|Year : 2014 | Volume
| Issue : 1 | Page : 9-12
Comparison of relation between visual function index and retinal nerve fiber layer structure by optical coherence tomography among primary open angle glaucoma and primary angle closure glaucoma eyes
Department of Glaucoma Services Head Glaucoma Services, Lakshmi Varaprasada Rao Prasad Eye Institute, Patia, Bhubaneswar, Orissa, India
|Date of Web Publication||1-Mar-2014|
LV Rao Prasad Eye Institute, Patia, Bhubaneswar, Orissa - 751 024
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Purpose: To compare the visual field index (VFI) in primary open angle glaucoma (POAG) and primary angle closure glaucoma (PACG) eyes, and to study the correlation with disc variables on optical coherence tomography (OCT) in all stages of severity.
Materials and Methods: Thirty POAG and PACG underwent Humphrey visual field 24-2 along with detailed examination. They also underwent stratus OCT imaging of the optic nerve and retinal nerve fiber layer (RNFL). The correlation of VFI with RNFL thickness was compared in POAG and PACG.
Results: The VFI significantly differed between POAG and PACG, with POAG eyes apparently having a better VFI at all severities of glaucoma. There were statistically significant differences in the superior max (Smax) and inferior max (Imax) in early and moderate POAG and PACG eyes. In early and moderate glaucoma, multivariate regression showed that maximum correlation of the VFI was seen with the mean deviation (b = 1.7, P < 0.001), average and superior RNFL thickness (b = 2.1, P < 0.001 and b = 1.8, P = 0.03, respectively), and age (b = 0.7, P = 0.04); while no correlation was seen with intraocular pressure (IOP), axial length, sex, or other clinical variables. VFI did not correlate well with RNFL thickness or other disc variables on OCT in severe glaucoma.
Conclusion: VFI may not serve as a useful indicator of visual function in severe glaucoma. More useful indicators are required to monitor glaucoma patients with severe damage.
Keywords: Primary angle closure glaucoma, primary open angle glaucoma, retinal nerve fiber layer thickness, visual field index
|How to cite this article:|
Rao A. Comparison of relation between visual function index and retinal nerve fiber layer structure by optical coherence tomography among primary open angle glaucoma and primary angle closure glaucoma eyes. Oman J Ophthalmol 2014;7:9-12
|How to cite this URL:|
Rao A. Comparison of relation between visual function index and retinal nerve fiber layer structure by optical coherence tomography among primary open angle glaucoma and primary angle closure glaucoma eyes. Oman J Ophthalmol [serial online] 2014 [cited 2020 Feb 24];7:9-12. Available from: http://www.ojoonline.org/text.asp?2014/7/1/9/127911
| Introduction|| |
Visual Field Index (VFI) is a measure of the patient's overall visual function as compared to an age-adjusted normal population.  As visual field loss progresses, the VFI value will fall reflecting the decrease in residual visual function. This is a recent advance in visual field examination that enables us to study the visual function of glaucoma patients at a glance and also provides us with a quick overview of the changes, either deterioration or improvement, over time in long term glaucoma management.  Of the currently available imaging techniques, optical coherence tomography (OCT) is most widely accepted tool for monitoring glaucoma management. ,,,,, It has proven useful for detecting early glaucoma cases and also monitoring progression over time. Since VFI is an indicator of visual function, we aimed to evaluate the structural differences on OCT with VFI in all stages of glaucoma.
| Materials and Methods|| |
Consecutive patients with adult primary open and closed angle glaucoma, attending the glaucoma service, were recruited for the study. Consecutive patients attending our outpatient department (OPD) services were also screened and 30 normals fulfilling all inclusion criteria of open angles on gonioscopy, intraocular pressure (IOP) < 21 mm Hg, and normal optic disc and visual field were also selected for the study.
A detailed medical and ocular history was obtained and all patients underwent an anterior segment slit lamp examination, central corneal thickness, stereoscopic fundus examination with a + 90 D lens, Goldmann applanation tonometry, gonioscopy, and perimetry.
Criteria for diagnosis of the primary glaucoma included, age of onset >35 years, IOP > 22 mm Hg on at least three separate occasions and glaucomatous optic neuropathy with visual field loss consistent with optic nerve damage in at least one eye. In primary open angle glaucoma (POAG), there were open angles on gonioscopy, and primary angle closure glaucoma (PACG) was diagnosed if the above criteria were met, in the presence of an occludable angle, having at least 180 degrees of synechial closure on indentation/manipulative gonioscopy, as per International Society of Geographical and Epidemiologic Ophthalmology (ISGEO) guidelines.
