Dina H. Erickson, O.D., a Denise Goodwin, O.D., a Michael Rollins, O.D., b Amber Belaustegui, O.D., c and Chad Anderson a

Optometry (2009) 80, 169-174 Comparison of dynamic contour tonometry and Goldmann applanation tonometry and their relationship to corneal properties, refractive error, and ocular pulse amplitude Dina H. Erickson, O.D., a Denise Goodwin, O.D., a Michael Rollins, O.D., b Amber Belaustegui, O.D., c and Chad Anderson a a Pacific University College of Optometry, Forest Grove, Oregon; b Nationwide Vision, Tucson, Arizona; and c Eye Care Associates of Nevada, Las Vegas, Nevada. KEYWORDS Intraocular pressure; Dynamic contour tonometry; Goldmann applanation tonometry; Central corneal thickness; Ocular pulse amplitude; Glaucoma; Myopia Abstract BACKGROUND: Accurate intraocular pressure (IOP) measurement is essential in diagnosing and managing glaucoma. Dynamic contour tonometry (DCT) is less dependent on corneal properties, such as thickness, elasticity, and rigidity, than Goldmann applanation tonometry (GAT). This study examined the relationship between GAT and DCT as well as their relationship with corneal properties and ocular pulse amplitude (OPA). METHODS: GAT, DCT, OPA, pachymetry, refractive error, and corneal curvature measurements were obtained on 115 healthy volunteers. RESULTS: Participants with thicker corneas (R580 mm) had higher IOP measurements with GAT than DCT (P 5 0.005). Those with thinner corneas (%520 mm) had lower IOP with GAT versus DCT (P 5 0.008). GAT and DCT readings did not differ significantly in corneas with average thickness (521 to 579 mm). A clinically significant IOP difference between DCT and GAT was found in 18.2% of subjects. A correlation was found between OPA and both refractive error and IOP (R 2 5.343, P, 0.0001). OPA was higher with increased IOP and decreased myopia. CONCLUSION: DCT provides IOP measurements that are less dependent on corneal factors than GAT, aiding in diagnosis and treatment of patients with ocular hypertension and glaucoma. Additional studies are necessary to examine the relationship between OPA, refractive error, and IOP and its possible association with increased incidence of glaucoma in myopic patients. Optometry 2009;80:169-174 Accurate measurement of intraocular pressure (IOP) is essential to the proper diagnosis and management of ocular hypertension and glaucoma. IOP is not only a major risk factor for glaucoma, its measurement is also an integral part of monitoring the treatment of the condition. Corresponding author: Dina H. Erickson, O.D., Pacific University College of Optometry, 2043 College Way, Forest Grove, Oregon 97116. E-mail: derickson@pacificu.edu Goldmann applanation tonometry (GAT) has been considered the gold standard for IOP measurement for several decades. However, the accuracy of GAT is limited because of the substantial effects of corneal properties, including thickness, elasticity, and rigidity. 1,2 Specifically, IOP as measured with GAT has been found to be underestimated with thin corneas and overestimated with thick corneas. Copt et al. 3 showed that after correcting IOP for central corneal thickness (CCT), 56% of the patients with ocular hypertension could be reclassified as normal, and 31% of 1529-1839/09/$ -see front matter Ó 2009 American Optometric Association. All rights reserved. doi:10.1016/j.optm.2009.01.013

170 Optometry, Vol 80, No 4, April 2009 patients with normal-tension glaucoma could be reclassified as having primary open-angle glaucoma. Several nomograms have been developed for adjusting GAT readings for varying CCT; however, these nomograms are not consistently accurate. 4-6 It is expected that the IOP is affected by CCT, but the exact numerical correction factor to account for CCT is uncertain. 7 The Pascal dynamic contour tonometer (DCT) is a slit lamp-mounted digital tonometer that has been introduced as an alternative method for measuring IOP independent of corneal properties. 8 The tonometer tip has a concave contour with a pressure sensor located at the tip that is thought to measure the IOP directly. The concave surface allows the cornea to assume its natural shape, theoretically creating equal pressure on both sides of the cornea. The sensor then acquires measurements of mean diastolic IOP. The DCT is also capable of measuring the ocular pulse amplitude (OPA). The OPA is the difference between the systolic and diastolic IOP readings. The OPA is thought to indirectly measure choroidal perfusion. Although the importance of this value in a clinical setting is not yet clear, a reduction in ocular blood flow may cause hypoxia and cell death, possibly contributing to diseases such as glaucoma. 