Comparability of icare pro rebound tonometer with goldmann applanation and noncontact tonometer in a

Original Paper Ophthalmic Res 2015;54:18–25 DOI: 10.1159/000381781

Received: December 10, 2014 Accepted after revision: March 17, 2015 Published online: May 28, 2015

Comparability of Icare Pro Rebound Tonometer with Goldmann Applanation and Noncontact Tonometer in a Wide Range of Intraocular Pressure and Central Corneal Thickness Nevbahar Tamçelik a Eray Atalay b Erdogan Cicik a Ahmet Özkök a Cerrahpasa Medical School, Istanbul University, and b Istanbul Medipol University, Istanbul, Turkey

Key Words Corneal thickness · Goldmann applanation tonometer · Icare rebound tonometer

Abstract Purpose: To evaluate the agreement between the reading values of the Goldmann applanation tonometer (GAT), Icare Pro rebound tonometer (IRT) and noncontact tonometer (NCT) in glaucoma patients. Methods: This cross-sectional study comprised 292 eyes of 292 patients selected from a glaucoma outpatient clinic. The intraocular pressure (IOP) was measured sequentially, at a 10-min interval each, in the following order: NCT, IRT and GAT. The central corneal thickness (CCT) was measured using Pentacam HR before the IOP measurements. Results: The mean IOPs measured by the GAT, NCT and IRT were 20.17 ± 6.73 mm Hg (range: 4–48), 19.77 ± 6.88 mm Hg (range: 3–46) and 19.30 ± 5.15 mm Hg (range: 7.30–44.5), respectively. The correlation coefficients of the GAT and IRT, NCT and IRT, and GAT and NCT measurements were r2 = 0.673, r2 = 0.663 and r2 = 0.938 (all p < 0.001), respectively. The IRT tends to overestimate in the low GATmeasured IOPs, whereas it underestimates in high GAT-measured IOPs. The measurements of all 3 devices were also correlated with the CCT at a statistically significant level (GAT:

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r2 = 0.063, NCT: r2 = 0.063, IRT: r2 = 0.058). Conclusion: The agreement between the IRT and GAT measurements is higher in the IOP range of 9–22 mm Hg, whereas significant discrepancies occur as the IOP deviates from normal values. The variability of the IRT and GAT measurements over a wide range of CCT is minimal. © 2015 S. Karger AG, Basel

Introduction

The measurement of intraocular pressure (IOP) is of paramount importance in managing glaucoma patients, as the reduction of the IOP is the only proven method of slowing down glaucomatous disease progression [1].

Many new diagnostic devices have emerged recently, to aid in a faster and reasonably more comfortable measurement of the IOP, yet the Goldmann applanation tonometer (GAT) is still considered the gold standard. Among the alternatives, the ICare rebound tonometer (IRT) has gained much attention owing to its accuracy and reliability comparable to that of the GAT, noncontact tonometer (NCT) and Tono-Pen [2–4]. The IRT device uses the impact rebound principle to measure the IOP. It is composed of a probe in a solenoid housing which is fired by a Nevbahar Tamçelik Tesvikiye Mah. Hakki Yeten Cd. Terrace Fulya Center 2 D: 13 Sisli TR–34098 Istanbul (Turkey) E-Mail nevbahartamcelik @ gmail.com

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Materials and Methods This study was designed as a cross-sectional study. The study was approved by the Institutional Review Board of the Cerrahpasa Medical Faculty of Istanbul University, and the protocol in the study adhered to the tenets of the Declaration of Helsinki. All patients were informed about the details of the study before being admitted, and a signed informed consent was obtained from all study subjects.

The patients were recruited from the Glaucoma Clinic of the Cerrahpasa Medical School. The exclusion criteria were: corneal edema, central corneal opacities, previous keratoplasty or refractive surgery, >3-dimensional corneal astigmatism, any history of laser or intraocular surgery within the previous 3 months and patients with absolute central scotoma.

