Purpose: The accuracy of Goldmann applanation tonometry (GAT) has been shown to depend on several biomechanical properties of the cornea. Newer tonometry devices (e.g., the Dynamic Contour Tonometer PASCAL? [DCT] and the Tono-Pen? XL [TP]) have been designed to accurately measure intraocular pressure (IOP) independent of corneal thickness (CCT) and pathology. This study investigates the influence of corneal pathologies on the accuracy of these IOP measuring devices, and compares this accuracy to that of direct intracameral IOP measurement. Methods: 8 eyes of 8 patients suffering from corneal pathologies scheduled for penetrating keratoplasty, and 10 eyes of 10 patients scheduled for cataract surgery (control group) were examined. Before surgery, the anterior chamber was cannulated at the temporal corneal limbus. In a closed system, the intraocular pressure (IOP) was directly set to 10, 20, and 30 mmHg with a manometric water column. Intraocular pressure measurements taken by GAT, DCT, and TP were compared to intracameral measurements obtained by a precision reference pressure sensor. Results: Control group: All three methods showed good agreement with the intracameral readings (mean deviation of all three devices, -0.9 mmHg). Group with corneal pathologies: The TP yielded the most exact IOP values in the group with corneal pathologies when taking all diagnoses into account. The mean deviations from the intracameral IOP measurements were -0.9 mmHg ± 3.2 mmHg (mean ± SD) for TP, -2.9 mmHg ± 3.3 mmHg for GAT, and -5.0 mmHg ± 7.9 mmHg for DCT. For bullous keratopathy, the most exact IOP readings were obtained by the TP (mean deviation -0.2 mmHg ± 3.5 mmHg). The TP and GAT devices underestimated IOP in the patients with Fuchs’ endothelial dystrophy; all 3 devices underestimated adjusted IOP after keratoplasty. DCT showed the greatest deviations from adjusted IOP in the case of non-herpetic scars. In the control group, none of the devices showed a statistically relevant dependency on CCT. Nevertheless, in the group with corneal pathologies, only TP showed no dependency on CCT. Conclusion: Our results suggest that the Tono-Pen XL? is the most accurate measurement device to determine IOP in patients with corneal pathologies, especially in patients suffering from corneal edema (bullous keratopathy). GAT yielded surprisingly exact IOP values in patients suffering from irregular corneal surface. DCT showed a high degree of deviation from the adjusted IOP, and should not be used to determine IOP in corneas with the disorders listed here.
For more than 50 years, Goldmann Applanation Tonometry (GAT) has been the gold standard for measuring intraocular pressure (IOP) [
Several new devices aiming for correct IOP measurement that is less dependent on the cornea’s biomechani- cal properties have been developed. Two of these devices are the Dynamic Contour Tonometer (PASCAL®, DCT; Ziemer Ophthalmic Systems AG, Port, Switzerland) and the Tono-Pen XL® (TP; Reichert, Depew, New York, United States).
The purpose of this study was to investigate the accuracy of GAT and these new devices compared to direct intracameral IOP readings in a prospective in vivo study in patients suffering from corneal pathologies.
8 patients (a total of 8 eyes) scheduled for penetrating keratoplasty due to corneal pathology were included in this study. Four patients were experiencing bullous keratopathy after cataract surgery; one patient had non-her- petic scars; one patient had Fuchs’ endothelial dystrophy; and two patients needed to undergo keratoplasty again due to graft failure or corneal scarring. All eyes included in this group were pseudophakic. In a control group, 10 patients without corneal pathology who were scheduled for cataract surgery were included.
The indications for penetrating keratoplasty and cataract surgery were independent from participation in this study. Exclusion criteria included any history of ocular surgery in the 4 weeks prior to penetrating keratoplasty or cataract surgery, or any active intraocular inflammation. Patients scheduled for penetrating keratoplasty had to be pseudophakic to participate in the study. The presence of any corneal pathology was an exclusion criterion for the control group.
Before surgery, central corneal thickness (CCT) was measured with an ultrasonic pachymeter (PachetteTM, DGH Technology Inc., Exton, Pennsylvania, United States).
For penetrating keratoplasty, patients were anesthetized with general anesthesia. Cataract surgery was per- formed either in general anesthesia or in retrobulbar anesthesia (injection of 5 ml mepivacaine [Scandicain] 2% 15 minutes prior to surgery; no oculopression was performed).
All IOP measurements were performed before surgery by one of the authors (MN). With the aid of a laser water scale, the tubing system used to manometrically determine the IOP was adjusted to the height of each pa- tient’s anterior chamber before the measurement was taken.
