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| ORIGINAL ARTICLE |
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| Year : 2000 | Volume
: 48
| Issue : 1 | Page : 25-32 |
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Tonometry in normal and scarred corneas, and in postkeratoplasty eyes : A comparative study of the Goldmann, the ProTon and the Schiotz tonometers.
AK Jain, JS Saini, R Gupta
Department of Ophthalmology, Post-Graduate Institute of Medical Education and Research [PGIMER], Chandigarh, India
Correspondence Address:
A K Jain
Department of Ophthalmology, Post-Graduate Institute of Medical Education and Research [PGIMER], Chandigarh
India
 Source of Support: None, Conflict of Interest: None  | Check |
PMID: 11271930 
PURPOSE: Clinical comparison of intraocular pressure (IOP) measured with the Goldmann applanation tonometer (GAT), the ProTon tonometer (PT), and the Schiotz tonometer (ST), in normal eyes, eyes with scarred corneas and postkeratoplasty eyes. MATERIAL AND METHODS: The IOP readings with GAT, PT, and ST were compared in 125 eyes with normal corneas (Group A), 17 eyes with scarred corneas (Group B), and in 21 postkeratoplasty eyes (Group C). The data were statistically analysed at 95% confidence interval; linear regression analysis and paired t-test were done. RESULTS: The mean differences and their standard deviation [SD] between GAT and PT readings, and GAT and ST readings respectively were: [1] in Group A: -0.23 [SD 2.75] mmHg and +0.24 [SD 3.18] mmHg respectively; [2] in Group B: -1.8 [SD 12.67] mmHg and -4.5 [SD 9.95 mmHg; and [3] in Group C: +0.24 [SD 8.72] mmHg and -0.12 [SD 8.7] mmHg. They were not statistically significant. In Group A the 95% confidence interval between GAT and PT readings was -5.27 mmHg to 5.73 mmHg, and between GAT and ST readings, -6.12 mmHg to 6.59 mmHg. Ninety six [77%] eyes with the PT and 84 [69%] eyes with ST measurements were within 3 mmHg of GAT pressure. The correlation coefficients [r] for PT and ST were 0.93 [P = 0.0000] and 0.88 [P = 0.0000] respectively. In Group B 95% confidence interval between GAT and PT readings was -27.17 mmHg to 23.51 mmHg, and between GAT and ST measurement, -24.37 mmHg to 15.44 mmHg. The correlation coefficients [r] for the PT and ST were 0.112 [P = 0.660] and 0.630 [P = 0.006] respectively. In group C, the 95% confidence interval between GAT and PT measurements was -17.20 mmHg to 17.67 mmHg, and between GAT and ST measurements, -17.51 mmHg to 17.27 mmHg. The correlation coefficients [r] for the PT and the ST were 0.780 [P = 0.0000] and 0.740 [P = 0.0001] respectively. CONCLUSIONS: In clinical practice PT appears to have a higher level of accuracy than ST in normal corneas. In scarred corneas and post-penetrating keratoplasty eyes, because of high SD for mean differences and wide confidence interval of 95%, both PT and ST are inaccurate in measuring IOP as compared to GAT in such eyes. Keywords: Adolescent, Cicatrix, pathology, physiopathology, surgery, Comparative Study, Confidence Intervals, Cornea, pathology, surgery, Corneal Diseases, pathology, physiopa
How to cite this article:
Jain A K, Saini J S, Gupta R. Tonometry in normal and scarred corneas, and in postkeratoplasty eyes : A comparative study of the Goldmann, the ProTon and the Schiotz tonometers. Indian J Ophthalmol 2000;48:25-32 |
Accurate measurement of intraocular pressure [IOP] plays a significant role in screening for and managing patients with glaucoma. Several methods measure IOP accurately in healthy eyes with a normal corneal surface. These include the Schiotz indentation tonometer [ST], the Goldmann applanation tonometer [GAT], the MacKay-Marg tonometer, the TonoPen and the non-contact and contact pneumotonometer. All these have some limitations. The GAT is considered the gold standard for measuring IOP in adults.
