CORRELATING OF THE VISUAL FIELD INDEX WITH MEAN DEVIATION AND PATTERN STANDARD DEVIATION IN GLAUCOMA PATIENTS

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1 CORRELATING OF THE VISUAL FIELD INDEX WITH MEAN DEVIATION AND PATTERN STANDARD DEVIATION IN GLAUCOMA PATIENTS Bui Thi Huong Giang, Pham Thi Kim Thanh Department of Ophthamology, Hanoi Medical University The purpose of our study was to evaluate the correlation between the new index - visual field index (VFI) and mean deviation index (MD) and pattern standard deviation (PSD) in patients with glaucoma. MD, PSD and VFI were calculated in data about 103 eyes obtained from a cross-sectional study in 103 eyes of 58 patients with mild to severe glaucoma or ocular hypertension. The correlation of VFI to MD, PSD was evaluated with linear regression models, and the coefficient of determination (r) was calculated. The result showed that the average values of VFI, MD and PSD were 78.76%, db and 4.56 db respectively. The VFI and the MD were linearly correlated with r = For the patients with VFIs below 90%, the correlation with the MD was better than for the patients with VFIs 90% and above (r = vs 0.571). There was no statistically significant difference in VFI value between the group with cataract and the group without cataract (p > 0.05) but MD varied significantly between these two groups (p < 0.05). For the PSD, the correlation for the patients with VFIs 90% and above was greater than for the patients with VFIs below 90% (r = vs ). In conclusion, VFI was linearly correlated with MD and PSD. VFI seems to be less affected by cataract than MD. Keywords: VFI, MD, PSD, visual field. I. INTRODUCTION In management of glaucoma patients, the visual field (VF) is the most important tool to determine the stage and progression of the disease [1]. VF data is summarized in global summary indices [2]. At this time, the mean deviation index (MD) and the pattern standard deviation (PSD) are the standard indices to evaluate for glaucomatous damage [3]. The Visual Field Index (VFI) is a Corresponding author: Bui Thi Huong Giang, Department of Ophthamology, Hanoi Medical University buihuonggiang@hmu.edu.vn Received: 03 June 2017 Accepted: 16 November 2017 new index, introduced by Bengtsson B and Heijl A in 2008 [4]. The VFI expresses the visual function as a percentage of normal age-corrected sensitivity. Therefore, the VFI of an eye with a completely normal visual field is 100% and the VFI of a perimetrically blind eye is 0%. The VFI was designed using the Humphrey 30-2 and 24-2 test point patterns, which are the most commonly used patterns in glaucoma management [3]. To avoid effects of cataract on the VFI, the pattern deviation probability map was used to identify test points with normal sensitivity (100%), having absolute defect (0%), and 68

2 those demonstrating relative loss. The sensitivity at these points were scored using the following formula: [(total deviationi/age-corrected normal threshold) x 100], where total deviation is the absolute value of the numerical total deviation value and age-corrected normal threshold is intercept - age coefficient x patient age. The VFI was also constructed to weigh the central points of the visual field more heavily than peripheral points. The VFI is the mean of all weighed scores in percent. The test point pattern was divided into five concentric rings of increasing eccentricity. Cortical magnification of any given visual field location is assumed to reflect both ganglion cell density and the number of neurons in an area of the visual cortex responsible for processing a stimulus of a given size [5]. The weights of the test points decreased with increasing eccentricity from 3.29, 1.28, 0.79, and 0.57 to 0.45 at the outermost ring. The VFI is automatically computed using in the current Statpac software of the Humphrey Field Analyzer (HFA II; Carl Zeiss Meditec, Inc., Dublin, California, USA) (Fig.1) [4]. Figure 1. Visual field of a glaucoma patient Mean Deviation (MD) is the average of these deviations across all test locations. Mean defect expresses the difference between observed and expected mean sensitivity. MD expresses the overall reduction in sensitivity, and is therefore decreased not only by increasing glaucoma, but also by cataract [6]. Pattern standard deviation (PSD), a depiction of focal defects, measures irregularity between the threshold value for each point and the average visual field sensitivity at each point. Thus, in patients with severely damaged visual fields the value of PSD is too low to be useful as an indicator of severity of disease [7]. Glaucoma is an ocular disease, in which characteristic visual field loss corresponds to the underlying anatomic arrangement of damaged retinal ganglion cells. Glaucomatous visual field characterized by loss is localized defects [8]. To stage glaucoma, most classification systems use MD, PSD, and the number of defective points in the visual field test. However, the MD is affected by both glaucoma and media opacities. Thus, cataract or media opacities can falsely inflate glaucoma severity. Bengtsson B and Heijl A used the pattern deviation probability maps in the visual field test to make the VFI test as resistant as possible to the effects of media opacities, while clearly depicting localized loss [4]. There are many studies concerning the VFI in the world [9;10]. The relationship between VFI and other visual field indices has not been studied in Vietnam. The objective of this study was to evaluate the correlation between VFI and other visual field indices (MD, PSD) in glaucoma patients. 69

