Ocular Response Analyzer in Subjects with and without Glaucoma

The effect of corneal biomechanical properties on rebound tonometer in patients with normal-tension glaucoma

PURPOSE

To evaluate the effects of corneal biomechanical properties on intraocular pressure (IOP) measured with the ICare, and to compare IOP readings obtained with ICare, Ocular Response Analyzer (ORA), and Goldmann applanation tonometry (GAT) in normal-tension glaucoma (NTG) and normal subjects.

DESIGN

Prospective, cross-sectional, comparative study.

METHODS

IOP was measured with ICare, ORA, and GAT. All subjects had corneal hysteresis (CH) and corneal resistance factor (CRF), which were measured with ORA; and central corneal thickness (CCT), axial length, spherical equivalent, and keratometry.

RESULTS

This study enrolled 97 eyes of 97 NTG patients and 89 eyes of 89 normal subjects. CCT, CH, and CRF in NTG patients were significantly lower than those in normal subjects (P = .033, P = .006, and P = .003). The difference in IOP between techniques was highly significant in NTG patients (P < .001), while there was no significant difference in IOP values between techniques in normal controls (P = .931). ICare readings were significantly lower than corneal-compensated IOP in NTG patients (P = .014). CH and CRF were significantly associated with IOP measurements with ICare in NTG and normal subjects (P < .001). The greater difference between IOPcc and ICare in NTG patients was significantly influenced by the lower CH (P < .001).

CONCLUSIONS

Since ICare is a convenient way to measure IOP, ICare is a reasonable option as an alternative tonometer in NTG patients. However, the clinician must consider that the corneal biomechanical characteristics in NTG can cause ICare to underestimate IOP.

A comparison of corneal biomechanical properties in patients with psoriasis and healthy subjects

OBJECTIVE

To evaluate the differences in corneal biomechanical properties between healthy subjects and patients with psoriasis using the ocular response analyzer (ORA).

METHODS

Fifty-nine eyes of 59 psoriasis patients and 66 healthy individuals were included in the study. Corneal biomechanical parameters were obtained using ORA. Ultrasound pachymetry was used to measure central corneal thickness (CCT). The main parameters assessed were corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg) measurement and corneal-compensated IOP (IOPcc) through ORA. The dry eye evaluation was performed with tear break-up time (TBUT) and Shirmer test.

RESULTS

The mean CH values in psoriasis and healthy subject eyes were 10.75±2.9 mm Hg, 11.97±3.6 mm Hg, respectively (P=0.047). The mean CRF values were 10.14±3.1 mm Hg and 11.61±3.3 mm Hg, respectively (P=0.012). The mean CCT were 539.1±36 μm and 536.3±28 μm, respectively (P=0.643). Mean TBUT values were 8.2±2.9 sec in psoriasis group and 10.4±3.6 sec in healthy subjects (P<0.001). Shirmer test values were less (8.9±3.8 mm/5 min) in psoriasis than in healthy subjects (13.1±3.6 mm) (P<0.001).

CONCLUSIONS

Psoriasis can influence the corneal biomechanical properties. Patients with psoriasis had lower CH and CRF, but higher IOPg and IOPcc values than healthy controls. These corneal biomechanical changes should be considered when determining IOP values.

Assessment of the Ocular Response Analyzer as an Instrument for Measurement of Intraocular Pressure and Corneal Biomechanics

PURPOSE

The purpose of this study is to provide better understanding of the Ocular Response Analyzer (ORA) and how reliable it is to produce intraocular pressure (IOP) measurements that are free of the effects of corneal stiffness parameters, and stiffness estimates that are independent of IOP.

MATERIALS AND METHODS

A numerical parametric study that closely represents the in-vivo conditions of the human eye and the ORA procedure was conducted to determine the correlation coefficient r(2) between ORA output and the values of true IOP and a number of stiffness parameters, namely corneal thickness, curvature and age. For the purpose of this exercise, the ORA output was put in the form k1P1+k2P2 where k1 and k2 were variables and P1 and P2 were ORA's measured applanation pressures. Two separate clinical datasets involving Moorfields Eye Hospital, London and the University of New South Wales, Sydney participants, respectively, were used to validate the numerical results.

RESULTS

The numerical study results show a strong association between (k1P1 + k2P2) and the true IOP over a wide range of k1 and k2 values apart from a narrow region approximately extending from (k1 = +2, k2 = -2) to (k1 = -2, k2 = +2). On the other hand, (k1· P1 + k2· P2) was found to have a strong association with CCT, R and age (the stiffness parameters) over the same narrow region, beyond which the association was weak. Similar trends were found with the two clinical datasets.

CONCLUSIONS

The results of this study show the potential of the ORA to provide reliable IOP measurements with weak dependence on the cornea's stiffness parameters and the considerably reduced reliability in producing stiffness estimates that are unaffected by IOP values.

Corneal biomechanics: a review

Biomechanics is often defined as 'mechanics applied to biology'. Due to the variety and complexity of the behaviour of biological structures and materials, biomechanics is better defined as the development, extension and application of mechanics for a better understanding of physiology and physiopathology and consequently for a better diagnosis and treatment of disease and injury. Different methods for the characterisation of corneal biomechanics are reviewed in detail, including those that are currently commercially available (Ocular Response Analyzer and CorVis ST). The clinical applicability of the parameters provided by these devices are discussed, especially in the fields of glaucoma, detection of ectatic disorders and orthokeratology. Likewise, other methods are also reviewed, such as Brillouin microscopy or dynamic optical coherence tomography and others with potential application to clinical practice but not validated for in vivo measurements, such as ultrasonic elastography. Advantages and disadvantages of all these techniques are described. Finally, the concept of biomechanical modelling is revised as well as the requirements for developing biomechanical models, with special emphasis on finite element modelling.

Automatic method of analysis and measurement of additional parameters of corneal deformation in the Corvis tonometer

INTRODUCTION

The method for measuring intraocular pressure using the Corvis tonometer provides a sequence of images of corneal deformation. Deformations of the cornea are recorded using the ultra-high-speed Scheimpflug camera. This paper presents a new and reproducible method of analysis of corneal deformation images that allows for automatic measurements of new features, namely new three parameters unavailable in the original software.

