Results

The mean age of the patients at examination was 65.5 years (range 30–85 years). Patients presented with a wide range of visual field defects in both eyes. The sample mean Humphrey MD (mean deviation) was −11.5 (SD 8.2) dB for the right eye and −10.6 (7.7) dB for the left eye. The sample mean Humphrey CPSD (corrected pattern standard deviation) was 7.1 (3.7) dB for the right eye and 7.0 (3.8) dB for the left eye.

The adequacy of perimetric performance was assessed using the Humphrey reliability indices.8 A visual field was considered unreliable if 33% or more was recorded for at least one of the false positive (FP), false negative (FN), and fixation loss (FL) indices. These limits have been previously recommended for identifying inadequate perimetric performance.9 10Patients were excluded from further analysis if at least one field examined on the 24-2 test or the EVFT was unreliable (FP and FN are also available on the EVFT). In total, 15 patients were excluded from further analysis because of inadequate perimetric performance in at least one of the tests. (Eleven patients were unreliable on at least one monocular field; two unreliable on EVFT; two unreliable on at least one monocular field and EVFT.)

Results from EVFT and binocular simulation analysis for example patients are shown in Figures 3 and 4. Classification by the EVFT and binocular simulation analysis of the reliable 44 patients as exhibiting “At least one defect (<10 dB)” or “No defects (<10 dB)” in the central field (20° from fixation) is shown in Table 1. The kappa coefficient of agreement was 0.81 (SE 0.09). This value indicates a substantial level of agreement between the two techniques.11 All patients that exhibited “At least one defect (<10 dB)” using the EVFT were correctly identified by the binocular simulation (100% sensitivity). Four of the 29 patients defined as having “No defects (<10 dB)” using the EVFT were classified by the binocular simulation as having a defect (86% specificity).

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Figure 3

EVFT output for a sample patient is shown in (A). A circle has been superimposed to denote the central 20°. Results from the binocular simulation implemented by the progressor for Windows software (Institute of Ophthalmology, London) are shown for the same patient in (B). The sensitivity of the left and right 24-2 fields are shown as Humphrey type grey scales. The lower panels show the results from the binocular simulation as Humphrey type grey scale with and without superimposed symbols denoting point by point EVFT type defects (<10 dB). The blue circle on the display indicates the central 20° area. This patient clearly demonstrates “At least one defect (<10 dB)” using the EVFT (A) and similarly “At least one defect (<10 dB)” using the binocular simulation (B).  

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Figure 4

EVFT output (A) and results from the binocular simulation (B) implemented by the progressor for Windows software (Institute of Ophthalmology, London) are shown for another sample patient. The lower panels show the results from the binocular simulation as Humphrey type grey scale with and without superimposed symbols denoting point by point EVFT type defects (<10 dB). This patient demonstrates “No defects” using the EVFT (A). This patient also demonstrates “No defects” using the binocular simulation (B) despite the presence of areas of visual field damage in each monocular field.  

Discussion

This study has illustrated a new method of simulating the central binocular field by merging results from monocular Humphrey fields. Using the higher sensitivity from either eye for each test location has been shown to be superior to other methods in combining monocular results to estimate a binocular field in glaucoma patients.12 Earlier studies, using results from Goldmann perimetry, have considered this concept but concluded that ideally a patient’s functional field capacity should only be accurately assessed via binocular viewing because of what is termed enhancement or sensitivity summation.1 Binocular enhancement occurs in areas where the two monocular fields overlap and has the effect of increasing the Esterman score (percentage of 120 EVFT locations where the stimulus was seen) above that predicted by merging the two monocular fields. Enhancement has been demonstrated in normal subjects.13 However, there is evidence that it occurs less often in patients with advanced glaucoma.2 14 15 The results from our study also suggest that enhancement occurs infrequently in the central field (20° from fixation) of a group of patients with bilateral glaucoma. Significant binocular enhancement would have manifested poor agreement between the simulation and the EVFT in identifying patients with at least one defect (<10 dB). This was not the case since high levels of agreement were observed.