9-11 In comparative studies of IOP on cadaver eyes, measurements with DCT were closer to the true manometric IOP than with GAT. 12,13 Additionally, the DCT has been shown to measure IOP without influence from CCT in patients who have undergone laser in situ keratomileusis (LA- SIK). 6,14-16 Although the majority of studies have found that DCT is less dependent on CCT than GAT, 15,17,18 some have found that DCT measurements depend as much on CCT as GAT readings. 11,19,20 Also, few studies address whether the difference between DCT and GAT is clinically significant in differentiating patients at risk for glaucoma. 21,22 Research on the differences and similarities between DCT and GAT is invaluable to eye care practitioners. It is important to determine if DCT is more accurate than current methods of IOP measurement and whether any differences are clinically relevant. The objective of this study was to examine the relationship between GAT, DCT, and CCT in healthy, nonsurgical eyes. We also examine the relationship between IOP, corneal curvature, refractive error, and OPA. Methods Data were obtained on 115 eyes of 115 healthy students enrolled at Pacific University College of Optometry. To reduce variability and not overestimate the effect of unquantifiable factors, such as corneal elasticity and rigidity, only right eyes were evaluated. The study was approved by the Institutional Review Board at Pacific University, and all participants gave informed consent before participating in the study. Each individual was screened before the study for the presence of abnormalities of the anterior segment that might compromise the results of the study. Individuals with a history of glaucoma, ocular inflammation, trauma, or corneal procedures that affect central corneal thickness were excluded from the study. Pregnant women and patients with sensitivity to anesthetic were also excluded from the study. Two trained examiners performed all testing. Automated keratometry and refractive error measurements were obtained using a Canon TX-F Auto Refractor/Auto Keratometer (Lake Success, New York). An anterior segment evaluation was then performed, followed by insertion of 1 drop of proparacaine HCl 0.5% in each eye. Three DCT readings using the Pascal tonometer (Swiss Microtechnology, Port, Switzerland) were obtained by 1 examiner followed by 3 GAT measurements and pachymetry by a second examiner. Prior studies have found that repeated measurements with DCT have minimal to no effect on IOP. 23 For this reason, DCT was performed before GAT. After each IOP measurement with the DCT, a quality assessment score is displayed digitally. The quality is graded from 1 to 5; a score of 1 is optimal. Scores of 2 and 3 are acceptable, and scores of 4 and 5 are unacceptable. Three DCT readings with a quality score of 3 or better were obtained. In accordance with manufacturer guidelines, a new protective cover was used on the DCT probe for each patient. The probe was left in place for 5 to 10 seconds to allow an accurate IOP and OPA measurement. A digital printout, including the IOP, OPA, and quality score was collected for each subject, and the readings were averaged for use in the statistical analysis. To avoid any influence on the findings, the DCT results were not revealed to the investigator obtaining GAT and pachymetry measurements. The GAT was set to 10 mmhg before each reading. The average from 3 GAT readings was used for statistical analysis. After the IOP measurements, 6 pachymetry readings were taken with the Sonogage pachymeter (Sonogage Inc., Cleveland, Ohio). In accordance with the manufacturer s guidelines, the first and last readings on each eye were eliminated, and the lowest of the remaining readings was used for statistical analysis. Statistical analysis was performed using the SPSS statistical software package, version 15.0 (SPSS Inc., Chicago, Illinois). A paired samples t-test was used to determine if a significant difference existed between DCT and GAT readings. Associations between variables were examined using the Pearson coefficient and P-value. Forward multiple regression was used to develop various models. A P-value less than 0.05 was considered statistically significant. Results The study included 115 right eyes of 115 healthy patients (62 women and 53 men). The mean age 6 SD of the patients was 26.4 6 4.4 years (age range, 22 to 46 years).