The TonoPen has been validated in human studies to be an acceptable measuring device in eyes with corneas that are normal enough to be measured with GAT.[1-3] The reliability of the TonoPen in pathologic corneas has also been studied.[4-7] The two electronic tonometers, ProTon (PT) and TonoPen, share similar measurement principles, developed on the basis of the MacKay-Marg tonometer. In normal corneas the ProTon tonometer appears to have a higher level of accuracy than the TonoPen XL tonometer, using the Goldmann applanation tonometry as a standard method.[8] The PT has a strain gauge that converts IOP into an electrical signal, transmitted to a microprocessor where it is analyzed for acceptability. The measurements are displayed on a liquid crystal panel. The instrument records multiple measurements and displays the average with a confidence interval. This instrument has certain advantages over GAT. It is portable and compact, easy to calibrate, has a disposable tip that eliminates contamination risks, can be used regardless of the patient's position and has a digital display that minimizes user bias. Because of its smaller contact area, the TonoPen is considered more accurate for the measurement of the IOP in eyes with irregular corneas.[4] The ST measures the depth of indentation of a plunger of a given weight sliding through a curved footplate and correlates this with the pressure in the eye. Comparative studies have been done of the IOP changes in eye bank eyes9 and the accuracy of IOP measurements in eyes of children10 obtained with the TonoPen, Perkins and Schiotz tonometers. The purpose of this study was to compare IOP measurements obtained with GAT, PT, and ST in. normal eyes, scarred corneas, and postkeratoplasty eyes. To the best of our knowledge this is the first clinical study comparing PT with GAT measurements of intraocular pressure in scarred corneas and post-penetrating keratoplasty eyes.
| Materials and Methods | |  |
Patients were selected from the cornea and glaucoma service of the Eye Department. Verbal consent for participation was obtained from each patient. Patients were divided into three groups (a total of 171 eyes). Group A had 125 eyes with normal corneas, Group B had 22 eyes with scarred corneas and Group C had 24 eyes which had undergone penetrating keratoplasty. Five eyes from group B and three eyes from group C precluded the use of the GAT, and were excluded from the study. Finally there were 17 eyes in group B and 21 eyes in group C. All the patients were 17 years or older.
In all groups of patients, 4% lignocaine was instilled in the eye. Tonometry examination was performed in uniform sequence in all eyes, using GAT first, followed in sequence by PT [Tomey technology, Inc. Cambridge, USA] and ST [Riester, Germany]. Three consecutive measurements were recorded with the instrument. Measurements were performed on each eye with GAT until three consecutive readings were within 1.0 mmHg. This was followed by PT. These readings were considered satisfactory only at the <5% range indicator. A new disposable latex membrane [Proto Tips, Tomey, USA] was applied on the transducer for each patient. The Schiotz tonometry was performed last, using 5.5g and 10.0g weights. The final pressure was obtained using the pressure and rigidity table for paired reading. The tests were performed by two ophthalmologists familiar with all three tonometers. One used PT exclusively, while GAT and ST were performed by the second. All the tonometers were calibrated according to the manufacturer's instructions each day before use.
The data were collected and statistically analyzed. The reading discrepancies between GAT and PT, and GAT and ST were compared for all categories in each group using the regression test and paired t-test. A p-value of less than 0.05 was considered statistically significant. 95% limits of agreement [mean difference {SD 1.96}] and 95% confidence intervals [CI] were also calculated.11 A variation of within ± 3mm Hg in the readings between the two tonometers was considered to be agreement between the instruments, and the GAT was used as the standard for comparison.
| Results | | |
The results for IOPs measured with GAT, PT, and ST in three groups are shown in [Table - 1]. The mean differences and their standard deviation [SD] between GAT and PT readings, and the GAT and ST readings respectively were: [1] in Group A: 0.23 [SD 2.75] mm Hg {range -7 to +5 mmHg} and +0.24 [SD 3.18] mmHg {range -6.6 to + 8.4 mmHg} respectively; [2] in Group B: -1.8 [SD 12.67] mmHg {range -44 to +19 mmHg } and -4.5 [SD 9.95] mmHg { range -40.4 to +2.6 mmHg}: [3] in Group C; +0.24 [SD 8.72] mmHg {range -20 to +15 mmHg} and -0.12 [SD 8.7] mmHg {range -16.6 to 20.1 mmHg}. They were not statistically significant [p>0.05]. Only in Group A, in the pressure interval of 16 mmHg to 30 mmHg [number of cases = 78], mean differences between GAT and PT [0.72 mmHg], and between GAT and ST [0.94 mmHg] were statistically significant [p 0.02 and 0.007 respectively]. The GAT-based IOP range was from 4 to 39 mmHg, from 10 to 42 mmHg, and from 4 to 42 mmHg in three groups respectively.
In Group A the 95% CI between GAT and PT readings was between -5.27 mmHg and 5.73 mmHg, and between GAT and ST readings between -6.12 mmHg and 6.59 mmHg, with the ST readings showing greater variance than PT readings. Ninety six of 125 [77%] eyes with PT and 84 of 125 [69%] eyes with ST measurements were within 3mm Hg of GAT pressure.
