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Keywords:

  • accuracy;
  • applanation resonance tonometry;
  • central corneal thickness;
  • Goldmann applanation tonometry;
  • intraocular pressure;
  • repeatability

Abstract.

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of interest
  8. References

Purpose:  To evaluate the repeatability and accuracy of the applanation resonance tonometer (ART) used in the automatic servo-controlled version, and to evaluate the influence of central corneal thickness (CCT) on the ART intraocular pressure (IOP) measurements.

Methods:  This prospective, randomized, single-centre study included one eye of 153 subjects (35 healthy volunteers and 118 patients with glaucoma). All participants underwent ultrasonic CCT measurement, followed by IOP evaluation with Goldmann applanation tonometer (GAT) and ART in random order. A single operator measured the IOP with each tonometer three times. Intra-examiner variability was evaluated using the coefficient of variation (CoV), intraclass correlation coefficient (ICC) and test–retest differences. Intermethod agreement was assessed using the Bland–Altman method. Linear regression analysis was used to evaluate the relationship between IOP measurements and CCT.

Results:  The mean IOP was 17.7 ± 4.4 mmHg with GAT and 20.6 ± 5.3 mmHg with ART (p < 0.001). CoV and ICC were, respectively, 5 ± 3% and 0.99 for GAT, and 8 ± 4% and 0.96 for ART (intermethods differences, p = 0.001). The ART test–retest differences significantly increased with increasing mean IOP (p = 0.003). The mean IOP difference (ART minus GAT) was 3.0 ± 4.0 mmHg, which increased with increasing mean IOP (p < 0.001). Both GAT IOP and ART IOP readings were significantly directly related to the CCT values (p = 0.03 and p = 0.004, respectively; intermethods difference, p = 0.32).

Conclusions:  The ART intra-examiner repeatability was excellent, although significantly lower than that of GAT, and decreased at higher IOP levels. ART significantly overestimated GAT IOP measurements, especially at higher IOP range. Both GAT and ART appeared similarly influenced by CCT value.


Introduction

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of interest
  8. References

Elevated intraocular pressure (IOP) has been shown to be one of the major risk factors for glaucoma (Coleman & Miglior 2008). Although the vulnerability of the optic nerve can vary amongst patients, longitudinal randomized controlled population-based trials have provided strong evidence that the reduction in the mean IOP of only 1 mmHg is significantly effective in preventing the development (Kass et al. 2002) and in delaying the progression (Heijl et al. 2002) of the glaucomatous damage. Accurate and reproducible IOP evaluation is therefore crucial with regards to the classification, management and follow-up of patients with ocular hypertension and glaucoma, and in patients that have undergone ocular surgical procedures.

Goldmann applanation tonometer (GAT) has been considered as the clinical gold standard for IOP measurement since it was introduced in the 1950s (Goldmann & Schmidt 1957; Wessels & Oh 1990). GAT is based on the applanation principle using the Imbert-Fick law, which states that the IOP can be calculated as the ratio between the force required to applanate a definite area of the cornea and contact area (Goldmann & Schmidt 1957). The Imbert-Fick law assumes that the cornea is infinitely thin, perfectly elastic, flexible and has a completely dry surface. The human cornea does not meet these theoretical conditions (Whitacre & Stein 1993). As expected, the accuracy of GAT has been shown to be significantly influenced by corneal properties, such as thickness, curvature, rigidity, viscosity, elasticity and hydration (Whitacre & Stein 1993; Doughty & Zaman 2000; Liu & Roberts 2005), which have shown to have high intra-and inter-individual variability and tend to be affected by corneal pathology and surgery (Luce 2005). Other GAT drawbacks include the need for a slit-lamp and local anaesthetic drops.

The increased awareness of the disadvantages and sources of errors of GAT has intensified the search for alternative clinically applicable tonometers, which mainly include new electronic applanation tonometers, such as the TonoPen (Minckler et al. 1987); noncontact tonometers, such as the ‘air-puff’ tonometers (Koçak et al. 1998) and the ocular response analyzer (ORA) (Luce 2005); the rebound tonometry (RT) (Kontiola 1997); and the dynamic contour tonometry (DCT) (Kaufmann et al. 2003).

