A study for the assessment of central corneal thickness and visual field defects in patients of primary open-angle glaucoma

Richa Dhiman; Gaurav Sharma; Mandeep Tomar; Mitasha Singh

doi:10.4103/sjopthal.sjopthal_7_19

ORIGINAL ARTICLE

Year : 2019 | Volume : 11 | Issue : 1 | Page : 25-30

A study for the assessment of central corneal thickness and visual field defects in patients of primary open-angle glaucoma

Richa Dhiman, Gaurav Sharma, Mandeep Tomar, Mitasha Singh
Department of Ophthalmology, Dr. Rajendra Prasad Government Medical College, Kangra at Tanda, Himachal Pradesh, India

Date of Submission	15-Mar-2019
Date of Decision	23-Jun-2019
Date of Acceptance	23-Jun-2019
Date of Web Publication	10-Oct-2019

Correspondence Address:
Dr. Richa Dhiman
Department of Ophthalmology, Dr. Rajendra Prasad Government Medical College, Tanda, Kangra, Himachal Pradesh
India

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/sjopthal.sjopthal_7_19

Abstract

Purpose: The aim of this study is to assess the central corneal thickness (CCT) and visual field (VF) changes in patients of primary open-angle glaucoma (POAG) and the influence of CCT on glaucomatous damage in glaucoma clinic of Dr. R. P. G. M. C Kangra at Tanda. Materials and Methods: This cross-sectional study conducted on 103 eyes of 58 patients of POAG- >40 years. All patients underwent a complete eye examination including Goldmann applanation tonometry, CCT measurement through ultrasonic pachymeter, stereoscopic disc photography, retinal nerve fiber layer (RNFL) photography, and automated perimetry. Correlation of CCT with age, gender, intraocular pressure (IOP), advanced glaucoma intervention study (AGIS) score, VF parameters (mean deviation, pattern standard deviation [PSD]), vertical cup-disc ratio, average RNFL, and number of antiglaucoma medications was analyzed. The patients were divided into two groups – thin and thick CCT taking a reference range of 529 μm derived from a previous study done in same area with larger sample size. Pearson correlation was used for correlation coefficient and a value of P < 0.05 was considered statistically significant. Results: Thin CCT was significantly correlated with IOP (r = 0.28, P = 0.004). Thin CCT was also significantly associated with worsened PSD of VF (r = 0.25, P = 0.02). The mean AGIS score was more (8.07 ± 5.52) in patients with thin CCT (<529 μm) in comparison (5.96 ± 6.10) to patients with thick CCT (>529 μm) which was statistically significant (P < 0.05). Conclusion: CCT is a significant risk factor for glaucomatous VF loss. Patients with thinner CCT are more likely to have advanced VF defects in comparison to patients with thick CCT.

Keywords: Central corneal thickness, primary open-angle glaucoma, visual field defects

How to cite this article:
Dhiman R, Sharma G, Tomar M, Singh M. A study for the assessment of central corneal thickness and visual field defects in patients of primary open-angle glaucoma. Sudanese J Ophthalmol 2019;11:25-30

How to cite this URL:
Dhiman R, Sharma G, Tomar M, Singh M. A study for the assessment of central corneal thickness and visual field defects in patients of primary open-angle glaucoma. Sudanese J Ophthalmol [serial online] 2019 [cited 2023 Mar 29];11:25-30. Available from: https://www.sjopthal.net/text.asp?2019/11/1/25/268796

Introduction

Glaucoma is the second leading cause of blindness globally^[1] and the third-most common cause in India.^[2],[3],[4] The reported prevalence for primary open-angle glaucoma (POAG) varies between 1.62% and 3.51%.^{[2],[3],[4],[5]}

Elevated intraocular pressure (IOP) remains the most important risk factor for the development of glaucomatous optic neuropathy^[6] besides other factors, such as race, advanced age, and positive family history. Recently, central corneal thickness (CCT) has been recognized as a significant factor for the progression of ocular hypertension to POAG in the ocular hypertension treatment study (OHTS).^[7] They found that a decrease in CCT of 40 μm added a 70% increased risk.