Patients with a visual acuity of less than 20/60, media opacities, any other ocular pathology, refractive errors > ±6 D, secondary glaucomas, or those unable to review periodically, were excluded from the study.
Full threshold achromatic 24-2 perimetry was performed using Humphrey field analyzer, (Model 750, Zeiss Humphrey systems, San Leandro). Perimetry was repeated at least twice, till a reliable and consistent visual field result was obtained. Reliability criteria for Visual field (VF) results were fixation loss ≤ 33%, false positive response ≤ 20%, and false negative response ≤ 20%. Severity was staged as early if mean deviation (MD) < -6 dB, moderate for MD -6 to − 12 dB, and severe if MD was worse than − 12 dB.
OCT measurements were performed using OCT (OCT 3 STRATUS, Zeiss Humphrey, Dublin CA). If both eyes fulfilled the inclusion criteria, eye with lower signal-to-noise ratio (SNR) was chosen for analysis. The basic principle and technical characteristics of the OCT have been described earlier. Scans were acquired under dilatation and the fast retinal nerve fiber layer (RNFL) thickness protocol and fast optic nerve head was used for the study. This uses three circular scans each of 3.46 mm in diameter centered on the optic disc. Mean RNFL thickness was calculated using the inbuilt RNFL thickness average analysis protocol.
Only good quality images with SNR of 10 microns were selected for the study. The RNFL thickness map was displayed along with its ratio to normative RNFL thickness. The global and four quadrant average RNFL thickness data (temporal, superior, nasal, and inferior) was collected and compared in both groups. The fast optic nerve head scan gave a volumetric measurement of the optic nerve head topography.
The variables selected for the study included the average RNFL thickness, superior max (Smax), inferior max (Imax), superior average, inferior average, vertical integrated rim area (VIRA), vertical cup-to-disc ratio, and the rim area. VFI and OCT variables among POAG, PACG, and normals eyes were compared.
Statistical analysis was done using Statistical Package for Social Science (SPSS) 10 and comparison between groups was done with unpaired Student's t-test. Statistical significance was defined as a P < 0.05. Multivariate regression analysis was done with VFI as the dependent variable and RNFL thickness, age, sex, and axial length as independent variable.
| Results|| |
Of 269 screened, 122 adult glaucoma patients were identified fulfilling inclusion criteria, which included 52 POAG and 70 PACG cases. Twenty POAG and 18 PACG patients with significant cataract were excluded. Two POAG and eight PACG patients refused OCT examination after recruitment. Four uncooperative PACG cases with poor quality images on OCT despite repeated attempts were excluded from the final analysis.
30 POAG and 30 PACG were finally selected for the study.
The mean age of the patients was 65 ± 34.3 years (r = 41-99 years), with no statistically significant difference between POAG and PACG eyes at all stages of glaucoma. Severe glaucoma cases were significantly older, P < 0.01 [Table 1].
|Table 1: Comparison of clinical, visual field indices (VFIs), and retinal nerve fibre layer (RNFL) thickness in primary open and closed angle glaucoma|
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The VFI significantly differed between POAG and PACG, with POAG eyes apparently having a better VFI at all severities of glaucoma [Table 1]. VFI was apparently better in males with 76 ± 31.9, females 54 ± 39.2, P = 0.02. Though age did not differ statistically in the two sexes, males: 54 ± 1.2 years and females 53 ± 2.1 years, P = 0.8.
The OCT variables differed across the spectrum of open and closed angle glaucoma [Table 1]. There were statistically significant differences in the Smax and Imax in early and moderate POAG and PACG eyes [Table 1]; this pattern was however not seen in eyes with severe glaucoma.
The average RNFL thickness in early glaucoma cases was 100 ± 10.7 and 96 ± 9.5 microns in POAG and PACG, respectively. The VFI in these eyes was 94 and 97% in PACG and POAG eyes, respectively [Table 1].
Moderate glaucoma cases had an average RNFL thickness ranging from 72-75 microns, while VFI ranged from 69-82% in PACG and POAG eyes, respectively.
