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1 From the Departments of Ophthalmology and 2 Neurology, College of Medicine, University of Iowa, Iowa City, Iowa.
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Abstract |
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METHODS. FDT and CAP were performed in 97 patients with sensory neuro-ophthalmic disorders and 42 subjects from the general population. The total and pattern-deviation probability plots for test loci common to the two perimetric tests were compared. The gold standard was an unequivocal clinical diagnosis.
RESULTS. The sensitivity of FDT was 81.3%, with a specificity of 76.2%. The difference in sensitivity and specificity of CAP, 87.5% and 81.0%, respectively, was not statistically significant (by 
2 test). In subjects with optic neuropathies, the similarity of the defect shown on FDT and CAP was judged good or fair in 62 of 72 cases. The extent of the defect as seen with FDT and CAP was equal in 41 of 72 cases, more extensive with FDT in 12, and more extensive with CAP in 19. In the patients with hemianopia, scattered abnormal test locations with FDT testing masked the hemianopic nature of the defect in 15 of 25 patients. Also, test locations along the vertical midline in densely hemianopic areas were seen with FDT testing in some patients with hemianopia, probably due to light scatter across the vertical midline and into the uninvolved hemianopic field.
CONCLUSIONS. FDT has sensitivity and specificity similar to that of CAP for detecting visual field defects in patients with optic neuropathies. However, defects in patients with hemianopias may be missed because of the presence of scattered abnormal test locations and failure to detect test locations along the vertical meridian. The defects demonstrated by both tests in patients with optic neuropathies are similar in number, extent, and shape of the defects. This suggests FDT may not be isolating the magnocellular (M) cells with nonlinear responses to stimulus contrast (My cells) in patients with visual loss.
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Introduction |
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The frequency-doubling percept has been attributed to a nonlinear response to contrast. The M-cell subset with nonlinear responses to stimulus contrast, the My cells, account for a small fraction of the total number of M cells. If the FDT method isolates these cells, a population of cells is tested that may have little anatomic and physiologic redundancy, although the amount of receptive field overlap of the My system is unknown. If there is not substantial overlap, there is probably not much redundancy, and visual system damage might be expected to show visual field defects early in a pathologic course. However, because subjects respond, whether or not they see the illusion, it is not clear whether this subset of M cells or the M cells, per se, are isolated.
FDT perimetry was developed for screening patients for evidence of glaucomatous damage to the optic nerve. It has been validated thoroughly for this purpose in normal subjects and in those with glaucoma.6 7 As a glaucoma-screening device, the sensitivity of FDT perimetry is similar to that of conventional automated perimetry (CAP), and its specificity is excellent.2 6 8 9 10 11 However, patients with nonglaucomatous damage to the optic nerve or damage to the optic chiasm or retrochiasmal pathways may have very different visual field defect morphology than patients with glaucoma. For example, patients with optic neuritis, anterior ischemic optic neuropathy, or compressive optic neuropathies may have cecocentral loss, in addition to arcuate nerve fiber bundle–like defects.
Patients with hemianopic defects present a different problem, in that those defects are considerably different from glaucomatous ones. First, in patients with hemianopias, the visual field defects usually respect the vertical midline. Therefore, if a stimulus is placed too close to the vertical meridian, with light scatter or fixation shifts, the procedure may fail to detect damage tested by locations along the vertical meridian. Second, defects in patients with hemianopias may be small with steep slopes, compared with the usually shallow-sloped defects in patients with glaucomatous visual field damage. Third, the defects have a homonymous character, occurring in approximately the same test locations in each eye. It is necessary that this homonymous character be obvious when examining test results. Because of these differences and the now widespread use of FDT perimetry as a general visual screening device, we undertook this study to determine whether FDT perimetry is sensitive as a screening test in patients with sensory neuro-ophthalmic disorders.
A secondary purpose was to test the hypothesis that by isolating the My cells, FDT perimetry would be a more sensitive test than CAP for the reasons just stated. If there is isolation of the My cell by FDT and the disease process damages the M cells more than the P cells (as suggested in idiopathic intracranial hypertension and optic nerve compression12 13 ), FDT-detectable defects should occur earlier and be more extensive. Alternatively, if the reduced redundancy hypothesis is true,14 and if there is not much redundancy in the My system, the sensitivity of FDT perimetry should be substantially higher than that of CAP.