The experimental setting for adjusting the IOP has been described elsewhere in detail [
Measurement with the DCT (the measurement principle for the DCT has been described in detail elsewhere [
Cannula (left) to be placed in the patient’s anterior chamber. A: Reference sensor for direct intracameral IOP reading. IOP was directly calibrated by changing the height of the water column in relation to the anterior chamber (open system) to the respective IOP level (10, 20, and 30 mmHg). After each IOP level was reached, the system was closed by turning the three way cocks (B and C), and three consecutive measurements (DCT, GAT, TP, and simultaneously with the reference sensor) were taken (closed system)
Dynamic contour tonometer measurement with a modified Per- kins tonometer. The intracameral cannula (red arrow) is fixed with sterile tapes to ensure its tension-free position during the measurement
(
The Tono-Pen® XL (TP; the way it works has been detailed elsewhere [
Three consecutive measurements with GAT were performed using the Perkins tonometer (HAAG-STREIT AG, Koeniz, Switzerland); the average of these three measurements was calculated.
In the following description, the term DCT measurement refers to the measurement with the handheld DCT; the term GAT refers to the measurement with the Perkins tonometer; the term TP refers to measurements taken with the Tono-Pen® XL; and the reference point is the DCT tip integrated into the tube system to obtain intra- cameral measurements.
All patients signed informed consent before entering the study. The study was performed in accordance with the Declaration of Helsinki and was approved by the institutional ethics committee of the Medical Department of the University of Freiburg.
Statistical analysis: We calculated the measurement errors from the difference between the intracameral ref- erence pressure and the respective measurement result. We computed average measurement errors for each de- vice and reference pressure separately. We plotted measurement errors against reference pressure for each de- vice, and performed a regression analysis by means of a linear model. Intercept and slope of the regression lines were used to rank the devices for validity. The regression and the dependency of the measurement results on CCT were tested for statistical significance by means of analysis of variance (ANOVA). All computations were performed with the R programme (www.r-project.org).
Control group (
Group with corneal pathologies (
. Intercept and slope of the regression lines of each device in the respective group (DCT: Dynamic contour tonometry; GAT: Goldmann applanation tonometry; TP: Tono-Pen)
Diagnosis/Group | Intercept | Slope | ||||
---|---|---|---|---|---|---|
DCT | GAT | TP | DCT | GAT | TP | |
Control group (Cataract) | 1.8 | 0.8 | 2.2 | 0.9 | 0.9 | 0.9 |
Keratoplasty (all diagnosis) | 6.1 | 1.3 | 2.9 | 0.4 | 0.8 | 0.8 |
Bullous keratopathy | −0.4 | 0.5 | 4.1 | 0.7 | 0.8 | 0.8 |
Fuchs’ endothelial dystrophy | 7.2 | 0.8 | −3.2 | 0.6 | 0.9 | 1.0 |
Post-Keratoplasty | 18.9 | 1.8 | 5.7 | 0.003 | 0.8 | 0.7 |
Nonherpetic scars | −0.4 | 4.0 | −2.7 | 0.4 | 0.7 | 1.0 |
Measurement error of dynamic contour tonometry (DCT), Goldmann applanation tonometry (GAT), and Tono-Pen XL (TP) against the reference sensor in the group with cor- neal pathologies (red) and the cataract group (blue). X-axis: respective device. Y-axis: mea- surement error in mmHg. Tono-Pen showed the best agreement with the adjusted IOP, while GAT and especially DCT underestimated the IOP. Dynamic contour tonometry also showed the greatest scattering of all three devices
Bullous keratopathy (
Fuchs’ endothelial dystrophy (
Post-keratoplasty (
Non-herpetic scars (
Central corneal thickness (CCT) (
The measurement error of dynamic contour tonometry (DCT), Goldmann applanation tonometry (GAT), and Tono-Pen (TP) against the reference sensor for the respective diagnosis. X-axis: respective device. Y-axis: measurement error in mmHg. Green, yellow, red box: adjusted IOP level (10, 20, 30 mmHg, respectively). DCT and GAT underestimate the adjusted pressure in patients suffering from bullous keratopathy, while all three devices yield quite accurate measurement results in Fuchs’ endothelial dystrophy. The TP and GAT yield exact IOP values on an irregular cornea surface (as with non-herpetic scars, and post- keratoplasty), while DCT underestimates the IOP at increasing IOP (non-herpetic scars) or shows a high degree of scattering (post-keratoplasty)
The gold standard for measuring IOP, Goldmann Applanation Tonometry (GAT), is known to depend on CCT and several other corneal properties (edema, tear film, astigmatism) [
In recent years, several methods to measure IOP independent from corneal properties have been developed. Dynamic Contour Tonometry (PASCAL®, DCT) is based on the physics of Pascal’s law. This device is said to measure IOP independent from CCT or other biomechanical corneal properties; this has been confirmed in sev- eral clinical trials [
Measurement error of dynamic contour tonometry (handheld, filled circle), Goldmann applanation tonometry (Perkins, open circle), and Tono-Pen (Tono-pen, filled square) against the reference sensor depending on CCT in the cata- ract group (a) and the group with corneal pathologies (b). X-axis: Central corneal thickness (CCT) in µm. Y-axis: Mea- surement error in mmHg. In the cataract group (a), none of the devices showed a statistically relevant dependency on CCT, while in the keratoplasty group only the TP showed no statistically relevant dependency on CCT. In this group, GAT and especially DCT underestimated the adjusted IOP at increasing CCT
viscoelastic properties should also be taken into account as another potential influencing factor. After penetrat- ing keratoplasty, Viestenz found that DCT yielded reproducible IOP values that were higher in comparison to those for GAT, and were disturbed by postoperative intrastromal sutures [
The Tono-Pen® XL (TP) is a handheld tonometer operating on the same principle as the Mackay-Marg tono- meter [
In or study, we wanted to examine which of the three devices for measuring IOP (GAT, TP, DCT) yielded the most exact IOP values in patients with corneal pathologies. We therefore used a closed system that allowed for adjustment of the IOP of each patient manometrically to 10, 20, and 30 mmHg, respectively, and for control of this adjusted IOP through a reference sensor. At each pressure level, we measured the IOP using each of the three devices.
