Correlating of the visual field index with mean deviation and pattern standard deviation in glaucoma patients

JOURNAL OF MEDICAL RESEARCH<br /> <br /> CORRELATING OF THE VISUAL FIELD INDEX WITH MEAN<br /> DEVIATION AND PATTERN STANDARD DEVIATION IN<br /> GLAUCOMA PATIENTS<br /> Bui Thi Huong Giang, Pham Thi Kim Thanh<br /> Department of Ophthamology , Hanoi Medical University<br /> The purpose of our study was to evaluate the correlation between the new index - visual field index<br /> (VFI) and mean deviation index (MD) and pattern standard deviation (PSD) in patients with glaucoma.<br /> MD, PSD and VFI were calculated in data about 103 eyes obtained from a cross-sectional study in<br /> 103 eyes of 58 patients with mild to severe glaucoma or ocular hypertension. The correlation of VFI<br /> to MD, PSD was evaluated with linear regression models, and the coefficient of determination (r) was<br /> calculated. The result showed that the average values of VFI, MD and PSD were 78.76%, -10.22<br /> dB and 4.56 dB respectively. The VFI and the MD were linearly correlated with r = 0.984. For the<br /> patients with VFIs below 90%, the correlation with the MD was better than for the patients with VFIs<br /> 90% and above (r = 0.986 vs 0.571). There was no statistically significant difference in VFI value<br /> between the group with cataract and the group without cataract (p > 0.05) but MD varied significantly<br /> between these two groups (p < 0.05). For the PSD, the correlation for the patients with VFIs 90% and<br /> above was greater than for the patients with VFIs below 90% (r = -0.982 vs -0.196). In conclusion,<br /> VFI was linearly correlated with MD and PSD. VFI seems to be less affected by cataract than MD.<br /> <br /> Keywords: VFI, MD, PSD, visual field.<br /> <br /> I. INTRODUCTION<br /> In management of glaucoma patients,<br /> the visual field (VF) is the most important<br /> tool to determine the stage and progression<br /> of the disease [1]. VF data is summarized<br /> in global summary indices [2] . At this time,<br /> the mean deviation index (MD) and the pattern<br /> standard deviation (PSD) are the standard<br /> indices to evaluate for glaucomatous<br /> damage [3]. The Visual Field Index (VFI) is a<br /> Corresponding author: Bui Thi Huong Giang, Department of Ophthamology, Hanoi Medical University<br /> Email: buihuonggiang@hmu.edu.vn<br /> Received: 03 June 2017<br /> Accepted: 16 November 2017<br /> <br /> 68<br /> <br /> new index, introduced by Bengtsson B and<br /> Heijl A in 2008 [4]. The VFI expresses the<br /> visual function as a percentage of normal<br /> age-corrected sensitivity. Therefore, the VFI<br /> of an eye with a completely normal visual<br /> field is 100% and the VFI of a perimetrically<br /> blind eye is 0%. The VFI was designed using<br /> the Humphrey 30 - 2 and 24 - 2 test point<br /> patterns, which are the most commonly<br /> used patterns in glaucoma management [3].<br /> To avoid effects of cataract on the VFI, the<br /> pattern deviation probability map was used<br /> to identify test points with normal sensitivity<br /> (100%), having absolute defect (0%), and<br /> JMR 111 E2 (2) - 2018<br /> <br /> JOURNAL OF MEDICAL RESEARCH<br /> those demonstrating relative loss. The<br /> sensitivity at these points were scored using<br /> the following formula:<br /> 100 - [(total deviationI/age-corrected<br /> normal threshold) x 100] ,<br /> where |total deviation| is the absolute<br /> value of the numerical total deviation value<br /> and age-corrected normal threshold is<br /> intercept - age coefficient x patient age.<br /> The VFI was also constructed to weigh<br /> the central points of the visual field more<br /> heavily than peripheral points. The VFI is<br /> the mean of all weighed scores in percent.<br /> The test point pattern was divided into five<br /> concentric rings of increasing eccentricity.<br /> Cortical magnification of any given visual<br /> field location is assumed to reflect both<br /> ganglion cell density and the number of<br /> neurons in an area of the visual cortex<br /> responsible for processing a stimulus of a<br /> given size [5]. The weights of the test points<br /> decreased with increasing eccentricity from<br /> 3.29, 1.28, 0.79, and 0.57 to 0.45 at the<br /> outermost ring. The VFI is automatically<br /> computed using in the current ‘‘Statpac<br /> software’’ of the Humphrey Field Analyzer<br /> (HFA II; Carl Zeiss Meditec, Inc., Dublin,<br /> California, USA) (Fig.1) [4].