Analysis of the distribution of differences between GAT readings and measurements with PT or ST revealed that these differences did not vary in any systematic manner over the range of measurements [Figure - 1] [Figure - 2]. The regression analysis of the Pro Ton or the Shiotz IOPs compared with the Goldmann IOPs in group A gave the regression equations as [y = 1.0574 x -1.2931] and [y = 0.935 x + 0.8208] respectively. The regression analysis revealed correlation coefficients [r] for PT and ST as 0.93 [p = 0.0000] and 0.88 [p = 0.0000] respectively.
In Group B 95% CI between GAT PT readings was -27.17 mmHg to 23.51 mmHg, and between GAT and ST measurement, between -24.37 mmHg and 15.44 mmHg [Table - 3], There was one case outlier in this group which measured 12 mmHg on GAT, and 56 and 52.4 mmHg on PT and ST respectively. If this case was deleted from analysis, then the 95% CI between GAT and PT readings is between -12.62 mmHg and 14.24 mmHg [mean difference {SD} 0.8 {6.7}mmHg], and between GAT and ST measurement between -9.75 mmHg and +5.31 mmHg [mean difference {SD} -2.22 {3.76) mmHg]. Eleven of 17[65%] eyes with the PT readings and 12 of 17[69%] eyes with ST measurements were within 3 mmHg of GAT pressure.
Analysis of the distribution of differences between GAT readings and measurements with PT or ST revealed that these differences did not vary in any systematic manner over the range of measurements [Figure - 3] [Figure - 4]. The regression analysis of ProTon or Shiotz IOPs compared with GAT IOPs in Group B gave the regression equations as [y = 0.1736 x +16.088] and [y = 1.1048 x + 2.6615] respectively. The regression analysis revealed correlation coefficients [r] for PT and ST as 0.112 [p=0.660] and 0.630 [p= 0.006] respectively.
In Group C the 95% CI between GAT and PT measurements was between -17.20 mmHg and 17.67 mmHg, and between GAT and ST measurements between -17.51 mmHg and 17.27 mmHg [Table - 4]. The PT underestimated the Goldmann IOP in pressure interval of 15-30 and >30 mmHg in postkeratoplasty eyes.
Eight of 21[38%] eyes with PT readings and 10 of 21[48%] eyes with ST measurements were within 3 mmHg of GAT pressure. Analysis of the distribution of differences between GAT readings and measurements with PT or ST revealed that these differences did not vary in any systematic manner over the range of measurements [Figure - 5] and [Figure - 6]. The regression analysis of PT or ST TOPs compared with GAT IOPs in Group C gave the regression equations as [y = 0.9019 x +1.9102] and [y = 0.7549 x + 5.4881] respectively. The regression analysis revealed correlation coefficients [r] for PT and ST as 0.780 [P=0.0000] and 0.740 [P=0.0001] respectively. There was no systematic error in any of the groups [mean difference ± 2 standard errors included zero].
| Discussion | | |
The ideal tonometer should be accurate, reproducible, convenient to use and maintenance free. Various instruments used for measuring IOP vary in terms of mechanism, design, ease of maneuverability, and accuracy. The Goldmann tonometer is considered the gold standard for measuring IOP. Studies in past have compared the TonoPen and the tonometers with the Goldmann applanation tonometer in normal as well as irregular corneas.[1-8] Low-displacement tonometers such as GAT and PT flatten a portion of the cornea causing minimum displacement of the intraocular fluid volume. The ST, an indentation tonometer, is a high-displacement tonometer which indents the cornea, causing large volume of intraocular fluid displacement.
This is the first clinical study to evaluate the accuracy of PT in eyes with scarred corneas and post penetrating keratoplasty eyes. In normal corneas PT appears to have a higher level of accuracy than the TonoPen XL tonometer using GAT as a standard method.[8] The results of this study showed a good agreement and correlation between PT and GAT in normal cornea eyes. The overall accuracy of PT was higher than ST in normal eyes.