The applanation resonance tonometer (ART), which uses a sensor that is based on the resonance technique, has recently been proposed for clinical use for measuring IOP (Eklund et al. 2000, 2003a). The ART estimates IOP by combining simultaneous continuous sampling of both parameters considered in the applanation principle, which include the force needed to applanate the cornea and the corresponding contact area (Eklund et al. 2000, 2003a). The ART uses the resonance technique to measure the contact area with the aid of a piezoelectric element that oscillates in resonance frequency, which produces a frequency shift that is proportional to the contact area (Eklund et al. 2000, 2003a). The IOP is calculated from the slope between force and contact area (frequency shift) (Eklund et al. 2003a).

The first ART prototype gave acceptable precision and accuracy in an in vitro porcine-eye model (Eklund et al. 2000, 2003a), although its application in human in vivo clinical studies showed poor agreement with GAT (Eklund et al. 2003b). After the recognition of some sources of errors, mostly related to off-centre applanation (Eklund et al. 2003a,b; Hallberg et al. 2004, 2006a), several technical implementation have been introduced (Eklund et al. 2003b; Hallberg et al. 2004, 2006b, 2007; Johannesson et al. 2012a). The current commercial version, known as the BioResonator ART, has been recently released and is available in a manual and automatic servo-controlled version (Johannesson et al. 2012a). There is only one prospective single-centre clinical study in literature by the inventors, which showed moderate accuracy with the BioResonator ART when GAT was taken as the reference standard (Johannesson et al. 2012a).

The purpose of our study was to evaluate the repeatability of the ART automatic servo-controlled version, and to compare ART IOP readings with those taken with GAT in a group of healthy subjects and patients with primary open-angle glaucoma (POAG). The influence of central corneal thickness (CCT) on the IOP measurements obtained with both tonometers was also analysed.

Materials and Methods

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of interest
  8. References

This prospective, observational, randomized, single-centre study included 154 subjects comprised of 36 healthy volunteers and 118 patients with POAG, having a mean age of 65 ± 13 years (range 30–87 years). If both eyes met the inclusion criteria, one eye from each subject was randomly selected using a computer-generated randomized number assignment. The study was in compliance with the tenets of the Helsinki’s Declaration, and informed consent was obtained from all participants prior to testing. The study was in compliance with institutional review boards (IRBs) and HIPAA requirements of the Azienda Ospedaliero-Universitaria ‘S. Maria della Misericordia’, Udine, Italy. We certify that all applicable institutional and governmental regulations concerning the ethical use of human volunteers and patients were followed during this research. Normal subjects were recruited from staff members and volunteers. Patients with glaucoma were recruited from the Glaucoma Outpatient Centre of the Department of Ophthalmology of the S. Maria della Misericordia Hospital, Udine, Italy. Each participant underwent the following examinations on the same day: complete ophthalmologic examination, including autorefractometry, best-corrected visual acuity evaluation, slit-lamp examination and fundus biomicroscopy with a 90D lens; corneal curvature (CC) measurements; ultrasonic CCT measurement, followed by IOP measurements with GAT and ART in random order.

Normal eyes were defined as: GAT IOP ≤21 mmHg; normal optic nerve head (ONH) and retinal nerve fibre layer (RNFL) appearance; normal standard automated perimetry (SAP) visual field (VF) results; no family history of glaucoma and other ocular pathologies. POAG was defined as reproducible ONH or RNFL thickness with typical glaucomatous changes and/or glaucomatous SAP VF defects.

Exclusion criteria included: poor or eccentric fixation; blepharospasm; nystagmus; buphthalmus; microphthalmus; corneal astigmatism higher than 3D; corneal scarring or history of intraocular surgery; contact lens wear; dry eye; or any other corneal or conjunctival pathology or infection.

All CCT measurements were performed by the same examiner (MLS). CCT was measured with an ultrasonic pachymeter (Altair pachymeter, Optikon 2000, Rome, Italy) after topical anaesthesia with 0.4% benoxinate hydrochloride eye drops. The pachymeter probe was placed on the centre of the cornea and the mean of three readings within a range of ±5 μm was considered for each eye.

