The effect of pupil size on the measurement of corneal birefringence properties: preliminary study.

We used a partial Mueller matrix polarimeter to measure the corneal anisotropic properties at three pupil sizes (dilated, natural, and constricted). The geometrical parameters of first order isochromes were described by quadrilaterals. These parameters are statistically significantly different between the three pupil sizes. The pupillary size changes do not influence the azimuth angle distribution α. The retardation R and birefringence distributions show asymmetry in the nasal-temporal cross-section. There are differences between pupil sizes for both nasal and temporal parts of the cornea for these distributions. Iridial light scattering and diffraction might be the reason for these differences.

Opto-mechanical self-adjustment model of the human eye.

The eye has specific optical and biomechanical properties that jointly regulate the eye's quality of vision, shape, and elasticity. These two characteristics are interdependent and correlated. Contrary to most currently available computational models of the human eye that only focus on biomechanical or optical aspects, the current study explores the inter-relationships between biomechanics, structure, and optical properties. Possible combinations of mechanical properties, boundary conditions, and biometrics were specified to ensure the opto-mechanical (OM) integrity to compensate for physiological changes in intraocular pressure (IOP) without compromising image acuity. This study evaluated the quality of the vision by analyzing the minimum spot diameters formed on the retina and drew how the self-adjustment mechanism affects the eye globe shape by adopting a finite element (FE) model of the eyeball. The model was verified by a water drinking test with biometric measurement (OCT Revo NX, Optopol) and tonometry (Corvis ST, Oculus).

Corneal Behavior during Tonometer Measurement during the Water Drinking Test in Eyes with XEN GelStent in Comparison to Non-Implanted Eyes.

Biomechanics of the cornea have significant influences on the non-contact measurement of the intraocular pressure. The corneal behaviour during tonometry is a fundamental factor in estimating its value. The aim of the study was to analyse the behaviour of the cornea during tonometric measurement with the forced change in intraocular pressure during the water drinking test. Ocular Response Analyser (Reichert) was used to the measurement. Besides four basic parameters connected with intraocular pressure (IOPg, IOPcc) and biomechanics (corneal hysteresis CH and corneal resistance factor (CRF), other parameters representing the behaviour of the cornea during a puff of air were analysed. There were 47 eyes included in the study, including 27 eyes with a XEN GelStent implanted and 20 without it. The eyes of people with monocular implementation were the reference group. The values of analysed parameters were compared before and after 10, 25, 40, and 55 min after drinking the water. The intraocular pressure increased by 2.4 mmHg (p < 0.05) for eyes with a XEN stent and 2.2 mmHg for eyes without a stent (p < 0.05) in the tenth minute after drinking of water. This change caused a decreasing of corneal hysteresis (p < 0.05) without significant changes in the corneal resistance factor (p > 0.05). Corneal hysteresis changed similarly in the reference group and the group with a XEN GelStent. The analysis of additional parameters showed a difference in the behaviour of the cornea in eyes with a XEN GelStent in comparison to the corneas of eyes without a stent. This was particularly visible in the analysis of the cornea's behaviour during the second applanation, when the cornea returns to its baseline state after deformation caused by air puff tonometry.

Does body position, age, and heart rate induce IOP's changes?

OBJECTIVES: The main goal of this research was to determine the differences between the values of intraocular pressure (IOP) in the supine and sitting positions, and to assess the effect of age and cardiovascular parameters. METHODS: Seventy-two healthy adults were enrolled and classified into age groups: 20-30 years (group A), 31-40 years (group B), and 41-71 years (group C). Corneal biometry and cardiovascular parameters, such as heart rate (HR), were measured. IOP measurements were taken in the sitting position (IOPS) and in the supine position (IOPL) using the iCare® Pro tonometer. RESULTS: A significant difference between the IOPS and IOPL in the entire cohort was found (p < 0.001). Regarding the age subgroups, a significant difference (p < 0.001) between the IOPS and IOPL was obtained in group A (2.6 ± 1.6 mmHg) and group C (1.5 ± 1.3 mmHg). There were no significant differences in the IOPS between groups. The highest IOP values were obtained for group A. The correlations between HR and IOPS are statistically significant for group A and group B, and for HR and IOPL-S for group B only. Multivariate analysis showed that HR has a significant influence on the difference in IOP in the two body positions. CONCLUSION: A statistically significant difference between the effect of age and the values of IOPS and IOPL was shown. Cardiovascular parameters showed some relevant statistical dependencies, but with a rather marginal significance in young people. The influence of body position for the measurement of IOP for healthy subjects does not seem to matter, despite the fact that there are some dependencies that are statistically significant.