In this study, we observed a tendency to underestimate the IOP using PT in pressure range of 4 to 42 mmHg with GAT. A possible explanation for that could be a decline of the IOP caused by the repeated measurements of IOP using three instruments in each eye. A decline of the IOP by 2-4.6 mmHg was reported to occur after several repeated readings.[12] Because the Goldmann applanation tonometry was performed first, the possible decline of the IOP as a result of repeated measurements with this instrument would influence all subsequent measurements with the other tonometers in the same manner. Another reason for doing tonometry in the specific sequence was that IOP changes are minimum with applanation type of tonometer followed by TonoPen and Schiotz tonometers.[9]
A clinical comparison study of the ProTon and the TonoPen XL tonometers with the GAT in normal corneas showed the 95% CI between the Goldmann applanation tonometry and the ProTon tonometry to lie between -4 mmHg and 5 mmHg.8 With the TonoPen the CI was between-3 mmHg and +8 mmHg.8 In the present study, in normal corneas, the 95% CI between GAT and PT was between -5.2 mmHg and 5.73 mmHg, and between GAT and ST between -6.5 mmHg and 7.27 mmHg. The limits of agreement between the values obtained with PT and GAT are less wide than those obtained with GAT and ST. The repeatability of clinical tonometry is poor. Several factors contribute to random variation. There are relatively large short-term pulsations of the IOP of each individual eye,[13] and there may be slight variation in the way two different measurements are carried out. The agreement between two methods of tonometry therefore is bound to be poor.[11]
Some investigators have suggested that an average error of more than ±3 mmHg cannot be tolerated in the diagnosis and treatment of vision-threatening diseases when more accurate techniques of measuring IOP are available. [14,15] The use of the TonoPen in glaucoma screening and case findings in general practice is discussed elsewhere.[16] There was agreement of 77% of PT readings and 67% of the ST.
In the present study there was good correlation between GAT and PT [r =0.93] in normal corneas. Also there was good correlation between GAT and ST [r =0.89]. In the same group, in the pressure range of 16-30 mmHg, mean difference of 0.72 mmHg [p=0.02] between the Goldmann and the ProTon, and mean difference of 0.94 mmHg [p= 0.007] between GAT and ST values was statistically significant, thereby indicating underestimation of GAT pressure in this pressure range. In normal corneas both PT and ST overestimated Goldmann tonometer pressure in the pressure range of 0-15 mmHg [mean diff. - 0.28 and - 0.86 mmHg respectively].
In six [1, 2, 14, 17-19] of nine [1-3, 5, 6, 14, 17-19] previous studies in post-mortem [3, 6, 17-18] or living [1-2, 5, 14, 17, 19] human eyes, the TonoPen tended to overestimate IOP <17 mmHg and to underestimate IOP >25 mmHg. One study[5] reported significant overestimation of IOP by the TonoPen compared to the GAT over the entire range of 0 - 45 mmHg in normal and postkeratoplasty living human eyes, the overestimate was more pronounced in the latter. Another study[6] analogously reported significant overestimation of manometrically set and measured IOP by the TonoPen over the entire IOP range of 0 - 65 mmHg immediately after same-sized or 0.5 mm oversized penetrating keratoplasty in postmortem human eye. Only one postmortem study[3] reported accurate measurements throughout the IOP range, based only on a mean of five separate measurements at five manometric pressures between 10 and 50 mmHg. One study[20] on Norway rats reported that the TonoPen overestimated manometric IOP <20 mmHg while underestimating IOP >25 mmHg. In the present study, in normal corneas the mean difference and the 95% limits of agreement between the GAT and the PT are comparable to earlier studies with TonoPen1, [19,20] and ProTon tonometers.[8]
In a comparative study among TonoPen, Perkins, and Schiotz tonometers[10] in paediatric patients, the Schiotz measurements were significantly higher than those obtained with the Perkins and the TonoPen tonometers. The overall correlation between the Schiotz and TonoPen tonometers, and the Schiotz and Perkins tonometers were 0.636 and 0.638, respectively. The correlation between the Perkins and TonoPen was higher [r = 0.867].
Whitcare et al[9] compared the TonoPen, Perkins, and Schiotz tonometers in an adult population and found that the Perkins tonometer has the fewest IOP deviations.
In the present study the mean IOP was higher in patients with scarred, irregular and postkeratoplasty corneas. It is precisely in these eyes where IOP measurement is most difficult and also requires close follow-up, because corneal opacity or associated media opacity precludes an accurate assessment of optic nerve head and visual field examination.
