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THE MULTIFOCAL VISUAL EVOKED POTENTIAL IN FUNCTIONAL VISUAL LOSS
KIMBERLY WOODWARD and MICHAEL WALL
Departments of Neurology and Ophthalmology, Veterans Administration Medical Center, and College of Medicine, University of Iowa, Iowa City, IA, USA
Abstract
Purpose: To investigate the usefulness of multifocal visual evoked potential (mfVEP) testing in patients with functional visual loss. Methods: Eight patients, aged between 16 and 40 years (mean age, 25.5 ± 8.83 years), with clinical presentations typical of functional visual loss, were tested with mfVEP. All patients had complete neuro-ophthalmological examinations. Goldmann or Humphrey perimetry examinations and mfVEP results were compared. Results: The mfVEP test demonstrated robust evoked potentials in areas of visual field ‘loss’ with Goldmann or Humphrey perimetry in all eight patients tested. All patients showed moderate to severe visual field loss with conventional perimetry. MfVEP was able to clearly record strong signals in the area of reported loss. Conclusion: MfVEP is a useful clinical tool for determining functional visual field loss in moderately cooperative patients.
Introduction
Multifocal visual evoked potential (mfVEP) testing has been successful in identifying visual field defects in glaucoma and other optic neuropathies.1-3 In these studies, conventional automated perimetry and Goldmann perimetry results from these patients correlated well with the mfVEP results.4 A common clinical problem faced in the neuro-ophthalmology clinic is patients who present with either functional overlay or ‘non-organic visual loss’. Separating functional overlay from organic-based loss makes treatment decisions difficult.
Nonorganic visual loss causes much unnecessary testing and added costs to medical care. Our goal was to determine whether the mfVEP would be a useful adjunct in the evaluation of patients with clinically apparent functional visual loss.
Methods
We tested eight patients with mfVEP with clinical presentations of functional visual loss. Their ages ranged from 16 to 40 years (mean age, 25.5 ± 8.83 years). All patients
Address for correspondence: Michael Wall, MD, University of Iowa, College of Medicine, Department of Neurology, 200 Hawkins Drive #2007 RCP, Iowa City, IA 52242-1053, USA
Perimetry Update 2002/2003, pp. 261–264
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
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had complete neuro-ophthalmological examinations. The right eye was involved in three patients, and both eyes in five patients. None of the patients had a relative afferent pupil defect, and slit lamp and ophthalmoscopy examinations were normal. Visual acuity was normal in five patients, between 20/63 and 20/400 in two patients, and one patient reported light perception only. On confrontation fields, three patients had tubular visual fields, two had inconsistent responses, two showed loss consistent with the Goldmann visual field, and one had full fields to confrontation. Six of the eight patients had MRI or CT scans of the brain, and all were negative. We compared the test results from manual kinetic (Goldmann) and static automated perimetry (Humphrey Visual Field Analyzer, SITA program) with those of mfVEP.
We recorded mfVEP using the Opera (Accumap) system. This uses an alternating dartboard stimulus. Recordings are made from a four-channel EEG electrode set-up using peak-to-trough amplitudes from 58 test locations. The mfVEP stimulus is cortically scaled and uses a pseudo-randomly reversing pattern. The Opera system records EEG activity, and fast-Fourier analysis is performed on the data. EKG and alpha rhythm spikes are removed, and the EEG is used to compute a scaling factor to adjust mfVEP amplitude so that interindividual differences are lessened. Each of the 58 separate checkerboard patches, with 16 checks each, is modulated in time according to a different m-sequence. A cross-correlation technique is then used to extract the signal at each of the sites.
Subjects were seated in a comfortable chair in order to relax their neck muscles. They looked at a monitor at 30 cm distance. The stimulus covered 26° surrounding fixation with two additional test locations covering 6° at the nasal horizontal. The signal was amplified 100,000 times and band-pass filtered between 3 and 30 Hz using a Grass four-channel amplifier. The sampling rate was 502 Hz.
We generated the visual stimulus on a 21-inch high-resolution monitor (Hitachi, Ltd.). In order to ensure good fixation, the numbers 3, 6, 8, or 9 were displayed in random sequence about every five seconds. The subject was asked to press a button whenever they saw a 3.
The Opera system currently uses a database of 100 normal subjects to compute the ranges for normal values. Based on the results of 30 normal subjects, we used the criteria of three contiguous points at p < 0.01 found in the amplitude deviation probability plot. The four most superior (points 52–55) and two nasal points (56 and 57) were excluded from analysis, due to poor signal-to-noise ratio in the normals. An example of normal results from an ocular normal subject is shown in Figure 1. Based
Fig. 1. Normal multifocal visual evoked potential results from a control subject.
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on our results in 30 normals, we used three non-edge, clustered test locations, abnormal at the p < 0.01 level in order to separate normals from patients with objective evidence.
Results
All eight patients had normal neuro-ophthalmological examinations and moderate to severe loss with both Goldmann and Humphrey perimetry. Seven of these were found to have normal results with mfVEP testing (Fig. 2), and one showed full recovery in her left eye and some improvement in her right eye on follow-up with conventional automated perimetry. One patient’s result was considered unreliable because of excessive and slow blinking, although it was useful clinically due to the good signal found in areas of reported loss on the Goldmann visual field.
Fig. 2. Example of a patient with a normal ophthalmological examination, hemianopic loss in the right eye and normal mfVEP results.
Discussion
Functional visual loss is a difficult clinical problem. Whether the abnormal results from psychophysical examinations are due to underlying psychosocial stresses, psychiatric illness or workmen’s compensation issues, these patients present a great challenge.
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We found mfVEP to be very useful in objectively confirming the presence of visual function in areas of the visual field that tested abnormally using conventional methods.
Miele et al. reported a patient with a functional quadrantanopia with normal mfVEP testing.5 Our results extend the application of mfVEP to the more commonly encountered functionally constricted visual field, and to those with a combination of organic and non-organic visual loss.
MfVEP is a reliable tool and provides objective evidence of visual function in the evaluation of patients with functional visual loss.
Conclusion
MfVEP is a useful clinical tool for determining functional visual field loss in at least moderately cooperative subjects.
Acknowledgment
This study was supported by VA Merit Review, an unrestricted grant to the Department of Ophthalmology from Research to Prevent Blindness, New York, NY, USA.
References
1.Hood DC, Odel JG, Zhang X: Tracking the recovery of local optic nerve function after optic neuritis. A multifocal VEP study. Invest Ophthalmol Vis Sci 41(12):4032-4038, 2000
2.Klistorner A, Graham SL: Objective perimetry in glaucoma. Ophthalmology 107(12):2283-2299, 2000
3.Kardon RH, Givre S, Wall M, Hood DC: Comparison of threshold and multifocal-visual evoked potential perimetry in recovered optic neuritis. In: Wall M, Mills RP (eds) Perimetry Update 2000/2001. Proceedings of the XIVth International Perimetric Society Meeting, pp 19-28. The Hague: Kugler Publ 2001
4.Graham SL, Klistorner A, Grigg JR, Billson FA: Objective perimetry in glaucoma: recent advances with multifocal stimuli. Surv Ophthalmol 43(Suppl 209): 1999
5.Miele DL, Odel JG, Behrens MM, Zhang X, Hood DC: Functional bitemporal quadrantanopia and the multifocal visual evoked potential. J Neuro-Ophthalmol 20(3):159-162, 2000