Effect of corneal thickness on intraocular ... - Wiley Online Library

C L I N I C A L

A N D

E X P E R I M E N T A L

OPTOMETRY

Effect of corneal thickness on intraocular pressure measurements with the Pascal dynamic contour, Canon TX-10 non-contact and Goldmann applanation tonometers in healthy subjects Clin Exp Optom 2009; 92: 1: 14–18 Aysel Pelit* MD Rana Altan-Yaycioglu* MD Aykut Pelit† PhD Yonca A Akova§ MD * Baskent University Faculty of Medicine, Department of Ophthalmology, Adana Teaching and Medical Research Center, Adana, Turkey † Cukurova University Faculty of Medicine, Department of Biophysics § Baskent University Faculty of Medicine, Department of Ophthalmology, Ankara, Turkey E-mail: [email protected]

DOI:10.1111/j.1444-0938.2008.00299.x Purpose: To investigate the effects of central corneal thickness (CCT) on intraocular pressure (IOP) measurements of the Pascal dynamic contour tonometry (DCT), Canon TX-10 non-contact tonometry (NCT) and Goldmann applanation tonometry measurements (GAT) in healthy subjects. Methods: IOP values of 135 eyes with normal corneas of 135 healthy volunteers were determined by DCT, NCT and by GAT. The CCT was measured using an ultrasonic pachymeter after all IOP determinations had been made. Results: When DCT measurements were compared (IOP = 17.52 ⫾ 2.0 mmHg) with NCT measurements (IOP = 16.54 ⫾ 2.77 mmHg) and GAT measurements (IOP = 15.07 ⫾ 2.35 mmHg), DCT measurements were significantly higher than NCT and GAT (p < 0.001). There was a significant correlation between CCT with both NCT (r = 0.260, p = 0.003) and GAT measurements (r = 0.257, p = 0.005). There was a weak correlation that was not statistically significant between CCT and DCT (r = 0.160, p = 0.079). Conclusion: The IOP measurements with DCT seem to be less dependent on CCT. NCT appears to be more affected by variation in CCT than GAT.

Submitted: 19 January 2008 Revised: 31 March 2008 Accepted for publication: 31 March 2008

Key words: corneal thickness, Goldmann applanation tonometry, intraocular pressure, non-contact tonometry, Pascal dynamic contour tonometry

Determination of the intraocular pressure (IOP) is important for diagnosis and management of glaucoma. The Goldmann applanation tonometer (GAT) has become the gold standard against which other tonometers have been compared. This is despite the fact that Goldmann recognised that the tonometer’s accuracy was questionable in corneas that were not of average thickness.1 Recent evidence has confirmed the wide variation in corneal Clinical and Experimental Optometry 92.1 January 2009

14

thickness and raised the possibility that corneas that are thicker or thinner than average might be associated with clinically significant over- or under-estimates of IOP.2–3 The reliability of other commonly used tonometers has also been assessed. The non-contact air tonometer (NCT)4–5 and the handheld tonometer (Tono-Pen; Mentor, Norwell, MA) have been reported to be more reliable, especially in corneas outside the average range in thickness,2,6

however, these observations have not always been confirmed by other studies.7,8 A new device, the Pascal dynamic contour tonometer (DCT), was recently introduced by Kanngiesser, Kniestedt and Robert.9 When this instrument is placed on the eye, the cornea takes the contour of the tip so that its biomechanical effects on IOP measurement are reduced. The objective of this study was to look for associations between the central © 2008 The Authors

Journal compilation © 2008 Optometrists Association Australia

IOP measurements with the DCT, NCT and GAT Pelit, Altan-Yaycioglu, Pelit and Akova

Mean Sex (female/number) Age (years) Eye (right/number) CCT (mm) Keratometry (mm) Corneal astigmatism (D) DCT (mmHg) NCT (mmHg) GAT (mmHg)

69 56 135 549 7.79 0.14 17.52 16.54 15.07

SD

Range

9.4

39–81

29.2 0.27 0.01 2.0 2.77 2.35

490–629 7.13–8.48 0.02–0.50 11.90–23.70 12.30–20.80 11.0–21.0

Table 1. Demographic data for the study group (n = 135)

corneal thickness (CCT) and IOP values provided by the DCT, GAT and NCT.

