|Year : 2013 | Volume
| Issue : 2 | Page : 158-162
Dynamic contour tonometry in primary open angle glaucoma and pseudoexfoliation glaucoma: Factors associated with intraocular pressure and ocular pulse amplitude
Sasan Moghimi1, Hamidreza Torabi1, Ghasem Fakhraie1, Nariman Nassiri2, Massood Mohammadi1
1 Department of Ophthalmology, Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran, Iran
2 Vanak Eye Surgery Center, Tehran, Iran
|Date of Web Publication||16-Apr-2013|
Farabi Eye Research Center, Farabi Eye Hospital, Tehran University of Medical Sciences, Qazvin Sq., Tehran
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Purpose: To compare the intraocular pressures (IOP) and ocular pulse amplitudes (OPAs) in patients with primary open-angle glaucoma (POAG) and pseudoexfoliation glaucoma (PXG), and to evaluate ocular and systemic factors associated with the OPA.
Materials and Methods: In this prospective study, on 28 POAG and 30 PXG patients, IOP was measured with the Goldmann applanation tonometry (GAT) and the Pascal dynamic contour tonometry (DCT). Other measurements included central corneal thickness (CCT), vertical cup-to-disc ratio (CDR), and systolic and diastolic blood pressure. Statistical significance was defined as P < 0.05.
Results: In each of the POAG and PXG groups, GAT IOP was correlated with CCT (r = 0.40, P = 0.03 and r = 0.35, P = 0.05, respectively), whereas DCT IOP and CCT were not correlated. In all patients and in the POAG group, OPA was positively correlated with DCT IOP (r = 0.39, P = 0.002). OPA was not correlated with CCT in the POAG (P = 0.80), nor in the PXG (P = 0.20) group, after adjusting for DCT IOP. When corrected for DCT IOP and CCT, there was a significant negative correlation between OPA and vertical CDR in all patients (r = −0.41, P = 0.002). There was no significant difference in OPA between groups (P = 0.55), even when OPA was adjusted for IOP and systolic and diastolic pressure (P = 0.40), in a linear regression model.
Conclusion: DCT IOP and OPA are not correlated with CCT. There is no significant difference between the OPA of PXG and POAG eyes. OPA is correlated with DCT IOP, and is lower in eyes with more advanced glaucomatous cupping.
Keywords: Dynamic Contour Tonometry, Ocular Pulse Amplitude, Primary Open-Angle Glaucoma, Pseudoexfoliation Glaucoma
|How to cite this article:|
Moghimi S, Torabi H, Fakhraie G, Nassiri N, Mohammadi M. Dynamic contour tonometry in primary open angle glaucoma and pseudoexfoliation glaucoma: Factors associated with intraocular pressure and ocular pulse amplitude. Middle East Afr J Ophthalmol 2013;20:158-62
|How to cite this URL:|
Moghimi S, Torabi H, Fakhraie G, Nassiri N, Mohammadi M. Dynamic contour tonometry in primary open angle glaucoma and pseudoexfoliation glaucoma: Factors associated with intraocular pressure and ocular pulse amplitude. Middle East Afr J Ophthalmol [serial online] 2013 [cited 2019 Dec 11];20:158-62. Available from: http://www.meajo.org/text.asp?2013/20/2/158/110606
| Introduction|| |
Dynamic contour tonometry (DCT) is a novel, digital, non-applanation method for measuring intraocular pressure (IOP). DCT measurements are very similar to true manometric IOP levels.  DCT is designed mitigate the effect of corneal curvature or corneal thickness which can reduce the accuracy of Goldmann applanation tonometry (GAT). ,
The continuous dynamic measurement of IOP with DCT, yields pressure curves from which the ocular pulse amplitude (OPA) can be determined. The OPA value is the difference between the average systolic IOP and the average diastolic IOP. Current theory states that OPA is generated by the bolus of blood that is pumped into the eye with each cardiac cycle, , thus, representing an indirect measure of intraocular pulsatile blood flow. 