<br /> <br /> Figure 1. Visual field of a glaucoma<br /> patient<br /> Mean Deviation (MD) is the average of<br /> JMR 111 E2 (2) - 2018<br /> <br /> these deviations across all test locations.<br /> Mean defect expresses the difference<br /> between observed and expected mean<br /> sensitivity. MD expresses the overall<br /> reduction in sensitivity, and is therefore<br /> decreased not only by increasing glaucoma,<br /> but also by cataract [6]. Pattern standard<br /> deviation (PSD), a depiction of focal defects,<br /> measures irregularity between the threshold<br /> value for each point and the average visual<br /> field sensitivity at each point . Thus, in<br /> patients with severely damaged visual fields<br /> the value of PSD is too low to be useful as<br /> an indicator of severity of disease [7].<br /> Glaucoma is an ocular disease, in which<br /> characteristic visual field loss corresponds<br /> to the underlying anatomic arrangement<br /> of damaged<br /> retinal<br /> ganglion<br /> cells.<br /> Glaucomatous visual field characterized<br /> by loss is localized defects [8]. To stage<br /> glaucoma, most classification systems use<br /> MD, PSD, and the number of defective<br /> points in the visual field test. However, the<br /> MD is affected by both glaucoma and media<br /> opacities. Thus, cataract or media opacities<br /> can falsely inflate glaucoma severity.<br /> Bengtsson B and Heijl A used the pattern<br /> deviation probability maps in the visual field<br /> test to make the VFI test as resistant as<br /> possible to the effects of media opacities,<br /> while clearly depicting localized loss [4].<br /> There are many studies concerning the VFI<br /> in the world [9;10].<br /> The relationship between VFI and other<br /> visual field indices has not been studied in<br /> Vietnam. The objective of this study was<br /> to evaluate the correlation between VFI<br /> and other visual field indices (MD, PSD) in<br /> glaucoma patients.<br /> 69<br /> <br /> JOURNAL OF MEDICAL RESEARCH<br /> <br /> II. SUBJECTS AND METHODS <br /> 1. Subjects<br /> Glaucoma patients who agreed to<br /> participate were examined. Patients<br /> performed the 24 - 2 threshold test<br /> (Humphrey SITA standard). The VF tests<br /> with reliability indices (Fixation losses,<br /> False positives, False negatives) > 20%<br /> were excluded.<br /> Inclusion criteria included a clinical<br /> diagnosis of primary glaucoma or ocular<br /> hypertension and absence of other retinal<br /> disease. They had a best corrected visual<br /> acuity (VA) equal to or better than 20/60 and<br /> refractive error within ± 5.00 D sphere and ±<br /> 3.00 D astigmatism.<br /> 2. Methods<br /> This cross-sectional study was performed<br /> between December 2015 and April 2016 at<br /> Glaucoma department, Vietnam National<br /> Institute of Ophthalmology.<br /> To calculate the sample size of this crosssectional study, we assumed that the rate of<br /> visual field loss in glaucoma patients was<br /> 85% [6]. The following formula was used:<br /> N = [(Z 1-α/2)2 p.q] / d2. To achieve 95%<br /> confidence intervals (CIs) and 7% error, 103<br /> eyes from 58 patients were examined.<br /> The MD was calculated as the weighted<br /> <br /> mean of the total deviation values, and<br /> the weight assigned to each location was<br /> the inverse of the variance in the healthy<br /> reference group. The PSD was determined<br /> by comparing the differences between<br /> adjacent points [2]. The VFI was calculated<br /> as described by Bengtsson and Heijl. At<br /> each location, the measured sensitivity was<br /> expressed as a percentage of the sensitivity<br /> expected in a healthy observer of the same<br /> age, and the VFI was calculated as the<br /> weighted mean of all locations with pattern<br /> deviation probability outside normal limits<br /> (< 5%) [11].<br /> The relationship between VFI, MD, and<br /> PSD was described with linear regression<br /> analysis. VFI was treated as the dependent<br /> variable; MD and PSD were treated as<br /> independent variables in all regressions. To<br /> evaluate the relationship between variables<br /> from a single patient, the correlation<br /> coefficient (r) was calculated by SPSS 20.0<br /> Statistics.<br /> 3. Ethics<br /> Research subjects were informed<br /> about the goals of the study and voluntarily<br /> agreed to participate. The study was<br /> approved by Vietnam National Institute of<br /> Ophthalmology.