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Glaucoma  |   January 2013
A New Tonometer—The Corvis ST Tonometer: Clinical Comparison with Noncontact and Goldmann Applanation Tonometers
Author Affiliations & Notes
  • Jiaxu Hong
    From the Department of Ophthalmology, Eye, Ear, Nose, and Throat Hospital, School of Shanghai Medicine, Fudan University, Shanghai, China; the
  • Jianjiang Xu
    From the Department of Ophthalmology, Eye, Ear, Nose, and Throat Hospital, School of Shanghai Medicine, Fudan University, Shanghai, China; the
  • Anji Wei
    From the Department of Ophthalmology, Eye, Ear, Nose, and Throat Hospital, School of Shanghai Medicine, Fudan University, Shanghai, China; the
  • Sophie X. Deng
    Cornea Division, Jules Stein Eye Institute, University of California, Los Angeles, California; and the
  • Xinhan Cui
    From the Department of Ophthalmology, Eye, Ear, Nose, and Throat Hospital, School of Shanghai Medicine, Fudan University, Shanghai, China; the
  • Xiaobo Yu
    From the Department of Ophthalmology, Eye, Ear, Nose, and Throat Hospital, School of Shanghai Medicine, Fudan University, Shanghai, China; the
  • Xinghuai Sun
    From the Department of Ophthalmology, Eye, Ear, Nose, and Throat Hospital, School of Shanghai Medicine, Fudan University, Shanghai, China; the
    State Key Laboratory of Medical Neurobiology, Institutes of Brain Science, Shanghai, China.
  • Corresponding author: Xinghuai Sun, Department of Ophthalmology, Eye, Ear, Nose, and Throat Hospital, School of Shanghai Medicine, Fudan University, 83 Fenyang Road, Shanghai 200031, China; xinghuaisun@gmail.com
Investigative Ophthalmology & Visual Science January 2013, Vol.54, 659-665. doi:10.1167/iovs.12-10984
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      Jiaxu Hong, Jianjiang Xu, Anji Wei, Sophie X. Deng, Xinhan Cui, Xiaobo Yu, Xinghuai Sun; A New Tonometer—The Corvis ST Tonometer: Clinical Comparison with Noncontact and Goldmann Applanation Tonometers. Invest. Ophthalmol. Vis. Sci. 2013;54(1):659-665. doi: 10.1167/iovs.12-10984.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose.: To compare intraocular pressure (IOP) measurements obtained using the Topocon noncontact tonometer (NCT), the Goldmann applanation tonometer (GAT), and the Corvis ST (CST), a newly developed tonometer with features of visualization and measurement of the corneal deformation response to an air impulse. A secondary objective was to assess the agreement among the devices.

Methods.: Fifty-nine participants, including glaucoma patients (36 cases) and control volunteers (23 cases), were enrolled. One eye was selected randomly for further study. IOP measurements were obtained with the CST, NCT, and GAT by two experienced clinicians. IOP values were compared. Intraobserver variability and interobserver variability were assessed by the coefficient of variation and intraclass correlation coefficient. Device agreement was calculated by Bland-Altman analysis.

Results.: Mean IOP for all examined eyes was 18.9 ± 5.8 mm Hg for CST, 21.3 ± 6.8 mm Hg for NCT, and 20.3 ± 5.7 mm Hg for GAT. There was no statistically significant difference in IOP measurements among the tonometers except between the CST and NCT. Correlation analysis showed a high correlation between each pair of devices (all P < 0.001). The CST displayed the best intraobserver variability and interobserver variability. Bland-Altman analysis revealed a bias between CST and GAT, CST and NCT, and GAT and NCT of −1.3, −2.4, and −1.1 mm Hg, with 95% limits of agreement of −6.2 to 3.5 mm Hg, −10.1 to 5.2 mm Hg, and −8.3 to 6.2 mm Hg, respectively.

Conclusions.: The CST offers an alternative method for measuring IOP. IOP measurements taken with these devices may not be interchangeable.