The measurement of CCT of adult eyes with primary open-angle glaucoma has a major impact on the clinician's assessment for two reasons: (1) applanation IOP readings are profoundly affected by the CCT (namely., thicker CCTs “overestimate” and thinner CCTs “underestimate” true IOPs), and (2) there is a significant risk factor for developing glaucoma damage, independent of IOP corrections, with thinner CCTs.^[8]

It is unclear whether the risk attributed to CCT is only the result of inaccuracies in the measurement of IOP or whether there are additional related factors, such as properties of the posterior sclera and lamina cribrosa, which can significantly influence the development and progression of glaucoma.

The present study was planned to find out the mean CCT among glaucoma patients and the visual field (VF) defects among them. It was done as a preliminary study to see the effect of CCT on VF damage. Hence, the estimation of CCT will not only help in the early diagnosis of glaucoma but also in optimizing therapy in each patient.

Materials And Methods

Study design

This was a cross-sectional study. After explaining the details of the study, written informed consent was obtained from all patients before enrolment. The study was approved by the Institutional Ethics Committee of Dr. RPGMC, Kangra at Tanda (Himachal Pradesh) and was carried out in accordance with the Declaration of Helsinki (1989) of the world medical association.

Patients

One hundred and three eyes of 58 patients aged >40 years with POAG diagnosed clinically during 1 year period were included in this study. The diagnosis of POAG was based on: IOP of 22 mmHg or higher at the initial visit, characteristic glaucomatous optic neuropathy with diffuse or focal optic rim thinning, hemorrhage, cupping or nerve fiber layer defects indicative of glaucoma and corresponding VF loss.

Each patient underwent a comprehensive ophthalmological examination, including review of medical history, best-corrected visual acuity, slit-lamp biomicroscopy, Humphrey 30–2 VF testing, IOP measurement using Goldmann applanation tonometer, gonioscopy using 2-mirror gonio lens, CCT measurement using ultrasonic pachymeter, calculation of corrected IOP, optic nerve head images using fundus camera, and retinal nerve fiber layer (RNFL) thickness measurement using Optovue spectral-domain optical coherence tomography (SD-OCT).

Exclusion criteria

Patients with corneal or retinal disease
Patients with secondary forms of open-angle glaucoma
Patients with a history of refractory or corneal surgery
Patients with VF defects resulting from nonglaucomatous entities such as retinal pathology, nonglaucomatous optic nerve pathology and visual tract compromise were excluded from the study.

Central corneal thickness evaluation

CCT was measured with ultrasonic pachymeter (DGH 500). For each patient, CCT was calculated as the mean of five readings obtained during the same visit of the patient.

Based on the CCT, corrected IOP was calculated for each patient.

Visual field testing

It was done by Humphrey automated 30–2 Swedish interactive threshold algorithm (SITA) standard perimetry protocol if their visual function allowed or by Humphrey automated 10–2 SITA standard perimetry if their glaucomatous damage was severe. VF global indices were recorded, including mean deviation (MD) and pattern standard deviation (PSD). Reliable Humphrey, automated perimetry was defined by having fewer than 2 of the following characteristics: Fixation losses <20%, false-positive responses <33% and false-negative responses <33%. This reliability criterion was adapted from the advanced glaucoma intervention study (AGIS) reliability ratings. To determine the severity of glaucomatous VF loss, VFs were graded using the AGIS numeric scoring criteria.^[9] It generates a numerical score of 0–20 in which zero equates to no VF loss, 1–5 equates to mild loss, 6–11 equates to moderate loss, 12–17 equates to severe loss, and 18–20 equates to end-stage visual loss. The outer circumference of points, except for the outermost nasal points, was excluded from the scoring process, when 30–2 VFs were analyzed.

Cup-disc ratio (CDR) was calculated as the mean of vertical and horizontal CDR as recorded in the chart.

RNFL thickness measurement was done using Opto-Vue Spectral Domain-OCT. A signal strength of ≥7 (maximum 10) in addition to uniform brightness across the scan circumference and being well centered at the optic nerve head was considered a good scan. Measurements of RNFL thickness from three scans were averaged to provide a mean measurement of the RNFL thickness average, as well as the following retinal regions: Temporal (316°–45° on a unit circle), superior (46°–135°), nasal (136°–225°), and inferior (226°–315°).