MATERIAL AND METHOD

The images subjected to processing had a resolution of 200 × 576 × 140 pixels. They were acquired from the Corvis tonometer and simulation. In total 14,000 2D images were analysed. The image analysis method proposed by the author automatically detects the edge of the cornea and sclera fragments. For this purpose, new methods of image analysis and processing proposed by the author as well as those well-known, such as Canny filter, binarization, median filtering etc., have been used. The presented algorithms were implemented in Matlab (version 7.11.0.584-R2010b) with Image Processing toolbox (version 7.1-R2010b) using both known algorithms for image analysis and processing and those proposed by the author.

RESULTS

Owing to the proposed algorithm it is possible to determine three parameters: (1) the degree of the corneal reaction relative to the static position; (2) the corneal length changes; (3) the ratio of amplitude changes to the corneal deformation length. The corneal reaction is smaller by about 30.40% compared to its static position. The change in the corneal length during deformation is very small, approximately 1% of its original length. Parameter (3) enables to determine the applanation points with a correlation of 92% compared to the conventional method for calculating corneal flattening areas. The proposed algorithm provides reproducible results fully automatically within a few seconds/per patient using Core i7 processor.

CONCLUSIONS

Using the proposed algorithm, it is possible to measure new, additional parameters of corneal deformation, which are not available in the original software. The presented analysis method provides three new parameters of the corneal reaction. Detailed clinical studies based on this method will be presented in subsequent papers.

Reliability of corneal dynamic scheimpflug analyser measurements in virgin and post-PRK eyes

Purpose

To determine the measurement reliability of CorVis ST, a dynamic Scheimpflug analyser, in virgin and post-photorefractive keratectomy (PRK) eyes and compare the results between these two groups.

Methods

Forty virgin eyes and 42 post-PRK eyes underwent CorVis ST measurements performed by two technicians. Repeatability was evaluated by comparing three consecutive measurements by technician A. Reproducibility was determined by comparing the first measurement by technician A with one performed by technician B. Intraobserver and interobserver intraclass correlation coefficients (ICCs) were calculated. Univariate analysis of covariance (ANCOVA) was used to compare measured parameters between virgin and post-PRK eyes.

Results

The intraocular pressure (IOP), central corneal thickness (CCT) and 1_st applanation time demonstrated good intraobserver repeatability and interobserver reproducibility (ICC≧0.90) in virgin and post-PRK eyes. The deformation amplitude showed a good or close to good repeatability and reproducibility in both groups (ICC≧0.88). The CCT correlated positively with 1_st applanation time (r = 0.437 and 0.483, respectively, p<0.05) and negatively with deformation amplitude (r = −0.384 and −0.375, respectively, p<0.05) in both groups. Compared to post-PRK eyes, virgin eyes showed longer 1_st applanation time (7.29±0.21 vs. 6.96±0.17 ms, p<0.05) and lower deformation amplitude (1.06±0.07 vs. 1.17±0.08 mm, p<0.05).

Conclusions

CorVis ST demonstrated reliable measurements for CCT, IOP, and 1_st applanation time, as well as relatively reliable measurement for deformation amplitude in both virgin and post-PRK eyes. There were differences in 1_st applanation time and deformation amplitude between virgin and post-PRK eyes, which may reflect corneal biomechanical changes occurring after the surgery in the latter.

Corneal biomechanical comparison of pseudoexfoliation syndrome, pseudoexfoliative glaucoma and healthy subjects

PURPOSE

To evaluate the differences in corneal biomechanical properties between healthy subjects and patients with pseudoexfoliation syndrome (PEX) and pseudoexfoliative glaucoma (PEXG) using the ocular response analyzer (ORA).

MATERIALS AND METHODS

One hundred eighteen eyes of 45 healthy, 43 PEX and 30 PEXG eyes were included in to the study. Corneal biomechanical parameters measurements were obtained using ORA. The main parameters assessed were corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated pressure measurement (IOPg) and corneal compensated intraocular pressure (IOPcc). Ultrasound pachymetry was used for measurement of central corneal thickness (CCT).

RESULTS

In healthy subjects, PEX and PEXG eyes' mean CH values were 10.3 ± 1.4, 8.2 ± 1.4 and 6.8 ± 1.7 mmHg, respectively. The difference in mean CH between the PEXG and other two groups were statistically significant (p < 0.001). Mean CRF values were 10.3 ± 0.7, 7.9 ± 1.6 and 7.9 ± 1.9 mmHg, in healthy subjects PEX and PEXG, respectively. The difference in mean CRF between the PEX and PEXG was not statistically significant (p = 0.630), however the mean CRF was significantly higher in healthy subjects, compared to other two groups. Mean CCT were 546.3 ± 28, 525.5 ± 35 and 509 ± 36 μ, in healthy subjects, PEX and PEXG, respectively. The differences on CCT were also significant among the three groups (p < 0.001).

CONCLUSION

In this study, the corneal biomechanical features of subjects with PEX were found to be changed as compared to healthy controls. In these patients; CH, CRF and CCT were decreased which was more obvious in patients with PEXG in comparison to PEX patients.

Air puff induced corneal vibrations: theoretical simulations and clinical observations

PURPOSE

To investigate air puff induced corneal vibrations and their relationship to the intraocular pressure (IOP), viscoelasticity, mass, and elasticity of the cornea based on theoretical simulations and preliminary clinical observations.

METHODS

To simulate the corneal movement during air puff deformation, a kinematic viscoelastic corneal model was developed involving the factors of corneal mass, damping coefficient, elasticity, and IOP. Different parameter values were taken to investigate how factors would affect the corneal movements. Two clinical ocular instruments, CorVis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) and the Ocular Response Analyzer (ORA; Reichert, Inc., Buffalo, NY), were employed to observe the corneal dynamical behaviors.

RESULTS

Numerical results showed that during the air puff deformation, there would be vibrations along with the corneal deformation, and the damping viscoelastic response of the cornea had the potential to reduce the vibration amplitude. With consistent IOP, the overall vibration amplitude and inward motion depths were smaller with a stiffer cornea.

CONCLUSIONS

A kinematic viscoelastic model of the cornea is presented to illustrate how the vibrations are associated with factors such as corneal mass, viscoelasticity, and IOP. Also, the predicted corneal vibrations during air puff deformation were confirmed by clinical observation.