All patients underwent a full ophthalmologic examination in the following sequence: assessment of visual acuity; central corneal thickness (CCT) measurement (Pentacam HR, Oculus, Wetzlar, Germany); measurement of the IOP by NCT (Topcon CT-80; Topcon Corporation, Tokyo, Japan), IRT (Icare Pro, Icare Finland Oy, Helsinki, Finland) and GAT (AT900; Haag Streit, Köniz, Switzerland); gonioscopic examination, and a dilated fundus examination. An interval of at minimum 10 min was given between the utilization of each IOP measurement device to enhance the accuracy of the sequential measurements [9]. The CCT measurements were performed prior to the IOP measurements to avoid the influence of instruments, topical anesthetics and the fluorescein dye on measurements. The IOP measurements were conducted by independent observers. Each observer was responsible for utilizing only 1 single device. These independent observers were masked from each other during the IOP measurement process. The investigators were also masked to the independent observers throughout the data collection phase. During the IOP measurement with IRT, the patients were asked to fixate on a distant target while the observer aligned the probe of the device 3–7 mm from the central cornea, perpendicular to the corneal plane. The observer then pressed the main button which allowed the probe to collide with the central cornea. During each procedure, 6 measurements were

Comparability of IRT with GAT and NCT

obtained, of which the highest and lowest were discarded by the software. The average of the 4 remaining measurements were then calculated and shown on the device display. The device also indicates if the intermeasurement numerical deviations fall between the expected standard deviations (SD), the green background indicating ‘within normal limits’ and the yellow and red background indicating ‘outside normal limits’. The averages of the measurements with normal deviations were recorded for each eye. A new disposable probe was used for each patient. The measurement of the IOP with the GAT was performed by the following protocol [10, 11]: a drop of 0.25% fluorescein with 0.5% proparacaine was administered topically in each eye, and the average of 2 sequential measurements were recorded; if the 2 measurements differed by >2 mm Hg, a 3rd measurement was made, and the median of the 3 measurements was recorded. Three consecutive measurements were made by the NCT, and the average was recorded. To avoid any statistical bias, only the left eyes of the patients were included in the analysis. In any cases where the left eye fulfilled any of the exclusion criteria or the disease had a unilateral involvement, the right eye was used for analysis.

The subjects were grouped according to their IOP levels. Eyes with IOP levels ≤8 mm Hg (ocular hypotony) were grouped as group AIOP, eyes with IOP levels within the range of 9–22 mm Hg were grouped as group BIOP (normal range), eyes with IOP levels within the range of 23–29 mm Hg were grouped as group CIOP, and eyes with IOP levels ≥30 mm Hg were grouped as DIOP. The measurements of the GAT were taken into consideration when grouping the patients.

The statistical analysis was performed using SPSS version 21.0

by IBM (SPSS Inc., Chicago, Ill., USA). Descriptive statistics were performed to calculate the demographic data of the study cohort.

Moreover, the mean difference of the measurements, the SD and the 95% confidence interval (CI) of the differences were calculated.

Correlations were performed using the Pearson correlation test.

The ANOVA test was used to compare the mean numerical variables. The ANCOVA test was used to compare the regression slopes of the measurements of the 3 devices. Bland-Altman graphs were used to display a ±2 SD agreement between the selected 2 methods. In the Bland-Altman graphs, the difference between each IOP measurement (e.g. GAT-IRT) was plotted against the mean of the 2 measurements [(GAT + IRT)/2]. p values <0.05 were considered statistically significant.

Results

In this study, 292 patients (152 female, 140 male) with a mean age of 62.21 ± 9.92 years (range: 45–77) were recruited. The age distribution between males and females was not statistically significant (p = 0.32). Of the 292 patients, 169 (57.87%) had primary open-angle glaucoma, 32 (10.95%) had pseudoexfoliation glaucoma, 30 (10.27%) were glaucoma suspect, 20 (6.84%) had chronic angle closure glaucoma, 15 (5.13%) had normal-tension glaucoma, 12 (4.10%) had pigmentary glaucoma, 8 (2.73%) had congenital glaucoma, and 6 (2.05%) had uveitic glaucoma. Of the 292 patients, 197 had been receiving antiglauOphthalmic Res 2015;54:18–25 DOI: 10.1159/000381781