All IOP measurements were taken by the same examiner (PB). In order to limit potential biases, the examiner was masked to the measurements taken, while a different observer (CT) read and recorded the IOP readings. All subjects underwent IOP measurements with GAT and ART tonometer in random order. In accordance with the ISO standard requirements (ISO 8612:2009), with a time interval of 5–30 min between each measurement to minimize the tonographic effect related to repeated applanation tonometry measurements. Three readings were taken with each instrument with a pause of at least 5 min between each measurement; the mean of the three readings was used for the intermethods comparison and analysis of the influence of the CCT value on the IOP measurements. To minimize the potential confounding effect of diurnal variation in IOP, all measurements were taken from 10.00 am to 11.30 am.

Goldmann applanation tonometer (Haag Streit International, Koeniz, Switzerland) has been extensively described in literature (Goldmann & Schmidt 1957). GAT was performed with a BQ 900 slit-lamp (Haag Streit, Bern, Switzerland) using local anaesthetic (0.4% benoxinate hydrochloride) and fluorescein sodium 2% strips. GAT was calibrated according to the manufacturer’s guidelines and performed at the slit-lamp in accordance to original description by Goldmann & Schmidt (1957). The same daily calibrated GAT was used throughout the study. Before each reading, the measurement drum was reset to approximately 6 mmHg. If IOP fluctuated during the cardiac pulse cycle, GAT measurements were taken in the middle of the pulsation amplitude.

The development of the ART system has been thoroughly described in literature (Lindahl et al. 1998; Eklund et al. 2000, 2003a,b). In brief, the commercially available ART device (BioResonator ART, Medical sensors and Instruments, BioResonator AB, Umea, Sweden) is composed of a sensor module containing a force transducer, which continuously measures the contact force, and a resonance sensor element, which measures the contact area between the sensor and the cornea. The resonance sensor element consists of a cylindrical piezoelectric element made of lead zirconate titanate (PZT; Morgan Electroceramics, Southampton, UK), which is powered to oscillate in its resonance frequency. At the end of the PZT a 1 mm pickup part is used to provide a feedback signal from the system. The signal is used in a feedback circuit that processes the signal from the pickup and powers the PZT element to sustain the oscillations in the resonance frequency. In order to provide the proper contact against the cornea, a plastic piece with a convex contact surface of 5 mm of diameter is glued to the end of the PZT element (Johannesson et al. 2012a). When the sensor is brought in contact with the cornea, the acoustic impedance of the cornea mechanically loads the sensor and modifies the resonance frequency, with a frequency shift which is proportionally to the contact area between sensor and cornea (Eklund et al. 2000, 2003a,b). Intraocular pressure is based on the relationship between the force and frequency in a specific frequency shift interval (corresponding to an interval applanation area between 4.9 and 11.0 mm2 (Eklund et al. 2000, 2003a; Hallberg et al. 2006b). Force and frequency are sampled with a sampling rate of 1000 Hz, and data analysis is then used to combine the results obtained during two different time intervals: the dynamic analysis elaborates the data taken during the indentation phase, ranging from the initial 40–300 ms (Hallberg et al. 2006b); the static analysis considers the data taken during 2 seconds of full corneal applanation (Goldmann-like analysis) (Johannesson et al. 2012a). The ART is self-calibrated.

The sensor module of the ART was attached to a standard biomicroscope in a similar position as the GAT probe. The ART probe was disinfected with 70% ethanol before each subject and local anaesthetic drops (0.4% benoxinate hydrochloride) were instilled in the eye before IOP measurements. ART is available in two versions: the manual one (BioResonator ART manual), in which the sensor is manually pushed towards the cornea; and, the automatic servo-controlled version (BioResonator ART servo) used in our study. In this newer version, the ART sensor module is mounted on a servomotor-controlled lever, and the sensor movement is generated by a miniature motor, controlled by feedback information from the continuously measured force and area, with a controlled velocity of 6.4 mm/s. To facilitate the positioning of the probe in the centre of the cornea, a LED is mounted on the sensor module. The instrument emits a sound when the ART reaches a frequency shift of −600 Hz, indicating that sufficient contact area has been achieved. The sampled data are automatically processed and the median value of the repeated measurements is displayed, together with a quality index (QI >2 were excluded), which is based on the SD of the repeated measurements. The mean of three consecutive ART IOP measurements was taken for the analysis.