Introduction
Intraocular pressure (IOP) measurement is an important parameter in the detection and monitoring of glaucoma. Therefore, precise IOP assessment plays a crucial role in the management of glaucoma patients. The ideal tonometer is expected to be accurate, to provide repeatable and reproducible results, and to be minimally invasive. Presently, the Goldmann applanation tonometer (GAT) is regarded as the gold standard for IOP measurement. However, it is common knowledge that the accuracy of IOP obtained with the GAT is affected by corneal properties. 1,2 The recently introduced rebound tonometer and the dynamic contour tonometer were developed in an attempt to address the shortcomings of the GAT and are designed so as not to applanate the cornea. However, it has been reported that the rebound response of the probe may reflect the viscoelastic properties of the cornea 3 instead of IOP; and, according to another study, it was difficult to obtain accurate IOP with the dynamic contour tonometer in pathologic corneas. 4  
The Corvis ST (CST) is a novel noncontact tonometer that allows investigation of the dynamic reaction of the cornea to an air impulse. 5 The CST gathers 4330 frames per second within a 100 ms period, therefore recording dynamic deformation of the cornea to calculate the IOP value. Its measurement range is from 1 to 60 mm Hg. For the present study, a high-speed Scheimpflug camera was equipped to record the movements of the cornea, which then were displayed on the built-in control panel in ultraslow motion, as shown in Figure 1 and Supplementary Videos (see Supplementary Material and Supplementary Videos). The camera could cover up to 8.5 mm of a cornea and provide excellent image resolution (640 × 480 pixels). The complete theory behind the CST has not been published yet, but it is designed to measure IOP as well as corneal thickness and biomechanical properties. 
Figure 1. 
 
Intraocular pressure measurement by CST. Top: Real-time information for a participant recorded immediately upon an air impulse. Bottom: Real-time information for a participant recorded at the highest concavity, indicating the largest deformation amplitude of the cornea. The CST system is designed to gather 4330 frames per second within a 100 ms period and therefore to record dynamic IOP. Its measurement range 1 to 60 mm Hg. A high-speed Scheimpflug camera was equipped to record the movements of the cornea, which then were displayed on the built-in control panel in ultraslow motion as shown in Supplementary Videos (see Supplementary Material and Supplementary Videos). This camera could cover up to 8.5 mm of the cornea and provide excellent image resolution (640 × 480 pixels). The output includes the IOP value, central corneal thickness, and corneal biomechanical properties (applanation time, applanation length, applanation velocity, and details of highest concavity).
Figure 1. 
 