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transient electrical current causing a projection of the probe onto the cornea. After the impact, the deceleration time of the probe from the corneal surface is measured, which has been found to be correlated with the IOP level [5]. This device offers an IOP measurement without local anesthetic because the measurement time is faster than the corneal reflex [6]. The well-known disadvantages of the GAT (the use of topical anesthetics and fluorescein dye and the dependence of a slit-lamp biomicroscope to measure the IOP) have been eliminated by the use of the IRT; however, although there is a general consensus that there is a good correlation between these techniques, several reports have suggested a discrepancy of the results in the high and low IOPs [6–8]. The aim of this study is to compare the results of IRT relative to that of the GAT and NCT in glaucoma patients exhibiting a wide range of IOP.

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coma treatment, whereas the remaining 95 patients were stable with no medication after glaucoma surgery. Topical antiglaucoma medications used by the treated patients included fixed combinations, prostaglandin analogues, β-blockers, α2-agonists and carbonic anhydrase inhibitors.

The mean IOPs measured by the GAT, NCT and the IRT were 20.17 ± 6.73 mm Hg (range: 4–48; 95% CI: 19.39–20.94), 19.77 ± 6.88 mm Hg (range: 3–46; 95% CI: 20

Ophthalmic Res 2015;54:18–25 DOI: 10.1159/000381781

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18.98–20.56) and 19.30 ± 5.15 mm Hg (range: 7.30–44.5; 95% CI: 18.71–19.90), respectively. The differences between the 3 modalities were not statistically significant (p > 0.05). The correlation coefficients of the GAT and IRT, NCT and IRT, and GAT and NCT measurements were r2 = 0.673, r2 = 0.663 and r2 = 0.938 (p < 0.001), respectively. Figure 1 illustrates the correlation plots and linear regression between the 3 devices.

Tamçelik/Atalay/Cicik/Özkök

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Fig. 1. Correlation plot and linear regression line between GAT and IRT (β = 1.07, r2 = 0.673; a), between NCT and IRT (β = 1.08, r2 = 0.663; b) and between GAT and NCT (β = 0.94, r2 = 0.938; c).

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Fig. 2. Bland-Altman plot of means against the difference between the IOPs measured by GAT and IRT (a) and those measured by NCT and IRT (b).

Mean Range SD 95% CI

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0.864 –12.5 – 21.2 3.87 0.41 – 1.31

0.465 –12.5 – 20.2 4.02 0.001 – 0.92

GAT-NCT 0.399 –6 – 6 1.70 0.20 – 0.59

Table 1 shows the means, SD and 95% CI of the differences between the 3 devices. The agreement between the measurements of the IRT and the measurements of the GAT and NCT are illustrated in figure 2 as Bland-Altman plots. The limits of agreement (±2 SD) for the GAT and IRT were –6.884 to 8.612 mm Hg. The limits of agreement were similar between the NCT and the IRT, measuring –7.579 to 8.509 mm Hg. The agreement between the GAT and the IRT seem to be influenced by the GATmeasured IOP, as illustrated in figure 3.

Figure 4 shows the bar graph distribution of the variation of ±2, ±3 and ±5 mm Hg between the GAT and the IRT in the AIOP, BIOP, CIOP and DIOP groups. When the IOP levels were ≤8 mm Hg, as described in group AIOP, the difference between GAT and IRT readings were never within the range of ±2 mm Hg. Out of the 7 measurements in group AIOP, 1 (14.3%) measurement was withComparability of IRT with GAT and NCT

in the range of ±3 mm Hg and 3 (42.9%) measurements were within the range of ±5 mm Hg. In group BIOP (IOP range: 9–22 mm Hg), 60.7% (130/214) of the measurements were within the range of ±2 mm Hg. In the higher IOPs, the agreement between the 2 devices diminished, as 34.7% (17/49) of the measurements in group CIOP and 13.6% (3/22) of the measurements in group DIOP fell within the range of ±2 mm Hg. The differences between the measurements were within the range of ±3 mm Hg in 49.0% (24/49) of the cases in group CIOP and in 13.6% (3/22) of the cases in group DIOP. An agreement of ±5 mm Hg was noted in 79.6% (39/49) of the cases in group CIOP and in 36.4% (8/22) of the cases in group DIOP. The mean CCT measurements of the study population was 554.36 ± 46.46 μm (range: 411–724). The measurements of all 3 devices were correlated with CCT at a statistically significant level. The correlation coefficient between the CCT and IOP measurements of the GAT, NCT and IRT were r2 = 0.063 (p < 0.001), r2 = 0.101 (p < 0.001) and r2 = 0.058 (p < 0.001), respectively. Figure 5 shows the correlation and linear regression of CCT with the IOP measurements of the 3 devices. The difference between the linear regression slopes of the 3 devices was not statistically significant.

