Comparison of central corneal thickness of primary open angle glaucoma patients with normal controls in South India.

BACKGROUND: Studies mainly in the western population have compared central corneal thickness in primary open angle glaucoma and normal individuals have found variable results. We did this study to compare the central corneal thickness of primary open angle glaucoma patients with normal controls in a south Indian population.
MATERIALS AND METHODS: This was a masked, cross-sectional study undertaken in a tertiary care center in South India. A total of 50 controls and 50 primary open angle glaucoma patients were studied. Central corneal thickness between the two groups was compared using Wilcoxon two sample test and the signed rank test.
RESULTS: The mean central corneal thickness in the control group was 536 μm (462-608 μm) and in the primary open angle glaucoma group was 531 μm (476-609 μm).
CONCLUSION: There was no significant difference in the central corneal thickness between primary open angle glaucoma patients and the normal controls.

Introduction

One of the most important risk factors for primary open angle glaucoma is an elevated intra-ocular pressure whose measurement by Goldmann applanation tonometry has been found to be influenced by the central corneal thickness (CCT) of the individual. The variation in central corneal thickness was not taken into account as Goldmann applanation tonometry assumes a standard 520 μm for all corneas.[1] The normal range in most studies was between 427–620 μm.[2] The central corneal thickness in primary open angle glaucoma has been studied, but mostly in the western population. Though most studies have quoted comparable central corneal thickness in primary open angle glaucoma and normal individuals,[34] some studies have found that central corneal thickness in primary open angle glaucoma patients is significantly lesser than in the normal population.[5] This may lead to the misclassification of many patients as normal tension glaucoma. This has led us to do this study to see if there was any significant difference in the central corneal thickness between primary open angle glaucoma patients and normal controls in south India.

Materials and Methods

Aim

To compare the central corneal thickness of primary open angle glaucoma patients and normal controls.

Objectives

To see if there were differences in the central corneal thickness with respect to age and sex in the two groups.

The design of this study was masked cross-sectional study. The study was approved by the local ethics committee. Informed consent was obtained from all participants before including them in the study. A total of 100 patients were enrolled in the study during a period of one year as per the inclusion and exclusion criteria, which is given below. There were two groups of patients, the control group and the primary open angle glaucoma patients group. All patients were aged 40 and above keeping in mind the age distribution in primary open angle glaucoma.

Control group

Consisted of people who had intra-ocular pressures <21 mm. of Hg. in both the eyes measured by Goldman’s applanation tonometer, had normal optic discs, open angles on gonioscopy, no suspicion of any form of glaucoma, no family history of glaucoma, and did not have any other eye disease.

Fifty primary open angle glaucoma patients were enrolled in the study during the same one year period after written informed consent. Individuals with primary open angle glaucoma had untreated intra-ocular pressures of 22 mm of Hg or higher, an open, normal angle, a glaucomatous optic disc, and glaucomatous visual field defects. The subjects were chosen randomly from the glaucoma services department of our hospital. Both newly diagnosed cases of primary open angle glaucoma and those on treatment were included.

People who had the following characteristics were excluded: Persons with evidence of any anterior segment pathology including corneal opacities, history of previous intraocular or corneal surgery, people with diabetes mellitus, people using contact lenses or any other condition that may affect corneal thickness, corneal edema, corneal astigmatism >2D and sphere >4D, any optic nerve or cranial disease, and people with evidence of pseudoexfoliation.

All patients enrolled in the study underwent the following tests. Their best corrected visual acuity, slit lamp bio microscopy to exclude corneal pathology, applanation tonometry, gonioscopy, dilated fundus examination and stereoscopic examination of the optic discs and the nerve fiber layer using + 90D lens with the slit lamp. All glaucomatous patients underwent automated perimetry prior to dilation using the 24-2 program of the Humphrey field Analyzer. Central corneal thickness was measured in both the eyes. The readings were taken using the PACSCAN 300p model of SONOMED Inc. The corneal velocity was preset at 1636 ms. A calibration check was performed before performing actual measurements. A measurement accuracy test was also performed periodically to ensure the functionality of the PACSCAN. This performs an internal calibration check, which should generate a reading of 500+/- 1 μm. Topical proparacaine 0.5% was instilled in both the eyes. The patients were seated, erect, and were all asked to look at a target fixed 1 m. away when the measurements were made. Three consecutive readings were taken for each eye by a single observer, an ophthalmologist, who was masked to the diagnosis. The numerical value of the readings taken was recorded by a technician.