With regards to the intra-examiner variability analysis, the repeatability of the three measurements obtained by a single operator with each tonometer was statistically assessed with the analysis of variance (anova), the intraclass correlation coefficient (ICC) (Patton et al. 2006), the coefficient of variation (CoV) (Bland & Altman 1986) and the test–retest differences calculation (expressed as absolute value). For each eye, a CoV was obtained as the ratio of the standard deviation of the three measurements and the corresponding mean. The ICC was calculated for the absolute agreement using a one-way random effect model (ICC 1,1). The ICC was defined as: excellent if >0.80; good between 0.61 and 0.80; moderate between 0.41 and 0.60; and poor to fair for ICC lower than 0.40. CoV and ICC intermethod differences were calculated using a bootstrap resampling technique (Patton et al. 2006). For each tonometer, linear regression analysis was used to evaluate the relationship between the differences between the first and second readings, expressed as absolute value, and means of repeated readings. Intermethod correlations were calculated using the Pearson correlation coefficient. Intermethod differences were evaluated with the paired t-test. Agreement between tonometers was assessed using the Bland–Altman method, in which the differences between readings were plotted with the mean measurements. (Bland & Altman 1986). Linear regression analysis was used to evaluate the influence of CCT on the IOP measurements and the IOP differences (ART minus GAT). Differences between linear regression coefficients were evaluated using the paired t-test. The statistical analysis was performed using spss 20.0 (Chicago, IL, USA). Statistical significance was defined as p < 0.05.

Results

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of interest
  8. References

The analysis showed one POAG eye as an outlier (IOP differences ART minus GAT >3 SD), and was thus excluded. The analysis was thus based on a total of 153 eyes (78 right and 75 left eyes) of 153 subjects made up of 35 healthy volunteers and 118 patients with glaucoma. The average CCT was 544.8 ± 40.0 μm (95% CI: 470.6–618.3 μm).

Considering that the SD of IOP differences between tonometers was approximately 4.0 mmHg, the power of our study in detecting a 1 mmHg difference (at the 5% significance level, with an error type II of 20%) was 83% in the cohort of 153 eyes.

Table 1 summarizes the IOP repeatability analysis. The differences amongst the three IOP readings were not significant for both tonometers (anova, p = 0.79 for GAT and 0.36 for ART, respectively). The CoV was significantly lower for GAT than for ART (paired t-test, p = 0.001); the ICC was almost perfect for both tonometers (ranging between 0.98 and 0.99 for GAT, and between 0.95 and 0.97 for ART) and appeared significantly higher for GAT than for ART (paired t-test, p = 0.001).

Table 1. Intraocular pressure measurements repeatability.
 Reading 1 (mmHg)Reading 2 (mmHg)Reading 3 (mmHg)Interreading comparison* (p)CoV (%) ICC
  1. CoV = coefficient of variation, ICC = intraclass correlation coefficient, GAT = Goldmann applanation tonometry, ART = applanation resonance tonometry.

  2. Data are expressed as mean ± standard deviation (confidence interval 95%).

  3. *anova.

  4. ^ Paired t-test.

  5. § Calculated by bootstrapping, with 1000 replications.

GAT17.9 ± 4.5 (11.0–28.2)17.5 ± 4.5 (10.0–27.4)17.7 ± 4.6 (10.0–29)0.795 ± 3 (0–10)0.99 (0.98–0.99)
ART20.7 ± 5.3 (10.0–30.5)20.6 ± 5.6 (10.2–32.4)20.5 ± 4.9 (10.8–30.0)0.368 ± 4 (2–17)0.96 (0.95–0.97)
Intermethod comparison^ (p)0.00010.00010.0001 0.001§0.001§

As shown in Fig. 1, the relationship between test–retest IOP difference and mean IOP was not significant for GAT (linear regression analysis, p = 0.72); whereas it appeared statistically significant for ART, with increasing test–retest differences with increasing mean IOP values (linear regression analysis, p = 0.003).

image

Figure 1.  Linear regression analysis between test–retest intraocular pressure (IOP) differences and mean IOP values. Scatter plot of the differences in the IOP estimate in the first and second readings against the mean of the IOP estimate in the two readings, with linear regression function and coefficient of determination (R²), using Goldmann applanation tonometer (A) and applanation resonance tonometer (B), respectively.

