|Ahead of print publication
Correlation of corneal hysteresis and central corneal thickness with intraocular pressure measured by ocular response analyzer and goldmann applanation tonometer
Amit Sethi1, Vani Puri2, Shrikant Waikar1
1 Department of Ophthalmology, INHS Asvini, RC Church, Mumbai, Maharashtra, India
2 Bombay City Eye Institute and Research Centre, Mumbai, Maharashtra, India
|Date of Submission||16-Jul-2020|
|Date of Decision||09-Aug-2020|
|Date of Acceptance||18-Sep-2020|
|Date of Web Publication||07-Apr-2021|
Bombay City Eye Institute and Research Centre, Babulnath Road, Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
Background: Ocular response analyzer (ORA) is a unique tonometer which can measure intraocular pressure (IOP) independent of corneal properties. In this study, we compared the corneal compensated IOP (IOPcc) as measured by ORA with IOP measured by Goldmann applanation tonometry (GAT) and the influence of corneal hysteresis (CH) and central corneal thickness (CCT) on IOP measurement. Setting and Design: This was an observational cross-sectional study. Methods: This study was conducted on 176 eyes of 88 patients aged 23–77 years. A full ophthalmic examination including GAT, IOPcc, CH, and CCT was obtained. Exclusion criteria included previous intraocular surgery, refractive surgery, glaucoma, or any other intraocular disease. Statistical Analysis: Windows SPSS software was used, and the level of significance was taken as P < 0.05. Results: The mean (± standard deviation [SD]) IOP with GAT was 16.9 mmHg (±4.04) (range: 5.8–30 mmHg). The mean (±SD) IOPcc was 18.0 mmHg (±4.49) (range: 10–41.6 mmHg), the mean (±SD) CH was 10.1 (±1.17) (range: 6.1–16.9), and the mean CCT (±SD) was 528.11 μm (38.75) (range: 451–644 μm). The mean difference between IOP GAT and IOPcc (±SD) was 1.1 mmHg (±5.87), which was statistically significant. However, there was no correlation between IOP GAT and IOPcc. None of the methods correlated with CH and CCT. Conclusion: IOP readings with ORA were higher than GAT, with no significant correlation between them. To date, it is not clear regarding the impact and association of CH on IOP measurement.
Keywords: Intraocular pressure, glaucoma, Goldmann applanation tonometer, ocular response analyzer, corneal hysteresis, central corneal thickness
|How to cite this URL:|
Sethi A, Puri V, Waikar S. Correlation of corneal hysteresis and central corneal thickness with intraocular pressure measured by ocular response analyzer and goldmann applanation tonometer. J Mar Med Soc [Epub ahead of print] [cited 2021 Apr 23]. Available from: https://www.marinemedicalsociety.in/preprintarticle.asp?id=313006
| Introduction|| |
Intraocular pressure (IOP) measurement is one of the prime criteria to diagnose and monitor glaucoma. Reduction in IOP is the only evidence-based therapy for glaucoma. Hence, precise measurement of IOP is important to evaluate response to treatment and monitor the risk of progression. Goldmann applanation tonometry (GAT) has been considered as the gold standard to measure IOP since the mid-1950s. However, it is well known to be influenced by factors related to corneal properties such as corneal curvature and central corneal thickness (CCT)., It is recommended to record CCT along with GAT for glaucoma workup to determine target IOP.,,,, In recent years, several new instruments have been developed for precise measurement of IOP. Ocular response analyzer (ORA, Reichert Ophthalmic Instruments, Buffalo, NY, USA) is among one of these. With ORA, a new variable of the cornea can be taken, namely “corneal hysteresis” (CH). It is measured by corneal indentation caused by a 25-millisecond air pulse and analyzed by an electro-optical system. The air pulse pushes the cornea inward, to pass a definite point of applanation (P1), resulting in a slight concavity. When the air pressure decreases, the cornea returns to its normal shape passing a second time the definite point of applanation (P2). The difference between the two pressure readings at the point of applanation during the inward and outward movement is mentioned as CH. Using the values of P1 and P2, ORA calculates IOPg and IOPcc. IOPg (Goldmann correlated) is the mean of P1 and P2. IOPcc (corneal compensated) is a measure of IOP that is free of corneal influence. It is derived from IOP and corneal biomechanical data.