MATERIAL AND METHODS The IOP values of 135 normal eyes of 135 healthy volunteers were determined by DCT (Swiss Microtechnology AG, Port, Switzerland), NCT (Canon tonometer TX-10, Japan) and GAT (Haag-Streit, Switzerland). The exclusion criteria were pregnancy, trauma, corneal abnormalities (such as oedema, scars, dystrophy, astigmatism greater than one dioptre), previous refractive surgery, history of ocular surgery, use of eye drops and glaucoma. Informed consent, according to the tenets of the Declaration of Helsinki, was obtained before examination. The study was approved by the local ethics committee [KA05/207]. To minimise systematic bias, only the right eye of each patient was included in the study. The IOP measurements were taken in random order to guarantee an independent measurement of all instruments. The measurements were always taken between 10:00 am and 11:00 am. Proparacaine hydrochloride 0.5% was used in the measurement of IOP. Each measurement performed by the same experienced physician (AP), who did not read the measurements taken. A different observer read and recorded the IOP measurements. Three readings were taken with each instrument and the mean was used for analysis. After each measurement

on each instrument, a rest period of 20 minutes was allowed to minimise the tonographic effects of applanation tonometry. For DCT, three readings of good quality score ‘Q’ were saved (scores up to three on a scale of five, as recommended by the manufacturer). Subsequently, another physician (RAY) carried out ultrasonic pachymetry (UP-1000 Ultrasonic pachymeter, Nidek Co, Japan). The mean of five measurements was recorded for analysis.

Statistical analysis The mean IOP measurements by each of the DCT, NCT and GAT instruments were compared by repeated Analysis of Variance (ANOVA). Significant mean and interaction effects were further explored using the method of Tukey. Data are expressed as mean and standard deviation (SD). Correlations were calculated by Pearson coefficient. Linear regression analysis was used to examine the role of CCT in IOP measurement by each method. Inter-method agreement between tonometers was assessed using the method devised by Bland and Altman, which included calculation of the mean difference between measurements, the standard deviation and the 95% confidence interval (CI) of the differences. Values of p less than 0.05 were considered to be statistically significant. Statistical analysis was performed using SPSS licensed version 15.0 (SPSS Inc, Chicago, IL, USA).

© 2008 The Authors Journal compilation © 2008 Optometrists Association Australia

RESULTS The mean age of the 135 subjects was 56.08 ⫾ 9.4 years (range, 39 to 81 years). IOP values were 17.52 ⫾ 2.0 mmHg (range, 11.9 to 23.70 mmHg) with DCT, 16.54 ⫾ 2.77 mmHg (range, 12.30 to 20.80 mmHg) with NCT, and 15.07 ⫾ 2.35 mmHg (range, 11.0 to 21.0 mmHg) with GAT. Table 1 summarises the demographic data of the study group. When DCT measurements were compared with NCT and GAT measurements, DCT measurements were significantly higher than NCT and GAT measurements (p < 0.001). Figure 1 shows a Bland-Altman scatterplot comparing DCT and GAT readings. The mean of the differences between corresponding measurements (DCT value minus GAT value) was 2.47 ⫾ 2.08 mmHg (95% CI, -1.69 to +6.63 mmHg). There was a low positive linear relationship between DCT and GAT (r2 = 0.03, p = 0.03). Figure 2 shows a Bland-Altman scatterplot for the agreement between DCT and NCT readings. The mean of the differences between corresponding measurements (DCT value minus NCT value) was 1.08 ⫾ 2.50 mmHg (95% CI, -3.92 to +6.08 mmHg). There was a significant negative linear relationship between DCT and NCT (r2 = 0.126, p < 0.001). Figure 3 shows a Bland-Altman scatterplot for the agreement between GAT and NCT readings. The mean of the differences between corresponding measurements (GAT value minus NCT value) was 1.39 ⫾ 2.72 mmHg (95% CI, -4.05 to +6.83 mmHg). There was a low negative linear relationship between GAT and NCT (r2 = 0.03, p = 0.03). The mean CCT was 549.23 ⫾ 29.2 mm (minimum 490 mm; maximum 629 mm). There was a significant correlation between CCT with both NCT (r = 0.260, p = 0.003) and GAT measurements (r = 0.247, p = 0.005). There was a weak correlation that was not statistically significant between CCT and DCT (r = 0.160, p = 0.079) (Figures 4, 5 and 6). Clinical and Experimental Optometry 92.1 January 2009