Corneal biomechanics as a function of intraocular pressure and pachymetry by dynamic infrared signal and Scheimpflug imaging analysis in normal eyes

PURPOSE

To evaluate corneal biomechanical deformation response using Ocular Response Analyzer (ORA) and Corvis ST data.

DESIGN

Prospective observational case-control study.

METHODS

A total of 1262 eyes of 795 patients were enrolled. Three groups were established, according to the corneal compensated intraocular pressure (IOPcc): Group I (10-13 mm Hg), Group II (14-17 mm Hg), and Group III (18-21 mm Hg). Each group included 3 subgroups, based on central corneal thickness (CCT): Subgroups 1 (465-510 μm), 2 (510-555 μm), and 3 (555-600 μm). In addition, similar groups of CCT were divided into subgroups of IOPcc. Corneal hysteresis (CH) and corneal resistance factor (CRF) were derived from ORA. The parameters of highest concavity with the parameters of first and second applanation were recorded from Corvis ST.

RESULTS

CH and CRF, applanation 1 time, and radius of curvature at highest concavity showed significant differences between CCT subgroups for each IOPcc group (P < .0001). CH, applanation 1 and 2 time, and applanation 2 velocity, as well as deformation amplitude (DA), showed significant differences by IOP subgroups for all CCT groups. IOPcc is correlated negatively with CH (r = -0.38, P < .0001). There are positive correlations of IOPcc with applanation 1 time, applanation 2 velocity, and radius and negative correlations with applanation 2 time (r = -0.54, P < .0001), applanation 1 velocity (r = -0.118, P < .0001), and DA (r = -0.362, P < .0001).

CONCLUSION

ORA and Corvis ST parameters are informative in the evaluation of corneal biomechanics. IOP is important in deformation response evaluation and must be taken into consideration.

Corneal biomechanical properties in different ocular conditions and new measurement techniques

Several refractive and therapeutic treatments as well as several ocular or systemic diseases might induce changes in the mechanical resistance of the cornea. Furthermore, intraocular pressure measurement, one of the most used clinical tools, is also highly dependent on this characteristic. Corneal biomechanical properties can be measured now in the clinical setting with different instruments. In the present work, we review the potential role of the biomechanical properties of the cornea in different fields of ophthalmology and visual science in light of the definitions of the fundamental properties of matter and the results obtained from the different instruments available. The body of literature published so far provides an insight into how the corneal mechanical properties change in different sight-threatening ocular conditions and after different surgical procedures. The future in this field is very promising with several new technologies being applied to the analysis of the corneal biomechanical properties.

Corneal hysteresis with intraocular pressure of a wide range: a test on porcine eyes

PURPOSE

To investigate the relationship between corneal hysteresis (CH) and intraocular pressure (IOP) using porcine eyes in the low to high IOP ranges.

METHODS

In vitro porcine eyes were used to investigate the relationship of CH and IOP. IOP was altered by changing the height of a drip stand within the dynamic range of 60 mm Hg. CH and IOP were measured with the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Depew, NY) at different heights. Second-order polynomial regression method was employed to assess the nonlinear correlation of CH and IOP.

RESULTS

CH demonstrated an initial plateau stage with low IOP, which then decreased as IOP increased to higher values up to 60 mm Hg. The maximum CH value of approximately 6 to 8 mm Hg was achieved when IOP ranged from 11 to 25 mm Hg. The nonlinear regression lines of Goldmann correlated IOP (IOPg) and CH can be described as CH = − 0.0029 × IOPg2 + 0.1005 × IOPg + 5.2824, R2 = 0.3676, P < .05.

CONCLUSIONS

CH was relatively constant for lower values of IOP and showed a decreasing relationship at higher values of IOP. This nonlinear relationship provides insight into understanding the viscoelastic nature of CH over a wider range of IOP values. The experimental data on porcine eyes may indicate that IOP should be taken into account when analyzing the deformation response of the cornea to an applied air puff.

Anterior-segment morphology and corneal biomechanical characteristics in pigmentary glaucoma

Purpose

The aim of the study reported here was to evaluate characteristics of the anterior-segment via anterior-segment optical coherence tomography (AS-OCT) and corneal biomechanical properties using an ocular response analyzer and their changes by peripheral laser iridotomy (PI) in patients with pigmentary glaucoma (PG).

Materials and methods

Seventeen eyes with PG were included consecutively. AS-OCT and ocular response analyzer measurements were taken before and 3 months after PI. Baseline morphology and change in morphology were analyzed by correlation and multiple linear regression analysis. The main parameters assessed were anterior-chamber (AC) angles and volume as well as corneal hysteresis (CH) and corneal resistance factor.

Results

AC angles were found to have decreased significantly in each quadrant after PI (P<0.001), with the highest effect seen in the temporal quadrant, which decreased from 57.0°±9.6° to 44.1°±5.2° (± standard deviation). Mean AC volume decreased significantly from 213.1±36.4 to 187.0±23.4 mm³ (P<0.001). CH and corneal resistance factor did not change after PI. CH was found to correlate with the preoperative superior and inferior angle width (Spearman’s rho 0.553 and 0.615, respectively, P<0.05). Biomechanical parameters showed no predictive value on the change of AC angles or volume.

Conclusion

PI in eyes with PG results in a highly significant reduction in the AC angles and volume as visualized by AS-OCT, with the largest effect seen in the temporal quadrant. CH is strongly positively correlated with the superior and inferior preoperative AC angles, emphasizing the importance of the biomechanical properties of the cornea for glaucoma pathogenesis in PG, but corneal biomechanical properties cannot predict PI-related AC changes.

Corneal biomechanical properties: precision and influence on tonometry

PURPOSE

To assess the precision and reproducibility of the corneal biomechanical parameters, and their relationships with the intraocular pressure (IOP) measured with the Goldmann tonometer and a noncontact tonometer.

METHODS

Readings for biomechanical properties and for IOP measured with the Goldmann and noncontact tonometers, were taken on one randomly selected eye of 106 normal subjects, on each one of two measurement sessions. Measurements with the ocular response analyzer (ORA) and the noncontact tonometer were randomized, followed by the measurement of central corneal thickness and with the Goldmann tonometer.