Intraocular pressure measurement by CST. Top: Real-time information for a participant recorded immediately upon an air impulse. Bottom: Real-time information for a participant recorded at the highest concavity, indicating the largest deformation amplitude of the cornea. The CST system is designed to gather 4330 frames per second within a 100 ms period and therefore to record dynamic IOP. Its measurement range 1 to 60 mm Hg. A high-speed Scheimpflug camera was equipped to record the movements of the cornea, which then were displayed on the built-in control panel in ultraslow motion as shown in Supplementary Videos (see Supplementary Material and Supplementary Videos). This camera could cover up to 8.5 mm of the cornea and provide excellent image resolution (640 × 480 pixels). The output includes the IOP value, central corneal thickness, and corneal biomechanical properties (applanation time, applanation length, applanation velocity, and details of highest concavity).
To assess the clinical application of the new-generation tonometer, we believed it was important to evaluate its intraobserver and interobserver variabilities as well as its IOP measurement in comparison to that obtained with the gold standard GAT. In addition, no previous report of this technique in the literature could be found. The purpose of this study was to compare IOP measurements obtained with the CST, a traditional noncontact tonometer (NCT), and GAT and to determine their intraobserver and interobserver variabilities. A secondary objective was to examine agreement among the three devices and to investigate potential factors that might account for the difference between CST and GAT. 
Methods
A prospective analysis was performed on 59 participants (23 normal volunteers and 36 glaucoma patients) at the Shanghai Eye, Ear, Nose, and Throat Hospital. The Medical Ethics Committee approved the study protocol before data collection began. The research complied with the tenets of the Declaration of Helsinki. Written informed consent was obtained from each participant before examination. Normal volunteers did not have a prior history of glaucoma. The protocol excluded subjects with pathologic corneal conditions such as Fuchs' endothelial dystrophy or keratoconus. 
One eye of each subject was selected randomly for further analysis. A full ophthalmic examination was performed on each eye, including a visual acuity measurement and slit-lamp biomicroscopy evaluation of the anterior and the posterior segment with a 90-diopter (D) lens. IOP was measured in a sitting position and in either a CST-NCT-GAT or a NCT-CST-GAT sequence. The measurement sequence was randomly chosen for each participant. The instruments used for IOP measurements were a newly developed noncontact tonometer (Corvis ST; Oculus Optikgeräte GmbH, Wetzlar, Germany), a conventional noncontact tonometer (Topcon CT-80A Computerized Tonometer; Topcon, Tokyo, Japan), and a Goldmann applanation tonometer mounted to a slit lamp (Haag-Streit, Bern, Switzerland). During a single visit, the IOP was measured three times with each device by two experienced masked clinicians. Although the clinicians were allowed to view their own IOP results, they were masked with regard to the IOP measurements from other devices. Participants were given a 2-minute pause between each measurement taken with the same tonometer and a 5-minute pause between measurements taken by different tonometers. All participants underwent axial length and central corneal thickness assessment after all IOP measurements using an ultrasonic biometer (model 820; Allergan-Humphrey, San Leandro, CA). 
Statistical analysis was performed using an SPSS statistical software package (SPSS for Windows, version 17.0; SPSS, Inc., Chicago, IL) and MedCalc software version 12.2.1.0 (MedCalc Software, Mariakerke, Belgium). Data are shown as mean ± standard deviation. For the intraobserver and interobserver variability analyses, a coefficient of variation (CV) and intraclass correlation coefficients (ICC) were calculated. 68 Student's t-test, a mixed-model analysis of variance (ANOVA) with post hoc least significant difference (LSD) (Fisher LSD) multiple comparisons, and nonparametric Spearman correlation were used to compare the IOP measurements by different devices. The agreement between devices was assessed through Bland-Altman plots, which used the mean IOP measurement from two examiners. 6,9 The IOP difference between the CST and GAT as the dependent variable, and age, axial length, corneal curvature, corneal astigmatism, central corneal thickness (CCT), and corneal biomechanical properties as covariates, were used to establish what factors may impact the CST measurement using Spearman correlation. Information on the corneal biomechanical properties was obtained by the CST. Corneal biomechanical properties include the applanation time, applanation length, applanation velocity, and highest concavity (Fig. 1). All P values were two-sided and were considered statistically significant when they were less than 0.05. 
Results
Patient Demographics
Demographic data are presented in Table 1. Fifty-nine eyes of 59 participants (27 male and 32 female) were included in this study. IOP was successfully measured using the CST, NCT, and GAT in all eyes. Table 1 also shows the mean and range of IOP obtained using each of the three tonometers. There was no statistically significant difference in IOP measurement among the tonometers (P = 0.104) except between the CST and NCT by ANOVA (CST versus GAT, P = 0.236; CST versus NCT, P = 0.03; GAT versus NCT, P = 0.345). In addition, correlation analysis showed a high correlation between CST and NCT (ρ = 0.871; P < 0.001), between CST and GAT (ρ = 0.896; P < 0.001), and between GAT and NCT (ρ = 0.839; P < 0.001) as seen in Figure 2
Figure 2. 
 
Correlations among different IOP measurement devices. Significant positive correlations were noted between CST and NCT (ρ = 0.871; P < 0.001) (top left), between CST and GAT (ρ = 0.896; P < 0.001) (top right), and between GAT and NCT (ρ = 0.839; P < 0.001) (bottom).
Figure 2. 
 
Correlations among different IOP measurement devices. Significant positive correlations were noted between CST and NCT (ρ = 0.871; P < 0.001) (top left), between CST and GAT (ρ = 0.896; P < 0.001) (top right), and between GAT and NCT (ρ = 0.839; P < 0.001) (bottom).
Table 1. 
 
Demographic Data for Participants Examined
Table 1. 
 
