Assessment of a new Goldmann applanation tonometer
- M Egli1,
- D Goldblum1,2,
- A Kipfer1,
- K Rohrer1,
- C Tappeiner1,
- M Abegg1,
- L Berger1,
- A Schoetzau2,
- M E Iliev1
- 1Department of Ophthalmology, University of Bern, Bern, Switzerland
- 2Department of Ophthalmology, University of Basel, Basel, Switzerland
- Correspondence to Dr Milko E Iliev, Department of Ophthalmology, University of Bern, Inselspital, Bern 3010, Switzerland;
- Accepted 12 February 2011
- Published Online First 3 April 2011
Background/aims The classic Goldmann applanation tonometer (GAT) has been further developed by Haag-Streit International. The applanation principle has been retained, while the internal force transmission and the pressure gauging have been optimised, the display of results digitised. The authors compared the GAT standard with the new GAT digital.
Methods Four fixed tonometer pairs were used. The protocol included: non-contact pachymetry, slit-lamp examination, three consecutive measurements with each tonometer with a 5 min interval in between, check for side effects in 15 min. Three groups (intraocular pressure (IOP) levels) were defined: (1) IOP≤16; (2) IOP>16 and <23; (3) IOP≥23 mm Hg.
Results 125 Patients (250 eyes) were evaluated. IOP (mm Hg), GAT standard versus GAT digital, for the rights eyes was: Group 1: 12.94±0.55 versus 13.11±0.53, p=0.71. Group 2: 18.26±0.59 versus 18.03±0.52, p=0.53; Group 3: 30.28±0.48 versus 30.42±0.41, p=0.97; all right eyes: 17.48±7.48 versus 17.73±7.4, p=0.99. For the left eyes, there was no significant difference, either. The correlation was very good and was not influenced by the IOP level. The Pearson coefficient for the right eye was 0.985, and for the left eye 0.994. In the Bland–Altman analysis, although there were two single readings that differed by as much as 5 mm Hg, GAT digital measures showed almost no skew, and the mean difference was 0.03±1.23 mm Hg (n=250). A multiple regression analysis showed no influence of order of measurement, eyeside or pachymetry.
Conclusions The new GAT digital is as reliable and safe as GAT standard. IOP values correlate well. It offers a digitised display and a wireless transfer of data. The display of values up to the first decimal digit is not necessarily associated with a more precise measurement, but may offer an additional comfort compared with the 2 mm Hg scale of the classic GAT.
Intraocular pressure (IOP) measurement is a routine procedure in ophthalmological examination and an important parameter in the diagnostic of glaucoma. Goldmann applanation tonometry is widely accepted as the international gold standard for IOP measurement.1–5 Recently, Haag-Streit International (Koeniz, Switzerland) has further developed the classic Goldman applanation tonometer (GAT) by optimising the internal force transmission and the pressure gauging while the principle of applanation has been retained. In the new device, an internal measurement control has been integrated, too. These features are supposed to help increase the stability and decrease the variability of IOP measurements. The display of IOP has been digitised. The IOP is displayed in steps of 0.5 mm Hg, but can also be displayed in steps of 0.1 mm Hg. As compared with the pure mechanical force transmission mechanism of GAT standard, the new combination of precision mechanics and electronic force monitoring (resistive sensor element) makes it possible to create a readout of the intraocular pressure measured in 0.1 mm Hg increments but also to correct production-tolerance-based non-linearity of the system.
The purpose of this study was to compare the standard GAT AT900 (GAT standard) with the new version of the GAT AT900D (GAT digital), and test the variability of consecutive measurements and the safety of the device.
Materials and methods
One hundred and fifty-seven patients were included in the study. Study participants were recruited among the regular patients in the Department of Ophthalmology, University of Bern, Switzerland. Inclusion criteria were age over 18 and willingness to participate. Exclusion criteria were: high corneal astigmatism (>2 dioptres), corneal scars, history of corneal surgery, microphthalmus, buphthalmus, monoculus, patients who wear contact lenses, dry-eye syndrome, blepharospasm, nystagmus, keratoconus, all other corneal pathology that might affect applanatory pressure measurements, known allergy to oxybuprocaine or fluorescein, pregnancy.
