Vfi (Artes, Iovs Apr11)

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Artes et al., Visual Field Index (IOVS 10-6905, uncorrected proof for personal use only)

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Properties of the Statpac Visual Field Index

Authors Paul H Artes PhD (corresponding author)Neil OLeary PhD, Donna Hutchison, Lisa Heckler MD,

Glen Sharpe MSc, Marcelo T Nicolela MD, Balwantray C Chauhan PhD

Keywords glaucoma, visual field, progression, mean deviation, index, scaling, vision,visual field index

Affiliation Ophthalmology and Visual Sciences, Faculty of MedicineDalhousie University, Halifax, NS, Canada

Correspondence Paul H Artes, PhD [[email protected]]Ophthalmology and Visual Sciences, Dalhousie University

Rm 2035, West Victoria, 1276 South Park St,

Halifax, Nova Scotia, B3H 2Y9, CanadaSupport Glaucoma Research Foundation (PHA)

Grant # MOP-11357, Canadian Institutes for Health Research (BCC)Prcis Global visual field indices are useful for estimating the speed of visual field

progression over time. In this report, compare the properties of the Visual

Field Index (VFI) to those of Mean Deviation (MD), in patients with

glaucoma followed for 10 years.


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Abstract

Purpose

To compare the properties of the Visual Field Index (VFI) to those of Mean Deviation (MD), in

patients with glaucoma.Methods

MD and VFI were calculated in data obtained from an ongoing longitudinal study in which patients

with glaucoma (n=109, 204 eyes) were followed for 9.8 years (median, 21 tests) with static automated

perimetry. MD and VFI were compared in one test of each eye, and a subset of 30 tests was selected

to compare the VFI to the judgments of 8 experts who judged the percentage of remaining visual field.

In series of tests obtained over time, rates of change, statistical significance, evidence of non-linearity,

and variability were compared between both indices.

Results

In single tests, MD and VFI were closely related (r=0.88, p


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Methods

Data

The data for this report were obtained from an ongoing prospective longitudinal study in which a

cohort of patients with open-angle glaucoma are being followed.16 The visual field indices MD and VFI

were calculated for the purposes of this report. Since the VFI is based on normative limits of total- and

pattern-deviation that are not in the public domain, these limits were estimated from a group of healthy

controls were followed within the same study.16

Patients were recruited consecutively from the clinics at the QEII Health Sciences Centre in Halifax,

Nova Scotia, Canada. Inclusion criteria were a clinical diagnosis of open-angle glaucoma based on optic

disc and visual field changes, an MD between -2.0 and -10.0 dB in at least one eye, and absence of

other ocular disease. Controls were recruited from patients relatives, church groups and a local

telephone company; they had a normal eye examination and an IOP


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most central locations closest to fixation. Since pattern deviation calculations are unreliable with highly

damaged visual fields, total rather than pattern deviation probability values were used with MDs worse

than -20.0 dB.15

Total and pattern deviation probability limits were estimated, separately for each location, from the

visual fields of our healthy control group. Because these limits are used to control for physiologic

between-subject variation in the visual field only a single test was used from each control; to minimize

potential artefacts from lack of experience with static perimetry, the third test was selected. MD and

VFI agreed closely with the values provided on the Statpac printouts (see the Appendix for a formal

comparison).

Comparison of MD and VFI in single tests (cross-sectional analysis)

The relationship between MD and VFI in single tests was estimated using the 3rd test in each series.

Because both indices are derived from the same measured threshold data, orthogonal regression18 was

used to derive an equation describing the relationship, irrespective of which index is selected to be the

dependent or independent variable.

A subset of 30 tests that covered a wide range of MD and VFI values were selected to assess how the

VFI compared with the judgment of experts. Statpac printouts of these visual fields, with the VFI

masked, were then distributed to 8 experts (see Acknowledgment section) who were asked to provide a

subjective judgment of the percentage of remaining visual field. Experts were at liberty to consider any

information on the printout they might regard as relevant, including greyscale plots, total and patterndeviation maps, and global indices (except the VFI, which had been masked). They were also instructed

that their judgment should reflect their beliefs of relative importance of central and peripheral, as well

as superior and inferior, visual field damage. The median and range of the expert judgments were then

compared with the VFI.

Comparison of MD and VFI in series of examinations (longitudinal analysis)

To compare rates of change with the two indices, separate linear regressions were performed on theentire series of visual fields available for each eye. Rates of change were estimated from the slope of the

regression line of MD (dB) and VFI (%) as dependent and age (years) as independent variable. The

slope of the regression line (in dB/y with MD, and %/y with VFI) gave an estimate of the average

speed of change over the entire follow-up.


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To determine if either MD or VFI were more sensitive to visual field deterioration, we compared the

number of eyes with statistically significant negative slopes at p-values of


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Results

Demographic details of patients and controls

Table 1 provides demographic details of the patients. In 95 of 109 patients (87%), both eyes met the

inclusion criteria (baseline MD better than -20.0 dB, at least 5 tests available for analysis) and were

entered in the analysis. In the remaining 14 patients one eye had advanced damage at baseline (MD

worse than -20.0 dB) such that only the better eye qualified for inclusion.

Table 1. Demographic details (median and interquartile range) for patients with glaucoma.N 109 patients (204 eyes)Age 61.8 (52.4, 71.8) yearsbaseline MD -4.5 dB (-6.9, -2.7) dBfollow-up 9.9 (5.4, 11.9) yearsnumber of tests 21 (12, 24)

The total and pattern deviation limits used in the VFI calculation were derived from 100 healthy

controls (median age 46.5 years, interquartile range [IQR] 38.1, 59.2 years).

