A new scoring program for quantification of the binocular visual field

A NEW SCORING PROGRAM FOR QUANTIFICATION OF THE BINOCULAR VISUAL FIELD

FRANCESCO MORESCALCHI,1 ENRICO GANDOLFO,1

FEDERICO GANDOLFO,1 LUCIANO QUARANTA1 and PAOLO CAPRIS2

Departments of Ophthalmology, 1University of Brescia, Spedali Civili di Brescia, Brescia, and 2University of Genova, Spedale S. Martino, Genova, Italy

Introduction

In the past, estimates of visual efficiency were often based on central acuity only, even though peripheral visual field loss can also severely limit everyday performance.

The assessment of visual disability must consider the quantification of visual field damage.

Recently, thanks to the activity of some associations (the Italian Group for Low Vision Study, the Italian Union of Blinds, the Italian Section of the International Agency for the Prevention of Blindness, the Italian Ophthalmologic Society, etc.), the Italian parliament passed a bill that recognized the importance of peripheral visual impairment.1 This new article gave legal value to the disability caused by visual field constriction and classified peripheral visual damage into six categories:

•no peripheral low vision: binocular visual field residual ≥ 60%;

•slight peripheral low vision: binocular visual field residual < 60 and ≥ 50%;

•moderate peripheral low vision: binocular visual field residual < 50 and ≥ 30%;

•severe peripheral low vision: binocular visual field residual < 30 and ≥ 10%;

•relative peripheral blindness (with residuum): binocular visual field residual <10 and ≥ 3%;

•absolute peripheral blindness: binocular visual field residual < 3%.

The legal assessment of visual peripheral impairment is based on the concept of a functional binocular field.

A perimetric program, able to test the binocular visual field and to obtain a percentile score for quantifying the residual visual field, is needed to classify peripheral disability. The Humphrey Field Analyzer II (HFA-II, San Leandro, CA) incorporates the Esterman binocular visual field test.2-4 The Esterman program was originally designed for manual perimetry, and then adapted to the most common automatic visual field analyzers. The pattern of the original Esterman test was designed to allow the

Address for correspondence: Francesco Morescalchi, MD, Clinica Oculistica Università degli Studi di Brescia, c/o Spedali Civili, Piazzale Spedali Civili 1, 25125 Brescia, Italy. Email: oculistica@genie.it

Perimetry Update 2002/2003, pp. 21–27

Proceedings of the XVth International Perimetric Society Meeting, Stratford-upon-Avon, England, June 26–29, 2002

edited by David B. Henson and Michael Wall

© 2004 Kugler Publications, The Hague, The Netherlands

Fig. 1. Esterman binocular visual field test.

overlap of two monocular grids of 100 units, to create a binocular grid of 120 units (Fig. 1).

With the Humphrey and Octopus perimeters, the Esterman program adopts the single stimulus screening strategy. The stimulus intensity used is 10 dB (1000 asb) for simulating the standard conditions of the Goldmann perimeter. In this case, the concepts of the manual perimetry have been applied to automated perimeters. Several investigations have used the Esterman binocular visual field test, as configured for the HFA-II, and have attempted to correlate its findings with the other measurements of visual function. Many of these studies failed to find any strong correlation.5-10

The purpose of this work was to develop an automated program which correlated better with visual impairment and which can run on the HFA-II perimeter.

Procedure

The new test, known as Visual Field percent (VF%), can easily be created as a custom program for the Humphrey HFA-II. The test pattern follows similar general assumptions to those adopted with the Esterman program, giving more value to the central area of the visual field (Fig. 2). For human activities, the most important visual field area is that between 5 and 30° of eccentricity. Therefore, it is assigned a value of 64% (64 points) for the central 30° area and 36% (36 points) for the peripheral field between 30 and 60°. The lower half of the visual field is functionally more important than the upper half. Therefore, 40% (40 points) are assigned to the upper hemifield and 60% (60 points) to the lower hemifield.11,12

Fig. 2. Visual field percent (VF%) custom binocular visual field test.

The VF% test is in two phases: the first phase tests the central 30° while the patient is wearing their optical correction for refractive errors and presbyopia; the second phase examines the peripheral visual field (between 30 and 60°) without any optical correction. The evaluation of the visual field considers not only absolute, but also relative loss of sensitivity. In both phases, a static supraliminal gradient and an agerelated strategy with quantification of defect depth to two levels (absolute and relative) is employed. Target III (4 mm2) is presented at a luminance 6 dB above the theoretical threshold at each location. When the stimulus is not perceived twice, it is presented again at maximum luminance, thus allowing the defect to be classified as relative or absolute (option: screening three zone strategy of the HFA).

The main differences between Esterman and VF% are summarized in Table 1 and in Figures 1 and 2. The points seen with a 6 dB supraliminal stimulus are classified as ‘normal’ and a value of ‘1’ is assigned to them. The points only seen at the maximum luminance (10,000 asb) are classified as points with ‘reduced sensitivity’ and have a value of ‘0.5’. Points not seen, even after the presentation at the maximum luminance, are considered to have absolute defects, and are assigned a value of ‘0’.

Table 1. Main differences between the Esterman and VF% binocular programs

Responses corresponding to these three levels (normal, relative and absolute) are represented with three different symbols. The computer calculates the sum of the relevant values and provides the percentage value of the perimetric residuum.