The study aimed to assess the utility of ORA in a clinical setting by correlating IOPcc with GAT and the effect of CH and CCT with respect to IOP measurement.
| Methods|| |
This was a cross-sectional observational study. It was approved by the ethics committee of the hospital and was performed in accordance with the tenets of the Declaration of Helsinki. This study included 176 eyes of 88 patients who attended the ophthalmology outpatient department over a period of 10 months from March 2018 to January 2019. Patients were enlisted in the study in a sequential manner. Both the eyes of the patient were included. Subjects were excluded if they had a history of intraocular surgery, refractive surgery, glaucoma, or any other intraocular disease. A full ophthalmic examination was carried out including slit-lamp biomicroscopy and fundus examination with a 90D lens. All subjects had CCT, corneal curvature, and axial length measurements done before the IOP measurement in the same visit. CCT measurements were obtained using a computerized tonometer with pachymeter (Topcon CT-1P, Topcon Medical Systems, Oakland, NJ, USA). ORA testing was done by a trained technician. Three readings were obtained for each eye, and the average of the three readings per eye was analyzed. Subsequently, intraocular pressure readings were obtained with Goldmann applanation tonometer. Two readings were obtained for each eye, and the average of the two readings per eye was considered for analysis. If the two measurements differed by more than 3 mmHg, a third measurement was taken, and the average of the two closest measurements was considered as the final value for analysis. The results were arranged, and the differences and correlations between IOP measured by GAT and IOPcc were made. Correlations of each of GAT and IOPcc were also made with CH and CCT. Windows SPSS software was used to make the statistical analysis. The level of significance was taken as P < 0.05.
| Results|| |
The study was done on 176 eyes of 88 patients, 65 males and 23 females. Their age ranged between 23 and 77 years, average (± standard deviation [SD]): 50.84 (±14.15) years [Table 1]. The mean CCT (±SD) was 528.11 μm (±38.75) (range: 451–644 μm) and mean CH (±SD) was 10.1 (±1.17) (range: 6.1–16.9) [Table 2]. The mean value of IOPcc (±SD) was 18.0 mmHg (±4.49) (range: 10–41.6 mmHg) and the mean value of GAT (±SD) was 16.9 mmHg (±4.04) (range: 5.8–30 mmHg). In most cases, the IOP value as recorded by GAT was less than IOPcc [Figure 1]. The mean difference between IOP GAT and IOPcc (±SD) was 1.1 mmHg (±5.87) [Figure 2]. The paired t-test calculated a significant difference (P < 0.05) between IOP GAT and IOPcc in a 95% confidence interval. Using Pearson correlation coefficient (P < 0.05), no correlation was found between IOP GAT and IOPcc. Similarly, there was no correlation between CH and each of GAT and IOPcc [Figure 3] and between CCT and each of GAT and IOP cc [Figure 4].
