Ocular Response Analyzer

Participants underwent testing with the Ocular Response Analyzer (software version 2.02, ORA; Reichert Ophthalmic Instruments). Only readings with a waveform score (WS) of ≥7 in a software-generated scale of 0 to 10 were included in data analysis. At least 2 measurements were obtained per eye and the one with the highest-quality WS was retained for the analysis. Briefly, the ORA uses an applied force-displacement relationship by delivering a collimated air pulse on the cornea. The difference between the inward and outward motion applanation pressures (P1 and P2, respectively) owing to the delivered air jet is called corneal hysteresis, and this is measured in millimeters of mercury (mm Hg). CH is assumed to reflect the viscous properties of the cornea as well as its dampening and energy absorption capacity. The ORA also provides the corneal resistance factor, which seems to be an indication of the overall “resistance” or elasticity of the cornea. It is derived from the formula (P1 − kP2), where k is a constant determined from an empirical analysis of the relationship between both P1 and P2 in studies evaluating eyes that underwent corneal change induced by laser in situ keratomileusis (LASIK). The device also provides a Goldmann-correlated intraocular pressure measurement (IOPg) (average of P1 and P2) and a corneal-compensated intraocular pressure (IOPcc). IOPcc is considered to be less affected by corneal properties such as the central corneal thickness.

Standard Automated Perimetry

To be classified as glaucomatous, patients had to have at least 2 consecutive, reliable (fixation losses and false negatives ≤33%, and <15% false positives) and repeatable abnormal standard automated perimetry (SAP) tests using the 24-2 Swedish Interactive Threshold Algorithm with either a pattern standard deviation (PSD) outside the 95% normal limits or a glaucoma hemifield test (GHT) result outside the 99% normal limits. Patients considered suspect for glaucoma had an IOP > 21 mm Hg and/or glaucomatous optic disc damage based on standardized assessment of simultaneous stereophotographs by 2 experienced graders with a minimum of 2 reliable SAP visual fields without evidence of repeatable visual field damage. The Visual Field Assessment Center (VisFACT) reviewed all visual fields and checked for the presence of artifacts such as lid and rim artifacts, fatigue effects, inattention, or inappropriate fixation.

Retinal Nerve Fiber Layer Thickness Measurements

Thickness of the RNFL was measured with the GDx enhanced corneal compensation (GDx ECC; Carl Zeiss Meditec, Dublin, California, USA) and Spectralis SD-OCT (Heidelberg Engineering, Heidelberg, Germany). When there were multiple images during the same visit, the one with the highest-quality scores was chosen for statistical analysis. Measurements with SAP, GDx ECC, SD-OCT, and ORA were taken within 6 months of each other. Since most participants had either SD-OCT or GDx ECC scans, we divided them into an SD-OCT and a GDx ECC subgroup.

Patients were imaged using a commercially available scanning laser polarimeter with enhanced corneal compensation (GDx ECC; Carl-Zeiss Meditec). The general principles of scanning laser polarimetry and the algorithms used for enhanced corneal compensation have been described in detail elsewhere. An experienced examiner masked to the subject’s identity and results of the other tests assessed the quality of the GDx images. A good-quality image required a focused and evenly illuminated reflectance image with a centered optic disc. The image quality score had to be greater than or equal to 7 and the typical scan score (TSS) greater than or equal to 80. RNFL retardation measurements were obtained on a 3.2-mm-diameter calculation circle around the optic nerve head. The GDx provides measurements of RNFL retardation, which are converted to estimated RNFL thickness using a fixed conversion factor.

The Spectralis OCT (Spectralis HRA+OCT, software version 5.3.0.0; Heidelberg Engineering) device uses a dual-beam SD-OCT and a confocal laser scanning ophthalmoscope that works by emitting a superluminescent diode light with a center wavelength of 870 nm and an infrared scan to simultaneously provide images of ocular microstructures. The Spectralis OCT device incorporates a real-time eye tracking system that couples confocal laser scanning ophthalmoscope and SD-OCT scanners to adjust for eye movements and to ensure that the same location of the retina is scanned over time. The protocol used was the RNFL circle scan, which consists of 1024 A-scan points from a 3.45-mm circle centered on the optic disc. All patients had their corneal curvature input into the machine before the examination. The examiner is required to manually place the scan around the optic disc. The acquisition rate of the device is 40 000 A-scans per second at an axial resolution of 3.9 μm. The instrument includes scan averaging to reduce noise, thus increasing the effective acquisition time. To be included, all images were reviewed for noncentered scans and had to have a signal strength >15 dB. The images were reviewed by the Information and Data Evaluation Analysis (IDEA) reading center, based at the University of California, San Diego.

Statistical Analysis

Data are presented as means and standard deviations (SD). Initially, the associations between variables were evaluated using scatterplots and locally weighted scatterplott smoothing (LOWESS). A linear regression modeling technique was used to study the effect of biomechanical properties of the cornea (CCT, CRF, and CH) on visual field mean defect (MD), PSD, and RNFL thickness measurements. Initially, univariable regression models were fitted with MD, PSD, or average RNFL thickness (for the SD-OCT and GDx ECC groups) as dependent variables and CH as independent variable. Similar models were built with CRF as independent variable. Subsequently, multivariable models were built to evaluate the effect of corneal biomechanical properties on visual field indices and RNFL thickness while adjusting for other parameters such as age, CCT, and axial length. As measurements from both eyes of the same individual are likely to correlate, the use of standard statistical methods for parameter estimation can lead to underestimation of standard errors and to confidence intervals that are too narrow. Therefore, to account for the fact that both eyes of some patients were included in the analyses, generalized estimating equations (GEE) with robust standard errors (Huber-White sandwich variance estimator) were used to adjust for these potential correlations.

All tests were 2-sided and a P value less than .05 was considered statistically significant. Statistical analyses were performed using Stata 10.0 (StataCorp, College Station, Texas, USA).