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Year : 2022  |  Volume : 15  |  Issue : 3  |  Page : 315-320  

Comparison of IOP obtained in different kind of eyes with contact and no contact tonometers

Department of Medical, Surgical and Dental Specialites, Campania University “Luigi Vanvitelli”, Napoli, Italy

Date of Submission10-May-2021
Date of Decision07-Jan-2022
Date of Acceptance30-Apr-2022
Date of Web Publication02-Nov-2022

Correspondence Address:
Michele Lanza
Department of Medical, Surgical and Dental Specialites, Università della Campania Luigi Vanvitelli, Via De Crecchio 16, Napoli
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ojo.ojo_147_21

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BACKGROUND: The aim of this study is to analyze and compare the intraocular pressure (IOP) values measured in three different kinds of participants such as healthy subjects (HS), keratoconus patients (KP), and those who underwent myopic photorefractive keratectomy (MPRK). The devices used in this study are the Goldmann Applanation Tonometry (GAT), the dynamic contour tonometry (DCT), the ocular response analyzer (ORA), and the Corvis ST (CST).
SUBJECTS AND METHODS: This research included 92 eyes of 92 h, 63 eyes of 63 KP, and 58 eyes of 58 MPRKM. Each participant underwent a complete ophthalmic evaluation and IOP measurement with GAT, DCT, ORA, and CST. A statistical analysis was conducted to detect possible differences and correlations.
RESULTS: First, according to the observed data, HS eyes displayed mean IOP values measured with GAT, DCT, ORA, and CST, respectively, 15.82 ± 2.74 mmHg, 17.63 ± 2.28 mmHg, 16.24 ± 3.14 mmHg, and 17.31 ± 3.21 mmHg. Then, KP eyes showed mean IOP values measured with GAT, DCT, ORA, and CST of, respectively, 14.89 ± 1.64 mmHg, 16.97 ± 2.08 mmHg, 13.09 ± 3.12 mmHg, and 13.78 ± 2.11 mmHg. Finally, MPRK eyes showed mean IOP values measured with GAT, DCT, ORA, and CST of, respectively, 13.92 ± 1.34 mmHg, 15.39 ± 2.86 mmHg, 16.63 ± 2.51 mmHg, and 15.06 ± 1.56 mmHg.
CONCLUSION: According to the observed data, ORA and GAT might be used interchangeably in HS, whereas GAT, ORA, and CST in KP eyes. Moreover, it has been noticed that in those eyes that previously undergone a myopic PRK, GAT provided lower values of IOP in comparison with other devices.

Keywords: Corvis ST, dynamic contour tonometry, ocular response analyzer, refractive surgery, tonometry

How to cite this article:
Sbordone S, Ragucci A, Iaccarino G, Scognamiglio G, Leone A, Gironi Carnevale UA, Lanza M. Comparison of IOP obtained in different kind of eyes with contact and no contact tonometers. Oman J Ophthalmol 2022;15:315-20

How to cite this URL:
Sbordone S, Ragucci A, Iaccarino G, Scognamiglio G, Leone A, Gironi Carnevale UA, Lanza M. Comparison of IOP obtained in different kind of eyes with contact and no contact tonometers. Oman J Ophthalmol [serial online] 2022 [cited 2023 Jan 27];15:315-20. Available from: https://www.ojoonline.org/text.asp?2022/15/3/315/360395

   Introduction Top

Intraocular pressure (IOP) is an extremely important and useful marker to diagnose and manage glaucoma disease.[1],[2] It is well known that the IOP measurement might be biased both by corneal morphological properties, such as the central corneal thickness (CCT) or the corneal curvature (CC),[3],[4] and by corneal biomechanical ones, such as hysteresis, viscosity, elasticity, hydration, and the connective tissue composition.[5]

The Goldmann applanation tonometer (GAT) is the current gold standard tool used for IOP evaluation.[6]

Past studies showed the effectiveness of applying a correcting factor to GAT values based on morphological corneal properties (CCT and CC),[3],[4],[6],[7] but there are no available formulas allowing to compensate the influence of biomechanical corneal properties on IOP measurements with GAT.[6],[7] This is one of the reasons why during the last years, it has been possible to observe the introduction of new devices suitable for evaluating the IOP with less or no influence on morphological or biomechanical eye's properties.