RESULTS

Repeatability coefficients for CCT, corneal hysteresis (CH) and corneal resistance factor (CRF) in Session 1 were ± 0.01 μm, ± 3.05 mmHg and ± 2.62 mmHg, respectively. The mean CCT, CH, CRF, Goldmann and noncontact tonometry did not vary significantly between sessions. Reproducibility coefficients for CCT, CH and CRF were ± 0.02 μm, ± 2.19 mmHg and ± 1.97 mmHg, respectively. Univariate regression analysis showed that CCT, CH and CRF significantly (P<0.0001) correlated with the IOP measured with the Goldmann and noncontact tonometers (and with the differences between tonometers) in Session 1. There were no significant correlations with the differences between tonometers in Session 2. Multivariate analysis revealed a minimal effect of CCT on Goldmann measurements but a significant effect on those of the noncontact tonometer.

CONCLUSIONS

Measurement of the biomechanical properties of the cornea, using the ORA, are repeatable and reproducible, affect Goldmann tonometry less than noncontact tonometry, and have a minimal influence on the difference in measured intraocular pressure between tonometers.

Keeping an eye on decellularized corneas: a review of methods, characterization and applications

The worldwide limited availability of suitable corneal donor tissue has led to the development of alternatives, including keratoprostheses (Kpros) and tissue engineered (TE) constructs. Despite advances in bioscaffold design, there is yet to be a corneal equivalent that effectively mimics both the native tissue ultrastructure and biomechanical properties. Human decellularized corneas (DCs) could offer a safe, sustainable source of corneal tissue, increasing the donor pool and potentially reducing the risk of immune rejection after corneal graft surgery. Appropriate, human-specific, decellularization techniques and high-resolution, non-destructive analysis systems are required to ensure reproducible outputs can be achieved. If robust treatment and characterization processes can be developed, DCs could offer a supplement to the donor corneal pool, alongside superior cell culture systems for pharmacology, toxicology and drug discovery studies.

Corneal deformation measurement using Scheimpflug noncontact tonometry

PURPOSE

To determine the intraexaminer repeatability and intersession reproducibility of corneal deformation measurement using Scheimpflug noncontact tonometry (Corvis ST) on normal subjects.

METHODS

Thirty-seven adults aged 20 to 48 years were invited to have their corneal deformation and curvature measurements taken using Corvis ST and Pentacam, respectively. Three consecutive measurements were taken for each instrument between 9:00 and 11:00 AM for intraexaminer repeatability analysis. Participants returned between 3:00 and 5:00 PM the same day for intersession reproducibility analysis.

RESULTS

The most repeatable corneal parameter measured by Corvis ST was central corneal thickness ([CCT] ICC, 0.96; precision, 10.85 μm; repeatability, 15.34 μm; CV, 1.01%), followed by deformation amplitude ([DA] ICC, 0.80; precision, 0.08 mm; repeatability, 0.13 mm; CV, 4.33%), first applanation time ([1st A-time] ICC, 0.77; precision, 0.22 milliseconds; repeatability, 0.31 milliseconds; CV, 1.42), and intraocular pressure ([IOP] ICC, 0.75; precision, 1.39 mm Hg; repeatability, 1.97 mm Hg; CV, 4.98). Other parameters showed poor repeatability. The DA and 1st A-time showed good intersession reproducibility. The 95% limits of agreement were +0.13 to -0.13 mm for DA and +0.27 to -0.33 milliseconds for 1st A-time. The DA was negatively correlated with central corneal thickness (r = -0.53, p < 0.001) but not with corneal curvatures (flattest curvature, r = 0.13, p = 0.46; steepest curvature, r = 0.05, p = 0.75).

CONCLUSIONS

Corneal deformation parameters DA and 1st A-time were repeatable and reproducible. A thinner cornea was associated with a higher corneal deformation. Measurement of DA serves as an indicator of corneal biomechanical properties.

Variation in corneal hysteresis and central corneal thickness among black, hispanic and white subjects

PURPOSE

To determine whether differences in corneal hysteresis (CH) and central corneal thickness (CCT) between black, Hispanic and white subjects exist independently of one another.

METHODS

Retrospective, cross-sectional data were reviewed for 807 eyes of 410 patients consecutively evaluated for glaucoma. Included patients had open angles, at least one reliable 24-2 perimetric examination and no evidence of nonglaucomatous vision loss. Patients underwent CH measurement with the ocular response analyzer followed by CCT measurement and full ocular examination. Patients were asked to self-classify their race or ethnicity. Statistical analyses were performed to identify characteristics that varied between black, Hispanic and white subjects and to explain this variation.

RESULTS

Of the 270 patients (511 eyes) included, 84 were black, 96 Hispanic and 90 white. There were no significant differences in diagnosis, sex, age, intraocular pressure or glaucoma severity between races/ethnicities (p ≥ 0.16). Blacks were found to have lower CCT (529.3 μm) and CH (8.7 mmHg) compared to Hispanics (544.7 μm, p = 0.008; 9.4 mmHg, p = 0.007) and whites (549.9 μm, p < 0.001; 9.8 mmHg, p < 0.001). On multivariable analysis, inter-racial/ethnic differences in CCT were not found to exist independent of CH (p ≥ 0.10), whereas the significant intergroup variation in CH remained after adjustment for CCT and other covariates (p ≤ 0.005).

CONCLUSIONS

Variation in CCT between races/ethnicities does not exist independent of CH. However, significant intergroup variation in CH is present independent of CCT. This finding suggests that CH may be a preferable measurement to evaluate intergroup differences in corneal properties and their relationship to open-angle glaucoma.

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 (β = 0.39, p < 0.001), corneal curvature (β = 0.16, p = 0.01), corneal resistance factor (β = 0.57, p < 0.001), A1C (β = 0.15, p = 0.01), mean defect (β = 0.29, p < 0.001), and retinal nerve fiber layer (β = 0.31, p < 0.001). Factors inversely related to CH were age (β = -0.22, p < 0.001), IOP (β = -0.29, p < 0.001), ocular pulse amplitude (β = -0.11, p = 0.04), CDR (β = -0.34, p < 0.001), and pattern standard deviation (β = -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.

Combining corneal hysteresis with central corneal thickness and intraocular pressure for glaucoma risk assessment

PURPOSE

To determine whether adjusting corneal hysteresis (CH) values for central corneal thickness (CCT) and intraocular pressure (IOP) improves its capability to differentiate primary open-angle glaucoma (POAG) from ocular hypertension (OH).