Demographic Data for Participants Examined
Total Subjects (n = 59; Healthy Subjects, n = 23; Glaucoma Patients, n = 36) Mean Standard Deviation Range
Age, y
 Healthy subjects 47.8 15.0 23.0–73.0
 Glaucoma patients 54.2 14.8 24.0–75.0
 Total 51.7 15.1 23.0–75.0
P value* 0.117
Male sex, n
 Healthy subjects 9
 Glaucoma patients 16
 Total 27
P value* 0.054
Axial length, mm
 Healthy subjects 22.9 0.9 21.5–24.8
 Glaucoma patients 22.7 0.8 21.0–24.5
 Total 22.8 0.9 21.0–24.8
P value* 0.411
Central corneal thickness, μm
 Healthy subjects 544.7 33.3 468.0–607.0
 Glaucoma patients 557.1 41.9 470.0–661.0
 Total 552.2 39.0 468.0–661.0
P value* 0.234
Corneal curvature, mm
 Healthy subjects 7.8 0.1 7.7–7.9
 Glaucoma patients 7.9 0.2 7.7–8.5
 Total 7.8 0.3 7.7–8.5
P value* 0.317
Corneal astigmatism, D
 Healthy subjects 0.43 0.29 0–1.00
 Glaucoma patients 0.67 0.56 0–2.73
 Total 0.56 0.48 0–2.73
P value* 0.116
Corvis ST tonometer, mm Hg
 Healthy subjects 14.6 1.6 11.0–17.8
 Glaucoma patients 21.7 5.9 13.0–36.8
 Total 18.9 5.8 11.0–36.8
P value* <0.001
Goldmann applanation tonometer, mm Hg
 Healthy subjects 15.5 2.8 10.0–20.0
 Glaucoma patients 23.3 5.1 13.0–38.0
 Total 20.3 5.7 10.0–38.0
P value* <0.001
Noncontact tonometer, mm Hg
 Healthy subjects 15.8 4.0 9.0–24.3
 Glaucoma patients 24.9 5.8 13.5–39.0
 Total 21.3 6.8 9.0–39.0
P value* <0.001
Intraobserver Variability and Interobserver Variability of IOP Measurements
The intraobserver variability of IOP measurements obtained with the three devices is shown in Table 2. Of the three tonometers, CST demonstrated the best intraobserver variability and interobserver variability. 
Table 2. 
 
Repeatability and Reproducibility of IOP Measurement Techniques
Table 2. 
 
Repeatability and Reproducibility of IOP Measurement Techniques
Method (n = 59) Repeatability Reproducibility
CV ICC CV ICC
CST 6.7% 0.90 8.9% 0.78
GAT 9.0% 0.81 9.8% 0.70
NCT 9.8% 0.61 10.8% 0.63
Agreement between Devices
In Figure 3, Bland-Altman plots illustrate the agreement between devices. The analysis showed that, on average, IOP measured by CST was approximately 1.3 mm Hg lower than that with GAT and 2.4 mm Hg lower than that with NCT. CST showed better agreement with GAT than NCT did. 
Figure 3. 
 
Bland-Altman scatter plot showing the agreement among different IOP measurement devices. Top left: between CST and GAT. Mean difference, −1.3 mm Hg; 95% limits of agreement, +4.8 mm Hg. Top right: between CST and NCT. Mean difference, −2.4 mm Hg; 95% limits of agreement, +7.6 mm Hg. Bottom: between GAT and NCT. Mean difference, −1.1 mm Hg; 95% limits of agreement, +7.2 mm Hg.
Figure 3. 
 