15


10.00

10.00

Mean +2 SD

0.00 Mean -2 SD

-5.00

Mean +2 SD

Difference DCT-NCT (mmHg)

Difference DCT-GAT (mmHg)

5.00

5.00

0.00

Mean -2 SD -5.00

-10.00

-10.00 10.00

12.00

14.00

16.00

18.00

20.00

10.00

22.00

12.00

14.00

Average DCT-GAT (mmHg)

16.00

18.00

20.00

22.00

Average DCT-NCT (mmHg)

Figure 1. Difference between GAT and DCT IOP measurements, plotted against the mean of the two measurements, for each subject together with mean difference and 95% confidence limits. y = 17.15 - 0.17x, r2 = 0.03

Figure 2. Difference between DCT and NCT IOP measurements, plotted against the mean of the two measurements, for each subject together with mean difference and 95% confidence limits. y = 17.73 - 0.29x, r2 = 0.12

24 10.00

22

Difference NCT-GAT (mmHg)

5.00

0.00

Mean -2 SD -5.00

Intraocular pressure (mmHg)

Mean +2 SD

20

18

16

14

-10.00

12 10.00

12.00

14.00

16.00

18.00

20.00

22.00

Average NCT-GAT (mmHg)

Figure 3. Difference between GAT and NCT IOP measurements, plotted against the mean of the two measurements, for each subject together with mean difference and 95% confidence limits. y = 15.97 + 0.14x, r2 = 0.03

Clinical and Experimental Optometry 92.1 January 2009

16

480

500

520

540

560

580

600

620

640

Central corneal thickness (µm)

Figure 4. Regression plot of CCT versus NCT with regression equation and coefficient of correlation. y = 4.05 + 0.021x, r2 = 0.06.



22

24

22



20

18

16

14

20

18

16

14

12

12

10 480

10 500

520

540

560

580

600

620

640

480

500


DISCUSSION Several instruments for measuring IOP are available. Goldmann applanation tonometry is widely accepted as the international gold standard for measurement of IOP and is the most commonly used method. There are several well-known sources of error in applanation tonometry, including central thickness and structural rigidity of the cornea.10 In healthy corneas, central thickness is related to the rigidity of the cornea having an impact on the force required to flatten the area that is used to estimate IOP in applanation tonometry. Therefore, CCT has been widely accepted as an important and confounding variable, when it comes to estimating the IOP by means of applanation tonometry.2

540

560

580

600

620

640


Figure 5. Regression plot of CCT versus GAT with regression equation and coefficient of correlation. y = 7.05 + 0.019x, r2 = 0.06.

Linear regression analysis showed a mean change of 0.21 mmHg in IOP measured by NCT and 0.19 mmHg in IOP measured by GAT per 10 mm variation in CCT. In contrast IOP measured by DCT showed a change of 0.10 mmHg per 10 mm change in CCT.

520

Figure 6. Regression plot of CCT versus DCT with regression equation and coefficient of correlation. y = 11.32 + 0.010x, r2 = 0.01.

In the current prospective study, we compared the IOP results of DCT, NCT, and GAT in 135 healthy eyes of 135 subjects with normal corneas and assessed the influence of CCT on these IOP measurements. In this study, the mean DCT measurement was 2.47 mmHg higher than the GAT mean. This is slightly higher than the findings of Duba and Wirthlin,11 who observed 2.0 mmHg higher IOP values for DCT than GAT in a small group, including 20 patients undergoing refractive surgery. Kaufmann, Bachmann and Thiel12 also reported that IOP readings obtained by DCT were 1.7 mmHg higher than the readings obtained by GAT. Recently, Salvetat and colleagues13 reported that the mean DCT measurement was 3.2 mmHg higher than the GAT mean in patients with glaucoma. The higher readings obtained by DCT could be attributed to the calibration of the DCT, which is based on manometrically controlled standard pressure. DCT measures dynamic IOP and gives an average of the systolic and diastolic IOPs. The authors are aware of only three published studies comparing IOP measurements with GAT and the Canon TX10