The study was approved by the local ethics committee. All patients signed an informed consent prior to study inclusion. Each patient received a routine ophthalmological examination of the anterior and posterior segment without pupillary dilation. Central corneal thickness (CCT) was measured with a non-contact optical pachymetry (OLCR-Pachymeter, Haag-Streit, Koeniz, Switzerland).6–10 For IOP measurements, four tonometer pairs were used. Each ‘pair’ consisted of one GAT standard (randomly picked up at the beginning of the study among the tonometers routinely used in the outpatient clinic, and assigned a study number) and one GAT digital. All eight tonometers were calibrated according to the manufacturer's guidelines. The pairs did not change throughout the study, and the slit-lamps (examination units) on which the tonometers were mounted remained the same.
The measurement protocol was as follows: interview, informed consent, refractometry, non-contact pachymetry, and examination of the anterior and posterior segment on the slit lamp. Eyes were then anaesthetised with one drop of oxybuprocaine 0.4% (Novesin, OmniVision AG, Neuhausen, Switzerland), and fluorescein was instilled in the inferior fornix using standard fuorescein paper strips (Haag-Streit). Three measurements were taken consecutively on each eye with one tonometer, starting with the right eye. The device to be used first was randomly chosen prior to seeing the next patient. The three readings were recorded, and the mean was used as the final value. After tonometry with the fist device, a pause of 3–5 min was made. Then, oxybuprocaine and fluorescein were instilled again, and the procedure was carried out with the second tonometer. After 15 min, the patient was interviewed and examined at the slit lamp again to rule out complications such as corneal erosions, inflammation and unpleasant sensations. The study was then finished for the patient. However, the participant was instructed to call if bothering symptoms occurred. All IOP measurements were carried out by ophthalmologists experienced with the procedure.
In addition to evaluating all right eyes, all left eyes and all eyes together, we also formed three IOP groups according to the pressure as measured with the reference tonometer, GAT standard, to determine whether IOP level had any influence on the results: Group 1, IOP≤16 mm Hg; Group 2, IOP>16 and <23 mm Hg; and Group 3, IOP≥23 mm Hg.
Mean values and SDs were calculated. An analysis was performed using SPSS for Windows, software version 11.5. The Wilcoxon rank sum test was used for the comparison of IOP readings. Bland–Altman plots were constructed to evaluate agreement and CI.11 A difference was considered significant when p<0.05.
Working with tonometer pairs (GAT standard and GAT digital), we computed the mean IOP difference between the two devices for each pair, in order to determine whether a particular skew was present.
To detect if there was any influence of tonometer type, eyeside, measurement sequence, age, spherical equivalent, diagnosis or pachymetry on the IOP, a multivariate linear mixed-effects model analysis with the dependent variable of IOP was performed. The fixed factors (predictors) were eyeside, measurement sequence, age, spherical equivalent, diagnosis or pachymetry. The subjects were treated as the random factor in this analysis.
The potential interactions between the two tonometer types and eyeside, measurement sequence, age, spherical equivalent, diagnosis, or pachymetry were also included in the model. Results are presented as differences of means with corresponding 95% CIs and p values. This multivariate analysis was carried out using the statistical software R version 220.127.116.11
The patients included were between 18 and 87 years old (mean age: 62 years±15.5). Eighty-tree (53%) of them were male, and seventy-four (47%) female. The demographic parameters and mean central corneal thickness values are shown in table 1.
Twenty-one patients had healthy eyes, 81 had glaucoma of different types, 12 had ocular hypertension, six had diabetic retinopathy, nine had age-related macular degeneration, and 28 had other pathologies of the eye not relevant for the completion of this study (table 2). Of the glaucoma patients, 48 were currently under treatment with topical hypotensive preparations, some were recently diagnosed and were still under observation, others had had recent surgery and were still on topical steroids, yet others had had surgery in the past and were void of medications.
Valid measurements with both tonometers were performed in all 314 eyes of 157 patients.
Three patients with IOP<5 mm Hg had to be excluded from the statistical analysis (all in group 1), because the test version of GAT digital used in this study did not display values below 5 mm Hg; instead, ‘low’ was indicated in the LED in such cases.
Another 29 patients (11 from Group 1, nine from Group 2 and nine from Group 3) had to be excluded for the following reason: at the beginning of the study, all tonometers were calibrated according to the recommendations of the manufacturer. At the end of study, tonometers were examined for technical defects and deviation in calibration. It turned out that one of the test tonometers (tonometer no 8) was slightly damaged (In the test series of the new device, the encoder on the main axis of the tonometer had been mounted during manufacturing using a force fit coupling. Checks at the end of the study revealed that the encoder was found to be slightly rotated in tonometer no 8, probably as a result of a strong external impact. In the new GAT digital series, following this study, the type of mounting was changed to prevent such rotation.) The eyes measured with tonometer no 8 had to be excluded from the statistics needed for validation of the new device, since we were unable to determine when the technical defect had happened: at the beginning or end of the study.