The variability of the IRT to the measurements of the GAT (IRT-GAT) over a wide range of CCT was also analyzed. The significance of the correlation was weak and nearly statistically significant between IRT-GAT and CCT (r2 = 0.014, p = 0.046) as depicted in figure 6. The Ophthalmic Res 2015;54:18–25 DOI: 10.1159/000381781

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Table 1. Means, ranges, SD and 95% CI of the differences between the 3 devices

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the agreements of ±2, ±3 and ±5 mm Hg between the AIOP (IOP ≤8 mm Hg), BIOP (IOP range: 9–22 mm Hg), CIOP (IOP range: 23–29 mm Hg) and DIOP (IOP ≥30 mm Hg) groups.

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regression equation suggests an increase in the deviation from the GAT of 0.097 mm Hg for every 10 μm of increase in CCT.

The IRT has gained much interest recently, due to its practical use, comparable measurements with the GAT, good reproducibility and a relatively more comfortable experience for the patients [6, 12–15]. Questions have arisen whether the IRT would produce comparable results with the GAT over a wide range of IOP and CCT. Several studies have been published to evaluate the accuracy of the IRT in healthy subjects as well as in subjects with glaucoma.

In general, it has been accepted that the IRT would generate slightly higher readings than the GAT, as opposed to the findings of the current study [2, 7, 15–18].

Iliev et al. [2] reported a mean difference of 1.0 ± 2.17 mm Hg (95% limit of agreement: –3.2 to 5.2) in their mixed group of patients with minor complaints and open-angle glaucoma; Kim et al. [7], in their study group with various pathologies, found a mean difference of 1.92 ± 3.29 mm Hg (95% limit of agreement: –4.52 to 8.37) between the IRT and the GAT; Salim et al. [15] reported a mean difference of 2.45 ± 2.12 mm Hg (95% limit of agreement: 22

Ophthalmic Res 2015;54:18–25 DOI: 10.1159/000381781

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Discussion

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Fig. 4. Correlation plot and linear regression line between the IOP obtained by GAT and the difference between IRT and GAT (IRTGAT). IRT measures higher than GAT in the low IOPs and lower than GAT in the high IOPs.

–1.79 to 6.69) in their study group consisting of glaucoma patients only; Martinez-de-la-Casa et al. [16] found a median difference of 1.8 ± 2.8 mm Hg (95% limit of agreement –3.7 to 7.3) in their group consisting of subjects Tamçelik/Atalay/Cicik/Özkök

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Fig. 3. The proportional distributions of

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Fig. 6. Influence of CCT on the deviation of IRT from GAT (IRT-

GAT) measurements (r2 = 0.014, β = –0.0097, p = 0.046).

with glaucoma and controls; in a study of normal population, Fernandes et al. [17] reported a mean difference of 1.34 ± 2.03 mm Hg (95% limit of agreement: –2.64 to 5.31) between the 2 devices. Conversely, the mean IRT was significantly lower than the GAT, with statistical sigComparability of IRT with GAT and NCT

nificance, in a study of normal eyes and eyes with primary open-angle glaucoma [19]. Eyes with glaucoma had a higher mean IOP difference between the GAT and the IRT than healthy eyes, in this study. The findings of the present study suggest an overall IRT reading lower than the GAT (with no statistical significance), lower variability between the IRT and the GAT with a mean difference of 0.864 ± 3.87 but a wider 95% limit of agreement from –6.884 to 8.612 mm Hg. The limits of agreement were similar between the NCT and the IRT, measuring –7.579 to 8.509 mm Hg. The wide limit of agreement in this study is probably due to the significant discrepancies between the IRT and the other 2 devices in the high and low IOPs (fig. 5).