VFI and MD in single visual fields (cross-sectional analysis)

In single visual fields, there was a close relationship between MD and VFI (Fig 1, Spearmans r=0.88,

p


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Figure 1

Relationship between MD and VFI in single

examinations. For eyes with MD worse than

-5.0 dB, the relationship appeared linear

(red line). The 95% prediction interval isshaded in grey. Histograms on the x- and y-

axes show the distribution of MD and VFI.

Except for 3 visual fields with MD > 0 dB, the VFI estimates of the percentage remaining visual field

were higher than the median judgment of the experts, on average by 12%. In 16 of the 30 fields (53%),

the VFI was higher than the largest individual judgment of the 8 experts (Fig. 2). Of note was the large

spread in the experts estimates. In all visual fields with MD


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VFI and MD in visual field series (longitudinal analysis)

The mean and median rates of global visual field change in the patients with glaucoma were -0.27 and

-0.18 dB/y with MD, and -0.84 and -0.26 %/year with VFI, respectively (Fig. 3). Rates of change

estimated from both indices were closely related (Spearmans = 0.79, p


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The number of eyes with statistically significant negative trends in MD and VFI were similar (Fig 4).

For example, 92 eyes (45%) showed a negative MD slope significant at the 5% level, while 87 eyes

(43%) had a negative VFI slope at the same significance (Fig 4a; =0.65). The close agreement on the

presence of a negative trend with MD and VFI remained similar when p-values of


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Figure 5

Probability limits for the differences between the level of MD and VFI at single tests and their best available

estimates (BAEs). For a specific level of BAE, the interval enclosed by the 2.5th and 97.5th percentiles (dark

grey shading) encompasses the range within which estimates from single tests are likely to fall 95% of the time.

Intervals for 90% and 80% are shown in medium and light grey, respectively.

To investigate if either MD or VFI would provide a more linear fit of visual field change over time,

statistical tests for non-linearity were performed by testing whether a quadratic or cubic polynomial

explained a significantly larger variance than a linear model. The proportions of eyes in which this was

the case were similar with the MD and the VFI (Fig. 6); thus, there was no evidence that the temporal

pattern of change was more linear with one index than the other.

Figure 6

Proportion of eyes in which the series of MD and VFI

appeared significantly non-linear, at p-values of 0.05,

0.01, and


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Case examples.

In Example 1 (Fig. 7), the visual field series showed a slow rate of global visual field change that was

statistically significant with both indices. Over a period of nearly 13 years, the MD changed from

-3.8 dB (first test) to -6.7 dB (last test), with a linear rate of change of -0.15 dB/y. The VFI changed

from 95% to 88%, at a linear rate of -0.3%/y. With the MD, the temporal pattern of visual field change

appeared non-linear (p=0.04), with a faster speed initially, but no evidence of non-linearity was detected

with the VFI (p=0.44).

Example 2 (Fig. 8)illustrates a case with a rapid rate of change (-0.8 dB/y with MD, and -3%/y with

VFI). During the early follow-up, the rate appeared relatively more rapid with MD than with the VFI,

most likely due to a ceiling effect with the latter index. Over the entire follow-up, the trend in both

indices was non-linear (p=0.005 with MD, p


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Figure 7) Example 1.

Figure 8) Example 2. See legend to Fig. 7 (page 13).


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Discussion

The aim of this report was to compare the MD and VFI indices for estimating rates of change in

glaucoma. Our results indicate that, in the large majority of patients, both indices provide equivalent

information on the speed of visual field progression (Fig. 3). In addition, there were no substantial

differences between the number of eyes in which MD and VFI showed statistically significant negativerates of change (Fig. 4), suggesting that both indices have similar capability to detect the presence of

global visual field progression.

The increase in the variability of MD observed in damaged visual fields is consistent with previous

reports.5, 25-27 Our analysis suggests that in visual fields with a true MD near 0 dB, MDs from single

tests are likely to fall into a 95% tolerance interval of 1 dB, suggesting that a deterioration by more

than 2 dB, from one test to the next, is unlikely to occur due to chance. When the true MD is near

-5 dB, the tolerance interval is much broader (-3 dB, -7 dB), suggesting that differences of up to 4 dB

from one test to the next may be explained by variability. A similar picture emerged with the VFI.

When the true value of the index was near 90%, the tolerance interval of single tests ranged from 85%

to 95%, but this expanded with VFIs near 70% to cover a range from 60% to 77%.

Because the VFI disregards loss of sensitivity at locations at which the pattern deviation is within

normal limits, this index may be more resistant than the MD to diffuse visual field changes caused by

cataract. This is an advantage for estimating the speed of glaucomatous change in patients who develop

significant lens opacity during follow-up. In our material, however, there were only very few patients in

whom surgery was delayed sufficiently long for cataract to lead to a substantial reduction in MD.

A previous report from this study population showed that, despite a 2-line post-operative improvement

in visual acuity, the mean improvement in MD after cataract surgery was


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Appendix

The visual field indices provided on the Statpac printouts are based on proprietary normative values

whereas the MD and VFI values in this report were calculated using data from healthy subjects that

have either been previously reported in the literature17 or have been derived from a group of healthy

controls followed in our center.

16

To compare the calculations of MD and VFI performed in thisreport to the Statpac analyses, we selected a subset of 100 visual fields covering a large spectrum of

visual field damage. MD and VFI were manually extracted from the Statpac printouts and compared

with the values derived from our analyses (Fig. A1).

Although the MD according to our calculations was systematically higher than that of Statpac, the

mean difference (-0.21 dB, 95% CI -0.27, -0.15 dB, p


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