Subjectively impaired vision without a detectable organic basis is termed functional visual loss (FVL). Patients with FVL pose significant diagnostic and therapeutic challenges to ophthalmologists and may also present to other medical specialties (1-4).
FVL can present as loss of visual acuity, visual field, or both (5). In all cases, an underlying organic cause must first be excluded. A common pattern of functional field loss is marked visual field constriction. The field area is often the same at different testing distances (a tubular field), a feature that helps distinguish it from a physiological conical field. In some patients, however, it can be difficult to be certain that marked field constriction is functional rather than organic.
The Goldmann perimeter is an ideal tool to demonstrate functional features in suspected nonorganic field loss. Overlapping of isopters, spiraling, stellate patterns, and centrifugal expansion are evidence of functional behavior (6,7). However, the Goldmann perimeter has also been used as a way of measuring ocular ductions in conditions such as thyroid eye disease (8-10) and chronic progressive external ophthalmoplegia (11). The quantitative record of the ocular motility obtained, which is plotted on standard kinetic perimetry paper, is called the Uniocular Field of Fixation (UFOF). The Goldmann perimeter provides the opportunity to switch from a visual task (perimetry) to a motor task (UFOF) without changing the testing environment.
In our study, we aimed to determine 1) if functional behavior observed on perimetry would be translated into functional motor behavior when the task was switched and 2) if the responses obtained might be helpful in distinguishing a functional cause from an organic cause of field constriction.
Ten patients undergoing kinetic perimetry as part of their standard ophthalmic workup were studied. All had previously demonstrated constricted visual fields with preserved central vision and no limitation of ocular motility. The first group (“organic”) comprised 5 patients with known disease thought to be wholly responsible for their visual field loss. Their characteristics are shown in Table 1. The second group (“functional”) comprised 5 patients with suspected FVL, based on typical functional features as shown in Table 2. Three of these functional patients (#6, 7, 8) had no concurrent organic ophthalmic or neurological disease and were considered to have pure FVL. In the remaining 2 patients, the diagnosis of FVL on a background of organic disease (functional overlay) had been made. Case 9 had long-standing hydrocephalus, and Case 10 had craniopharyngioma resection earlier in life.
Immediately following conventional perimetry, the right eye of each patient was retested using the procedure described below. Testing was performed in all cases by an experienced perimetrist (N.A.).
The testing method was designed to be quick, simple to understand, repeatable, and easily incorporated into a routine kinetic perimetry session. It consisted of 2 steps: the first visual and the second motor. The first step was a simplified visual field plot using a single stimulus target (V4e). The patient was instructed to “look at the central spot all the time and press the buzzer when the light first appears.” The target was moved toward the center along each of the 8 cardinal meridians in random order, and the responses were plotted in the standard way. This was termed the “visual field.”
The second step was a modified UFOF and followed immediately. The patient was instructed, “Now the target is going to start in the centre and move outwards, but this time you should follow it with your eyes as it moves. Press the button as soon as it disappears.” The V4e target was then moved centrifugally from the center along each of the 8 cardinal meridians, and the responses were plotted. This was termed the “motor field.”
For quantitative analysis of the visual and motor fields, the mean radial distance from the center for each of the plotted responses was measured in millimeters, and the 8 distances summed to give a total field score. T tests (5% level, 2 tailed), paired for within group and independent for between group, were performed on SPSS (ver 17).
The visual and motor field plots for each patient are shown in Figure 1. Table 3 shows the visual field and motor field scores. All patients had markedly constricted visual fields, the size of which was not significantly different between the 2 groups (mean field scores: organic, 139 ± 44; functional, 111 ± 30; P = 0.61).
For the organic group, the motor fields were significantly larger than the visual fields (mean field scores: visual, 139 ± 44; motor, 446 ± 48; P = 0.02). For the functional group, the motor fields were markedly constricted and were, on average, the same size as the visual fields (mean field scores: visual, 111 ± 30; ocular motor, 142 ± 30; P = 0.27). Comparing the motor fields between the 2 groups, the difference was highly significant (mean field scores: organic, 446 ± 48; functional, 142 ± 30; P = 0.001).
Most of the techniques for demonstrating functional features in patients with suspected nonorganic visual loss are tricks (7). They rely on the patient's inadequate understanding of visual physiology. In the technique described here, the patient is led to believe that step 2 is another visual field task when in reality, because the target is foveated as it moves, it is a test of ocular motility. We found that patients with suspected FVL demonstrated a functional gaze paresis, which mapped quantitatively in visual space to their perceived field of vision, a feature that can distinguish them from patients with organic field loss.
Other techniques have been described to expose functional field loss. A commonly used method is to tell the patient to make saccades to targets in the supposed blind field, disguising the test as an assessment of eye movements (7). This has recently been examined quantitatively for its value in discriminating organic from nonorganic visual loss (12). More objective tests, which demonstrate intact visual pathways and are less reliant on patient cooperation, include pupil perimetry (13,14) and measuring multifocal visual evoked potentials (15).
The technique described in this study differs from these in that no attempt is made by the examiner to prove sight in the blind field. Rather the goal is to demonstrate additional unequivocally functional behavior, in this case restriction of ocular ductions where none exists. This is a limitation of the technique since other tests would still be needed to prove a scotoma is factitious.
The advantages of our technique are that it is simple to perform and can be used within the setting of the Goldmann perimeter as soon as functional perimetry features are suspected. In addition, in this preliminary series of patients, there were no false-positive results. This technique may prove to be a useful way of distinguishing organic from nonorganic field loss. However, larger patient groups are required to validate sensitivity and specificity and to see whether patients with pure FVL differ from patients with functional overlay. We are planning additional studies to address these issues.
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