To the best of our knowledge, there is limited information about the relationship between OPA and ocular blood flow, as well as factors that may contribute to or alter these pulsations. Some investigators have studied the clinical significance of OPA measurement in glaucoma, and one report  suggests that increased OPA might correlate with less severity of glaucoma. Although others  have reported a lower pulsatile ocular blood flow (POBF) in pseudoexfoliation glaucoma (PXG), few studies have actually measured OPA in these patients, or compared it to OPA in patients with primary open-angle glaucoma (POAG). 
In this study, we measured the DCT IOP and OPA in patients with PXG and POAG. We also evaluated the factors that affected OPA in each group, and tried to correlate these factors with the severity of the disease.
| Materials and Methods|| |
A prospective study was conducted between September 2008 and September 2009 at the Farabi Eye Hospital, Tehran, Iran. The study protocol was reviewed and approved by the institutional ethics committees, and adhered to the tenets of the Declaration of Helsinki. Written informed consent was obtained from all participants.
Patients with POAG and PXG were recruited into the study. Glaucoma was defined by the presence of characteristic optic disc changes and visual field loss, and an untreated IOP of 21 mmHg or more. In patients with unilateral disease, the eye with glaucoma was included in the study, and in bilateral cases, only the right eye was included. Exclusion criteria were a history of ocular trauma, intraocular surgery including laser and incisional glaucoma procedures, and eye diseases other than glaucoma. All patients were dilated for detecting pseudoexfoliation material on the lens or pupillary border. Topical anti-glaucoma therapy was not discontinued during the study.
Patient characteristics included age, gender, and type of glaucoma (POAG or PXG). All patients underwent a complete ophthalmic examination and blood pressure (BP) measurement (in the sitting position). Ophthalmic examination included best-corrected visual acuity (BCVA), slit-lamp funduscopy using a + 78 diopter lens, tonometry, central corneal thickness (CCT) measurement, gonioscopy, and OPA measurement.
The BCVA was measured using a Snellen chart calibrated for a 20-foot distance by the line assignment method. CCT was measured by pachymetry (UP-1000 Nidek; Nidek Co, Tokyo, Japan). In each patient, IOP was measured first using the GAT (AT-900; Haag-Streit AG, Koniz, Switzerland) mounted on a slit lamp, and then with the Pascal DCT (Pascal DCT; Swiss Microtechnology AG, Port, Switzerland).
Both types of tonometry, GAT and DCT, were performed by one examiner (Hamidreza Torabi). All measurements were performed between 8 A.M and 10 A.M. All measurements were repeated twice, and their average was recorded. There was a 15-min break between GAT and DCT. Only DCT measurements with quality 1 and 2 were included in data collection. For CCT, the ultrasonic probe was placed on the center of the cornea and the mean of five readings within a range of ± 5 μm was used for analysis. After these examinations and measurements, an experienced examiner (Sasan Moghimi) who was masked to the IOP readings and OPA, performed dilated slit-lamp funduscopy and determined the vertical cup-to-disc ratio (CDR). After the GAT and DCT measurements, systolic and diastolic BPs and CCT were measured and recorded.
Data were analyzed using SPSS Software (Version 17, SPSS Inc., Chicago, Illinois, USA). Demographic data were compared using the t-test and Chi square test. Pearson correlation and partial correlation were used do define the correlation between variables. A linear regression model was used to verify the effect of different variables, such as gender, age, GAT IOP, type of glaucoma, systolic and diastolic pressures and pulse pressure on OPA. P < 0.05 was considered statistically significant.
| Results|| |
Fifty-eight eyes of 58 patients were recruited for this study. The female/male ratio was 31/27. [Table 1] summarizes the baseline characteristics of all patients (both POAG and PXG) in the study. Twenty eight patients had POAG and 30 had PXG. [Table 2] presents the patient characteristics in each group. Six patients in the POAG group (21%) and eight patients in the PXG group (27%) were on systemic antihypertensive medications (P = 0.43). There was no statistically significant difference in baseline values between groups, except for the mean systolic and diastolic BPs, which were higher in PXG patients, and for the mean CCT, which was lower in PXG eyes (P < 0.05, all comparisons).