Bland-Altman scatter plot showing the agreement among different IOP measurement devices. Top left: between CST and GAT. Mean difference, −1.3 mm Hg; 95% limits of agreement, +4.8 mm Hg. Top right: between CST and NCT. Mean difference, −2.4 mm Hg; 95% limits of agreement, +7.6 mm Hg. Bottom: between GAT and NCT. Mean difference, −1.1 mm Hg; 95% limits of agreement, +7.2 mm Hg.
Potential Effect of Patient Factors on Difference between CST and GAT
The difference in IOP value between CST and GAT did not correlate significantly to patient age (ρ = 0.117; P = 0.377), axial length (ρ = 0.028; P = 0.833), corneal curvature (ρ = 0.137; P = 0.3), corneal astigmatism (ρ = 0.039; P = 0.739), central corneal thickness (ρ = 0.035; P = 0.793), applanation time (ρ = −0.240; P = 0.067), applanation velocity (all P > 0.05), or highest concavity (all P > 0.05). It was significantly associated with applanation length only upon an air impulse (ρ = 0.283; P = 0.037). 
Discussion
An accurate assessment of IOP is of great importance for diagnosis and decision making regarding treatment modalities in patients with glaucoma. Delay in detection and treatment of elevated IOP in these eyes may cause visual impairment because of damage to the optic nerve and deterioration of the visual field. In this study, we compared the performance of a novel noncontact tonometer, the CST, with two traditional and widely used tonometers, GAT and NCT, both in healthy subjects and in glaucoma patients. The data presented show no statistically significant difference in the IOP values among all three devices. Of the three devices tested, the CST had the best intraobserver variability and interobserver variability. To the best of our knowledge, this is the first study that directly compares IOP measurements among these three tonometry devices. 
Both IOP readings of the CST and the NCT showed correlation with GAT readings, and the CST showed a better correlation than the NCT. When measurements were compared within subjects, although no statistically significant difference was detected between the CST and the GAT, the CST seemed to result in a lower IOP value than the NCT (NCT, 21.3 ± 6.8 mm Hg; CST, 18.9 ± 5.8 mm Hg; P < 0.05). The exact reason for this discrepancy is unknown. One possible explanation is that the two devices use different calculation methods to obtain the IOP. In addition, such post hoc P values for IOP comparisons should be interpreted with caution because the study was not powered to detect small differences in IOP and no adjustments for multiplicity were made. A number of previous studies have compared NCT versus GAT with respect to IOP measurement.10–12 Two studies found that IOP values measured with NCT were significantly higher than those measured with GAT. 7,8 In contrast, Tonnu et al. reported that the NCT significantly underestimated GAT measurements at lower IOP and overestimated those at higher IOP. 9 The authors speculated that this paradoxical phenomenon may be explained by the variance in CCT among subjects. It should be noted that CST could give a lower IOP than the other two instruments as shown in the Bland-Altman plots, which could lead to a delay in the detection and treatment of glaucoma. 
CST intraobserver variability in the present study was better than that for GAT and NCT. There are two possible explanations for this finding. First, the CST IOP measurement technique is more objective. Unlike GAT, which requires the operator to determine proper applanation and then read it on the knob scale, the CST notifies the operator when the tonometer is properly placed and reports the IOP automatically. Second, it is possible that some of the external factors that affect IOP measurements also vary between measurements. 1315 For example, if the biomechanical properties fluctuate between measurements, a tonometer that is minimally affected by these factors would show less variability between tests. Whether the CST is less affected by biomechanical properties than other devices or not needs further investigation. Interestingly, it seemed that the CV and ICC for intraobserver variability of the CST were also better than those for the ocular response analyzer (ORA) as reported in previous studies. 10,11 It has been found that the intraobserver variability of ORA IOP measurements was affected by the magnitude of the ocular pulse amplitude and the waveform quality. 12  
This study also examined the interobserver variability of the devices. Of the three devices, the CST had the lowest CV and the highest ICC; those for the GAT and NCT were similar to values in previous studies. 8,11,18 Differences may result from investigator and participant group variabilities. 
There was notable bias among the three tonometers, with the CST reading on average 1.3 mm Hg lower than the GAT reading and 2.4 mm Hg lower than the NCT reading. An analysis of potential factors explaining the difference in measurements between CST and GAT was performed. Interestingly, we found that this difference did not correlate to patient age and axial length, which is similar to findings from previous studies. 13,14 In addition, CCT did not explain any of the IOP measurement differences in our study. In contrast, Medeiros et al. showed that the difference in IOP between GAT and NCT measurements was significantly influenced by corneal thickness. 13  
Noncontact devices measure IOP within 5 ms, equating to approximately 1/500 of the cardiac cycle. As a result, applanation time becomes a significant factor affecting the difference in IOP readings. 16 With the CST, the separation between the two applanations is approximately 15 ms. It is plausible, therefore, that the difference in IOP measurements between the CST and GAT may be greater in patients with a longer applanation time. In fact, our study found that the association between applanation time and IOP difference was statistically marginal (P = 0.067). It has also been reported that eyes with large ocular pulse aptitudes were associated with high IOP measurement variability. 14,17 Whether the applanation time affects the intraobserver variability and interobserver variability of the CST requires further investigation. 