Statistical analysis

The data were entered in Microsoft Excel sheet, tabulated, and statistically analyzed using the SPSS version 21.0. (Armonk, NY, IBM Corp., USA). The categorical variables were presented as numbers and proportions, whereas the continuous variables were presented as means. The test of significance applied to test the difference between categorical variables was Chi-square and for continuous variables was t-test. The strength of the association between two continuous variables has been represented using Pearson correlation coefficient. The level of statistical significance was set as P < 0.05.

Results

A total of 103 eyes of 58 POAG patients who met the inclusion criteria were included in the study. There were 65 male eyes and 38 female eyes. Mean age of the presentation was 62.37 ± 10.12 years with a range between 40 and 80 years of age. The mean CCT of the patients was 514.58 ± 33.44 μm. To assess the impact of CCT on structural and functional damage in glaucoma, patients were divided into two groups: Group A with CCT <529 μm and Group B with CCT >529 μm based on a cutoff value for CCT of 529 μm which was taken from a study done in same area on normals with a larger sample size by Chauhan et al.^[10]

The demographic characteristics of the POAG patients who were included in the study are shown in [Table 1]. There were no significant differences in the demographic characteristics of the patients between the two groups as shown in the table.

Table 1: Demographic characteristics of primary open-angle glaucoma patients

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The ophthalmic characteristics of POAG patients in the two groups are shown in [Table 2]. There was a significant difference between the PSD values of the two groups. Since the PSD is more specific for glaucoma, we can interpret that the VF loss was more in patients with thin CCT while the patients with thick central corneas have less VF loss.

Table 2: Ophthalmic characteristics of primary open-angle glaucoma patients

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Based on the MD, VF defects were graded according to the AGIS score to see the severity of field defects in each group as shown in [Table 3].

Table 3: Advanced glaucoma intervention study scoring in each group

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Thin CCT (<529 μm) group had more advanced VF defects in comparison to thick CCT (>529 μm) group which was statistically significant (P< 0.05).

Moderate and severe loss was more pronounced in thin CCT Group A accounting for 23.59% and 13.48%, respectively, in comparison to thick CCT Group B which had moderate and severe field losses as 5.62% and 2.25%, respectively, as shown in [Figure 1].

Figure 1: Grading of field defects in each group according to advanced glaucoma intervention study score

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[Table 4] shows that there was a statistically significant positive correlation of CCT with IOP, which indicates that as the as the CCT increases, there is falsely elevated IOP.

Table 4: Correlation of central corneal thickness with different parameters

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When CCT is taken into consideration, the correlation between CCT and IOP becomes negative. Hence, with rise in CCT, there is decrease in the actual reading of IOP.

When the correlation between CCT and VF parameters was studied, it was found that there was a negative correlation between CCT and PSD and AGIS score. This indicates that as the CCT decreases, the PSD and AGIS score increases. Hence, patients with thin CCT had worse VF parameters. Although CCT was also negatively correlated to CDR, it was not statistically significant. [Figure 2], [Figure 3], [Figure 4] displays a case with POAG showing fundus photography, corresponding visual field defect and ocular coherence tomography retinal nerve fibre layer changes.

Figure 2: Color and red-free fundus photograph of right eye of a patient showing optic cup-disc ratio of 0.8 with inferior notch and corresponding nerve fibre layer wedge defect

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Figure 3: Humphrey visual field of the right eye of same patient with superior arcuate defect corresponding to the inferior notch of optic disc

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Figure 4: Spectral domain optical coherence tomography of the right eye of same patient showing retinal nerve fiber layer thinning in superior and inferior quadrants (shown by red color)

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Discussion

Glaucoma is a chronic progressive optic neuropathy which is characterized by morphological changes occurring at the optic nerve head and RNFL along with the functional deficit with or without rise in IOP. There may be significant dissociation between the two measures of change, i.e., structural and functional signs in glaucoma. Therefore, it is important to evaluate both structural and functional measures as indicators of the stage of the disease.