The aim of the present study is to analyze the IOP evaluation in healthy subjects (HS), in patients affected by keratoconus (KP), and in those who previously undergone myopic photorefractive KP (MPRK) through the use of four devices: Goldmann applanation tonometry (GAT), dynamic contour tonometry (DCT), ocular response analyzer (ORA), and Corvis ST (CST).

These eyes categories were selected aiming to study the eventual differences in IOP answers using different tonometers evaluating eyes with different corneal structures.

This is one of the first studies that aims to compare simultaneously IOP values provided by GAT, DCT, ORA, and CST in HS, KP, and MPRK.

   Subjects and Methods Top

In this retrospective study, a total of 213 eyes of 213 subjects have been analyzed as follows: 92 eyes of 92 HS (48 females); 63 eyes of 63 patients (37 females) affected by KC (Stage 1, 2, and 3, according to Amsler classification); and 58 eyes of 58 subjects (26 females) that underwent myopic PRK with a refractive error measured as spherical equivalent, ranging from −8.25 to −1.37 D (mean: −4.72 ± 2.45 D). Demographic and ocular data of the participants are summarized in [Table 1].
Table 1: Mean, standard deviation, minimum and maximum age, spherical equivalent, corneal curvature, and corneal pachymetry at pupil center in healthy subjects, subjects that underwent myopic photorefractive keratectomy, and subjects affected by keratoconus

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Each subject affected by a systemic and/or ocular disease that might influence the IOP measurement and the corneal evaluation (for example, diabetes, connective tissue disorders, dry eye, corneal dystrophies, previous trauma, etc.) was excluded from the study. Subjects wearing contact lenses were asked to discontinue their use for at least 14 days before the visit. Moreover, also subjects with glaucoma, glaucoma suspect, or ocular hypertension were excluded by this study because it was focusing on influence of corneal structure on IOP measurements using different tonometers.

During the process of examination, each patient has been analyzed according to the following fixed order: a complete eye visit, an Oculus Pentacam HR scan (Oculus, Wetzlar, Germany), and three consecutive IOP measurements for each instrument (ORA, CST, DCT, and GAT). The final mean of the resulted values was used for the statistical analysis.

Because the applanation determined by GAT might involve some errors both in the next IOP evaluations and in the biomechanical corneal analysis, the following fixed sequence of acting has been adopted to reduce the influence that some devices might have on IOP assessments.

During the study, to avoid operator related bias, a different physician was assigned to each type of tonometer, unaware of the other results. On the other side, a fifth one was conceived to collect and analyze all the data. A 10 min pause was observed between each IOP estimation aiming to reduce bias in IOP measurement precision because some of the devices used to cause a corneal distortion that can modify the precision of the following measurement. Ophthalmic assessments were performed between 2:00 and 4:00 pm, aiming to reduce the variability of IOP values related to circadian rhythm of aqueous humor production and to obtain more homogeneous measurements; after having measured the IOP; both the slit-lamp evaluation and the Pentacam scan were repeated for each eye at the end of visits to check their condition.

The DCT (Swiss Microtechnology AG, Port, Switzerland) is a tonometer capable of measuring the IOP according to the Blaise Pascal principle, and it is characterized by a pressure sensor embedded centrally and concavely into the tonometer tip.[8],[9]

The ORA is a noncontact tonometer that measures the biomechanical response of the eye using an air jet directed to the cornea to obtain a corneal compensated IOP measure.[10]

The CST (Oculus, Wetzlar, Germany) is a noncontact tonometer that, thanks to an air impulse, examines the dynamic reaction of the cornea and quantifies the corneal deformation's properties to calculate the IOP.[11],[12],[13]

The Oculus Pentacam (Oculus, Wetzlar, Germany) is a corneal tomopgraph that uses a rotating Scheimpflug camera and a monochromatic slit light source (blue led at 475 nm). The simultaneous rotation of the two tools around the optical axes of the eye enables to calculate a three-dimensional model of the anterior segment and to receive data regarding the anterior and posterior corneal topography and pachymetry measurements of the anterior chamber depth, lens opacity, and lens thickness.[14]

In this study, the option to use 25 images per scan was chosen. The parameters provided by Pentacam and evaluated in this research were CCT at the pupil center and at the anterior CC measured with Sim-K (CC).