Statistics

The median of the three values was noted. If two out of the three readings were similar, then the mode was taken as the working value, which would also be the median value. As the distributions of the measurements were non-normal and badly skewed, taking the median value was found to be more appropriate. A histogram was constructed with the measured values. The distribution of the central corneal thickness measurements in controls and primary open angle glaucoma had a non-normal or an asymmetric distribution. Wilcoxon two sample test and the signed rank test were the two non-parametric tests used in this study to compare the central corneal thickness measurements in primary open angle glaucoma and controls.

Results

Studying the age distribution in the two groups, it was found that the maximum numbers of patients in the control group were between the ages of 40–50 years while in the primary open angle glaucoma group, they were between the ages of 50–60 years. The mean central corneal thickness for each patient was calculated by averaging the right and left eye values. This was done because we did not find any statistically significant difference in the central corneal thickness values between the right and left eye. This was done using non-parametric testing. In the controls, the difference in the central corneal thickness between the two eyes was not significant (signed rank test, P = 0.62). In the primary open angle glaucoma group also, the difference in the central corneal thickness between the two eyes was not significant (signed rank test, P = 0.49). The distribution of the difference in the right and left eye central corneal thickness in both the groups showed that most of the values were within -12 to +12 μm. The mean central corneal thickness in the control group was 536 μm with a range of 462-608 μm. In the primary open angle glaucoma group, the mean was 531 μm with a range of 476–609 μm. Thus, the mean central corneal thicknesses in the two groups were comparable. By non-parametric testing using the Wilcoxon two samples test, it was found that there was no statistically significant difference in the mean central corneal thickness measurements in primary open angle glaucoma and the control group (P = 0.16). The central corneal thickness was sub-grouped using a range of 10 μm for each subgroup, and the number of controls and primary open angle glaucoma patients falling into each group was assessed as showed in Table 1. The maximum number (i.e., eighteen patients) in the control group belonged to the subgroup of central corneal thickness between 511–520 μm. In the primary open angle glaucoma group, the maximum number (i.e., 23 patients) belonged to the subgroup of central corneal thickness 531–540 μm. By non-parametric testing using the Wilcoxon two sample test, P value was 0.16, and thus, there was no statistically significant difference between the two groups. The mean and the range of central corneal thickness in the various age groups were studied and depicted in Table 2. There was no statistically significant difference in the central corneal thickness among the various age groups between the control group and the primary open angle glaucoma group. We also did not find any significant trend in the central corneal thickness as the age advanced. Among the controls, 28 were males and 22 females. Among glaucoma group, there were 40 males and 10 females. Non-parametric testing revealed no significant statistical difference in the distribution of central corneal thickness between males and females in the control group (P = 0.20) and the glaucoma group (P = 0.21).

Table 1

Distribution of central corneal thickness

Table 2

Age-wise distribution of central corneal thickness

Discussion

In our study, we found a central corneal thickness of 536 um in the normal control group. A meta-analysis of corneal thickness literature done previously had found that the mean central corneal thickness in normal white adults was 535 um,[6] whereas a study done in rural central India found a mean central corneal thickness of 514 um, which was thinner compared to our values.[7] Studies done on this topic previously have found significant inter-racial variation in central corneal thickness.[89] Some studies have even found intra-racial differences in the central corneal thickness values.[1011] Hence, there seem to be differences in the central corneal thickness in various populations and sometimes different in various parts of the same country. In our study, there was no significant difference between the central corneal thicknesses of normal group compared to that of primary open angle glaucoma. Similar observations were reported in studies done in Korea and Barbados.[1213] However, the mean central corneal thickness was found to be significantly lower in open angle glaucoma group compared to normal people in the study done by Aghaian E et al.[14] No significant differences in central corneal thicknesses were found between males and females in our study. Similar findings were found in a studies by Casson RJ et al. and Bron AM et al.,[1516] whereas other studies found the central corneal thickness in males to be significantly greater than females.[1718] We did not find any significant differences in the central corneal thicknesses between the two eyes. Similar findings were noted by La Rosa Fa et al.[19] We did not find any significant trend in the central corneal thickness with increasing age in our study. Noche CD et al. in their study also did not find any significant effect of age on the central corneal thickness,[20] whereas a few studies showed a tendency for the central corneal thickness to decrease with age.[2122]

Conclusion

We did not find any significant difference in the central corneal thickness between primary open angle glaucoma patients and normal individuals. The mean central corneal thickness found in our study was comparable to that of western populations.

Limitation

This study was done on patients in a tertiary care center, and the results may not necessarily be applicable to that of the general population, for which a population-based study will be needed.