In the scarred eyes though agreement between GAT and PT was 65%, there was very poor correlation [ r = 0.11]. The PT slightly overestimated the IOP [mean difference {SD} - 1.8 {12.7} mmHg]. Frenkel and associates found the TonoPen to be inaccurate at IOPs over 30mm Hg and recommended that IOPs over 30mm Hg be remeasured with a GAT or other tonometer.[19] Olson et al[21] used the MacKay-Marg tonometer in eye bank eyes with and without epikeratophakia lenticules and / or bandage contact lenses. They also measured the IOP in rhesus monkey with an epikeratophakia lenticule using the MacKay-Marg and the Perkins tonometer. The IOP in all their measurements was monitored with a pressure transducer in the anterior chamber. They found that, with an epikeratophakia lenticule in place, the MacKay-Marg tonometer was not accurate with IOPs under 20mm Hg, but GAT was accurate over the entire range of pressures tested. Rootman et al4 compared the TonoPen with the MacKay-Marg in eyes with scarred corneas, and in eyes after keratoplasty and epikeratophakia. Since the TonoPen design is based on the design of the MacKay-Marg tonometer, they expected similar readings from the two tonometers. The authors concluded that the TonoPen was as accurate as the MacKay-Marg in these conditions; however, no manometer was used to determine the actual pressure in the eyes studied. So this does not prove the precision of the TonoPen, since both tonometers operate the same principle. The agreement between the GAT and the ST was 70% and correlation [r = 0.63], but the 95% limits of agreement were very wide in scarred corneas.
In the group with scarred irregular corneas there was poor correlation between the Goldmann and the PT measurements. In such eyes Schiotz and Goldmann tonometers are inaccurate and misleading.[22-26] In such eyes TonoPen tonometer may be a useful device to monitor IOP especially when fundus and visual field examination is precluded. No study is available where ProTon or TonoPen measurements of IOP have been compared to manometrically set and measured IOP readings in scarred irregular corneas. Indirect evidence is available in studies [24,27] which compared MacKay-Marg tonometer results to anterior chamber cannulation studies in patients with scarred, irregular, oedematous corneas and postkeratoplasty patients, and found excellent correlation over a wide range of IOP, and further, TonoPen tonometer has been reported to be as accurate as the MacKay-Marg tonometer in measuring IOP after keratoplasty, epikeratophakia and in scarred corneas.[4]
Postkeratoplasty eyes often have large and irregular astigmatism. The measurement of IOP after penetrating keratoplasty is difficult with the GAT because of inaccuracy induced by the sutures interfering with the tip of the instrument, corneal surface irregularity, and corneal oedema.[22] The accuracy of a miniature version of the Goldmann applanation tonometer in normal Cynomolgus monkey eyes, human eyes and postmortem grafted human eyes has been proved. [6,28] Because of small contact diameter the TonoPen was recommended for the measurement of IOP in eyes with irregular corneas.[4] The TonoPen is a modern development of the MacKay-Marg tonometer, which is no longer commercially available. The TonoPen is reported to be as accurate as the MacKay-Marg tonometer in measuring IOP after keratoplasty and in scarred corneas in which Goldmann or Schiotz tonometer is not useful,[4] where the MacKay-Marg has been shown previously to be accurate after penetrating keratoplasty,23-25] or in the presence of corneal irregularity or oedema.[26-28]
In our study there was a good correlation between GAT and PT tonometers[r =0.78], and GAT and ST [r = 0.74] in postkeratoplasty eyes. Also, the correlation of 0.93 and 0.78 between PT and GAT in normal and postkeratoplasty eyes respectively in our study is comparable to a correlation of 0.83 and 0.79 in normal and postkeratoplasty eyes respectively in another study.[5] In our study PT under estimated the Goldmann IOP in pressure interval of 15-30 and >30 mmHg in postkeratoplasty eyes. There is an average overestimation of 3-6 mmHg over GAT IOP by TonoPen after keratoplasty.[5] In keratoplasty eyes the correlation between GAT and ST was [r = 0.74] and the agreement was 48%, but the 95% CI was very wide. In the scarred cornea group and post penetrating keratoplasty eyes group the 95% limits of agreement were very wide. This is because of the high SD of the mean differences. The SD of differences is a measure of the total variability of the study and includes variability due to operator, instruments, physiologically related factors and any possible effect one measurement may have on the next.
In normal corneas it was concluded that the ProTon tonometer appears to have a higher level of accuracy than the Schiotz tonometer in clinical practice. In scarred corneas and the keratoplasty eyes, because of the wide limits of agreement, and disagreement of 35% between GAT and PT, and 30% between GAT ST in scarred corneas, and disagreement of 62% between GAT and PT and 52% between GAT and ST in keratoplasty eyes, it was concluded that PT and ST are inaccurate in comparison to GAT, in eyes with such conditions. The GAT measurements should be done whenever possible.
Further comparative studies should be done between the ProTon or TonoPen and the Goldmann applanation tonometer, and manometrically set and measured IOP readings in scarred irregular corneas.
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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6]
[Table - 1], [Table - 2], [Table - 3], [Table - 4]
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