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Table 2 lists the IOP measurements obtained with GAT and ART and the intermethods differences, correlations and comparisons. GAT and ART IOP readings appeared highly correlated (Pearson correlation coefficients, p < 0.001). The IOP measurements were statistically significantly higher with ART than with GAT (paired t-test, p < 0.001). The ΔIOP were not significantly different between right and left eyes (data not shown, paired t-test, p = 0.34).

Table 2. Intraocular pressure measurements.
 GAT (mmHg)ART (mmHg)ART minus GAT (mmHg)Intermethod correlation§ (r)Intermethods comparison* (p)
  1. SD = standard deviation, CI = confidence interval 95%, GAT = Goldmann applanation tonometry, ART = applanation resonance tonometry.

  2. § Pearson correlation coefficient (p value).

  3. * Paired t-test.

Mean ± SD17.7 ± 4.420.6 ± 5.33.0 ± 4.0  
CI 95%(11–27.6)(10–32.2)(−5 to 11)0.70 (0.0001)0.0001

The Bland–Altman scatter plot for the agreement between GAT and ART tonometry readings is shown in Fig. 2. The mean of the ΔIOP between corresponding measurements (ART minus GAT value) was 3.0 ± 4.0 mmHg; the ±2 SD interval ranged from −5 to 11 mmHg. The ΔIOP (ART minus GAT) significantly increased with increasing mean intermethod IOP values (linear regression analysis, p < 0.001).

image

Figure 2.  Bland–Altman plots of the agreement between Goldmann applanation tonometer (GAT) and applanation resonance tonometer (ART) measurements. Bland–Altman analysis showing the distribution of differences in intraocular pressure (IOP) (ART value minus GAT value, mmHg) (y-axis) and the mean IOP value of the tonometers (x-axis) for each eye measured, with linear regression function and coefficient of determination (R²).

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As shown in Fig. 3, both tonometer IOP readings were significantly directly related to CCT (linear regression analysis, p = 0.03 and p = 0.004, respectively for GAT and ART), which appeared slightly higher for ART (difference not statistically significant; paired t-test, p = 0.32). According to the linear regression formula, a 10 μm change in CCT resulted in a deviation of 0.20 mmHg (95% CI of 0.02–0.37 mmHg) for the GAT IOP value and of 0.30 mmHg (95% CI of 0.1–0.5 mmHg) for the ART IOP value. The relationship between the ΔIOP (ART minus GAT) and CCT values was not statistically significant (Fig. 4; linear regression analysis, p = 0.6).

image

Figure 3.  Linear regression analysis between intraocular pressure (IOP) measurements and central corneal thickness (CCT) values. Scatter plot of applanation resonance tonometer and Goldmann applanation tonometer IOP measurements against CCT value, with linear regression function and coefficient of determination (R²).

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image

Figure 4.  Linear regression analysis between intraocular pressure (IOP) intermethod differences and central corneal thickness (CCT) values. Scatter plot of Applanation resonance tonometry minus Goldmann applanation tonometer differences in IOP against CCT, with linear regression function and coefficient of determination (R²).

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Discussion

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of interest
  8. References

Applanation resonance tonomettry is a new tonometry method that seems to have several theoretical advantages over GAT. While GAT measurements are determined with a single-point method, which uses only one reading to calculate the force necessary to reach a constant applanation area (Goldmann & Schmidt 1957), ART IOP measurements are based on a multipoint approach, which are determined by a continuous sampling of both force and contact area. Theoretically, IOP measurements with ART should only be dependent on the slope of the curve resulting from the relationship between force and area, thus less influenced than GAT from constant forces that imply a parallel move of the slope, such as corneal rigidity and capillary forces from the tear fluid surface tension (Eklund et al. 2000, 2003a). Some studies have claimed that ART, especially in its servo-controlled version, is less operator-dependent than GAT considering that, to obtain a standardized circular contact area, the GAT operator has to interpret an optical pattern and manually adjust the force accordingly, which is not required with ART (Johannesson et al. 2012a). Moreover, in the presence of high astigmatism, the GAT operator has to adjust the optical setting to approximate a circular contact area from the mean diameter of an elliptic contact area, whereas the corneal contact area can theoretically be measured with ART regardless of shape and thus should not be as sensitive to astigmatism. ART offers practical advantages compared with GAT in that fluorescein is not required.