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Methods |
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Perimeters
CAP was performed with a visual field analyzer (Humphrey) according to the manufacturer’s recommendations. We used a 4-mm2 Goldmann size III stimulus (0.43°) on a dim background (31.5 apostilb). The differential light sensitivity threshold was found at each test location. The patients’ appropriate near correction was used. Rest breaks were allowed when requested. The 24-2 or 30-2 test program (Humphrey) presents stimuli on a 6° spaced grid encompassing the central 21° or 27° of the visual field.
FDT perimetry was performed after conventional perimetry testing with at least a 15-minute rest period between, in an attempt to diminish the fatigue effect. Testing was performed in a darkened room (the test can be taken with normal room lighting) using an FDT device (FDT threshold C-20 test, version 2.6; Welch-Allyn; Skaneateles, NY). This protocol determines the minimum contrast necessary to detect the stimulus for each of the 17 target locations in the stimulus display (Fig. 1) . This is accomplished by means of a staircase bracketing procedure. If a stimulus is detected, the contrast is decreased for the next presentation; if the stimulus is not detected, contrast is increased. FDT perimetry uses a modified binary search (MOBS) type of staircase. The FDT software tabulates the status of each MOBS staircase result for the individual stimulus locations and signals the end of the test when all are completed. That is, the test continues until the stimulus with the least contrast is detected at each test location. The test time for the C-20 procedure is approximately 4 minutes per eye.
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With the C-20 stimulus presentation pattern, 17 stimulus locations are tested. This pattern has four stimulus test locations in each quadrant, each approximately 10° in diameter, and a central 5° radius target. This provides a test area of approximately 40° x 40° or a 20° radius surrounding fixation. It is by far the largest stimulus size commonly used in clinical perimetry.
Visual Field Defects
To qualify as a visual field defect for CAP, three adjacent abnormal points at the P < 0.05 level or two adjacent points with one abnormal at the P < 0.01 level were needed. The defects also must occur in a clinically suspect area. For data using FDT perimetry, to meet criteria for a visual field defect, we required two adjacent abnormal points at P < 0.05 or one homonymous point with a probability of occurrence in a normal population of less than 1%. The abnormalities had to appear in a clinically suspect area. We also required that, to qualify as a hemianopic defect, the abnormal test points be along the vertical meridian and homonymous and that there not be other abnormal test locations that would obviously obscure the homonymous pattern. That is, there had to be a reasonable minimum of scattered abnormal test locations from noninvolved (normal hemifield) test locations to be able to clearly see the hemianopia. We did not accept a single abnormal test location as a visual field defect in one eye, because with 17 test locations, one abnormal test location would be expected to occur, by chance alone.
Data from the patients’ total-deviation probability plots and pattern-deviation plots were separately analyzed for normal subjects, patients with optic neuropathy, and patients with hemianopias. Lastly, the test results were analyzed using all the data supplied (including the total-deviation probability plots and pattern-deviation plots), as a clinician would for a screening visual field test.
For the total-deviation probability plot and pattern-deviation plot analyses, we compared the extent of the defects using the tests’ total and pattern-deviation probability plots. If the defects’ extent was more than 25% more in area, that perimetric examination was tabulated as showing a more extensive defect in that patient. Otherwise, the extent was tabulated as the same. We also judged the similarity of the topographic pattern of the visual field defects. The similarity was categorized as good when the same type of defect was present in both, fair when the defects were different but some overlap was present, and poor when the areas of visual loss did not coincide.
Statistical Analysis
Sensitivity and specificity measures were calculated in the traditional manner. The gold standard for classification of disease was the patient’s clinical diagnosis. The 2 and Fisher exact tests were used to test for differences between groups (FDT and CAP).
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Results |
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We found the sensitivity of FDT in all patients to be 81.3%, with a specificity of 76.2%. The sensitivity of CAP was 87.5%, with a specificity of 81.0% (NS, 2).