Finally, 125 patients (250 eyes) were eligible for statistical analysis of IOP. However, for the sake of completeness and transparency, we present in this paper the results prior to exclusion of tonometer no 8 (154 patients) and after its exclusion (125 patients). As shown in table 3 there was no influence on the results.
IOP results are shown in table 3. We performed an analysis for all right eyes and for all left eyes separately, and for all eyes together. IOP readings showed a high concordance between GAT standard and GAT digital for low pressures as well as for high pressures (p values between 0.53 and 0.99).
The order in which the devices were used did not seem to influence the results. Of a total of 132 eyes (66 patients) measured first with GAT standard, in 50.8% (61 eyes) the reading with the second tonometer was lower (mean difference 0.015 mm Hg, SD 1.06). Of a total of 118 eyes (59 patients) measured first with GAT digital, in 44.9% the reading with the second tonometer was lower (mean difference of 0.07 mm Hg, SD 1.36).
The overall multivariate analysis for all eyes including or excluding tonometer no 8 showed no influence and interaction between tonometer type and measurement sequence, eyeside, spherical equivalent, diagnosis, or pachymetry (p>0.58, all eyes; p>0.81, tonometer eight excluded). This indicates that there was no systematic error resulting from the different tonometer types regarding these parameters. Table 4 summarises the results as differences of means with corresponding 95% CIs and its p values.
The correlation between GAT standard and GAT digital was excellent and did not seem to be influenced by the IOP level (figures 1 and 2). The trend line for the right eye was: y=1.0222x−0.4714, and for the left eye: y=1.0004x−0.2548, for a range of pressures between 5 and 60 mm Hg. The Pearson correlation coefficient for the right eye was 0.985, for the left eye 0.994.
In the Bland–Altmann analysis, GAT digital readings showed almost no skew when compared with GAT standard (figures 3–5). The mean difference in the IOP readings of all eyes was 0.03±1.23 mm Hg (n=250, Tonometer no 8 excluded), resp. 0.01±1.19 mm Hg (n=308, tonometer no 8 included)
When comparing the mean differences GAT digital/GAT standard in IOP readings between the four tonometer pairs, no skew in the direction of a particular tonometer pair was observed (table 5; tonometer no 8 included; n=154 patients).
The control exam of the cornea after measurements to verify tonometry safety showed no corneal damage in any of the patients. None of the participants reported any unpleasant sensations or pain.
Goldmann applanation tonometry has been routinely used since 1957, and continues to be the reference and the most reliable method for IOP measurement. The new, ‘digital’ version of the device is denoted for the following features according to the manufacturer: retained principle of applanation and handling, optimised internal force transmission and pressure gauging; an internal measurement control has been integrated; the display of IOP has been digitised; values in steps of 0.5 or 0.1 mm Hg can be displayed.
The results of this study showed that the new GAT digital is safe. A very good correlation in IOP measurements between GAT standard and GAT digital was observed. The correlation was preserved in eyes with low and with high pressures. The Bland–Altman plot shows no skewness trend in the readings.
We measured IOP using two different types of tonometers. The order of examination was chosen randomly. However, this cannot fully exclude the possibility that the order of measurement might have influenced the results. In order to test for such an influence, a multivariate analysis of the interaction of tonometer type with measurement order, eyeside, pachymetry and other variables was performed. Also, the difference within each of the four tonometer pair was analysed. Neither of these statistical models showed any significant influence.
No difficulties were experienced handling the new device because the principle of operation is the same as with GAT standard.
Even if pressures lower than 5 mm Hg are rare in clinical practice, the fact that GAT digital does not display values below 5 mm Hg can be seen as a handicap. In the commercial series model, following this study, the lower limit of IOP readings in GAT digital can optionally be set to 3 mm Hg.
In summary, the new GAT digital is as convenient to use and as reliable as GAT standard but offers a digitised display and a wireless transfer of data to a digital patient record. The display of IOP values up to the first decimal digit is not necessarily associated with a more precise measurement but may offer additional comfort compared with the 2 mm Hg scale of the classic GAT.
Funding Haag-Streit International, Koeniz, Switzerland.
Competing interests None.
Patient consent Obtained.
Ethics approval Ethics approval was provided by the University of Bern, Bern, Switzerland.
Provenance and peer review Not commissioned; externally peer reviewed.