It is of utmost importance for a tonometry device to prove its reliability over a wide range of IOPs. The findings of this study have shown that agreement between the IRT and the GAT is significantly impaired in the very low and very high IOPs. In the ocular hypotony group (group AIOP), only 1 measurement of a total of 7 measurements (14.9%) were within the range of ±3 mm Hg, while none of the IRT and GAT measurements were in the agreement of ±2 mm Hg; however, the number of cases in this group was very low. In the normal IOP range (group BIOP), 60.7% of the measurements were within the range of ±2 mm Hg. With increasing IOPs, the agreement between the 2 devices became significantly weaker, but not as Ophthalmic Res 2015;54:18–25 DOI: 10.1159/000381781

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r2

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Ophthalmic Res 2015;54:18–25 DOI: 10.1159/000381781

for the IRT and the GAT, respectively. In the present study, the deviation of the IRT readings from the GAT is only 0.097 mm Hg for every 10 μm of CCT and is comparable with the work of Salvetat et al. [19] who reported a deviation of 0.09 mm Hg for every 10-μm increase in CCT. Perhaps, biomechanical properties of the cornea, namely corneal hysteresis and corneal resistance factor, are more influential on the IRT measurements than CCT [20, 31–33]. Interestingly, Neuburger et al. [34], in their in vitro model of a donor cornea coupled with an artificial chamber, reported the IRT and Tono-Pen XL to produce more accurate results than the GAT in edematous corneas.

In summary, the IRT seems to overestimate IOP in low GAT-measured IOPs, and underestimate it in high GATmeasured IOPs. Likewise, the agreement between the IRT and GAT measurements is higher in the range of 9–22 mm Hg, whereas significant discrepancies occur as the IOP deviates from normal values. In general, the deviation of the IRT from GAT as a function of CCT is very minimal.

This study is limited by not having evaluated some of the measurable corneal biomechanical properties, as it would be interesting to show their interactions with IOP measurements. It would also be of interest to investigate the comparability of various tonometers in different subgroups of glaucoma.

Disclosure Statement None of the authors has any conflict of interest with this submission.

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much as in the very low IOPs. Similarly to this study, 1 study [6] grouped their study subjects according to their IOPs and found that 64% of all measurements of the GAT and IRT (IOP range: 7–22 mm Hg) were within the range of ±3 mm Hg. The variability between the 2 devices was higher (60% within the range of ±3 mm Hg) in their group of subjects who had an IOP within the range of 23–60 mm Hg. It is also clearly visible in figure 3 that there is a linear relationship between the GAT-measured IOP and the difference between the IRT and GAT (IRTGAT). The IRT tends to overestimate in the low GATmeasured IOPs, whereas it underestimates in the high GAT-measured IOPs. A comparable relationship also exists for the IRT and NCT. Kim et al. [7] also found a decrease in IOP differences between the IRT and GAT (IRT-GAT) as the GAT-measured IOP increased. In contrast, Chui et al. [20] in their study with 125 normal healthy subjects, reported a higher IRT reading at higher IOP and a lower IRT reading at a lower IOP compared to the GAT. A similar finding was observed by Pakrou et al.

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As indicated in figure 5, the IOP measurements of all tonometers were positively correlated with CCT, and the NCT seemed to be influenced the most over a wide range of CCT (r2 = 0.101), followed by the GAT (r2 = 0.063) and the IRT (r2 = 0.058). The NCT is estimated to exhibit a deviation of 0.10 mm Hg from the GAT for every 10-μm increase in CCT. Previous studies have reported a deviation of 0.22–0.98 mm Hg for a similar change of CCT [22–24]. There is a significant controversy regarding the influence of CCT on IRT measurements compared to the GAT. Some studies have suggested an overestimation of the IOP by the IRT relative to the GAT as the CCT increased [8, 21, 25–28], and some have suggested that the GAT and IRT are equally affected by CCT [2, 16, 20, 29, 30]. The results regarding the deviation of the IRT from the GAT as a function of CCT are extremely variable.

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