METHODS

This prospective, observational, cross-sectional study included 169 eyes of 169 subjects with a diagnosis of POAG (n=81) or OH (n=88). We utilized the Ocular Response Analyzer (ORA), Pascal Dynamic Contour Tonometer (DCT), Goldmann applanation tonometer (GAT), and ORA ultrasound pachymeter to obtain CH, IOP, and CCT values. Correlational, regression, and t-test analyses were conducted before and after the sample was divided into low, intermediate, and thick CCT subgroups.

RESULTS

In the full sample, CH and CCT were moderately correlated (r=0.44, P<0.001). Although both were related to diagnosis in univariate regression analysis, only CH was independently related to glaucoma diagnosis in multivariate analysis. After the sample was divided into CCT tertiles, CH was significantly lower in POAG vs OH eyes within all three CCT subgroups, and CH was the only multivariate variable that differentiated POAG from OH in each CCT subgroup. Moreover, the relationship between CH and diagnosis was more robust within the CCT subgroups compared with the full sample, suggesting that integrating CCT into CH interpretation is beneficial. Adjusting CH for IOP did not aid diagnostic precision in this study.

CONCLUSION

Our findings suggest that combining CH and CCT for glaucoma risk assessment improves diagnostic capability compared to using either factor alone. Conversely, adjusting CH for IOP provided no clear clinical benefit in this study.

Measuring corneal hysteresis: threshold estimation of the waveform score from the Ocular Response Analyzer

Background

The aim of this study was to determine a threshold waveform score (WS) for the best score value (BSV) in the Ocular Response Analyzer (ORA).

Methods

Retrospective study. One hundred and thirty-three healthy adults were recruited. Measurements were done with the ORA 2.04.

Results

Two hundred and sixty-six eyes were analyzed. Mean age was 56.49 ± 15.97 years. The mean waveform score of the BSV was 7.39 ± 1.32. The waveform scores ranged from 2.8 to 9.7. Kolmogorov–Smirnov test for normality was significant (p ≤ 0.0001).

Linear regression showed a significant positive correlation between IOPg (measured with the ORA) and IOP measured with Goldmann applanation tonometry (p ≤ 0.0001), as well as significant negative correlation between the difference IOPg–IOP Goldmann and waveform score of the BSV values. Threshold estimation considering 95 % confidence interval was 7.23. Meanwhile, threshold estimation considering the difference IOPg–IOP Goldmann, for 3 mmHg, was 6.7.

Conclusions

When using the ORA device, we recommend that clinicians try to obtain several waveform score measurements of 7 or above. Waveform scores lower than 7 may render less reliable results.

Relationship between corneal biomechanical properties, central corneal thickness, and intraocular pressure across the spectrum of glaucoma

PURPOSE

To evaluate corneal biomechanical properties across the glaucoma spectrum and study the relationship between these measurements and intraocular pressure measured by Goldmann applanation tonometry (GAT-IOP) and central corneal thickness (CCT).

DESIGN

Prospective cross-sectional study.

METHODS

SETTING

Tertiary-care teaching institute.

STUDY POPULATION

A total of 323 eyes of 323 participants (71 normal, 101 glaucoma suspect [GS], 38 ocular hypertension [OHT], 59 primary angle-closure disease [PACD], 36 primary open-angle glaucoma [POAG], and 18 normal-tension glaucoma [NTG]) who had received no ophthalmic treatment.

OBSERVATION PROCEDURES

Corneal hysteresis (CH), corneal resistance factor (CRF), corneal-compensated IOP (IOPcc), and Goldmann-correlated IOP (IOPg) measured by the Ocular Response Analyzer (ORA). GAT-IOP and CCT recorded in all subjects.

MAIN OUTCOME MEASURES

Regression analysis used to determine the relationship between GAT-IOP, CCT, age, CRF, and CH. Bland-Altman plots used to assess agreement between IOP measured by GAT and the ORA (IOPg).

RESULTS

CH measurements were significantly less in POAG and NTG compared to normal subjects (P = .034 and P = .030 respectively), regardless of the IOP. The CRF was significantly less in NTG and maximum in POAG and OHT. Regression analysis with CH as dependant variable showed significant association with GAT-IOP and CRF (P < .001) but not CCT, persisting on multivariate analysis (adjusted R(2) = 0.483). GAT-IOP correlated strongly with Goldmann-correlated IOP on the ORA (IOPg) (r = 0.82; P < .001), but limits of agreement between the measurements were poor.

CONCLUSIONS

CH and CRF may constitute a pressure-independent risk factor for glaucoma. CRF appears to influence GAT-IOP measurements more than simple geometric thickness measured by CCT. However, IOP measurements from the ORA are not interchangeable with, and are unlikely to replace, Goldmann applanation tonometry at the present time.

Assessment of intraocular pressure measured by Reichert Ocular Response Analyzer, Goldmann Applanation Tonometry, and Dynamic Contour Tonometry in healthy individuals

AIM

To investigate the accuracy of intraocular pressure (IOP) as measured by a Reichert Ocular Response Analyzer (ORA), as well as the relationship between central corneal thickness (CCT) and IOP as measured by ORA, Goldmann applanation tonometry (GAT), and dynamic contour tonometry (DCT).

METHODS

A total of 158 healthy individuals (296 eyes) were chosen randomly for measurement of IOP. After CCT was measured using A-ultrasound (A-US), IOP was measured by ORA, GAT, and DCT devices in a randomized order. The IOP values acquired using each of the three tonometries were compared, and the relationship between CCT and IOP values were analyzed separately. Two IOP values, Goldmann-correlated IOP value (IOPg) and corneal-compensated intraocular pressure (IOPcc), were got using ORA. Three groups were defined according to CCT: 1) thin cornea (CCT<520µm); 2) normal-thickness cornea (CCT: 520-580µm); and 3) thick cornea (CCT>580µm) groups.

