneal resistance factor (CRF) decreased with age, and in initial POAG. We also revealed a significant correlation between the CH and TIMP-4 levels in tear fluid (r=-0,510; p=0,01), which suggests the presence of age associated cornea-scleral capsule restructuring resulting in an increase of its stiffness. Significant increase of the systemic level of MMP-9 may play a key role in the launch of pathological changes in the cornea-scleral capsule of the eye and be one of the major factors leading to the development of POAG.