<br /> <br /> III. RESULTS<br /> A total of 77 eyes with glaucoma and 26 eyes with ocular hypertension were studied. The<br /> VFI ranged from 100% (normal visual field) to 1%. The MD showed the overall depression<br /> ranged from - 33.56 dB to - 0.93 dB. PSD ranged from 1.03 to 13.35 dB.<br /> <br /> 70<br /> <br /> JMR 111 E2 (2) - 2018<br /> <br /> JOURNAL OF MEDICAL RESEARCH<br /> Table 1. Average visual field indices<br /> <br /> Average ± SD<br /> Min - max<br /> <br /> VFI (%)<br /> <br /> MD (dB)<br /> <br /> PSD (dB)<br /> <br /> 78.76 ± 28.75<br /> <br /> - 10.22 ± 8.71<br /> <br /> 4.56 ± 3.45<br /> <br /> 1 – 100<br /> <br /> (- 33.56) – (- 0.93)<br /> <br /> 1.03 – 13.35<br /> <br /> Chart 1. Comparison of MD and VFI<br /> Correlation coefficient (r) is 0.984<br /> The two tailed P value is < 0.0001<br /> In single visual fields, there was a close relationship between MD and VFI. A significant<br /> correlation between MD and VFI was shown in all eyes (r = 0.984, P < 0.0001) which was<br /> positive linear. This relationship was described by the equation: VFI = 111.4 + 3.2 x MD (%).<br /> This model predicts VFIs of 92%, 73%, and 47% for visual fields with MDs of − 6, − 12, and −<br /> 20 dB, with prediction intervals of approximately ± 8%.<br /> <br /> Chart 2. Comparison of PSD and VFI<br /> Correlation coefficient(r) is -0.667<br /> The two tailed P value is < 0.0001<br /> Chart 2 showed the linear regression between VFI and PSD with r = - 0.667, p < 0.0001.<br /> JMR 111 E2 (2) - 2018<br /> <br /> 71<br /> <br /> JOURNAL OF MEDICAL RESEARCH<br /> Table 2. The correlation between VFI and MD, PSD in the patients with VFI ≥ 90%<br /> and VFI < 90%<br /> VFI (≥ 90%) - MD<br /> <br /> VFI (< 90%) - MD<br /> <br /> VFI (≥ 90%) - PSD<br /> <br /> VFI (< 90%) - PSD<br /> <br /> p<br /> <br /> < 0.001<br /> <br /> < 0.01<br /> <br /> < 0.001<br /> <br /> > 0.05<br /> <br /> R<br /> <br /> 0.986<br /> <br /> 0.571<br /> <br /> - 0.982<br /> <br /> - 0.196<br /> <br /> The VFI was more closely correlated with MD than it was with PSD. PSD was significantly<br /> correlated observations of VFI ≥ 90%, but not with observations where VFI < 90%<br /> Table 3. MD and VFI average patients without glaucomatous defects<br /> in visual field test<br /> Cataract<br /> <br /> VF index<br /> <br /> MD (dB)<br /> <br /> VFI (%)<br /> <br /> No cataract<br /> <br /> - 3.22 ± 1.28 (26)<br /> <br /> 98.73 ± 0.96 (26)<br /> <br /> Cataract<br /> <br /> - 5.34 ± 2.63 (12)<br /> <br /> 98.83 ± 0.72 (12)<br /> <br /> 0.014<br /> <br /> 0.09<br /> <br /> p<br /> <br /> In 50 eyes with MD > - 6dB, 38 eyes had no glaucomatous defects in visual field test<br /> (stage 0 – classification Hodapp, Parish and Anderson - 1993) [12]. We compare the average<br /> MD and average VFI between the group with cataract and the group without cataract. There<br /> was no significant difference in the VFI between two the groups (p > 0.05) but was significant<br /> difference in the MD between two the groups (p < 0.05).<br /> <br /> IV. DISCUSSION<br /> Global indices of VFs have always<br /> played an important role in summarizing<br /> the severity of glaucoma. MD and PSD are<br /> the two most popular global indices used<br /> in clinical practice. However, both of them<br /> have limitations. MD is affected by media<br /> opacities and by other causes of generalized<br /> depression of visual function in addition to<br /> glaucoma. PSD is less affected by media<br /> opacities, but has the disadvantage that<br /> it falsely improves as the severity of VF<br /> loss increases. VFI was meant to address<br /> some of the limitations of MD and PSD [13].<br /> The results of our study show that a linear<br /> regression between VFI and MD, and PSD<br /> 72<br /> <br /> with the correlation coefficient is 0.984 and<br /> -0.667.<br /> The increase in the variability of MD<br /> observed in damaged visual fields is<br /> consistent with previous reports [13,14].<br /> Our analysis suggests also an equation<br /> for estimating VFI from MD.<br /> In this study, we compared the MD and<br /> the VFI in the patients without glaucomatous<br /> defects in visual field test. So that, MD<br /> decreased only by cataract or by media<br /> opacities. There was no statistical difference<br /> in the VFI between group with cataract and<br /> group without cataract. These findings are<br /> similar to those of previous studies on visual<br /> fields [15,16]. This result demonstrated that<br /> JMR 111 E2 (2) - 2018<br /> <br />