|Table 1: Demographic and baseline characteristics of all study subjects |
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|Table 2: Demographic and baseline characteristics of patients in the primary open-angle and pseudoexfoliation glaucoma groups |
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GAT IOP, DCT IOP and CCT
In all patients in both groups, DCT IOP was correlated to GAT IOP (r = 0.88, P < 0.001). In all patients of both groups, GAT IOP was correlated with CCT (r = 0.23, P = 0.02). There was no statistical correlation between DCT IOP and CCT (P = 0.23). In each of the POAG and PXG groups separately: GAT IOP was also correlated with CCT (r = 0.40, P = 0.03 and r = 0.35, P = 0.05, respectively); DCT IOP and CCT were not significantly correlated (P = 0.07 and P = 0.65, respectively). DCT IOP significantly overestimated GAT IOP in the POAG and PXG groups (P < 0.001 and P = 0.01, respectively). The difference between DCT IOP and GAT IOP were not statistically significantly different between groups (P = 0.44).
OPA with DCT IOP, CCT and CDR
In the entire study population, and in the POAG group, OPA was positively correlated with DCT IOP (r = 0.39, P = 0.002; r =0.62, P < 0.001, respectively). The correlation between OPA and DCT IOP were not significant in PXG patients (P = 0.22). [Table 3] presents the results of univariate correlation of OPA and different variables in the PXG and POAG groups. OPA was not correlated with CCT in the entire study population (P = 0.18), the POAG group (P = 0.80) or the PXG group (P = 0.20), after adjusting for DCT IOP.
|Table 3: Correlation (univariate) of different variables with ocular pulse amplitude in primary open-angle (n = 28) and pseudoexfoliation (n = 30) glaucoma groups |
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When corrected for DCT IOP and CCT, there was a significant negative correlation between OPA and vertical CDR in all patients (r = −0.41, P = 0.002).
Correcting for DCT IOP, there was a significant negative correlation between OPA and vertical CDR in the POAG and PXG groups (r = −0.22, P = 0.05 and r = −0.57, P = 0.002 for the POAG and PXG groups, respectively).
OPA and systolic, diastolic and pulse pressure
Using univariate analysis, there was no correlation between OPA and mean pulse pressure (P = 0.09), diastolic pressure (P = 0.29), and systolic pressure (P = 0.14) in all patients. Using a linear regression model, age, gender, DCT IOP, CCT, type of glaucoma, diastolic and systolic BPs, and pulse pressure were investigated. Linear regression indicated the variables that were found to affect OPA were DCT IOP (β = 0.09, P = 0.006) and pulse pressure (β = 0.03, P = 0.04). There was no significant differences in OPA between groups (Student t-test, P = 0.55). This was also the same if OPA was corrected for IOP, systolic pressure and diastolic pressure (P = 0.40) in the linear regression model. Factors that affected OPA with multiple regression analysis, were DCT IOP (β = 0.15, P = 0.007) and age (β = −0.07, P = 0.02) in the POAG group, and pulse pressure in the PXG group (β = 0.08, P = 0.01).
| Discussion|| |
Our study identified factors that can influence DCT IOP and OPA measurements. In this study, DCT IOP was significantly higher than GAT IOP in all the patients (P < 0.001 and P = 0.01 in POAG and PXG, respectively). Other studies have also consistently reported DCT IOP to be higher than GAT IOP. ,, Punjabi et al.  evaluated the difference between DCT IOP and GAT IOP in patients with PXG, POAG, ocular hypertension (OHT), normal-tension glaucoma (NTG) and normal controls (NC). The difference was statistically significant in all groups (POAG, NTG, PXG, and NC), except in OHT. The PXG group had the highest difference in the tonometer readings, and this difference was significantly larger than the other groups (PXG vs. POAG, P = 0.02; PXG vs. NC, P = 0.005). They proposed that the difference might be due to a difference in corneal rigidity in PXG patients. The difference between GAT IOP and DCT IOP were correlated with ocular rigidity changes in eyes under treatment with latanoprost.  However, we found that DCT IOP significantly overestimated GAT IOP in the POAG and PXG groups, yet, the difference between DCT IOP and GAT IOP was not significant between groups (P > 0.05). This difference between the previous study  and ours may be due to the smaller sample size in our study.
We also found a positive correlation between CCT and GAT IOP levels. This outcome concurs with the assumption that GAT overestimated IOP in thicker corneas.  However, DCT IOP did not show a significant correlation with CCT in our cases (P > 0.05).
