Xu S, Meyer D, Yoser S, Mathews D, Elfervig J. Pattern visual evoked potential in the diagnosis of functional visual loss. Ophthalmology 2001;108:76–81.
Original study reprint requests: Shizhao Xu, Vitreoretinal Foundation, 825 Ridge Lake Blvd., Memphis, TN 38120.
To study the pattern visual evoked potential (P-VEP) in the diagnosis of functional visual loss.
Retrospective, observational case series.
University of Tennessee, Department of Ophthalmology, Memphis, TN.
Crippled Children Vitreoretinal Research Foundation and the University of Tennessee, Department of Ophthalmology, Memphis, TN.
A retrospective study was performed on 138 subjects suspected of having functional visual loss and referred to the electrophysiologic laboratory at the Vitreoretinal Foundation for P-VEP from 1991 to 1999. Patients were scheduled for follow-up at 1 to 2 weeks the first month, every 3 months times 1 year, and every 6 months for 3 years.
The Universal Testing and Analysis System, Electrophysiologic 2000 (UTAS-E 2000, LKC Technologies, Inc., Gaithersburg, MD) was used to record the P-VEP stimulated by reversal checkerboard patterns.
To compare the P-VEP estimated acuity to the initial subjective visual acuity (VA) and to the best-performed VA.
Seventy-two subjects with functional visual loss had normal P-VEPs. The initial subjective VA was 20/50 in 9 subjects and ≤20/200 in 42 subjects. After clinical examination and reassurance, the best-performed VA was ≥20/50 in 53 subjects and ≤20/200 in 8 subjects. The discrepancy between the P-VEP estimated acuity and the best-performed VA was less than 3 lines of Snellen acuity in 63 of 72 (87.5%) subjects and more than 4 lines in 6 subjects. These six subjects were three women with loss of vision of unknown origin and three men with injury-related visual loss.
P-VEP has the advantage of objectively predicting VA and is a useful test in the diagnosis of functional visual loss.
Of 138 subjects suspected of having functional visual loss, the authors analyze 72 subjects who had normal pattern evoked potentials. The authors conclude: “Pattern visual evoked potential has the advantage of objectively predicting visual acuity and is a useful test in the diagnosis of functional visual loss.” The statement suggests that by analyzing the pattern evoked potential, one can obtain an objective measurement of vision. To prove that, one would need to demonstrate a range of abnormal visions with a range of responses to the specific check sizes. Because all of the patients in this series had “normal pattern evoked potential,” there was no range over which one could assess correlation. The most the authors should be able to conclude would be that pattern evoked potential can objectively identify patients with normal vision and is therefore a useful test in the diagnosis of functional visual loss. However, even for that statement to be true, the authors should have looked at a random population. This would ideally include both patients with abnormal vision and those with normal vision. Because the authors restrict themselves to looking at patients who had normal visual evoked potential, they are unable to comment on those patients in whom the visual evoked potential was abnormal. How many of those patients actually had objective evidence of abnormal visual system function? If, in fact, all 138 patients were truly functional, then 48% of the patients were able to fool this test.
Follow-up for these 72 patients indicated that 53 later demonstrated visual acuity of equal to or better than 20/50. This would indicate that in this series, 73% of patients identified as having normal pattern evoked potential were, by other measurements, normal. It does not, however, tell us whether the remaining 27% of patients who were still “abnormal” has other reasons for reduced vision, in which case there is a 27% false-positive rate, or whether these patients were malingering and simply could not be fooled by other clinical tests.
What can we conclude from this study? Based on the authors' experience in patients who are otherwise clinically normal, at least 70% of patients who did have normal visual evoked potentials could be expected to be found to be normal with subsequent testing. Thus, the finding of a normal visual evoked potential in a patient with presumed functional disease would help give additional support for that diagnosis.
In addition, and perhaps more importantly, the article does not address what happens when one finds an abnormal visual evoked potential. As the authors reference, previous studies have been quite concerned about the ability to create abnormal P-VEPs by defocusing or looking away. Thus, in those patients with presumed functional visual loss, we cannot comment on the implications of an abnormal P-VEP. This is particularly compounded by the fact that functional patients often have some degree of abnormality and the functional aspects are an overlay or exaggeration of existent disease.
The authors would like to conclude that pattern evoked potential offers the ability to quantitatively assess central visual function, and although that may be true, it is not supported by the data presented in this article. This issue is discussed by Dr. Frohman in his accompanying discussion. It is interesting that both Dr. Frohman and the authors conclude the article by pointing out the importance of the clinical examination and the examiners' effort in dealing with these challenging patients. Although it would be ideal if we had an objective physiologic test that could replace the subjective aspect of the more frequently used psychophysical tests, we have yet to have credible data that pattern evoked potential fulfills this role. In a similar article, Steele and colleagues reported normal quantitative pattern evoked potentials in 17 patients with functional vision loss. 1 They did appropriately point out “there are limitations, however, in applying visual evoked potential measures of acuity. If an uninterpretable response is obtained, one can never be sure of the reason. We have shown that both refractive blur and ocular disease can reduce the validity of the visual-evoked potential acuity. Likewise, fixating at areas other than the center of the stimulus can produce uninterpretable results. Control testing disclosed that when a normal individual fixated on a corner of the monitor's screen, no interpretable visual-evoked potential results were obtained. It is also possible that voluntary alterations in accommodation can sufficiently blur the stimulus patterns to render visual-evoked potentials invalid.”
1. Steele M, Seiple WH, Carr RE, et al. The clinical utility of visual-evoked potential testing. Am J Ophthalmol 1989; 108:572–577.