Erickson et al Issue Highlight 171 Table 1 Summary of statistics of the evaluated parameters Mean SD Minimum Maximum Pachymetry, mm 553.46 34.33 438.00 636.00 Flat keratometry, D 43.43 1.53 39.99 46.94 Steep keratometry, D 44.40 1.60 41.26 48.98 Corneal astigmatism, D.98.66.10 3.49 Spherical equivalent, D 22.72 2.79 213.25 1.75 GAT, mmhg 14.86 2.69 8.33 26.33 DCT, mmhg 14.90 1.90 10.50 20.83 OPA, mmhg 2.09.71.90 5.33 Difference DCT-GAT, mmhg 0.04 2.23 28.07 5.40 D, diopters. The majority of patients were white (n 5 95), followed by Asian (n 5 8), Hispanic (n 5 3), East Indian (n 5 2), and Filipino (n 5 2). Five subjects did not report ethnicity. Table 1 lists the mean, standard deviation, and range of the evaluated parameters. DCT readings were, on average, 0.04 mmhg higher than GAT measurements. Measurements with the 2 devices correlated significantly with each other (r 5 0.574, P, 0.0001). When comparing the reliability of the 3 GAT readings that were averaged for statistical analysis, the intraclass correlation coefficient (also referred to as reliability coefficient) demonstrated excellent reliability (ICC 5 0.963 with 95% confidence interval of 0.950 to 0.974). A high intraclass correlation coefficient was also found when comparing the DCT readings that were averaged for statistical analysis (ICC 5 0.942 with 95% confidence interval of 0.920 to 0.959). Table 2 shows the correlation coefficients for measured values. GAT showed a significant correlation with CCT (r 5 0.328, P, 0.0001). DCT did not show a significant correlation with CCT (r 520.38, P 5 0.687). GAT was not significantly influenced by corneal curvature or refractive error (r 5 0.040 and 20.046; P 5 0.669 and 0.628, respectively). DCT did show a significant correlation with corneal curvature (r 5 0.227, P 5 0.015) but not with refractive error (r 520.181, P 5 0.053). The impact of CCT on GAT was shown in the regression of DCT and CCT on GAT. The amount of variance accounted for by DCT alone on GAT was 33% (R 2 5 0.330, P, 0.001). Adding CCT to the equation added an additional independent amount of variance (R 2 5 0.122, P, 0.001), resulting in a model that accounts for 45% of the variance. DCT and GAT measurements were separated into groups based on corneal thickness: thick corneas (R580 mm), corneas with average central thickness (521 to 579 mm), and thin corneas (%520 mm). The data were evaluated with a paired t-test to determine if a significant difference existed between the three groups (see Table 3). Patients with an average corneal thickness (n 5 69) showed no statistically significant difference in IOP measured with DCT and GAT, with a mean difference of 0.12 mmhg (P 5 0.616). Those with thicker corneas (n 5 29) did have significantly higher IOP as measured with GAT versus DCT, with the mean difference of 1.19 mmhg (p 5 0.005). Patients with thinner corneas (n 5 17) were more likely to have a lower IOP with GAT compared with DCT, with a mean difference of 1.84 mmhg (P 5 0.008). These data are shown graphically in a Bland-Altman plot (see Figure 1). Eleven of the 115 participants (9.7%) had an IOP R3 mmhg higher when measured with GAT compared with DCT, 7 of whom had a CCT.580 mm. Ten subjects were found to have GAT lower than DCT by at least 3 mmhg (8.7%). Seven of these subjects had CCT,520 mm. The OPA showed moderate to substantial correlation with refractive error, DCT, and GAT (r 5 0.241, 0.481, and 0.282; P 5 0.010,,0.001, and 0.002, respectively). The OPA was higher in participants with increased IOP and lower with increasing myopia. A stepwise regression analysis was performed to predict the variables that most affect OPA. Two predictors (DCT and refractive error) account for 34.3% of the variance in the OPA (R 2 5 0.343, P, 0.0001). Table 2 The correlation coefficients for evaluated variables Pachymetry Flat K reading Corneal astigmatism Spherical equivalent GAT DCT OPA DCT - GAT Pachymetry 1 Flat keratometry reading 20.149 1 Corneal astigmatism 0.097 20.109 1 Spherical equivalent refractive error 20.025 20.173 20.142 1 GAT 0.328* 0.040 20.018 20.046 1 DCT 20.038 0.227 0.095 20.181 0.574* 1 OPA 20.069 0.165 0.022 0.241* 0.282* 0.481* 1 DCT - GAT 20.428* 0.145 0.102 20.099 20.719* 0.157 0.069 1 * Correlations significant at a 5 0.01 level. Correlations significant at a 5 0.05 level.