RESULTS

In normal subjects, IOP measurements were 14.95±2.99mmHg with ORA (IOPg), 15.21±2.77mmHg with ORA (IOPcc), 15.22±2.77mmHg with GAT, and 15.49±2.56mmHg with DCT. Mean differences were 0.01±2.29mmHg between IOPcc and GAT (P>0.05) and 0.28±2.20mmHg between IOPcc and DCT (P>0.05). There was a greater correlation between IOPcc and DCT (r=0.946, P=0.000) than that between IOPcc and GAT (r=0.845, P=0.000). DCT had a significant correlation with GAT (r=0.854, P=0.000). GAT was moderately correlated with CCT (r=0.296, P<0.001), while IOPcc showed a weak but significant correlation with CCT (r=-0.155, P=0.007). There was a strong negative correlation between CCT and the difference between IOPcc and GAT(r=-0.803, P=0.000), with every 10µm increase in CCT resulting in an increase in this difference of 0.35mmHg. The thick cornea group (CCT>580µm) showed the least significant correlation between IOPcc and GAT (r=0.859, P=0.000); while the thin cornea group (CCT<520µm) had the most significant correlation between IOPcc and GAT (r=0.926, P=0.000). The correlated differences between IOPcc and DCT were not significant in any of the three groups (P>0.05).

CONCLUSION

Measurement of IOP by ORA has high repeatability and is largely consistent with GAT measurements. Moreover, the ORA measurements are affected only to a small extent by CCT, and are likely to be much closer to the real IOP value than GAT.

Corneal biomechanical properties in normal-tension glaucoma

PURPOSE

To investigate the intraocular pressure (IOP) and corneal biomechanical properties of normal and normal-tension glaucoma (NTG) eyes.

METHODS

This study included 83 normal and 83 NTG eyes. We measured corneal-compensated IOP (IOPcc), Goldmann-correlated IOP (IOPg), corneal resistance factor (CRF), corneal hysteresis (CH) and central corneal thickness (CCT) three times each for normal and NTG eyes using an Ocular Response Analyzer (ORA).

RESULTS

No significant difference in CCT was seen between normal eyes (541.4 ± 26.8 μm) and NTG eyes (535.4 ± 24.9 μm; p = 0.16). IOPcc was significantly higher in NTG eyes (16.1 ± 2.6 mmHg) than in normal eyes (15.1 ± 2.9 mmHg; p = 0.01), while IOPg was significantly lower in NTG eyes (14.1 ± 2.7 mmHg) than in normal eyes (15.1 ± 3.0 mmHg; p = 0.04). CRF and CH were significantly lower in NTG eyes (CRF, 8.9 ± 1.5 mmHg; CH, 9.2 ± 1.3 mmHg) than in normal eyes (CRF, 10.6 ± 1.4 mmHg; CH, 10.8 ± 1.3 mmHg; p < 0.0001 each).

CONCLUSION

IOPcc was significantly higher in NTG eyes than in normal eyes. The ORA may be useful for distinguishing between the IOPcc of NTG eyes with normal IOP and that of normal eyes. In addition, the ORA enables CRF and CH to be measured in vivo, and weakness of the lamina cribrosa may be clinically inferred from the fact that CRF and CH were reduced in NTG eyes in our study. Low CRF and CH may be clues to the pathology of NTG.

Reproducibility of viscoelastic property and intraocular pressure measurements obtained with the Ocular Response Analyzer

PURPOSE

To analyse the reproducibility of corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg) and corneal-compensated intraocular pressure (IOPcc) obtained with the ocular response analyzer (ORA).

METHODS

This is a prospective study, nonmasked, of eight successive examinations with the ORA device in 60 normal eyes. Using 30 eyes (one eye per subject), the reproducibility was assessed by comparing the first series of four examinations to the second four and by calculating the within-subject coefficient of variation. The correlation and difference with the fellow eye were analysed, respectively.

RESULTS

The mean values were 10.7 ± 1.8 mmHg, CRF; 10.6 ± 1.6 mmHg, CH; 15.9 ± 3.9 mmHg, IOPg and 16.2 ± 3.7 mmHg, IOPcc. The reproducibility was significantly different for CRF (5.2 ± 5.9%), CH (7.3 ± 8.6%), IOPg (7.7 ± 6.7%) and IOPcc (10.1 ± 8.0%); p < 0.001. Considering the reproducibility, CRF correlated with CH (rs = 0.55; p < 0.001) and showed to be independent of IOPg and IOPcc. The score spread was best for CRF (2.6 ± 1.5 mmHg; 24.0%) compared to IOPg (4.3 ± 1.5 mmHg; 28.1%) and CH (3.1 ± 1.7 mmHg; 29.9%) and worst for IOPcc (5.5 ± 2.5 mmHg; 34.4%). The lowest difference with the fellow eye was observed for CRF (5.0%; p = 0.09). The correlation with the fellow eye was high, especially for IOPcc and CRF (rs > 0.9; p < 0.001) followed by IOPg and CH (rs > 0.8; p < 0.001).

CONCLUSION

The ORA device provides reproducible information on viscoelastic properties of the cornea in normal eyes notably CRF and CH. IOPcc was less reproducible. Four measurements per eye were necessary to reach a 10% precision and six for 5%.

Corneal biomechanics measured with the ocular response analyser in patients with unilateral open-angle glaucoma

PURPOSE

To evaluate the relationship between biomechanical properties of the cornea and intraocular pressure (IOP) and the role of biomechanical properties in eyes of patients with unilateral primary open-angle glaucoma (POAG).

METHODS

The biomechanical properties of corneal hysteresis (CH) and the corneal resistance factor (CRF) were measured with the ocular response analyser (ORA). In an experimental setting, three human donor eyes with Schiotz-tonometry-controlled IOP were investigated. In addition, a series of patients with unilateral POAG were evaluated. Main outcome measures were CH, CRF, corneal-compensated IOP (IOPcc), standard automated perimetry parameters mean defect (MD) and pattern standard deviation, central corneal thickness, Goldmann applanation tonometry (GAT), and cup-to-disc ratio.

RESULTS

A highly significant linear correlation between CH and the corneal-compensated IOP (IOPcc, r = -0.926; p < 0.001) was found. The correlation between IOP(CC) and CRF was not significant (r = 0.335; p = 0.08). In total, 36 eyes of 18 patients with unilateral POAG were examined. Regarding uncorrected CH (mean 7.73 ± 1.46 mmHg glaucomatous eye and 9.28 ± 1.42 mmHg fellow eye), there was a highly significant difference between both eyes. This difference disappears, when CH was corrected for IOP (9.44 ± 3.78 mmHg and 9.97 ± 3.22 mmHg, respectively).