3 II. SUBJECTS AND METHODS 1. Subjects Glaucoma patients who agreed to participate were examined. Patients performed the 24-2 threshold test (Humphrey SITA standard). The VF tests with reliability indices (Fixation losses, False positives, False negatives) > 20% were excluded. Inclusion criteria included a clinical diagnosis of primary glaucoma or ocular hypertension and absence of other retinal disease. They had a best corrected visual acuity (VA) equal to or better than 20/60 and refractive error within ± 5.00 D sphere and ± 3.00 D astigmatism. 2. Methods This cross-sectional study was performed between December 2015 and April 2016 at Glaucoma department, Vietnam National Institute of Ophthalmology. To calculate the sample size of this crosssectional study, we assumed that the rate of visual field loss in glaucoma patients was 85% [6]. The following formula was used: N = [(Z 1-α/2)2 p.q] / d2. To achieve 95% confidence intervals (CIs) and 7% error, 103 eyes from 58 patients were examined. The MD was calculated as the weighted mean of the total deviation values, and the weight assigned to each location was the inverse of the variance in the healthy reference group. The PSD was determined by comparing the differences between adjacent points [2]. The VFI was calculated as described by Bengtsson and Heijl. At each location, the measured sensitivity was expressed as a percentage of the sensitivity expected in a healthy observer of the same age, and the VFI was calculated as the weighted mean of all locations with pattern deviation probability outside normal limits (< 5%) [11]. The relationship between VFI, MD, and PSD was described with linear regression analysis. VFI was treated as the dependent variable; MD and PSD were treated as independent variables in all regressions. To evaluate the relationship between variables from a single patient, the correlation coefficient (r) was calculated by SPSS 20.0 Statistics. 3. Ethics Research subjects were informed about the goals of the study and voluntarily agreed to participate. The study was approved by Vietnam National Institute of Ophthalmology. III. RESULTS A total of 77 eyes with glaucoma and 26 eyes with ocular hypertension were studied. The VFI ranged from 100% (normal visual field) to 1%. The MD showed the overall depression ranged from db to db. PSD ranged from 1.03 to db. 70

4 Table 1. Average visual field indices VFI (%) MD (db) PSD (db) Average ± SD ± ± ± 3.45 Min - max ( ) (- 0.93) Chart 1. Comparison of MD and VFI Correlation coefficient (r) is The two tailed P value is < In single visual fields, there was a close relationship between MD and VFI. A significant correlation between MD and VFI was shown in all eyes (r = 0.984, P < ) which was positive linear. This relationship was described by the equation: VFI = x MD (%). This model predicts VFIs of 92%, 73%, and 47% for visual fields with MDs of 6, 12, and 20 db, with prediction intervals of approximately ± 8%. Chart 2. Comparison of PSD and VFI Correlation coefficient(r) is The two tailed P value is < Chart 2 showed the linear regression between VFI and PSD with r = , p <