|Table 2: Statistical data (intraocular pressure corneal compensated, corneal hysteresis, and central corneal thickness)|
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|Figure 1: IOPcc and GAT values; median values with 95% CI in mmHg (n = 176). IOPcc: Intraocular pressure corneal compensated, GAT: Goldmann applanation tonometer|
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|Figure 2: Bland–Altman analysis of IOPcc and GAT (P < 0.05) (n = 176). IOPcc: Intraocular pressure corneal compensated, GAT: Goldmann applanation tonometer|
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|Figure 3: GAT and IOPcc in relation to CH (n = 176). IOPcc: Intraocular pressure corneal compensated, GAT: Goldmann applanation tonometer|
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|Figure 4: GAT and IOPcc in relation to CCT (n = 176). IOPcc: Intraocular pressure corneal compensated, GAT: Goldmann applanation tonometer|
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| Discussion|| |
The aim of this study was to find the effect of CH and CCT on IOP measurement. In spite of Goldmann applanation tonometer being considered the gold standard, it does have some shortcomings. Goldmann admitted that IOP was measured for a CCT of 520 μm and the IOP measurement for corneas thicker or thinner than 520 μm would be less accurate. Several studies were done that brought out various formulas and nomograms to compensate for the effect of CCT on GAT.,,,,, Several tonometers have been introduced to avoid these limitations. ORA is one of these. It measures the viscoelastic properties of the cornea, namely CH. It is a noncontact method and requires less expertise and is easier to operate than GAT.
There are various studies which have brought out the mean values of CH in a normal population: 9.6 mmHg in 339 normal eyes with a mean age of 28 years, 10.6 ± 2.29 mmHg in 156 normal eyes, and in diurnal variation values between 12.2 and 12.7 mmHg in normal eyes (mean age 39.8 years, n = 42). The average hysteresis of glaucoma patients was reported to be 8.8 ± 2.1 mmHg in 48 eyes. In our study of normal eyes, the mean hysteresis is 10.0 ± 1.17 mmHg, with an average age of 51 years.
In our study, IOPcc values are higher than the values of GAT with a mean difference of 1.1 mmHg. Martinez-de-la-Casa et al. found a much higher difference between IOPcc and GAT, with a mean difference of 8.3 ± 4.0 mmHg in glaucoma patients. Medeiros and Weinreb examined 153 subjects and measured IOPcc and GAT and found that the mean difference between them was not significantly different but was varying with CCT changes. Hager et al. showed a mean difference of 1.6 mmHg between IOPcc and GAT in a normal population. Kynigopoulas et al. and Ehongo et al. found that IOPcc values were higher than IOP GAT. In a study by Lam et al. in a Chinese population, they found out a significant correlation but no significant difference between IOP GAT and IOPcc.
In the study by Nader Hussein et al., a comparative study of ORA and GAT, there was no correlation between CCT and GAT, similar to our study. This disagrees with the previous studies,, which show a significant correlation of GAT with CCT. This contradiction could be related to the fact that CCT values in our study were clustered around the mean value for which GAT is calibrated (528 μm) with minimal SD (38.75). This led to an apparent independence of GAT from CCT.
Our study reveals a significant difference between GAT and IOPcc but no correlation between the two readings. Furthermore, there is no correlation between each of GAT and IOPcc with CCT and CH.
The presence of no correlation between each of GAT and IOPcc with CH brings out a possibility that IOP measurement with each of ORA and GAT is independent of corneal biomechanical properties. However, since these measurements were taken in a normal population, the findings could have been different in glaucomatous patients as the corneal and scleral properties could be different in glaucoma patients.
No correlation between CCT and IOPcc can again have the same explanation. However, explanation for no correlation between GAT and CCT has been explained earlier in view of CCT values in our study very close to the mean value for which GAT is calibrated.
IOP measurements with ORA are significantly higher than GAT which is not completely understood. It has been brought out in various studies that noncontact tonometer gives higher readings than GAT. The readings are higher in eyes with thicker corneas., A probable explanation of high IOPcc readings could be due to ORA based on noncontact method in contrast to applanation method.
| Conclusion|| |
It is to be understood that CCT is not the only sole criterion affecting IOP. Other corneal factors such as viscoelastic properties, rigidity, and hydration also need to be considered. A unique feature of ORA is that it measures CH and IOPcc. IOPcc measurements take CH into account. To date, it is not clear regarding the association and impact of CH on IOP measurements. Hence, it is essential to understand other corneal properties over and above CCT for accurate IOP measurement.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]