Comparative analysis of the corneal birefringence pattern in healthy children and adults.

PURPOSE: To undertake a comparative analysis of the corneal shape, thickness and isochromatics in the eyes of children and adults in order to determine the extent of similarities and differences between the cohorts. METHODS: The study involved 24 children (aged 8 years) and 37 young White adults (aged between 22-24 years) with no apparent or known health or ocular conditions. Measurements were made of corneal radius of curvature, both central (CCT) and paracentral (PCT) corneal thickness and intraocular pressure (IOP). Images of the isochromatics were captured using a slit lamp and a circular polarizer. The geometry of fringe I and II of the isochromatics was analysed. RESULTS: Statistically significant differences were found between CCT and PCT in nasal and temporal regions for both the children and adult cohorts. The same trends were observed in the radii of the cornea. Statistically significant differences between side lengths and angles of isochromatic fringes were found. No differences in asymmetry of shape for fringe I between adults and children were detected; greater symmetry was seen in fringe II in children than for adults. CONCLUSIONS: The asymmetry in corneal shape and curvature contributes to the shape of the isochromatic fringes. This is likely linked to the orientation and parameters of the collagen fibres and to the muscles' forces, and be relevant for surgical procedures such as corneal transplantation.

Birefringent properties of the cornea measured by a Mueller type polarimeter in healthy adults and children.

Mueller type polarimeter was used for in vivo measurements of the anisotropic parameters (retardation and azimuth angle) of corneas. To determine birefringence, corneal thickness was measured with a Scheimpflug camera (Corvist ST). The retardation distributions in the nasal-temporal cross-section in both children (N=7) and adults (N=38) groups occurred asymmetrical. The asymmetry in birefringence distributions was observed only in adults group. The geometrical analysis of the first order isochromes in both age groups showed the asymmetry of its shapes. The changes of symmetry in birefringent properties with age may have potential relationship with changing corneal biometry.

[Evaluation analysis of blood flow of peripapillary area in glaucoma patients using scanning laser Doppler flowmetry]. / Ocena okolotarczowego przeplywu krwi u chorych na jaskre za pomoca skaningowego przeplywomierza laserowego.

PURPOSE: To evaluate capillary blood flow and velocity in retinal peripapillary area in primary open angle glaucoma (POAG), normal tension glaucoma (NTG), pseudoexfoliation glaucoma (PEXG) and age matched control group (C) using scanning laser Doppler flowmetry (Heildelberg Retina Flowmeter). MATERIAL AND METHOD: One randomly selected eye of 17 POAG patients (mean age 59.3 +/- 2.6), 18 NTG patients (mean age 56.4 +/- 2.4), 17 PEXG patients (63.3 +/- 2.8) and 15 healthy subjects (mean age 55.8 +/- 3.8) underwent examination with scanning laser Doppler flowmetry. The measurements were performed at the superior and inferior temporal peripapillary retinal area. The measured parameters were capillary blood flow and velocity of the moving erythrocytes. Each image was analysed using the program AFFPIA. This software calculates the hemodynamic parameter flow of each pixel. Comparison of value obtained from patients and control participants were made using Student's t test. RESULTS: Scanning laser Doppler flowmetry results showed a significant reduction in blood flow at the superior and inferior temporal peripapillary area in POAG and NTG patients compared to control group (p < 0.04). The POAG and NTG patients had also significantly decreased velocity at the inferior temporal peripapillary retinal area compared to control group. No statistical difference was found between blood flow and velocity in POAG compared with NTG. When compared with the control participants, patients with PEXG showed significant decreases in the mean blood flow at the inferior temporal quadrant (p < 0.05), while the differences in the mean flow and velocity at the superior quadrant between those groups were not statistically significant. CONCLUSIONS: Our findings indicate a reduction in ocular blood flow at the temporal peripapillary retinal area in patients with POAG and NTG compared to age matched control participants. The peripapillary retinal microcirculation is not significantly altered in patients with PEXG compare with POAG and JNC. The blood flow parameters in the capillaries are lower in JNC retinas, the difference however is not significant. Further clinical investigation is required having groups homogeneous with respect to stage of glaucoma and controlled for factors known to effect perfusion pressure.