It is widely accepted that IOP measurement is affected by the corneal biomechanical properties. 18,19 The applanation time and corneal concavity, that is, corneal deformation amplitude, were not associated with IOP measurement variability. However, the variability tends to increase in eyes with longer applanation length upon an air impulse, meaning that taking the average of multiple repeated measurements is important for reliable measurement in such eyes. Our findings reinforce the importance of corneal biomechanical properties in obtaining accurate transcorneal estimates of IOP. Sullivan-Mee et al. found that the most consistent confounders of IOP measurement agreement were the ORA-measured corneal parameters, corneal hysteresis, and corneal resistance factor. 20 Our data suggest that the CST is not completely unaffected by corneal biomechanical effects. 
The present study had several limitations. First, the relatively small sample size made it difficult to carry out further analyses with sufficient statistical power and completely avoid a possible selection bias. Second, we did not take into account other factors that may influence IOP readings, such as corneal curvature. However, the main aim of the study was to evaluate the usefulness of IOP measurement by the CST in routine clinical practice rather than to investigate the influence of various ocular structural properties on different IOP measurements. In addition, the myopic refractive error of all examined participants was less than −2.0 D. The corneal curvature may not have influenced our conclusions. Third, when IOP is measured repeatedly, it may fluctuate with each measurement. The factors that contribute to fluctuations in IOP are diurnal variations, body posture, exercise, eye movement, Valsalva maneuver, and the consumption of various foods and/or drugs. 21 However, because all IOP measurements were completed within 1 or 2 hours, this potential effect may be very limited. Finally, due to financial limitations, agreement between the CST and other recently developed tonometers, such as the rebound tonometer, dynamic contour tonometer, and ocular response analyzer, was not evaluated in this study. It would be interesting to identify correlations among IOP measurements using these devices. 
In conclusion, the results of this study suggest that the CST shows excellent consistency in IOP measurement and might be less affected by corneal properties. However, our data do not support that the CST provides an accurate IOP measurement; it may tend to yield lower IOP measurements. In addition, IOPs measured by different devices may be not interchangeable. 
Supplementary Materials
References
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Rosentreter A Athanasopoulos A Schild AM Rebound, applanation, and dynamic contour tonometry in pathologic corneas [published online ahead of print January 10, 2012]. Cornea . doi:10.1097/ICO.0b013e31823f0977 .
Oculus Optikgeräte GmbH. Corvis ST pocket book. Available at: http://www.oculus.de/en/sites/detail_ger.php?page=597 . Accessed June 28, 2012.
Bland JM Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet . 1986; 1: 307–310. [CrossRef] [PubMed]
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Schiano Lomoriello D Lombardo M Tranchina L Repeatability of intra-ocular pressure and central corneal thickness measurements provided by a non-contact method of tonometry and pachymetry. Graefes Arch Clin Exp Ophthalmol . 2011; 249: 429–434. [CrossRef] [PubMed]
Tonnu PA Ho T Sharma K A comparison of four methods of tonometry: method agreement and interobserver variability. Br J Ophthalmol . 2005; 89: 847–850. [CrossRef] [PubMed]
Moreno-Montañés J Maldonado MJ García N Reproducibility and clinical relevance of the ocular response analyzer in nonoperated eyes: corneal biomechanical and tonometric implications. Invest Ophthalmol Vis Sci . 2008; 49: 968–974. [CrossRef] [PubMed]
Wang AS Alencar LM Weinreb RN Repeatability and reproducibility of Goldmann applanation, dynamic contour, and ocular response analyzer tonometry [published online ahead of print June 22, 2011]. J Glaucoma . doi:10.1097/IJG.0b013e318 2254ba3 .
Kotecha A White E Schlottmann PG Garway-Heath DF. Intraocular pressure measurement precision with the Goldmann applanation, dynamic contour, and ocular response analyzer tonometers. Ophthalmology . 2010; 117: 730–737. [CrossRef] [PubMed]
Medeiros FA Weinreb RN. Evaluation of the influence of corneal biomechanical properties on intraocular pressure measurements using the ocular response analyzer. J Glaucoma . 2006; 15: 364–370. [CrossRef] [PubMed]
Xu G Lam DS Leung CK. Influence of ocular pulse amplitude on ocular response analyzer measurements. J Glaucoma . 2011; 20: 344–349. [CrossRef] [PubMed]
Fabian ID Barequet IS Skaat A Intraocular pressure measurements and biomechanical properties of the cornea in eyes after penetrating keratoplasty. Am J Ophthalmol . 2011; 151: 774–781. [CrossRef] [PubMed]
Vernon SA. Intra-eye pressure range and pulse profiles in normals with the Pulsair non-contact tonometer. Eye (Lond) . 1993; 7: 134–137. [CrossRef] [PubMed]
Kotecha A Elsheikh A Roberts CR Corneal thickness- and age-related biomechanical properties of the cornea measured with the ocular response analyzer. Invest Ophthalmol Vis Sci . 2006; 47: 5337–5347. [CrossRef] [PubMed]
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Footnotes
 Supported by grants from the Key Clinic Medicine Research Program, Ministry of Health, China (2010–2012); Doctoral Fund, Ministry of Education, China; National Science and Technology Research Program, Ministry of Science and Technology, China (2012BAI08B01); National Natural Science Foundation of China (81170817, 81200658); and the Scientific Research Program, Science and Technology Commission of Shanghai Municipality, Shanghai (11231200602, 11DZ2260900). The sponsor or funding organization had no role in the design or conduct of this research. The authors declare no financial relationship with the organization that sponsored the research exists. The authors alone are responsible for the content and writing of the paper.
Footnotes
 Disclosure: J. Hong, None; J. Xu, None; A. Wei, None; S.X. Deng, None; X. Cui, None; X. Yu, None; X. Sun, None
Figure 1. 
 