pneumotonometer but no clinical studies comparing GAT, DCT and the pneumotonometer readings in the same eyes. Stabuc Silis and Hawlina14 found that in keratoconic patients, the IOP measured by NCT was significantly lower than that measured by GAT. They concluded that the thin cornea had more influence on IOP measurements with NCT than GAT. Tonnu and associates15 found that NCT significantly underestimated GAT measurements at lower IOP and overestimated these at higher IOP in a study including 105 eyes with ocular hypertension or glaucoma. Sanchez-Tocino and co-workers16 determined a statistically significant difference (p < 0.001) between the measurements using NCT (15.6 ⫾ 2.9 mmHg) and GAT (15.4 ⫾ 2.7 mmHg). The mean of the differences between the two tonometers was 0.24 mmHg. In our study, the IOP measured by NCT was significantly (p < 0.001) higher than GAT (mean = 1.39 mmHg) and lower than DCT (mean = 1.08 mmHg) in healthy subjects. Linear regression analysis showed a low positive correlation between DCT and GAT measurements. There was a significant negative linear relationship between Clinical and Experimental Optometry 92.1 January 2009

17


DCT and NCT and there was a low negative linear relationship between GAT and NCT measurements. Several studies have pointed at the interindividual variability of central corneal thickness as a source of error for conventional GAT.17 Studies conducted on cadaver eyes and eyes of patients after laser in situ keratomileusis showed a significantly lower correlation of CCT with DCT than with GAT.18 In the current prospective study, the relationship between IOP measured by DCT and CCT approached but did not achieve significance (r = 0.160, p = 0.079). This is consistent with previously published data.19–21 NCT and GAT measurements showed a statistically significant relationship with corneal thickness. This is similar to the findings of Tonnu and associates.15 Kniestedt and colleagues22 found that DCT is not significantly influenced by CCT, pneumatonometry appears not to be affected by CCT and GAT is significantly influenced by CCT. This study included patients with glaucoma and suspected glaucoma, and all of the patients with glaucoma were receiving treatment to lower the pressure. They found a significant correlation between GAT and CCT with a 0.25 mmHg change per 10 mm. Correspondingly, we found a significant correlation between GAT and CCT with a 0.19 mmHg change per 10 mm. In our study, we also found a significant correlation between NCT and CCT with a 0.21 mmHg change per 10 mm. This study demonstrates that DCT measurements are minimally dependent on CCT in a group of normal eyes. This is probably due to its ability to measure IOP transcorneally, without the need to applanate the corneal surface. Our study suggests that DCT measurements in healthy subjects seem to provide an estimate of IOP that is less influenced by corneal thickness than those provided by GAT and NCT. ACKNOWLEDGEMENTS

The study is registered at actr.org.au with the clinical trial registration ACTRN12607000411448. Clinical and Experimental Optometry 92.1 January 2009