172 Optometry, Vol 80, No 4, April 2009 Table 3 The means and statistical significance level of DCT and GAT for various classifications of corneal thickness Corneal thickness classification Discussion N Mean DCT Mean GAT Mean DCT-GAT P-value (2-tailed) Thin (%520 mm) 17 15.13 13.29 1.84 0.008 Average (521-579 mm) 69 14.96 14.84 0.12 0.616 Thick (R580 mm) 29 14.64 15.83 21.19 0.005 IOP measurement is essential in the diagnosis and management of ocular hypertension and glaucoma. Goldmann applanation tonometry is the current gold standard for IOP measurement; however, it has been well-established in the literature that the accuracy of this method is limited by corneal thickness and other corneal properties. 12,13,24 Therefore, GAT may produce inaccurate readings with eyes that have atypical corneal thickness or have undergone LASIK, creating several clinical implications. Patients with normal IOP may be misclassified as having ocular hypertension caused by thicker corneas producing an artificially high IOP as measured with GAT. Furthermore, there may be a delay in diagnosing glaucoma in patients with thinner corneas because of an artificially low IOP. 3 It is, therefore, important to determine whether alternative tonometry methods that minimize the effect of CCT and other corneal factors will be an accurate substitute for GAT in clinical practice. Several interesting findings emerged from the current study. Both thin and thick corneas showed a statistically significant difference between DCT and GAT, with GAT having higher IOP readings in thick corneas and DCT having higher IOP readings in thin corneas. This finding is in agreement with those of other studies. 25,26 As expected, there was no statistically significant difference between DCT and GAT in participants with corneas of average thickness. A 3-mmHg or greater difference in IOP with GAT and DCT was found in 18.2% of patients. A difference of 3 mmhg was chosen because it was felt to represent a clinically significant change in IOP measurement. Eleven of the 115 participants (9.7%) had an IOP R3 mmhg higher when measured with GAT compared with DCT, 7 of whom had a CCT.580 mm. This result is similar to that found by Francis et al. 21 who reported that 7.7% of Hispanic subjects had a clinically significant GAT reading that was higher (R3 mmhg) than that measured with DCT. The same study found that 32.7% of Hispanic subjects had a GAT measurement lower than DCT by at least 3 mmhg. 21 The results of the current study found 10 subjects with a clinically significant difference in which GAT was lower than DCT by at least 3 mmhg (8.7%). There was a significant correlation between CCT and GAT but not CCT and DCT, indicating that corneal Difference DCT-GAT (mmhg) 5.00 2.50 0.00-2.50-5.00-7.50 10.00 Corneal Thickness 12.00 14.00 16.00 18.00 20.00 Average of DCT and GAT (mmhg) thin average thick 22.00 Figure 1 Bland-Altman plot comparing the mean and difference between DCT and GAT. thickness affects IOP as measured by GAT considerably more than DCT. There was a moderate to substantial correlation between CCT and the difference between DCT and GAT, representing the increase in difference between DCT and GAT readings with thicker and thinner corneas. Of particular interest are 6 study participants who displayed a clinically significant difference between DCT and GAT measurements despite a CCT between 521 and 579 mm. The IOP of these subjects ranges from a GAT reading 8.07 mmhg higher than DCT, to an IOP 4.47 mmhg lower with GAT versus DCT. This large difference in IOP cannot be accounted for solely by corneal thickness. These cases may illustrate the potential effect of corneal biomechanical properties, independent of CCT, as influencing factors on the accuracy of GAT measurements. 