Previous studies have shown that ART provided acceptable precision and accuracy in the IOP determination in an in vitro porcine-eye model (Eklund et al. 2000, 2003a,b; Hallberg et al. 2006a,b). Recent studies regarding the commercial version of ART in a clinical setting showed that only the manual version, but not the servo-controlled one, met the ISO standard requirements for tonometer accuracy (Johannesson et al. 2012a). Considering that the clinical application of the new ART methods is currently limited, it appears thus important to determine whether it is sufficiently accurate to be used as a substitute of GAT in patient management and whether its accuracy is influenced by the CCT value.

To the best of our knowledge, this is one of the first studies in literature that aims at assessing the repeatability of automatic servo-controlled ART in a clinical setting, especially conducted independently from the inventors, which evaluates the accuracy of ART in comparison with GAT.

The automatic servo-controlled ART appeared to be simple, user friendly and accepted by patients in a daily clinical setting. With regards to ART repeatability, previous authors reported that the intra-eye IOP variation was <1 mmHg using an ART prototype in an in vitro porcine model (Hallberg et al. 2006b). Our analysis showed that the repeatability of ART was high; it appeared, however, significantly lower than that of GAT, especially at high IOP levels, which may result in low precise measurements for monitoring IOP changes over time compared with GAT. The results of our study are in agreement with the conclusions of a recent systematic review of the literature showing that GAT seems to provide lower levels of variability than most of the other tonometers that are currently used in a clinical setting (Cook et al. 2012). Previous studies have demonstrated that the intra-observer variability of GAT was significantly better than that provided by the TonoPen (Tonnu et al. 2005), ‘air-puff’ tonometers (Tonnu et al. 2005) and ORA (Kotecha et al. 2010; Wang et al. 2013), comparable to that of RT (Johannesson et al. 2008) and lower than that of DCT (Johannesson et al. 2008; Wang et al. 2013). The GAT repeatability found in our study was within the expected standards for the use of GAT under ideal conditions (Weinreb et al. 2007) and better than that found by previous authors (Tonnu et al. 2005; Kotecha et al. 2010; Wang et al. 2013). Differences in populations or in inclusion criteria may explain these different results. Considering that ART is an automated method (however, dependent on the position of the probe on the cornea), whereas GAT relies on subjective interpretation of the operator, the lower repeatability of ART in comparison with GAT was unexpected. The lower precision of ART may be related to the variation of factors affecting the tonometer between measurements, such as corneal viscoelastic properties. Differences in centration of the ART probe between measurements must also not be excluded.

In agreement with previous reports (Hallberg et al. 2004, 2006a, 2007), our results showed that GAT and ART IOP measurements were significantly correlated but not interchangeable. The Bland–Altman scatter plot showed an overall moderate agreement between GAT and ART IOP measurements, as suggested by the wide SD and 95% CI of the intermethod differences. The IOP readings were significantly higher with ART than with GAT and, as shown by the slope of the regression line in Fig. 1, the overestimation in IOP measurements obtained with ART relative to GAT tended to be greater as IOP increased. These results are in accordance with those found by other authors in an in vitro porcine-eye model (Hallberg et al. 2006a), but in disagreement with a previous clinical study showing that the mean of the residuals between GAT and ART approximated the zero and that GAT underestimated ART for lower IOP levels (approximately <21 mmHg) and overestimated ART IOP measurements for higher IOP levels (Hallberg et al. 2007). It is important to note that this study, however, has used an ART prototype that has been calibrated against GAT (Eklund et al. 2000, 2003a), which eliminates the systematic differences between the instruments. Considering that the IOP measured by GAT has been shown to be 1.2–2 mmHg lower than the IOP measured manometrically in human eyes in vivo (Feltgen et al. 2001), the higher readings obtained with ART compared to GAT could be expected because ART calibration is based on a manometrically controlled standard pressure (even if in porcine-eye models) (Eklund et al. 2000, 2003a,b; Hallberg et al. 2006a,b), rather than on a GAT pressure reading. Possible explanations to account for the increased overestimation with ART in eyes with higher IOPs can be due to the alteration of corneal biomechanical properties not related to CCT as IOP rises, which could have different influences on ART and GAT.