18

The contents of this article were presented at the International Congress of Ophthalmology, Sao Paulo, Brazil, 19–24 February 2006. REFERENCES 1. Goldmann H, Schmidt T. Studien mittels Applanationstonographie. Doc Ophthalmol 1966; 20: 184–213. 2. Bhan A, Browning AC, Shah S, Hamilton R, Dave D, Dua HS. Effect of corneal thickness on intraocular pressure measurements with the pneumotonometer, Goldmann applanation tonometer and Tono-pen. Invest Ophthalmol Vis Sci 2002; 43: 1389–1392. 3. Brandt JD, Beiser JA, Kass MA, Gordon MO. Central corneal thickness in the Ocular Hypertension Treatment Study (OHTS). Am J Ophthalmol 2001; 108: 1779–1788. 4. Langham ME, McCarthy E. A rapid pneumatic applanation tonometer: comparative findings and evaluation. Arch Ophthalmol 1968; 79: 389–399. 5. Moses RA, Grodzki WJ. The pneumotonograph: A laboratory study. Arch Ophthalmol 1979; 97: 547–552. 6. Duch S, Serra A, Castanera J, Abos R, Quintana M. Tonometry after laser in situ keratomileusis treatment. J Glaucoma 2001; 10: 261–265. 7. Morgan AJ, Harper J, Hosking SL, Gilmartin B. The effect of corneal thickness and corneal curvature on pneumotonometer measurements. Curr Eye Res 2002; 25: 107–112. 8. Rao VJ, Gnanaraj L, Mitchell KW, Figueiredo FC. Clinical comparison of ocular blood flow tonometer, Tonopen and Goldmann applanation tonometer for measuring intraocular pressure in postkeratoplasty eyes. Cornea 2001; 20: 834–838. 9. Kanngiesser HE, Kniestedt C, Robert YC. Dynamic contour tonometry: presentation of a new tonometer. J Glaucoma 2005; 14: 344–350. 10. Whitacre MM, Stein R. Sources of error with use of Goldmann-type tonometers. Surv Ophthalmol 1993; 38: 1–30. 11. Duba I, Wirthlin AC. Dynamic contour tonometry for post-LASIK intraocular pressure measurements. Klin Monatsbl fur Augenheilkd 2004; 221: 347–350. 12. Kaufmann C, Bachmann LM, Thiel MA. Intraocular pressure measurements using dynamic contour tonometry after laser in situ keratomileusis. Invest Ophthalmol Vis Sci 2003; 44: 3790–3794. 13. Salvetant ML, Zeppieri M, Tosoni C, Brusini P. Comparisons between Pascal dynamic contour tonometry, the TonoPen and Goldmann applanation tonometry in patients with glaucoma. Acta Ophthalmol Scand 2007; 85: 272–279.

14. Stabuc Silih M, Hawlina M. Influence of corneal thickness on comparative intraocular pressure measurements with Goldmann and non-contact tonometers in keratoconus. Klin Monatsbl Augenheilkd 2003; 220: 843–847. 15. Tonnu PA, Ho T, Newson T, El Sheikh A, Sharma K, White E, Bunce C, GarwayHeath D. The influence of central corneal thickness and age on intraocular pressure measured by pneumotonometry, noncontact tonometry, the Tono-Pen XL and Goldmann applanation tonometry. Br J Ophthalmol 2005; 89: 851–854. 16. Sanchez-Tocino H, Bringas-Calvo R, Iglesias-Cortinas D. Comparative study between the non-contact pneumotonometer Canon TX10 and the Goldmann tonometer. Arch Soc Esp Oftalmol 2005; 80: 643–650. 17. Herndon LW, Choudhri SA, Cox T, Damji KF, Shields MB, Allingham RR. Central corneal thickness in normal glaucomatous and ocular hypertensive eyes. Arch Ophthalmol 1997; 115: 1137–1141. 18. Kniestedt C, Nee M, Stamper RL. Dynamic contour tonometry a comparative study on human cadaver eyes. Arch Ophthalmol 2004; 122: 1287–1293. 19. Barleon L, Hoffmann EM, Berres M, Pfeiffer N, Grus FH. Comparison of dynamic contour tonometry and Goldmann applanation tonometry in glaucoma patients and healthy subjects. Am J Ophthalmol 2006; 142: 583–590. 20. Ku JYF, Danesh-Meyer HV, Craig JP, Gamble GD, McGhee CNJ. Comparison of intraocular pressure measured by Pascal dynamic contour tonometry and Goldmann applanation tonometry. Eye 2005; 4: 1–8. 21. Siganos DS, Papastergiou GI, Moedas C. Assessment of the Pascal dynamic contour tonometer in monitoring intraocular pressure in unoperated eyes and eyes after LASIK. J Cataract Refract Surg 2004; 30: 746– 751. 22. Kniestedt C, Lin S, Choe J, Bostrom A, Nee M, Stamper RL. Clinical comparison of contour and applanation tonometry and their relationship to pachymetry. Arch Ophthalmol 2005; 123: 1532–1537.

Corresponding author: Dr Aysel Pelit Baskent University Faculty of Medicine Department of Ophthalmology Adana Teaching and Medical Research Center Dadalog˘lu Mh 39 sok No 6 Yüreg˘ir-Adana 01250 TURKEY E-mail: [email protected] © 2008 The Authors

Journal compilation © 2008 Optometrists Association Australia