Relative importance of factors affecting corneal hysteresis measurement.

PURPOSE: To evaluate the relative influences of several demographic, ocular, and systemic parameters on corneal hysteresis (CH). METHODS: This is a prospective, observational, cross-sectional study using subjects recruited from consecutive Albuquerque VAMC eye clinic patients. We classified eligible subjects as primary open-angle glaucoma (POAG), ocular hypertension, glaucoma suspect, or normal. We used the Ocular Response Analyzer, Pascal Dynamic Contour Tonometer, and Goldmann applanation tonometer to obtain intraocular pressure (IOP), CH, corneal resistance factor, and ocular pulse amplitude values. We also obtained corneal curvature, central corneal thickness (CCT), axial length, retinal nerve fiber layer thickness, clinical cup/disc ratio (CDR) estimates, and standard automated perimetry metrics (mean defect, pattern standard deviation). We gathered glycosylated hemoglobin (A1C) data through chart review. Multivariate regression analyses were used to determine independent relationships between CH and the other parameters. RESULTS: Three hundred seventeen eyes in 317 subjects were studied (116 POAG, 87 ocular hypertension, 47 glaucoma suspect, and 67 normal). In univariate regression analysis, CH varied directly with CCT (ß = 0.39, p < 0.001), corneal curvature (ß = 0.16, p = 0.01), corneal resistance factor (ß = 0.57, p < 0.001), A1C (ß = 0.15, p = 0.01), mean defect (ß = 0.29, p < 0.001), and retinal nerve fiber layer (ß = 0.31, p < 0.001). Factors inversely related to CH were age (ß = -0.22, p < 0.001), IOP (ß = -0.29, p < 0.001), ocular pulse amplitude (ß = -0.11, p = 0.04), CDR (ß = -0.34, p < 0.001), and pattern standard deviation (ß = -0.29, p < 0.001). CH was lower in POAG compared with the other diagnostic groups. In multivariate analysis, CH was independently associated with age, IOP, CCT, A1C, glaucoma diagnosis, and CDR. Of these factors, CCT and IOP demonstrated twice as much influence on CH compared with the other four factors. CONCLUSIONS: Although this study identified six separate variables that independently influence CH values, the overall r value indicates that these variables together only explain 40% of CH variability. These results suggest that other significant sources of variability exist and deserve investigation.

Clinical comparison of pascal dynamic contour tonometry and goldmann applanation tonometry in asymmetric open-angle glaucoma.

PURPOSE: To investigate and compare the relationships between glaucomatous visual field loss and intraocular pressure (IOP) as measured by both Pascal dynamic contour tonometry (DCT) and Goldmann applanation tonometry (GAT). PATIENTS AND METHODS: All primary open-angle glaucoma and normal tension glaucoma patients seen between July 2005 and June 2006 with at least 2 sets of good-quality, bilateral DCT and GAT measurements were retrospectively identified. Additional inclusion criteria required that all subjects had repeatable, asymmetric glaucomatous visual field loss that corresponded with asymmetric glaucomatous optic neuropathy. After mean IOP values were computed and visual fields were scored using Advanced Glaucoma Intervention Study (AGIS) criteria, paired-eye comparisons were conducted using right versus left eyes and higher versus lower AGIS-score eyes. RESULTS: Sixty-seven (42 primary open-angle glaucoma, 25 normal tension glaucoma) subjects met all criteria for study inclusion. Per paired t test, mean DCT-IOP was significantly higher in the higher AGIS-score eyes compared with the lower AGIS-score eyes (16.3 vs. 15.5 mm Hg, P=0.004), whereas GAT-IOP was not significantly different in these same eyes (14.5 vs. 14.4 mm Hg, P=0.56). Mean IOP difference between the 2 methods was significantly larger in higher versus lower AGIS-score eyes (P<0.001), and 72% of the subjects demonstrated larger intermethod IOP differences in their higher AGIS-score eye compared with their lower AGIS-score eye (P<0.001; 95% confidence interval: 0.59-0.82). Multivariate linear regression analysis revealed that AGIS-score differences between eyes were independently associated with both intermethod IOP differences between eyes (P=0.004) and central corneal thickness (CCT) differences between eyes (P=0.04). CCT, however, was not associated with intermethod IOP differences within or between eyes. CONCLUSIONS: These findings suggest that DCT-IOP is correlated with glaucomatous damage, and moreover, DCT-IOP is more closely related to extent of glaucoma damage than is GAT-IOP. The most likely explanation for these results is that GAT-IOP systematically underestimates IOP compared with DCT-IOP. Our findings also support the hypothesis that corneal biomechanical factors other than CCT are major confounders of applanation tonometry measurements.

Relationship between central corneal thickness and severity of glaucomatous visual field loss in a primary care population.

