How a dynamic optical system maintains image quality: Self-adjustment of the human eye.

The eyeball is continually subjected to forces that cause alterations to its shape and dimensions, as well as to its optical components. Forces that induce accommodation result in an intentional change in focus; others, such as the effect of intraocular pressure fluctuations, are more subtle. Although the mechanical properties of the eyeball and its components permit mediation of such subtle forces, the concomitant optical changes are not detected by the visual system. Optical self-adjustment is postulated as the mechanism that maintains image quality. The purpose of this study was to investigate how self-adjustment occurs by using an optical model of the eyeball and to test the requisite optical and biometric conditions.

Analysis of torsional eye movements using the corneal birefringence pattern.

The literature mentions several invasive methods to measure the degree of the compensatory torsional eye movement during a head-tilt. Nevertheless, none of them have yielded universally clinical tests. This study focuses on an optical system (with a circular polarizer) for noninvasive acquisition of corneal birefringence patterns (isochromes). The acquired isochromes are quadrangular in shape and unique for each eye, as well as independent of the head-rotation angle. The results obtained suggest that isochrome orientation analysis could be an effective method to accurately measure the degree of compensatory torsional eye movement.

Clinical utility of spectral analysis of intraocular pressure pulse wave.

BACKGROUND: To evaluate the clinical utility of spectral analysis of intraocular pressure pulse wave in healthy eyes of a control group (CG), patients having glaucomatous optic disc appearance or ocular hypertension, and patients with primary open angle glaucoma or primary angle closure glaucoma. METHODS: This is a prospective study that enrolled 296 patients from a single glaucoma clinic. Age matched CG consisted of 62 individuals. Subjects underwent comprehensive clinical diagnostic procedures including intraocular pressure (IOP) measurement with dynamic contour tonometry (DCT) and Goldmann applanation tonometry (GAT). DCT time series were analyzed with custom written software that included signal preprocessing, filtering and spectral analysis. An amplitude and energy content analysis, which takes into account non-stationarity of signals but also provides methodology that is independent of IOP and ocular pulse amplitude (OPA) levels, was applied. Spectral content up to the 6th harmonic of the pressure pulse wave was considered. Statistical analyses included descriptive statistics, normality test, and a multicomparison of medians for independent groups using Kruskal-Wallis test. RESULTS: GAT IOP showed statistical significance (Kruskal-Willis test p < 0.05) for three out of 10 considered multiple comparisons, DCT IOP and OPA showed statistically significant results in five and seven cases, respectively. Changes in heart rate and central corneal thickness between the groups were statistically significant in two cases. None of the above parameters showed statistically significant differences between CG and the suspects with glaucomatous optic disc appearance (GODA). On the other hand, spectral analysis showed statistically significant differences for that case. CONCLUSIONS: Spectral analysis of the DCT signals was the only method showing statistically significant differences between healthy eyes and those of GODA suspects.

Intraocular pressure in a cohort of healthy eastern European schoolchildren: variations in method and corneal thickness.

BACKGROUND: Intraocular pressure (IOP) in the developing eye of a child is not always easy to measure and there is no technique that is known to be the most accurate for the young eye. Measurements are needed on many cohorts of children with different tonometers to determine how the values correlate between instruments, whether corneal parameters affect readings and whether correlations between age and IOP values can be discerned. The aim of this study was to undertake a comparative analysis of three different tonometers on a group of healthy children to see whether differences exist and whether these may be related to central corneal thickness and/or radius of curvature. In addition, the study adds to the relatively small body of literature on IOP in the growing eye which will collectively allow trends to be identified and ultimately norms to be established. METHODS: IOP was measured on 115 eyes in a group of Polish children, aged between 5-17 years (mean±standard deviation [SD] 11.3±3.0 years) using three different tonometers: non-contact (NCT), the ICare and Goldmann applanation (GAT). Readings obtained were compared between instruments and with central corneal thickness and radius of curvature. RESULTS: The ICare tonometer provided statistically higher IOP values (16.9±3.4 mmHg) than the GAT (14.7±2.9 mmHg) regardless of corneal thickness and whether or not a correction factor was applied. A correlation was found between central corneal thickness (CCT) and IOP values obtained with all three tonometers but only the IOP values detected with the ICare tonometer showed a statistically significant correlation with radius of curvature (p<0.004). No correlations with age or gender were found for IOP values measured with any of the instruments. CONCLUSIONS: IOP measurements on children vary significantly between instruments and correlations are affected by the corneal thickness. Further studies on children are needed to determine which instrument is most appropriate and to derive a normative IOP scale for the growing eye.