Statistical analysis

The normal data distribution has been verified with the Kolmogorov–Smirnov test. For those data that did not meet the normality assumption, specific nonparametric tests have been performed to evaluate both differences and correlations.

In particular, the comparison among measurements provided by different devices was evaluated using parametric Student's t-test or nonparametric Wilcoxon test for paired data. Moreover, the correlation between IOP measurements and the corneal anatomical-structural parameters was evaluated using parametric (Pearson) and nonparametric tests.

Before the visit, each patient was asked to sign an informed consent to use their data for scientific purposes.

The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of Università della Campania Luigi Vanvitelli (approval code 0014459/i) as a retrospective study.

   Results Top

In HS [Figure 1]a, GAT values showed a good alignment with ORA ones, while DCT and CST displayed higher IOP values.
Figure 1: (a) Comparison among IOP values provided by the different devices in healthy subjects;(b) Comparison among IOP values provided by the different devices in subjects that underwent MPRK; (c) Comparison among IOP values provided by the different devices in eyes affected by KP. IOP: Intraocular pressure values, MPRK: Myopic photorefractive keratectomy, KP: Keratoconus patients

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In particular, the differences between DCT versus GAT (mean = +1.82 mmHg; P < 0.0001) and CST versus GAT (mean = +1.49 mmHg; P < 0.003) were remarkable and statistically significant. Furthermore, the differences between ORA versus DCT (mean= −1.39 mmHg; P < 0.0001) and ORA versus CST (mean= −1.06 mmHg; P < 0.005) were considerable, whereas the difference between DCT versus CST (mean = +0.32 mmHg; P = 0.072) was not.

In the MPRK group [Figure 1]b, IOP values provided by ORA were significantly higher than the ones obtained from GAT (mean = +2.71 mmHg; P < 0.001) and CST (mean= +1.57 mmHg; P < 0.001).

In the third group of patients affected by keratoconus [Figure 1]c, DCT values were higher than GAT (mean= +2.08 mmHg; P < 0.001), ORA (mean = +3.61; P < 0.001), and CST ones (mean = +2.78; P < 0.00.1). Meanwhile, no significant difference has been observed between ORA and CST measurements (mean= −0.74 mmHg; P = 0.081).

The IOP values displayed some correlations with corneal anatomical-structural parameters [Table 2].
Table 2: Correlation between intraocular pressure values measured by ocular response analyzer, Goldmann applanation tonometry, dynamic contour tonometry, and Corvis ST and corneal anatomical-structural parameters, corneal curvature, and central corneal thickness, in healthy subjects, subjects that underwent myopic photorefractive keratectomy, and in eyes affected by keratoconus

Click here to view

Statistically significant correlations observed are: in HS, ORA and CST measures were positively correlated only with the corneal pachymetry at pupil center (CCT); in MPRK eyes, IOP values provided by DCT were inversely correlated with CC, whereas CST values were positively correlated with both CC and CCT, while in KP eyes, on the one hand, ORA and CST measures showed an inverse correlation with CC, on the other hand, CST was also positively correlated with CCT.

Furthermore, in HS [Table 3], some inter-device IOP differences were correlated with CCT.
Table 3: Correlation among differences in intraocular pressure obtained from the tested devices versus corneal curvature and central corneal thickness, in healthy subjects, subjects that underwent myopic photorefractive keratectomy, and in eyes affected by keratoconus

Click here to view

Conversely, in the MPRK eyes correlations have been found between IOP differences and CC or CCT, whereas in patients affected by keratoconus, only correlations with CC were found.