We then divided the patient sample into those with optic neuropathies and those with hemianopias. The sensitivity and specificity results are found in Tables 1 and 2 . Of the patients with optic neuropathies, 62 (86%) of 72 had visual field defects, with the total-deviation probability plot results by conventional perimetry and 56 (78%) by FDT. The results of the pattern-deviation tabulation were 61 (85%) with CAP and 57 (79%) with FDT. All patients with anterior ischemic optic neuropathy had characteristic visual field defects to both total and pattern-deviation plot analyses, except one patient who had normal results with both tests and one patient with a cecocentral defect by CAP testing who had normal FDT results. Patients with compressive optic neuropathies had similar results with total and pattern-deviation probability plot analyses. Three of the four patients had characteristic visual field defects. Eleven (62.5%) of 16 patients with idiopathic intracranial hypertension had defects with both CAP testing and FDT with total-deviation probability plot analysis. Pattern-deviation analysis revealed 62.5% and 69% respectively. Of the 23 patients with optic neuritis, 87% had defects shown in the total-deviation plot analysis whereas 78% had defects with FDT. In this group, the sensitivity for the pattern-deviation plot analysis were 70% for CAP testing and 65% for FDT.
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Most revealing was the clinical interpretation that was performed as if the patient were having a visual field screening test performed as part of a patient visit. Although there were no meaningful differences in the patients with optic neuropathy, there were large differences in the patients with hemianopia (Table 2) . There were two factors that appeared to be causing hemianopic defects to be missed. First, there were numerous scattered abnormal test locations with FDT testing that masked the hemianopic nature of the defect (15/25 patients). Second, the main reason for failure to demonstrate hemianopic defects in three patients was that test stimuli along the vertical midline related to the hemianopic field were seen at the non-hemianopic vertical test locations likely due to either light scatter across the vertical or shifts in fixation. Another apparent factor was that homonymous hemianopic defects could more easily be seen with the Humphrey gray-scale graphic than with the probability plots. No gray-scale graphic is provided with FDT.
The analysis for extent and similarity of visual field defects is found in Table 3 . Often, especially in the patients with optic neuropathy, there was excellent agreement between the results of the two tests. For example, Figure 2 shows the results in a patient with idiopathic intracranial hypertension. The inferior arcuate defect was shown similarly by both testing procedures. Figure 3 shows results in a patient with anterior ischemic optic neuropathy who had a cecocentral scotoma present with CAP; FDT results were normal. A 28-year-old woman with idiopathic intracranial hypertension (Fig. 4) had a large inferior arcuate defect present on CAP. The defect was present and more extensive on FDT perimetry. A 53-year-old woman with a temporal lobectomy for control of epilepsy had a right superior homonymous hemianopia present on both tests (Fig. 5) . A 78-year-old man with a left occipital stroke had a congruous, macula-sparing, right inferior quadrantanopia present on CAP testing (Fig. 6) ; FDT perimetry showed some loss in the same area but scattered abnormal test locations obscured the hemianopic morphology of the visual field defect. A 76-year-old man had incongruous right homonymous hemianopia (Fig. 7) . FDT perimetry did not show the abnormality along the vertical meridian on the right side, because of light scatter or fixation shifts.
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Discussion |
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Alternatively, there is evidence that there may be selective large retinal ganglion cell type loss in glaucoma18 19 20 and, possibly, idiopathic intracranial hypertension.12 13 If the My cells are selectively lost, the visual field deficit may occur, because the spared cells of other types cannot easily detect the stimulus. Therefore, there is a second theoretic basis that psychophysically isolating the My cell for perimetry may allow for early detection of visual loss.
In the current study, FDT was similar in sensitivity and specificity to CAP in patients with optic neuropathies. The shape of the field defects demonstrated by both perimetric methods were similar. Similarly, the extent of the abnormality, on average, appeared to be similar between the two tests. Therefore, if the My cell is being isolated, it does not appear to result in an increase in the sensitivity of FDT perimetry.