A recent study, comparing Esterman score against mean deviation of monocular fields, reported lower than expected number of EVFT defects in glaucoma patients with bilateral field loss.16Moreover, the automated binocular EVFT has been described by others as “less stringent” and “more forgiving” than other perimetric tests.4 These findings may of course be explained in part by binocular enhancement but other explanatory factors are likely to be more significant. For example, the test strategy employed by the EVFT on the Humphrey field analyser is not optimal. Retesting only a selected sample of test locations where the stimulus was not seen provides a simple evaluation of the binocular field. However, in the presence of the inherent variability in patient response to the light stimulus, this test strategy is biased.17 Results from a study investigating the performance of this type of test strategy indicated that the true number of binocular defects can be underestimated by approximately 20%.17

There is no direct monitoring of fixation on the Humphrey EVFT. In contrast, the binocular simulation is based on results from the Humphrey monocular fields where fixation is monitored throughout the test by a method that presents stimuli at the site of the predetermined blind spot. Control of fixation is an important component of perimetric testing and reliability of response.18 While the lack of fixation monitoring on the EVFT may mimic a “real” field of view it may also inflate the number of points seen compared with results from merging the monocular fields.

The obvious limitation of the binocular simulation is that it can only provide an assessment of the central field whereas the EVFT tests the full peripheral field across 130°. It has, however, been suggested that the distribution of points on the Esterman grid does not fully reflect the claimed functional significance.19 For example, it has been shown that the central 10° may provide the input for up to half of the primary visual cortex.20 It may be that a functional assessment based on central areas may have a wider role in assessing visual ability.

The binocular simulation may provide an indication that a patient with bilateral defects should be further investigated in terms of legal fitness to drive. This is particularly relevant in the UK where currently the recommended definition for the standard of the minimum field of vision for safe driving includes, “... no significant [field] defect in the binocular field which encroaches within 20° of fixation [either] above or below the [horizontal] meridian.”4 The high level of sensitivity (100%) and specificity (86%) associated with the binocular simulation in determining whether a glaucomatous patient with bilateral defects will have at least one defect (<10 dB) on the EVFT suggests this may be a useful guide for clinicians. Patients demonstrating defects on the binocular simulation are likely to exhibit defects in the central area of the EVFT and may warrant further investigation. Equally, patients presenting with no defects on the binocular simulation are likely to exhibit no defects in the central area of the EVFT. The utility of this indicator that a glaucomatous patient may fail the criteria for legal fitness to drive in the UK may be proved by establishing a link between defects in the central field and significant defects in the peripheral field. An investigation is under way but actual testing of the peripheral binocular field is likely to remain central to assessing legal fitness to drive. Nevertheless, the simulation provides the clinician with a rapid and routine assessment of the central binocular field. This may be particularly useful since it has been suggested that alerting elderly drivers to deterioration in their visual performance, so that they can make compensatory changes in their driving behaviour, may be important.21

A recent study demonstrated that while glaucoma is a leading cause of visual disability, in many cases visual field progression is not sufficient to lead to bilateral blindness.22 The study concluded that glaucoma management should be closely tailored to the confirmed rate of visual field loss in an individual. A framework for the analysis of the rate of field loss at individual test locations has been developed.23 24 By using this rate of change in a given series of fields it is possible to predict the sensitivity at subsequent fields. Used in tandem with the binocular simulation described in this study it may be possible to predict the future status of a patient’s central binocular field. These predictions of future changes to the functional field may prove to be useful in the management of glaucoma and demonstrate the “relevance” of the insidious behaviour of glaucomatous progression to individual patients. Such forecasts may be impeded by the inherent variability that exists between visual field results25 but improved perimetric test strategies26 27 and variability reduction techniques28 may reduce this limitation.

In summary, a new method for simulating binocular visual field status in glaucoma by merging results from Humphrey monocular fields has been demonstrated. Results are computed quickly, and displayed in an easy to interpret manner on a computer screen. Substantial agreement exists between the simulated binocular results and EVFT in classifying glaucomatous patients with central binocular defects. In contrast with the EVFT, results from the simulation are based on sensitivity values from a full threshold test strategy and are controlled for fixation. The simulation cannot currently estimate the status of the binocular field beyond the central area. However, it may be a clinically useful indicator for determining whether a patient is likely to fulfil the visual field requirement for legal fitness to drive in the UK. In conclusion, this new method provides a rapid estimate of a patient’s central binocular field and visual functional capacity. This can be ascertained in a routine clinical environment without the need for extra perimetric examination.