This finding was compatible with previous reports that demonstrated IOP measurements by DCT were theoretically less dependent on ocular biomechanical properties, compared to GAT. , We found the same results for the POAG and PXG groups, separately. Similarly, Grammenandi et al.  reported that the DCT IOP was less dependent on CCT than GAT IOP in the PXG group.
In the present study, we found a positive correlation between OPA and DCT IOP when the entire study population was investigated and in the POAG group. This positive correlation between OPA and IOP had been previously reported in both healthy and glaucomatous subjects. ,,, Stalmans et al.  explanation for this observation was that identical volume changes in a sphere (caused by the blood volume pumped into the eye during each cardiac cycle) can cause greater pressure changes (OPA) within this sphere (eye) when the pressure is higher, which is due to the elastic properties of the eye wall. As reported in previous studies, our data indicates that CCT does not have a significant influence on the OPA as it may theoretically affect OPA through its impact on IOP measurement. , The properties of the sclera are likely to have an important influence on OPA. , In fact, scleral properties (rigidity) may vary with the level of IOP, which may contribute to the fact that OPA increases with higher IOP.
Using various techniques, numerous studies on the vascular aspects of glaucoma have led to the consensus that ocular perfusion plays a role in the pathogenesis of glaucoma. ,,, Several studies using different methodologies have indicated that the pulsatile ocular blood flow (POBF) is reduced in patients with glaucoma. ,, This reduction in glaucomatous patients is not likely due to atherosclerosis, but rather to a systemic and partially primary vascular dysregulation, leading to both low-perfusion pressure and insufficient autoregulation. ,, This in turn may lead to unstable ocular perfusion and thereby to ischemia and reperfusion damage.  In our study, we detected a negative correlation between the OPA and the degree of structural damage as measured by the vertical CDR in all patients as well as in each group separately. Similarly, Weizer et al.  reported that increased OPA was significantly associated with smaller mean vertical and horizontal CDRs. Vulsteke et al.  showed that there was a significant relationship between OPA and mean deviation and pattern standard deviation, with and without correction for IOP in different types of glaucoma. They concluded that a small OPA was correlated with moderate to severe glaucomatous visual field loss and might be a risk factor for the development of glaucomatous visual field defects. In contrast, only one study  demonstrated that OPA could not be a reliable indicator of glaucoma severity; however, in their study OPA was not corrected for IOP.
Blood flow parameters have been measured in several studies in eyes with pseudoexfoliation and reported to be lower in PXG compared to POAG.  There were a few studies that measured OPA in eyes with pseudoexfoliation. Punjabi et al.  evaluated OPA in these eyes and did not find any statistically significant difference in OPA between PXG and POAG groups. Our results concur with Punjabi et al.
We did not detect any correlation between OPA and mean pulse pressure, diastolic pressure and systolic pressure. Pourjavan et al.  previously reported that the OPA was not correlated with BP in normal healthy eyes. Grieshaber et al.  also showed that the OPA readings measured with DCT in healthy subjects were not related to BP levels and amplitude when BP and OPA were measured simultaneously. They concluded that the OPA strongly depended on the time-course of cardiac contraction. Regulating mechanisms in the carotid system, as well as scleral rigidity might be responsible for dampening of the direct effect of BP variations. Detry-Morel et al.  reported that systolic BP was positively correlated with OPA, whereas diastolic BP was negatively correlated with OPA. However, in their study, BP was measured neither continuously nor concomitantly with the OPA recording. Moreover, they used a heterogeneous group consisting of both glaucoma patients on treatment and healthy subjects, in a linear regression model.
We found that pulse pressure along with DCT IOP affected OPA and age had a borderline effect. In multiple regression analysis, factors that affected OPA were DCT IOP and age in the POAG group, and pulse pressure in the PXG group. Pourjavan et al.  did not find any correlation between OPA and age.
The relatively small number of patients and the non-randomized design may be considered as potential weaknesses of this study. Additionally we did not measure BP and ocular pressure simultaneously which is another drawback of this study.
conclusion, we did not find a significant difference in OPA between the PXG and POAG groups. OPA was correlated with DCT IOP but not with CCT. In both POAG and PXG, OPA decreases as the severity of glaucoma increases.
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[Table 1], [Table 2], [Table 3]