There are many investigations of sensitivity and specificity comparing FDT and CAP in glaucoma.7 8 9 10 21 The studies are difficult to compare because (1) different criteria for abnormality were used; (2) full threshold testing was performed in some studies, whereas the fast screening protocols were used in others; (3) population-based normal subjects were used in some studies, whereas ocular normal subjects were used in others; and (4) samples of varying degrees of glaucoma were reported. Of the five studies listed, the sensitivity of FDT and CAP appear equivalent in four. A study by Sample et al.,21 in which criteria were set for a variety of tests at a specificity of 90% and the test results compared, showed FDT to be more sensitive in patients with glaucomatous optic neuropathy. Our results show that there is no improvement in sensitivity for FDT over CAP for nonglaucomatous optic neuropathies, and it is controversial whether there is improvement in glaucoma detection. Therefore, we conclude it is unlikely that the My cell is being psychophysically isolated in FDT perimetry.
To date, the only report of FDT testing in patients with neurologic disease is by Fujimoto and Adachi-Usami.22 They reported results in 14 patients with recovered optic neuritis. They compared deficits by zone (central, paracentral, and peripheral) and found deficits with both CAP and FDT. With CAP, eight patients had normal findings and six had localized defects by Optic Neuritis Treatment Trial criteria. They did not report the FDT results in individual eyes. Ours is the first report of other nonglaucomatous optic neuropathies and hemianopias.
It is clear that FDT perimetry (version 2.60; Welch-Allyn) failed to detect hemianopias in a substantial number of patients. A variety of mechanisms is likely to be responsible for this. An important consideration is scatter of light from the stimulus from the nonseeing into the seeing hemifield. Clearly, test locations along the vertical in a densely hemianopic field can be missed with FDT. We have tested a group of six subjects with a version of FDT where the stimulus was offset 2° away from the vertical without improvement in the results. Testing with a 3° offset improved the results. This experiment is the subject of another article currently in preparation. Another potential reason for this failure to detect test locations is shifts in fixation. In any event, this problem can be mostly resolved with a change in the FDT stimulus size and placement.
A problem that is more difficult to remedy is the presence of scattered abnormal test locations obscuring the homonymous hemianopic character of the defect (Fig. 6) . The reason for the presence of this probability plot noise is unclear. It can also occur in CAP, but because there are at least 52 test locations, the effect of a few scattered abnormal test locations is much less problematic (Fig. 7) . A possible explanation is that FDT perimetry was performed after CAP. However, all patients had at least a 15-minute break, and most had a much longer break between the tests.
Although the gray-scale printout, when used exclusively, can lead to diagnostic errors,23 it can also be useful in selected instances. For example, the four superior-edge test locations used in CAP with a 6° spaced grid have a very high variability because of interference by the lids and eye lashes. Subtle bitemporal hemianopias, present on the gray-scale printouts, can be missed if only the probability plots are used for those test locations. In all our patients, the hemianopias were obvious on the gray scale. It would probably be useful if this graphic representation were available on FDT perimetry.
In summary, we found FDT and CAP to have similar specificities and sensitivities in patients with nonglaucomatous optic neuropathies. In addition, the abnormalities found were of similar appearance and extent. This suggests the FDT stimulus does not adequately isolate the My cells, nor it is likely that the test method would be more sensitive than CAP. Patients with hemianopias present another problem. There is failure to identify hemianopic defects because of detection of the stimulus by the receptive fields of the uninvolved hemifield and the presence of scattered abnormal test locations. In conclusion, however, we found FDT perimetry to be an excellent visual field screening test for nonglaucomatous optic neuropathies. The addition of a gray-scale printout and a change in the position, size, and number of stimuli appear to be needed for FDT to function as well as CAP for detection of hemianopias.
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Footnotes |
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Submitted for publication August 27, 2001; revised November 30, 2001; accepted December 13, 2001.
Commercial relationships policy: F.
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. 
1734 solely to indicate this fact.
Corresponding author: Michael Wall, Department of Neurology, University of Iowa, College of Medicine, 200 Hawkins Drive 2007 RCP, Iowa City, IA 52242-1053; michael-wall{at}uiowa.edu
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