   Discussion Top

In several studies, there is evidence that an accurate IOP measurement is crucial to diagnose and manage the glaucoma disease.[15],[16],[17],[18] Higher values of IOP are associated with the progression of glaucoma, whereas their reduction can provide a stabilization of the illness.[15],[18]

The well-known limits of the GAT tonometry to measure IOP in eyes characterized by anomalies in corneal thickness and curvature suggest developing new devices characterized by different principles of working to provide more accurate IOP values.[11],[12],[13],[19],[20],[21],[22],[23],[24]

These innovations have become necessary since it was even harder to obtain reliable IOP measurements with GAT in eyes with corneal alterations caused by diseases (e.g., KC) or by previous corneal surgeries (e.g., myopic PRK).

To offer a proper and most complete evaluation possible, distinct tonometers have been used in this study to evaluate different eyes.

The analysis of the results observed in this study suggested that in HS, GAT and DCT provided IOP values less affected by CCT compared to the ones provided by ORA and CST.

In MPRK patients, GAT and ORA provided IOP values not influenced by the corneal parameters evaluated, whereas the measurements of DCT and CST influenced by CC and CST evaluations were affected by CCT.

In the KP group, GAT and DCT values did not show significant correlations neither with CC nor with CCT, whereas ORA and CST values pointed out a significant inverse correlation with CC; at the same time, CST values showed a significant direct correlation with CCT.

The results obtained in this study provide different results compared to the ones appearing in previously published papers.

First, Martinez-de-la-Casa et al.[19] highlighted an overestimation of IOP provided by ORA compared with GAT. Probably, this is due to the fact that the authors performed IOP evaluations in a random order, whereas in the present study, less invasive procedures have always been performed before the more invasive ones. Hence, the differences pointed out in the two studies might be caused by changes implemented in the procedural protocols.

Then, Kotecha et al.[20] obtained higher IOP values using DCT and ORA rather than GAT: what might explain the dissimilarity of the results with the present study is the involvement of a smaller group of participants and the random order of the examinations conducted by the above mentioned authors.

Hong et al.[12] found no statistically significant difference between GAT and CST; however, their population included both HS and glaucoma patients.

Finally, Reznicek et al.[24] made a comparison of IOP measurements performed with GAT and CST between patients affected by glaucoma and 36 HS. Their results are similar to the ones obtained in the present study, even though less patients were analyzed and a different statistical analysis was performed. Moreover, in the abovementioned study, it was provided neither the comparison between DCT and ORA nor the evaluation of KC or MPRK subjects.

A previously published study compared the tested devices only on healthy eyes.[25] In the present study, findings observed previously were confirmed in a larger cohort of HS; moreover, other eyes categories were evaluated, and interesting differences and correlations were detected.[25]

During this research, the observed data showed that GAT and ORA might be used interchangeably in healthy eyes since they provide analogous IOP values. On the other side, DCT and CST provide significant higher measurements and these differences are affected by CCT.

In MPRK, GAT provides the lowest IOP values, whereas ORA provides the highest ones. this difference might be due to the dissimilar influence of CCT on these two devices. While in the same group of patients, DCT and CST provide similar IOP measurements.

In KC patients, IOP obtained by DCT is higher than the one measured with other devices; ORA gives the lowest value and this difference is affected by CC.

Among the limitations of the present study, this is a retrospective one. a prospective research could have provided more useful information. Furthermore, it is possible to include the sample size of the evaluated groups of eyes that should be increased in further studies. Moreover, MPRK patients were analyzed only after the surgery without any previous evaluation: it might be necessary to conduct a prospective study to have additional information regarding these types of eyes.

To sum-up, the present study does not state the supremacy of a device over the others in the measurements of IOP in HS, KP, and MPRK. Instead, it provides some indications whether these devices might be used interchangeably or rather in a complementary way to assess IOP in different groups of subjects.

In addition, it has been pointed out that not only CCT, as a corneal parameter, needs to be considered in the measurement of IOP but also CC, especially in patients with corneal alterations caused by corneal diseases or refractive surgeries.

Moreover, the data observed in this study might be employed to recognize some limits in IOP evaluation through the use of the tested devices.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

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  [Figure 1]

  [Table 1], [Table 2], [Table 3]


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