Intraocular pressure measurement by CST. Top: Real-time information for a participant recorded immediately upon an air impulse. Bottom: Real-time information for a participant recorded at the highest concavity, indicating the largest deformation amplitude of the cornea. The CST system is designed to gather 4330 frames per second within a 100 ms period and therefore to record dynamic IOP. Its measurement range 1 to 60 mm Hg. A high-speed Scheimpflug camera was equipped to record the movements of the cornea, which then were displayed on the built-in control panel in ultraslow motion as shown in Supplementary Videos (see Supplementary Material and Supplementary Videos). This camera could cover up to 8.5 mm of the cornea and provide excellent image resolution (640 × 480 pixels). The output includes the IOP value, central corneal thickness, and corneal biomechanical properties (applanation time, applanation length, applanation velocity, and details of highest concavity).
Figure 1. 
 
Intraocular pressure measurement by CST. Top: Real-time information for a participant recorded immediately upon an air impulse. Bottom: Real-time information for a participant recorded at the highest concavity, indicating the largest deformation amplitude of the cornea. The CST system is designed to gather 4330 frames per second within a 100 ms period and therefore to record dynamic IOP. Its measurement range 1 to 60 mm Hg. A high-speed Scheimpflug camera was equipped to record the movements of the cornea, which then were displayed on the built-in control panel in ultraslow motion as shown in Supplementary Videos (see Supplementary Material and Supplementary Videos). This camera could cover up to 8.5 mm of the cornea and provide excellent image resolution (640 × 480 pixels). The output includes the IOP value, central corneal thickness, and corneal biomechanical properties (applanation time, applanation length, applanation velocity, and details of highest concavity).
Figure 2. 
 