In accordance with a previous report (Johannesson et al. 2012a), the accuracy of the automatic servo-controlled ART in comparison with GAT was moderate. The accuracy of ART reported in our study was similar to that found for other newly introduced tonometers. In comparison with GAT, the SD of the differences between corresponding measurements ranged from 1.12 to 3.97 mmHg for the noncontact techniques (Lam et al. 2004; Tonnu et al. 2005; Martinez-de-la-Casa et al. 2011); 2.29–4.5 mmHg for TonoPen (Bafa et al. 2001; Tonnu et al. 2005; Salvetat et al. 2007); 1.5–2.86 mmHg for Pascal (Doyle & Lachkar 2005; Martinez-de-la-Casa et al. 2006; Salvetat et al. 2007); and 2.17–3.92 mmHg for iCare (Brusini et al. 2006; Iliev et al. 2006; Martinez-de-la-Casa et al. 2006, 2011).

In agreement with previous studies regarding GAT (Whitacre & Stein 1993; Whitacre et al. 1993; Doughty & Zaman 2000) and ART (Eklund et al. 2003a), our data indicate that CCT value affects both tonometers IOP readings. The linear regression analysis precisely showed that a 10 μm change in CCT yielded a 0.30 and 0.20 mmHg deviation in the ART and GAT readings, respectively.

The influence of the CCT values on GAT accuracy has been widely demonstrated (Whitacre & Stein 1993; Whitacre et al. 1993; Doughty & Zaman 2000) and has several clinical implications (Copt et al. 1999; Shaikh et al. 2002). The correction factor of 0.20 mmHg in GAT IOP per 10 μm increase in CCT found in our study is comparable with those reported in the literature, which report ranges from 0.19 to 0.7 mmHg for each 10-μm difference in CCT compared with mean values (Doughty & Zaman 2000). With regards to ART, and in agreement with our results, a statistically significant relationship between CCT and ART IOP measurements has been already demonstrated in an in vitro porcine-eye model, in which an increase of 10% in corneal thickness translates to a contribution of 0.75 mmHg of the ART IOP (Eklund et al. 2003a).

The relationship between ΔIOP and CCT values was not significant, suggesting a similar behaviour of both tonometer errors relative to the CCT. These results are in agreement with previous studies showing that the variable CCT did not significantly contributed to explain the differences between GAT and ART in a general linear model (Hallberg et al. 2007). The hypothesis that ART could be less sensitive than GAT to corneal properties, such as CCT, curvature and rigidity, is supported by previous reports showing that the underestimation of IOP after myopic laser refractive surgery was higher for GAT than for ART both in an in vitro porcine-eye model (Hallberg et al. 2006c) and in a clinical human investigation (Johannesson et al. 2012b).

Studies have suggested that ART could be more sensitive than GAT to changes in corneal surface structure, such as the integrity of the corneal epithelium and the degree of moistening, which could partly explain the differences found in comparison with GAT. Corneal epithelium removal, which did not influence GAT IOP readings, was found to induce a significantly IOP underestimation with ART, likely related to a more effective acoustic contact between the sensor tip and the underlying tissue (Hallberg et al. 2006c); insufficient moistening of the cornea, on the other hand, can decrease the acoustic contact inducing an IOP overestimation with ART (Eklund et al. 2003a; Hallberg et al. 2006c).

Our study has several limitations, including the fact that the ocular intrinsic differences amongst patients that can affect IOP readings (i.e. axial length, corneal astigmatism, corneal biomechanical properties, tears film characteristics, etc.) were not assessed in our analysis. Moreover, subjects on topical medication with antiglaucomatous or steroid drops were not excluded, which may have influenced results due to their effect on corneal biomechanics and CCT.

In conclusion, our study showed that the intra-examiner repeatability of the automatic servo-controlled ART was high, although significantly lower than that of GAT, and decreased at higher IOP levels. The agreement between GAT and ART appeared moderate. ART significantly overestimated GAT readings, especially at higher IOP ranges. GAT and ART were similarly significantly influenced by CCT, which must be considered as an important variable when interpreting both tonometer IOP readings. Additional studies are required to further assess the repeatability and accuracy of ART measurements and to determine how corneal characteristics and other eye morphological parameters can influence ART IOP readings, especially in comparison with intracameral manometry.

References

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of interest
  8. References