27,28 Measurements of IOP with both DCT and GAT appeared to be unaffected by refractive error. This finding is consistent with those of other studies. 26,29 GAT did not correlate with corneal curvature, but there was a statistically significant correlation between corneal curvature and DCT. Francis et al. 21 also found a correlation between corneal curvature and DCT but not between corneal curvature and GAT. This study hypothesized that the relationship between these variables may be caused by steeper corneas needing to flatten to conform to the curvature of the DCT probe. This deformation may cause an increase in IOP. Other studies have found no correlation between corneal curvature and GAT or DCT. 4,23,26 An interesting finding in our study was the correlation between OPA and refractive error as well as IOP measured with either DCT or GAT. Participants with increased IOP had higher OPA, and those with increased myopia had

Erickson et al Issue Highlight 173 lower OPA. The OPA is thought to indirectly measure the choroidal perfusion. 30 A reduced blood flow may cause hypoxia and cell death initiating diseases such as glaucoma. 31 Similarly, Kaufmann et al. 8 found an inverse relationship between axial length and OPA. This relationship is thought to occur because an equal volume of blood flow in a larger myopic globe will represent a smaller relative volume than that in a shorter emmetropic eye. Studies have also reported that eyes with higher IOPs tend to have a higher OPA. 8,32,33 With increased IOP, the tension of the scleral wall increases. An increase in blood volume will then cause the IOP to further increase rather than cause extension of the already stretched globe. 8 Weizer et al. 11 found that increased OPA is associated with smaller vertical and horizontal cup-to-disc ratios and decreased severity of glaucoma. Likewise, Schwenn et al. 10 found OPA to be an independent risk factor for normal-tension glaucoma. The results of this study add important detail to the association between OPA, myopia, and glaucoma and may support the vascular theory of glaucomatous damage in myopic patients. Additional studies are needed to determine if the relationship between high OPA, axial length/ refractive error, and IOP might be related to the increased incidence of glaucoma in myopic patients. Future studies should examine measurements of the axial length directly in addition to measuring refractive error to determine if the relationship between axial length and OPA is similar to the relationship between refractive error and OPA. Several observations were noted while using the DCT for this study. First, there is a learning curve associated with using the device and obtaining adequate quality scores. Second, practitioners have to rely on sound as well as sight to take accurate readings. DCT also requires a longer applanation period lasting approximately 5 to 10 seconds. This was not a problem for most study participants but did make measurements slightly more difficult to obtain than with GAT. The study is limited by the lack of independent reference, which would allow the measurement of the true IOP. In vivo manometric studies would be helpful to further elucidate the relationship between DCT, GAT, and CCT. Unfortunately, the invasive nature of this technique makes this information difficult to obtain. Conclusion Dynamic contour tonometry provides an IOP measurement that is less dependent on corneal factors than GAT. Patients with thicker corneas tended to have higher GAT IOP measurements when compared with DCT, and patients with thinner corneas tended to have lower IOP measurements with GAT than DCT. This difference is clinically significant in 18.2% of cases. 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