BACKGROUND: Although the ability of central corneal thickness (CCT) to predict development of primary open-angle glaucoma has become increasingly well recognized, the ability of CCT to predict severity of glaucoma remains uncertain. This study was designed to expand the available knowledge about the relationship between CCT and glaucoma severity. METHODS: Retrospective identification of all patients with a clinical diagnosis of either primary open angle glaucoma (POAG) or ocular hypertension who were seen from September 2002 through May 2003 at the Albuquerque VA Medical Center eye clinic was completed. Eligible subjects were segregated into no, mild, moderate, or advanced visual field loss groups based on Advanced Glaucoma Intervention Study (AGIS) visual field scoring criteria. Following statistical analyses comparing the visual field groups, the sample was divided into thin, intermediate, and thick CCT groups, and further analysis was performed. RESULTS: Mean CCT was significantly higher in the no field loss group compared with all 3 groups with glaucomatous visual field loss. Mean CCT was not statistically different, however, between the mild, moderate, and advanced visual field loss groups. In linear regression analyses, no significant relationship was found between CCT and severity of visual field loss. CONCLUSIONS: Although CCT was associated strongly with development of POAG-related visual field loss, CCT was not associated with severity of visual field loss in this study. These findings suggest that glaucoma patients with thinner corneas are just as likely to have advanced levels of field loss as glaucoma patients with thicker corneas. Prospective studies are needed to validate these findings.

Central corneal thickness and normal tension glaucoma: a cross-sectional study.

BACKGROUND: Recently published evidence has identified thinner central corneal thickness (CCT) as a strong predictive factor for the conversion from ocular hypertension (OHT) to primary open-angle glaucoma (POAG). The association between CCT and development of normal-tension glaucoma (NTG), however, is less clear. Accordingly, we designed this cross-sectional study to further explore the relationship between CCT and NTG. PATIENTS AND METHODS: All patients with a clinical diagnosis of NTG and NTG suspect (NTGS) who were seen from September 2002 through May 2003 at the Albuquerque VA Medical Center eye clinic were identified retrospectively. After eligible subjects were categorized into no, mild, moderate, and advanced visual field loss groups, analysis of variance (ANOVA) and regression analyses were used to determine group differences for several IOP variables, several systemic variables, and CCT. Additional analyses were completed after eligible subjects were recategorized into thin, intermediate, and thick CCT groups. RESULTS: Eighty-four eyes in 84 NTGS subjects and 56 eyes in 56 NTG subjects were studied. Mean CCT was significantly thicker in the no field loss group (NTGS) when compared with all 3 groups with glaucomatous visual field loss (NTG). In multivariate regression analysis, the association between CCT and the presence of NTG-related visual field loss was robust and independent. Conversely, no relationship was found between CCT and severity of NTG-related visual field loss. CONCLUSIONS: In eyes characterized by statistically normal intraocular pressure (IOP) measurements as measured by Goldmann applanation tonometry, we found a significant relationship between CCT and the presence, but not severity, of glaucomatous visual field loss. A prospective study is required to further explore and confirm these relationships.

The relationship between central corneal thickness-adjusted intraocular pressure and glaucomatous visual-field loss.

BACKGROUND: Although measurement of central corneal thickness (CCT) is increasingly becoming an important component of glaucoma risk analysis, significant controversy exists regarding the benefit of calculating a corrected intraocular pressure (IOP) value from measured IOP and CCT data. METHODS: Three hundred forty-four male subjects were identified from a VA eye clinic with one of the following clinical diagnoses: ocular hypertension (OHT), primary open-angle glaucoma (POAG), normal tension glaucoma (NTG), and normal tension glaucoma suspect (NTGS). Using one eye per subject, multivariate logistic regression and correlational analyses were performed to determine relationships between glaucomatous visual-field loss and several glaucoma risk factors, including adjusted IOP values. RESULTS: Multivariate logistic regression analysis did not identify CCT-adjusted IOP values as independent risk factors for development of either NTG or POAG-related glaucomatous visual-field loss. CCT, however, was found to be strongly associated with both NTG and POAG-related visual-field loss. Correlational analysis revealed a weak correlation between Ehlers-adjusted pre-treatment IOP and severity of POAG-related visual-field loss, but no other adjusted IOP values significantly correlated with severity of visual-field loss in either POAG or NTG. CONCLUSIONS: Our results suggest that adjusted IOP, as calculated using current algorithms, is not useful within glaucoma risk analysis, since adjusted IOP was unable to predict either presence or severity of glaucomatous visual-field loss in this study. CCT, conversely, was found to be a robust and independent predictor of glaucomatous visual-field loss. These findings, while supporting routine CCT measurements for all glaucoma suspects, do not support routine clinical computation of adjusted IOP values using current algorithms.

Factors influencing intermethod agreement between goldmann applanation, pascal dynamic contour, and ocular response analyzer tonometry.