CONCLUSIONS

Corneal hysteresis but not corneal resistance factor is dependent on IOP. In patients with unilateral POAG, IOP is higher in the affected eye. When CH is corrected for IOP, corneal biomechanical properties do not differ in both eyes of patients with unilateral POAG.

Determinants of corneal biomechanical properties in an adult Chinese population

PURPOSE

To investigate variations in corneal hysteresis (CH) and corneal resistance factor (CRF) and their ocular and systemic associations in Chinese adults.

DESIGN

Population-based, cross-sectional study.

PARTICIPANTS

We included 1136 subjects of Chinese origin from an ongoing population-based study.

METHODS

All subjects in this population-based study underwent a standardized ocular examination including keratometry (corneal radius of curvature), intraocular pressure (IOP) measurement with Goldmann applanation tonometry, central corneal thickness (CCT), and axial length (AL) assessments. The CH and CRF were measured with the Ocular Response Analyzer. Participants underwent a detailed interview and laboratory investigations that included estimation of nonfasting serum glucose, glycosylated hemoglobin, and lipid profile.

MAIN OUTCOME MEASURES

We assessed CH, CRF, and their associations with demographic, ocular, and systemic factors.

RESULTS

The mean age of study subjects was 55.3±8.4 years and 51.2% were females. The mean CH and CRF were 10.6±1.5 and 10.1±1.6 mmHg, respectively. Women had higher mean CH (10.8 vs 10.4 mmHg; P<0.001) and CRF (10.4 vs. 9.8 mmHg; P<0.001) than men. After adjusting for age, gender, IOP, CCT, keratometry, AL, and diabetes, CH was negatively associated with age (β = -0.034; P<0.001), IOP (β = -0.037; P = 0.01), corneal radius of curvature (β = -0.963; P<0.001), and AL (β = -0.106; P = 0.001); and positively associated with female gender (β = 0.308; P<0.001) and CCT (β = 0.020; P<0.001). The CRF showed a negative association with age (β = -0.023; P<0.001) and corneal radius of curvature (β = -0.771; P<0.001), and was positively associated with female gender (β = 0.368; P<0.001), IOP (β = 0.134; P<0.001) and CCT (β = 0.024; P<0.001). Subjects with diabetes had a higher CH (β = 0.324; P<0.001) and CRF (β = 0.396; P = 0.002) compared with those without diabetes.

CONCLUSIONS

With advancing age, the CH and CRF in adult Chinese decreased. Women and subjects with diabetes had greater CH and CRF. Corneal biomechanical properties of hysteresis and resistance factor are significantly influenced by IOP, CCT, corneal radius of curvature, and AL.

Evaluation of the intraocular pressure measured with the ocular response analyzer

PURPOSE

Comparison of the magnitude and repeatability of the intraocular pressure (IOP) measured with the Ocular Response Analyzer (ORA) to that measured with the Goldmann tonometer.

METHODS

Two sets of IOP measurements were made, for 89 eyes of eighty-nine subjects, approximately 1-week apart. Goldmann tonometry was performed subsequent to non-contact tonometry, in which the order of measurement was randomized between the ORA and the Topcon CT80 non-contact tonometer (CT80). Each method was assessed twice for intrasession repeatability. The limits of agreement between each non-contact pressure and that measured with the Goldmann tonometer were assessed once per session. The level of statistical significance was 0.05.

RESULTS

The mean differences between the ORA-corneal compensated, Goldmann-correlated, and CT80-IOP (ORA-IOPcc; ORA-IOPg and CT80-IOP) versus the Goldmann IOP were -0.3 +/- 2.7 mmHg (mean +/- SD), -0.3 +/- 2.2 mmHg and -0.3 +/- 2.1 mmHg, respectively for session 1 and 0.3 +/- 3.0 mmHg, 0.2 +/- 2.2 mmHg, and -0.5 +/- 2.2 mmHg, respectively, for session 2. The repeatability coefficients were +/- 5.3 mmHg, +/- 4.2 mmHg, +/- 2.5 mmHg, and +/- 1.9 mmHg, respectively for ORA-IOPcc, ORA-IOPg, CT80-IOP, and Goldmann IOP in session 1 and +/- 3.8 mmHg, +/- 3.6 mmHg, +/- 1.6 mmHg, and +/- 1.9 mmHg, respectively for session 2.

CONCLUSION

The repeatability indices for the ORA were poorer than those with the Goldmann tonometer and the CT80 in both sessions. However, the average IOP measured with the ORA did not vary significantly from those measured with the other two tonometers in either session. The ORA provides valid, repeatable measures of IOP.

Evaluation of corneal biomechanical properties following penetrating keratoplasty using the ocular response analyzer

Purpose

To evaluate corneal biomechanical properties in eyes that had previously undergone penetrating keratoplasty (PK) using the ocular response analyzer (ORA).

Methods

We recruited 26 patients who had received unilateral PK. Corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg), and cornea-compensated intraocular pressure (IOPcc) were measured with the ORA and were compared to the measurements from the contralateral eyes that did not undergo PK.

Results

The CH was 8.95±2.59 mmHg in eyes that underwent PK and 9.78±1.45 mmHg in the contralateral eyes that did not undergo PK (p=0.077). The CRF was 10.26±2.64 mmHg in post-PK eyes and 9.75±1.45 mmHg in the contralateral eyes (p=0.509), and the CH-CRF was significantly smaller in post-PK eyes (-1.31±2.32 mmHg in post-PK eyes vs. 0.03±0.88 mmHg in fellow eyes, p=0.016). The IOPg and IOPcc were significantly higher in the PK group than they were in the control group. The IOPcc's were 20.81±7.81 mmHg and 16.27±2.49 mmHg in post-PK and control eyes, respectively (p=0.011); and the IOPg's were 19.22±7.34 mmHg and 15.07±3.03 mmHg in post-PK and control eyes, respectively (p=0.019). The IOPcc-g's were 1.59±2.81 mmHg and 1.21±1.30 mmHg in post-PK and control eyes, respectively (p=0.412), and the central corneal thickness (CCT)'s were 489.11±90.60 µm and 556.24±42.84 µm in post-PK and control eyes, respectively (p=0.068).