Short-Time Changes of Intraocular Pressure and Biomechanics of the Anterior Segment of the Eye during Water Drinking Test in Patients with XEN GelStent.

PURPOSE: Little is known about short-term changes in intraocular pressure (IOP) following minimally invasive glaucoma surgeries, such as post-XEN GelStent implantation. Although the importance of corneal biomechanics in glaucoma diagnostics has been reported, little work has been conducted on postoperative description of changes when the structure of the anterior segment is altered. The aim of presented study was to evaluate the changes in the biomechanical parameters of the anterior segment of the post-XEN GelStent implantation eyes. PATIENTS AND METHODS: This investigator-initiated, open-label, prospective, single-center study recruited patients. Patients with primary open-angle glaucoma (POAG) after XEN GelStent implantation versus matched POAG controls (considered as control group/CG) treated pharmacologically were screened. Water loading was conducted using 10 mL of water per kilogram of body weight for ≤5 min. Goldmann applanation tonometry (GAT), corneal hysteresis (CH), and corneal resistance factor (CRF) were performed before water loading and after every 15 min up to 1 h. RESULTS: The water drinking test (WDT) was positive in 3.7% (1 out of 27) of patients in the post-XEN group compared with 22.7% (5 out of 22) of patients in the control group (CG; p < 0.05). Mean fluctuations in GAT during the WDT were higher in the CG group (3.6 ± 2.5 mmHg vs. 2.9 ± 1.3 mmHg, p < 0.001). CRF and CH changed significantly only in the post-XEN group. The mean peak of CH and CRF occurred at 15 and 30 min of the test in the post-XEN group (p = 0.001). CONCLUSION: WDT is important to assess the ability of compensation mechanisms to reduce fluctuations in IOP after water upload. The relationship between biomechanics of the anterior segment and glaucoma may have substantial impact on surgical outcome evaluation.

[Clinical evaluation of the ICare tonometer in measuring intraocular pressure]. / Kliniczna ocena nowej metody pomiaru cisnienia wewnatrzgalkowego tonometrem ICare.

PURPOSE: To compare the intraocular pressure (P), values obtained with the ICare rebound tonometer with the Goldmann applanation tonometer (GAT), and to evaluate the influence of central comeal thickness (CCT), and radius of the corneal curvature (R) on IOP measurements. MATERIAL AND METHODS: Eighty four eyes of 48 subjects (65 eyes of glaucoma patients and 19 eyes of healthy volunteers), were examined with ICare and Goldmann tonometers. Central corneal thickness was determined by ultrasound pachymetry. Corneal radius of curvature was determined using corneal topography. RESULTS: There was no significant difference between IOP readings with ICare and GAT. Intraocular pressure measurements were found to be higher with the ICare tonometer, both in glaucoma patients (15.3 +/- 4.0 mmHg, and normal subjects (14.4 +/- 3.0 mmHg). ICare-GAT mean difference was 1.1 +/- 3.6 mmHg (95% CI, -6.1-8.3 mmHg), for glaucoma patients and 1.8 +/- 3.2 mmHg (95% CI, -4.6-8.2 mmHg), for healthy volunteers. Using CCT patients were divided into 2 groups: (1) thinnest corneas--CCT < or =556 microm and (2) thickest coneas--CCT >556 microm. In the group of thinnest comeas there was no correlation between IOP (CCT and R. In thickest corneas there was significant correlation between IOP readings obtained by ICare and CCT (r = -0.4, p < 0.01), and mean radius (R) (r = 0.5, p < 0.001). CONCLUSIONS: Measurements of IOP using the ICare rebound tonometer are in good agreement with Goldmann applanation tonometer, although influenced by CCT

Finite element simulation of Goldmann tonometry after refractive surgery.