PURPOSE: To examine factors that influence intraocular pressure (IOP) measurement agreement between Goldmann applanation (GAT), Ocular Response Analyzer (ORA), and Pascal Dynamic Contour tonometers (DCT). PATIENTS AND METHODS: In subjects who were diagnosed with primary open-angle glaucoma, ocular hypertension, glaucoma suspect, and normal, we used ORA, DCT, and GAT to obtain corneal hysteresis (CH), corneal resistance factor (CRF), ocular pulse amplitude, and 4 IOP values (ORA-IOPcc; ORA-IOPg; DCT-IOP; and GAT-IOP.) We also obtained corneal curvature, corneal thickness, axial length, retinal nerve fiber layer thickness, visual field parameters, diabetes diagnostic status, and topical IOP-lowering treatment data. Analysis of variance, Bland-Altman, and regression analyses were used to examine IOP agreement and associated factors. RESULTS: In 243 eyes of the 243 subjects, mean DCT-IOP (18.73±4.92) was not different from mean ORA-IOPcc (18.96±5.41) but both were significantly higher than ORA-IOPg (16.97±5.49) and GAT-IOP (16.37±4.97). In multivariate regression models, intermethod differences between IOPg, IOPcc, and DCT-IOP were explained almost completely by variations in CH, CRF, and level of IOP (r(2)=0.98 to 0.99); conversely, intermethod variability between GAT-IOP and the other 3 IOP metrics was only partially explained by the factors evaluated in this study (r(2)=0.31 to 0.65). CONCLUSIONS: Consistent with other studies, we found that the 4 IOP variables examined in this study are not interchangeable. The most consistent confounders of IOP measurement agreement were the ORA-measured corneal parameters, CH and CRF. Thus, accounting for these factors may be important in efforts to obtain accurate transcorneal estimates of IOP.

Repeatability and reproducibility for intraocular pressure measurement by dynamic contour, ocular response analyzer, and goldmann applanation tonometry.

PURPOSE: To evaluate and compare the intraocular pressure measurement variability between Goldmann applanation tonometry (GAT), Pascal dynamic contour tonometry (DCT), and ocular response analyzer (ORA) tonometry. METHODS: Subjects were prospectively recruited from consecutive Albuquerque VA Medical Center eye clinic patients that were previously diagnosed with ocular hypertension, glaucoma suspect, primary open-angle, or normal pressure glaucoma. Two sets of intraocular pressure measurements (3-4 ORA, 2 DCT, and 2 GAT) were obtained approximately 15 minutes apart. Each set was obtained by 1 of 2 examiners using random examiner sequences. ORA was measured first in both eyes, followed by either DCT or GAT, which were obtained in random order. Repeatability was assessed by examining variability of consecutive measurements by the same examiner, and reproducibility was examined by assessing variability between the first and second measurement sets. RESULTS: One hundred and twenty eyes of 60 subjects were included. Mean age was 64.1+/-9.6 years and 58/60 (97%) were male. Intraobserver repeatability was highest for GAT, followed closely by DCT, and then ORA. Intersession reproducibility was similar for all methods, although a tonographic effect may have corrupted GAT and DCT reproducibility results. We found no repeatability or reproducibility differences between eyes, between examiners, or between measurement sets. CONCLUSIONS: Although some intermethod variability differences were identified, all 3 methods in this study demonstrated clinically acceptable measurement repeatability and reproducibility. This result, in conjunction with the finding that variability was not different between eyes, examiners, or measurement sets, suggests that DCT and ORA are reliable enough to be clinically useful.

Ocular Response Analyzer in subjects with and without glaucoma.

PURPOSE: The Ocular Response Analyzer (ORA) is a newly introduced tonometer that uniquely measures and then integrates corneal biomechanical data into its intraocular pressure (IOP) estimates in an effort to improve accuracy of IOP assessment. This study was devised to investigate whether ORA-derived IOP and corneal biomechanical variables might be useful in discriminating between subjects with and without primary open-angle glaucoma (GLC). METHODS: All patients seen in the Albuquerque VAMC eye clinic over a 10-week period who demonstrated acceptable ORA signal profiles were retrospectively identified. In subjects classified as normal (NML), ocular hypertension (OH), glaucoma suspect (GS), and GLC, the following variables were compared: age, ethnicity, Goldmann IOP, central corneal thickness (CCT), and ORA-derived data: Goldmann-correlated IOP (IOPg), corneal-compensated IOP (IOPcc), corneal resistance factor (CRF), corneal hysteresis (CH), and difference between IOPcc and IOPg (DIOP; IOPcc - IOPg). RESULTS: Right eyes in 71 NML, 58 OH, 70 GS, and 99 GLC subjects were studied. Using analysis of variance, higher mean age, higher mean DIOP, and lower mean CH were found in the GLC group compared with OH, GS, and NML groups. In multivariate regression analyses, factors that independently discriminated between groups were: age, IOPcc, and DIOP (GLC vs. NML); age and IOPcc (GLC vs. GS); age and CRF (GLC vs. OH). When DIOP was left out of the models, CH replaced DIOP in the GLC vs. NML analysis with nearly equal statistical power. CONCLUSIONS: Our results suggest that ORA-generated parameters may be useful for differentiating subjects with and without GLC. Furthermore, the discriminatory power of each ORA variable seems to depend on the diagnostic groups that are being compared. Finally, our findings also suggest that measured IOP may be significantly underestimated in glaucoma patients compared with non-glaucoma patients.