Correlations among different IOP measurement devices. Significant positive correlations were noted between CST and NCT (ρ = 0.871; P < 0.001) (top left), between CST and GAT (ρ = 0.896; P < 0.001) (top right), and between GAT and NCT (ρ = 0.839; P < 0.001) (bottom).
Figure 2. 
 
Correlations among different IOP measurement devices. Significant positive correlations were noted between CST and NCT (ρ = 0.871; P < 0.001) (top left), between CST and GAT (ρ = 0.896; P < 0.001) (top right), and between GAT and NCT (ρ = 0.839; P < 0.001) (bottom).
Figure 3. 
 
Bland-Altman scatter plot showing the agreement among different IOP measurement devices. Top left: between CST and GAT. Mean difference, −1.3 mm Hg; 95% limits of agreement, +4.8 mm Hg. Top right: between CST and NCT. Mean difference, −2.4 mm Hg; 95% limits of agreement, +7.6 mm Hg. Bottom: between GAT and NCT. Mean difference, −1.1 mm Hg; 95% limits of agreement, +7.2 mm Hg.
Figure 3. 
 
Bland-Altman scatter plot showing the agreement among different IOP measurement devices. Top left: between CST and GAT. Mean difference, −1.3 mm Hg; 95% limits of agreement, +4.8 mm Hg. Top right: between CST and NCT. Mean difference, −2.4 mm Hg; 95% limits of agreement, +7.6 mm Hg. Bottom: between GAT and NCT. Mean difference, −1.1 mm Hg; 95% limits of agreement, +7.2 mm Hg.
Table 1. 
 
Demographic Data for Participants Examined
Table 1. 
 
Demographic Data for Participants Examined
Total Subjects (n = 59; Healthy Subjects, n = 23; Glaucoma Patients, n = 36) Mean Standard Deviation Range
Age, y
 Healthy subjects 47.8 15.0 23.0–73.0
 Glaucoma patients 54.2 14.8 24.0–75.0
 Total 51.7 15.1 23.0–75.0
P value* 0.117
Male sex, n
 Healthy subjects 9
 Glaucoma patients 16
 Total 27
P value* 0.054
Axial length, mm
 Healthy subjects 22.9 0.9 21.5–24.8
 Glaucoma patients 22.7 0.8 21.0–24.5
 Total 22.8 0.9 21.0–24.8
P value* 0.411
Central corneal thickness, μm
 Healthy subjects 544.7 33.3 468.0–607.0
 Glaucoma patients 557.1 41.9 470.0–661.0
 Total 552.2 39.0 468.0–661.0
P value* 0.234
Corneal curvature, mm
 Healthy subjects 7.8 0.1 7.7–7.9
 Glaucoma patients 7.9 0.2 7.7–8.5
 Total 7.8 0.3 7.7–8.5
P value* 0.317
Corneal astigmatism, D
 Healthy subjects 0.43 0.29 0–1.00
 Glaucoma patients 0.67 0.56 0–2.73
 Total 0.56 0.48 0–2.73
P value* 0.116
Corvis ST tonometer, mm Hg
 Healthy subjects 14.6 1.6 11.0–17.8
 Glaucoma patients 21.7 5.9 13.0–36.8
 Total 18.9 5.8 11.0–36.8
P value* <0.001
Goldmann applanation tonometer, mm Hg
 Healthy subjects 15.5 2.8 10.0–20.0
 Glaucoma patients 23.3 5.1 13.0–38.0
 Total 20.3 5.7 10.0–38.0
P value* <0.001
Noncontact tonometer, mm Hg
 Healthy subjects 15.8 4.0 9.0–24.3
 Glaucoma patients 24.9 5.8 13.5–39.0
 Total 21.3 6.8 9.0–39.0
P value* <0.001
Table 2. 
 
Repeatability and Reproducibility of IOP Measurement Techniques
Table 2. 
 
Repeatability and Reproducibility of IOP Measurement Techniques
Method (n = 59) Repeatability Reproducibility
CV ICC CV ICC
CST 6.7% 0.90 8.9% 0.78
GAT 9.0% 0.81 9.8% 0.70
NCT 9.8% 0.61 10.8% 0.63
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