This cross-sectional study was done to assess the CCT and VF defects in patients of POAG. A total of 103 eyes of 58 POAG patients who presented in the Department of Ophthalmology, Dr. R. P. G. M. C, Kangra at Tanda (Himachal Pradesh) during a period of 1 year were included in the study. All patients met the inclusion criteria as per the protocol. The patients were divided into two groups based on a cutoff value of CCT = 529 μm – Group A (CCT <529 μm) and Group B (CCT >529 μm).

In the current study, the thin CCT was not significantly associated with increased number of medications, though there was a negative correlation between them, i.e., with increase in the number of medications, there was decrease in CCT. Herndon et al.^[11] and Mokbel et al.^[12] found that lower CCT was significantly associated with increased number of medications. For an increase of 10 years of age, the number of glaucoma medications increased by 0.23. Since this was a cross-sectional study, follow-up was not done. Hence, the long-term influence of glaucoma medications on CCT cannot be commented on.

There was a significant difference in the mean IOP of the two groups with Group B (19.73 mmHg) having higher IOP in comparison to Group A (16.05 mmHg). Similarly, there was a significant difference in the mean CCT of the two groups, which is in concordance with the findings of OHTS whose results demonstrated that a 40 μm thinner cornea represented approximately a 70% increase in the risk of developing POAG.

Hence, when the CCT was taken into consideration in the present study, the difference between the corrected IOP of two groups became nonsignificant with Group A (19.51 mmHg) having higher corrected IOP in comparison to Group B (18.01 mmHg). Similar results were found by Kniestedt et al.^[13] who stated that the reported impact of CCT on Goldmann applanation tonometry covers a wide span, with changes in IOP reading ranging from 0.11 to 0.71 mmHg for each 10 μm of change in CCT.

The VFs are a functional marker of the progression of glaucoma. In the present study, we utilized AGIS scoring to interpret the VFs and found a significant difference in the AGIS score [Table 3] of the Group A (CCT <529 μm) and Group B (CCT >529 μm). It was found that thin CCT eyes had more advanced VF defects in comparison to thick CCT eyes. This is in accordance with the study done by Jonas and Holbach^[14] who showed that the progression of glaucoma was independent of CCT, although thinner CCT was associated with more advanced glaucoma. Similar were the findings of Lin et al.^[15] who found that the eyes with greater CCT had better VF indices than did those with thinner CCT (−6.91 vs. −9.17 dB).

This study demonstrates the lack of a significant relation between CCT and CDR in POAG patients. The mean CDR in Group A (CCT <529 μm) was 0.688 ± 0.132, whereas in Group B (CCT >529 μm) was 0.673 ± 0.128. The lack of association between CCT and vertical CDR may be attributed to the more complex interaction between structural and functional damage in glaucoma. However, there are studies showing correlation between CCT and vertical CDR such as Pakravan et al.,^[16] Herndon et al.,^[11] Gunvant et al.,^[17] and Mokbel et al.^[12] who reported that thinner than average CCT values appear to be associated with larger and deeper optic disc cup.

The thickness estimates of posterior sclera based on measurements of anterior sclera or cornea may be inaccurate. While our data did not find an association between CCT and vertical CDR, it does not rule out the importance of scleral or laminar thickness in glaucoma or the search for a reliable noninvasive method for its measurement.

RNFL thickness assessment has superseded VF assessment for early diagnosis of glaucoma. It is believed that assessment of RNFL gives direct evidence of structural damage before VF defects are elicited. We did not find any statistically significant difference in the RNFL thickness in the two groups [Table 2] as measured by SD-OCT which is similar to the findings of Sarfraz et al.^[18] This is in contrast to the findings by Wangsupadilok et al.^[19] who found a significant positive correlation between CCT and RNFL thickness. Hence, the direct impact of CCT on RNFL thickness is still controversial and needs further verification by clinical trials.

Conclusion

Estimation of CCT helps not only in the early diagnosis of glaucoma but also in optimising therapy in each patient. Those with lower CCT may need further lowering of IOP to meet the target IOP (corrected for CCT). The understanding that CCT <529 μm means more functional damage, such patients may also require more frequent follow-up (e.g., VFs need to be checked more frequently) than those with thicker CCTs. Thus, this study provides useful information about the impact of CCT on functional damage in glaucoma which can aid the ophthalmologist to treat the disease early.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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