5 Table 2. The correlation between VFI and MD, PSD in the patients with VFI 90% and VFI < 90% VFI ( 90%) - MD VFI (< 90%) - MD VFI ( 90%) - PSD VFI (< 90%) - PSD p < < 0.01 < > 0.05 R The VFI was more closely correlated with MD than it was with PSD. PSD was significantly correlated observations of VFI 90%, but not with observations where VFI < 90% Table 3. MD and VFI average patients without glaucomatous defects in visual field test Cataract VF index MD (db) VFI (%) No cataract ± 1.28 (26) ± 0.96 (26) Cataract ± 2.63 (12) ± 0.72 (12) p In 50 eyes with MD > - 6dB, 38 eyes had no glaucomatous defects in visual field test (stage 0 classification Hodapp, Parish and Anderson ) [12]. We compare the average MD and average VFI between the group with cataract and the group without cataract. There was no significant difference in the VFI between two the groups (p > 0.05) but was significant difference in the MD between two the groups (p < 0.05). IV. DISCUSSION Global indices of VFs have always played an important role in summarizing the severity of glaucoma. MD and PSD are the two most popular global indices used in clinical practice. However, both of them have limitations. MD is affected by media opacities and by other causes of generalized depression of visual function in addition to glaucoma. PSD is less affected by media opacities, but has the disadvantage that it falsely improves as the severity of VF loss increases. VFI was meant to address some of the limitations of MD and PSD [13]. The results of our study show that a linear regression between VFI and MD, and PSD with the correlation coefficient is and The increase in the variability of MD observed in damaged visual fields is consistent with previous reports [13,14]. Our analysis suggests also an equation for estimating VFI from MD. In this study, we compared the MD and the VFI in the patients without glaucomatous defects in visual field test. So that, MD decreased only by cataract or by media opacities. There was no statistical difference in the VFI between group with cataract and group without cataract. These findings are similar to those of previous studies on visual fields [15,16]. This result demonstrated that 72

6 VFI seems less affected by cataract than MD. Bengtsson B and Heijl A used the pattern deviation probability maps of the standard Statpac program of the Humphrey perimeter to make the VFI as resistant as possible to the effects of media opacities. They realized that MDI is only very weakly center weighted, and therefore is not as well correlated to patient visual function as may be desired [4]. The pattern deviation concept has been widely accepted and applied to perimetric analysis and is used as a way of reducing the effects of developing cataract. The pattern deviation was only used as a tool for identifying abnormal test points. The extent of the field loss at those locations, then, is based on total deviation values. In addition, locations in the center of the visual field are more heavily weighted and therefore make a greater contribution to the VFI than do those in the periphery [4,15,17]. For the patients with VFIs 90% and above, the correlation with the MD and with the PSD was better than for the patients with VFIs below 90% (r = vs ). The stronger correlation of higher VFI values with higher MD may reflect differing patterns of local and diffuse VF loss of glaucoma. In group with VFI below 90%, the patients had more severe glaucoma. These eyes were often accompanied by cataract (due to intraocular hyertension). The MD on these eyes were higher than the true value reflecting the glaucoma stage due to MD that reflect localized defects of glaucoma but also reflects diffuse defects due to cataract. While the VFI only reflects localized defects due to glaucoma [15]. This explains that the correlation between MD and VFI is less strict. This result confirmed that VFI seems less affected by media opacities. V. CONCLUSION In conclusion, VFI had linear correlation with both MD and with PSD. This correlation was stronger in patients with high VFI values. VFI seems to be less affected by cataract than MD. Our study suggest that VFI may become a useful and simple tool for accurately determining glaucomatous visual field defects, especially in patients that have both glaucoma and cetaracts. Acknowlegements We would like to express our sincere thanks to the doctors and medical staff at the Glaucoma department at Vietnam National Institute of Ophthalmology for their support during this study. REFERENCES 1. Paul N Shacknow andjohn R Samples (2010), The Glaucoma Book: a practical, evidence-based approach to patient care, chủ biên, Springer Science & Business Media, Heijl A, Lindgren G, Olsson Jet al (1992), "On weighted visual field indices", Graefes Arch Clin Exp Ophthalmo. 230, Advanced Glaucoma Intervention Study. 2. (1994), Visual field test scoring and reliability, ed, Vol. 101, Ophthalmology. 4. Bengtsson B andheijl A (2008), "A visual field index for calculation of glaucoma rate of progression", Am J Ophthalmol. 145,

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