BACKGROUND: The problem of accuracy of intraocular pressure (IOP) measurement using Goldmann Applanation Tonometry (GAT) after a refractive surgery is still unresolved. In this study a numerical simulation of refractive surgery and GAT is proposed as a method to evaluate accuracy of the correction formula for GAT reading after changes in corneal curvature and thickness. METHODS: In the finite element analysis the effect of the corneal shell buckling was taken into account. This effect is responsible for the nonlinear relation of Goldmann applanation tonometer measurement and intraocular pressure. Goldmann applanation tonometry was simulated for different ablation depths and values of intraocular pressure levels. FINDINGS: The results of the simulation indicate a strong relation between the Goldmann applanation reading and the intraocular pressure level for values greater than nominal (about 16 mmHg). The estimation error of the intraocular pressure after changes in corneal geometry, is lower than 1 mmHg for IOP = 32 mmHg and lower than 3 mmHg for IOP = 48 mmHg. INTERPRETATION: The proposed correction formula of Goldmann applanation tonometry occurred to be useful in the cases after the corneal refractive surgery regardless of the intraocular pressure values and ablation depth.

A Comparative Analysis of Goldmann Tonometry Correction.

PURPOSE: The measurement of intraocular pressure (IOP) by Goldmann applanation tonometry (GAT) is based on assumptions about corneal parameters. To correct for variations in corneal curvature and thickness, a number of equations have been proposed. This study evaluates the in vivo accuracy of these equations from individuals with primary open-angle glaucoma and compared them with measurements taken using the Pascal dynamic contour tonometer (DCT), which makes no assumptions about the corneal geometry or biomechanics. PARTICIPANTS AND METHODS: The study included 108 participants with primary open-angle glaucoma (47 male and 61 female) with an age range of 39 to 81 years. Participants were recruited from the Glaucoma Clinic at Wroclaw Medical University. A full ophthalmologic examination was conducted on all participants. Participants were divided into 3 groups depending on IOP as measured by GAT. Six formulae were applied and the results were compared with measurements taken with DCT. MAIN OUTCOME MEASURES: To determine as to which formula provides the closest value to IOP measured with DCT. RESULTS: For IOP values ≤29 mm Hg, 2 of the formulae showed the smallest and comparable mean differences and SDs between corrected IOP values obtained with GAT and those measured with DCT. For IOP≥30 mm Hg, the formula derived from the model of corneal applanation that takes into account corneal buckling showed the closest agreement with measurements taken using DCT. CONCLUSIONS: Correction formulae provide widely varying results and their appropriateness can depend on the IOP values.

Age-Related Changes in Ocular Blood Velocity in Suspects with Glaucomatous Optic Disc Appearance. Comparison with Healthy Subjects and Glaucoma Patients.

PURPOSE: To evaluate retrobulbar blood flow characteristics of glaucoma suspects with glaucomatous optic disc appearance (GODA) in comparison to healthy control group (CG) and primary open angle glaucoma patients (POAG) and assess the effect of age. METHODS: 145 patients from a single glaucoma clinic were enrolled and classified into two diagnostic groups (GODA and POAG). Third group of subjects consisted of 67 age matched individuals (CG). Retrobulbar blood velocity measurement in central retinal artery was performed using color Doppler imaging (CDI). CDI images were processed in custom software leading a range of parameter estimates from a continuous waveform signal. The effect of age on the estimated parameters was evaluated with the stepwise forward regression and ANCOVA in which age was used as a continuous factor. One-way ANOVA was used to test for the differences in the CDI parameters between the three considered groups. Correlation between restive index (RI) and pulsatility index (PI) was assessed with a bilinear fitting guaranteeing no discontinuities in RI intercept estimate. Fisher test was used to assess the applicability of a bilinear PI/RI relationship, while the statistics of the RI intercept estimate were evaluated using the bootstrap. RESULTS: ANCOVA showed significant interaction between age and group (p<0.05) for five out of nine considered CDI parameters. The RI intercept for CG and GODA groups was 0.602±0.047, and 0.574±0.044 respectively, while the RI intercept of 0.934±0.066 was found for the POAG. CONCLUSIONS: The observed similarity of CG and GODA group and dissimilarity between GODA and POAG groups in terms of PI/RI relationship is remarkable. Age may play some role in the different mechanisms occurring in blood velocity dynamics in GODA and POAG subjects but it is not a strongly determining factor.