Eyes with severe glaucoma had significantly thinner RNFL and lower VFI as compared to moderate glaucoma [Table 1]. While the VFI significantly differed between POAG and PACG eyes, RNFL thickness and other OCT variables were not statistically significant between the two [Table 1].
There was significant correlation of the visual function index with age, average RNFL, Smax and Imax, rim area, cup-to-disc ratio in POAG and PACG eyes, P = 0.01. This correlation was weaker in PACG (r = 0.4, P = 0.01) than POAG (r = 0.8, P = 0.001).
Multivariate regression showed that maximum correlation of the VFI was seen with the mean deviation (b = 1.7, P < 0.001), average and superior RNFL thickness (b = 2.1, P < 0.001 and b = 1.8, P = 0.03, respectively), and age (b = 0.7, P = 0.04); while no correlation was seen with IOP, axial length, sex, or other clinical variables [Table 2]. This relation held true in early and moderate glaucoma, while the VFI did not seem to correlate well with the RNFL thickness in severe glaucoma (b = 0.9, P = 0.08).
|Table 2: Multivariate regression of visual field index with clinical variables in primary adult glaucoma|
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| Discussion|| |
Our study identified significant differences in the correlation of VFI with the RNFL thickness across different severities of glaucoma with a poor correlation in severe glaucoma as also in PACG as compared to POAG. To our knowledge, this is the first study evaluating structure-function relationship between VFI and RNFL thickness in different severities of glaucoma.
Several studies have demonstrated significant differences in optic nerve head parameters in POAG and PACG using other imaging devices. ,,,,,,, A possible reason for the observed disparity in structure-function correlations between POAG and PACG is different pathophysiological process of the two disease entities.
Kalaboukhova and associates showed that there was significant difference in parameters like cup area, rim area, and cup shape measure among the stable and progressed group of POAG or ocular hypertensive eyes progressing to POAG.  In a study comparing the optic disc and VF alterations in 110 POAG patients and 36 PACG patients, Boland and associates found that the average RNFL thickness as measured by OCT was significantly greater in PACG than that in POAG even after controlling the mean deviation and axial length.  Their findings suggest different structure-function relationships between these two forms of glaucoma. We found a poor correlation of VFI with the RNFL in PACG eyes as compared to POAG, which concurred with results from other studies using other imaging modalities.
VFI was found to correlate significantly with early and moderate glaucoma, while in severe glaucoma such a significant correlation was not found. This suggests that the VFI is a poor measure of visual function in severe glaucoma. This also implies that use of this index for assessing progression may not be reliable in severe glaucoma since the indices would be a poor measure of actual visual reserve in advanced glaucoma with generalized loss of sensitivity.
The problem partially may be due to the logarithmic scaling of visual sensitivity, which maximizes sensitivity changes at low dB levels. It has been shown that RNFL thickness reaches a plateau at a visual sensitivity loss beyond 15 dB, after which there is not much change in the RNFL thickness. ,, It is unclear if the VFI also is subjected to similar truncation effect in advanced glaucoma making it unreliable for reflecting the true visual reserve as the disease progresses.
RNFL thickness is known to Decreases with age , VFI in this study correlated significantly with age. It is unclear if this strong correlation holds true even at later age with advanced RNFL loss and if this relationship also is subject to the same ceiling effect seen with its relation to RNFL thickness.
Medeiros et al.,  has proposed a new combined index of structure and function (CSFI) for detecting preprimetric glaucoma as well as differentiating different stages of glaucoma. Wheat et al., proposed a modified model to correct the disparities between subjective measure of function like visual sensitivities and objective measures of function on OCT.  Contrary to these results, the study by Harwerth et al., has shown that RNFL thickness may be a more sensitive measurement for early stages and perimetry a better measure for moderate to advanced stages of glaucoma. 
The limitations of this study include a small sample cohort which was hospital based. Though data on progression on the eyes with severe glaucoma is available for some patients, we did not study the progression indices on visual field and OCT since that was not the primary objective. Nevertheless, our study suggests the need to develop methods other than VFI for detecting glaucoma progression in eyes with severe glaucomatous damage.
| Conclusion|| |
Though VFI is a good quantitative measure of the residual visual function in POAG and PACG eyes at all severities of glaucoma, caution is required in analyzing the VFI in severe POAG eyes, where OCT may help indicate the true amount of optic nerve damage.
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[Table 1], [Table 2]