Conclusions

Following PK, CH tended to decrease while CRF tended to increase, significantly decreasing CH-CRF. A significantly higher intraocular pressure and a thinner CCT following PK may have contributed to the observed changes in these corneal biomechanical parameters.

Evaluation of agreement between intraocular pressure measurements using Goldmann applanation tonometry and Goldmann correlated intraocular pressure by Reichert's ocular response analyser

PURPOSE

To compare agreement of intraocular pressure (IOP) measurements using Goldmann applanation tonometry (GAT) and Goldmann correlated intraocular pressure generated (IOPg) by the Reichert ocular response analyser (ORA).

METHODS

Consecutive patients presenting for glaucoma evaluation underwent ORA assessment followed by examination including GAT. For each ORA assessment, measurements were taken until a waveform score (WS) of 6.5 was obtained or until five measurements were obtained per eye. The relationship between GAT and IOPg and the influence of the WS upon this relationship was evaluated. A Bland-Altman plot and linear regression were used to determine agreement between GAT and IOPg.

RESULTS

A total of 518 eyes of 260 patients were included in the final analysis. Increasing WS was found to predict a smaller difference between GAT and IOPg (β=-0.2, P≤0.001). Selecting the highest WS among ORA assessments of each eye, WS continued to predict concordance between GAT and IOPg (β=-0.2, P=0.006). The mean IOP difference between methods was 0.1 mm Hg (±0.3), which was found to be statistically insignificant (P=0.391). This relationship between GAT and IOPg was successfully validated using a second distinct data set of 100 eyes. GAT and IOPg measurements varied by 2 mm Hg or less in 53.9% of eyes and 5 mm Hg or less in 92.3% of eyes.

CONCLUSION

In clinical practice IOPg is strongly related to GAT. Although higher WS is indicative of greater IOPg/GAT concordance, its influence is minimal. This study does not support the use of a specific WS cutoff to determine quality of an IOPg measurement.

Ocular response analyzer measurements in keratoconus with normal central corneal thickness compared with matched normal control eyes

PURPOSE

To compare corneal hysteresis (CH) and corneal resistance factor (CRF) in eyes with keratoconus with a central corneal thickness (CCT) ≥ 520 μm with CH and CRF in matched controls, and to estimate the sensitivity and specificity of these parameters for discriminating between the two groups.

METHODS

This prospective, comparative case series comprised 19 eyes of 19 patients with keratoconus with CCT ≥ 520 μm and 19 eyes of 19 healthy sex-, age-, and CCT-matched patients who underwent a complete clinical eye examination, corneal topography, tomography, and biomechanical evaluation. The receiver operating characteristic (ROC) curve was used to identify cutoff points that maximized the sensitivity and specificity for discriminating between groups.

RESULTS

Central corneal thickness was 543.1 ± 13.9 μm (range: 520 to 568 μm) in the keratoconus group and 545 ± 12.5 μm (range: 527 to 575 μm) in the control group (P=.6017). Corneal hysteresis was 9.22 ± 1.44 mmHg (range: 6.2 to 11.35 mmHg) in the keratoconus group and 10.58 ± 1.91 mmHg (range: 7.34 to 13.53 mmHg) in the control group (P=.0075). Corneal resistance factor was 8.62 ± 1.52 mmHg (range: 5.60 to 11.20 mmHg) in the keratoconus group and 10.30 ± 1.92 mmHg (range: 6.95 to 14.12 mmHg) in the control group (P=.0049). The ROC curve analyses showed a poor overall predictive accuracy of CH (cutoff, 9.90 mmHg; sensitivity, 78.9%; specificity, 63.2%; test accuracy, 71.05%) and CRF (cutoff, 8.90 mmHg; sensitivity, 68.4%; specificity, 78.9%; test accuracy, 73.65%) for detecting keratoconus in the eyes studied.

CONCLUSIONS

Corneal hysteresis and CRF were statistically lower in the keratoconus group compared with the control group. Given the large overlap, both CH and CRF had low sensitivity and specificity for discriminating between groups.

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.

Intraocular pressure measured by dynamic contour tonometer and ocular response analyzer in normal tension glaucoma

BACKGROUND

To investigate intraocular pressure (IOP) measurement values in normal tension glaucoma (NTG) eyes using two different types of tonometer that are supposed to be little affected by corneal biochemical properties.

METHODS

This study included 30 normal eyes of 16 healthy subjects and 30 eyes of 16 patients with NTG. IOP was measured with a Goldmann applanation tonometer (GAT), a Pascal dynamic contour tonometer (DCT), and a Reichert ocular response analyzer (ORA) three times each for normal and NTG eyes. The main measures were GAT-IOP, DCT-IOP, corneal-compensated IOP (IOPcc), Goldmann-correlated IOP (IOPg), and central corneal thickness (CCT).

RESULTS

In normal eyes, GAT-IOP was 13.2 +/- 1.4 mmHg; DCT-IOP, 13.0 +/- 1.6 mmHg; IOPcc, 13.6 +/- 2.0 mmHg; and IOPg, 12.4 +/- 2.0 mmHg. Multivariate analysis revealed no significant differences between the four measurements (p = 0.08). CCT was 524.6 +/- 27.3 microns. In NTG eyes, GAT-IOP was 13.1 +/- 1.3 mmHg; DCT-IOP, 13.7 +/- 1.3 mmHg; IOPcc, 15.2 +/- 2.0 mmHg; and IOPg, 12.7 +/- 2.0 mmHg. Multivariate analysis showed significant differences between the four measurements (p < 0.01). Sheffé's test showed that IOPcc was significantly higher than GAT-IOP, DCT-IOP, and IOPg (GAT-IOP vs IOPcc: p < 0.0001; DCT-IOP vs IOPcc: p = 0.01; IOPcc vs IOPg: p < 0.0001). CCT was 515.4 +/- 32.9 microns, with no significant difference between normal and NTG eyes (p = 0.15).

CONCLUSIONS

We investigated the values of IOP in NTG eyes as measured by the DCT and ORA. IOPcc was significantly greater than GAT-IOP, DCT-IOP and IOPg in NTG eyes, suggesting the possibility that IOP values may be underestimated.