This study was performed in 8 patients scheduled for penetrating keratoplasty due to bullous keratopathy (n = 4), Fuchs’ endothelial dystrophy (n = 1), non-herpetic scars (n = 1); or after keratoplasty (n = 2), as well as in 10 patients scheduled for cataract surgery who served as the control group.
In the control group, we found all three devices to yield exact IOP values. Each device showed a mean devia- tion from the IOP value of −0.9 mmHg. These results indicate the reliability and accuracy of our measurement configuration. None of the three devices in the control group showed a statistically relevant dependency on CCT. For the DCT, this is in accordance with most studies; the literature has generally reported that DCT is indepen- dent of CCT [
In the group with corneal pathologies, the TP yielded the most accurate IOP measurements for all three de- vices, followed in accuracy by GAT and DCT. For the entire group of patients with corneal pathologies, the IOP measurement results for TP were just as accurate as those for the control group. Several other studies have re- ported similar results, with the TP yielding exact IOP values after penetrating keratoplasty [
Despite the small number of patients, a statistically significant negative correlation existed between the CCT and the IOP (GAT) and especially the IOP (DCT) measurement results in the group with corneal pathologies for all diagnoses. Most patients in the study group suffered from pathologies that led to edematous swelling of the cornea (Fuchs’ endothelial dystrophy, bullous keratopathy, graft failure). Indeed, mean CCT in the group with corneal pathologies was significantly higher than in the control group (559 µm vs. 616 µm; P < 0.05). Our re- sults are therefore in accordance with Simon and also Oh, who recorded significantly lower IOP (GAT) meas- urement results in edematous corneas [
Due to the small number of patients in the respective group suffering from Fuchs’ endothelial dystrophy, non- herpetic scars, bullous keratopathy, and post-keratoplasty, an interpretation of our results with respect to each individual diagnosis is difficult (
One possible explanation for the accuracy of the IOP measurement results obtained by TP may be the small contact area of this device with the cornea, and the minor distortion of the cornea during measurement. Possible biomechanical properties of the cornea, such as edema or a higher degree of stiffness due to scarring, therefore may have less influence on the measurement procedure. The higher resistance against distortion caused by cor- neal scarring, or the lower resistance due to corneal edema, might explain the worse results obtained by GAT and especially by DCT in our study.
The small number of patients in the study group was a major important limitation of our study. This small number makes statistically significant statements regarding the respective diagnoses difficult. Nevertheless, when taking all diagnoses into account, the results concerning the high accuracy of TP and its independence from CCT measurements as well as the dependance of GAT and DCT from CCT measurements were all statis- tically significant. Regarding DCT, the measurement procedure with the modified Perkins tonometer was prob- lematic, as the device had to be held in a stable position over the supine patient for several seconds. It was therefore sometimes complicated to obtain measurement results of adequate quality. Nevertheless, as only measurement results with a quality grade of 3 or better were provided in the statistical analysis, the difficulties that occurred during the measurement procedure could not have played a role in the interpretation of the poor quality of the measurement results obtained by this device.
In summary, our results suggest that the Tono-Pen XL® is the most accurate measurement device for the de- termination of IOP in patients with corneal pathologies, especially in patients suffering from corneal edema (bullous keratopathy), probably due to its relatively small contact area with the cornea. These results are in ac- cordance with several other groups reporting on its accuracy compared to GAT after penetrating keratoplasty. To our knowledge, our study is the first to reveal this accuracy when comparing the obtained IOP values to di- rect intracameral readings.
GAT yielded surprisingly exact IOP values in patients with an irregular corneal surface (non-herpetic scars, post-keratoplasty), and underestimated IOP in patients suffering from corneal edema. Apart from Fuchs’ endo- thelial dystrophy, DCT showed great deviations from the adjusted IOP in all patient groups and should not be used to determine IOP in corneas with any pathologies or disorders.