Patients and methods

Thirty-six patients (16 females and 20 males) with bilateral visual field loss due to advanced glaucoma, retinitis pigmentosa or optic neuritis were examined. The patients’ ages ranged from 45 to 79 years (average, 65.6 years). All these patients had prior experience in performing automatic visual field tests. Monocular and binocular visual field tests were obtained. All visual field tests were conducted using a Humphrey Field Analyzer II.

For the binocular tests, patients sat with the chin rest positioned in the midline, the binocular field was plotted without occluding either eye. During the examination, the patient was instructed to fixate on the central target with both eyes. Some patients complained of mild diplopia, which usually disappeared or was well tolerated during the examination. During the first phase of the test, when near correction was needed, patients were asked to use their reading spectacles. If these were not available, or were not correct for the test distance (+3.25 D), a modified pediatric trial frame (half frames) with the optimal lens correction placed before each eye was used. This trial frame could easily be worn while the patient performed the perimetric test. Its shape minimized the likelihood of the trial frame and lenses obstructing the field of view of both eyes during testing. It was adjusted to account for differences in interpupillary distance so that the trial lenses were properly centered for each eye.

During binocular testing, patients were aligned to the perimeter by adjusting the vertical head position and centering on the bridge of the nose in the fixation monitor. This precluded the ability to monitor fixation during binocular visual field testing. However, all patients had prior experience of visual field examination, and those with a history of poor fixation were excluded from the study.

The proposed binocular scoring strategy (VF%) and the Esterman binocular test were performed during the same visit, after a rest period of at least ten minutes. Monocular threshold visual field tests (program 24-2 SITA fast algorithm) were conducted on both eyes within three months of the binocular tests.

The results of the monocular threshold visual field tests were used as a reference, as suggested by Nelson-Quigg et al.13 This study assumed that the eye with best overall visual field sensitivity, as determined by mean deviation (MD), determines the binocular visual field properties of patients with glaucoma. It was found that MD of the best eye correlated better with quality-of-life measures than MD of the worse

eye.13-16

Both monocular and binocular visual field data were collected and statistically analyzed in order to detect the difference between the two binocular programs. Subjective visual disability was assessed during a subsequent interview by means of a questionnaire developed to determine difficulty across a range of 35 mobility situations.17 Trained interviewers administered the questionnaire to all patients. All items were scored on a five-level scale (see Appendix). The scores of the questionnaire and of the two binocular visual fields were statistically analyzed in order to find a correlation between patient perceived disability and residual visual field.

Results

The results of the mean score, reliability indexes, and execution time are summarized in Table 2.

The visual field scores were similar but not equivalent. There was no statistical difference between the global scores (Wilcoxon test: p > 0.06) but a significant trend (Wilcoxon test: p < 0.06) for the Esterman program to attribute a higher score in the central 30°.

The Esterman program attributed higher scores in patients with relative visual field defects inside the central 30°. In the central 30°, the VF% program was more accurate in revealing relative defects. The Esterman program produced global scores > 60% (= no visual disability) in more than 61% of our patients, while only 44.4% of patients tested with VF% had normal global scores. The Estermann program revealed a slight loss of peripheral vision in (binocular visual field residual < 60 and > 50%) in 11.1%, a moderate loss (binocular visual field residual < 50 and > 30%) in 22%, a severe loss (binocular visual field residual < 30 and >10%) in 5.5%, and a relative or absolute peripheral blindness in no patients. The VF% program reported a slight peripheral loss in 5.5%, a moderate loss in 33.3 %, a severe loss in 11.1%, and a relative peripheral blindness in 5.5 % of patients (Table 3). The VF% score, but not the Esterman score, correlated with MD of the better eye (Spearmann rank sum correlation: VF% versus MD: p = 0.001; Estermann versus MD: p = 0.052). Good correlation was also found between the questionnaire and the VF% scores (Spearmann rank sum correlation p = 0.01), while only a weak correlation was found between the score of the patient-based assessment of disability and the Esterman score (p = 0.055).

Table 2. Comparison between results obtained with the Esterman and VF% binocular tests

F.N. = false negative; F.P. = false positive

Table 3. Classification of the same group of patients with the Esterman and VF% binocular tests

26 F. Morescalchi et al.

Discussion

The Esterman program uses the single stimulus intensity of 10 dB; this means that every pattern location that has a retinal threshold higher than 10 dB is assumed to be normal. It has been stressed that, for human activities, the central 30° is the most important area of the visual field. In this area, a mean sensitivity of 10-15 dB is not compatible with normal vision. In a binocular Esterman visual field test, the central visual field could appear normal despite large relative defects that impair performance in everyday life.5,6 The stimulus luminance of 10 dB is too bright to test the paracentral retinal sensitivity, and the relative weight of this area (48.3% of test locations) could underestimate the real visual impairment. This is correlated with a higher number of false negative visual fields.

In the custom-made program VF%, the three-zone screening strategy was more accurate, and the relative weight of the central area (64% of the test location) seemed to be more realistic. Using this program, 27.2% of our patients, classified normals by the Esterman test, were shown to have a peripheral visual disability.

The mean score of this test correlated well with the mean deviation of the better eye and with the score of the questionnaire regarding the difficulty of performing 35 mobility situations. The same correlations were not found with the Esterman ‘global score’ in our group of patients.

In conclusion, the custom binocular program VF% appears to be well correlated to the reported visual impairment of low vision patients and is probably more effective in quantifying peripheral visual impairment for legal purposes.

References

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Appendix

Mobility questionnaire

Read each mobility situation below and circle the number which best expresses the level of difficulty you feel in the situation without any assistance. On a scale of 1 to 5, 1 represents no difficulty and 5 represents extreme difficulty. N/A represents not applicable.