Pattern Visual Evoked Potentials in Malingering

Abstract

Objectives 

We previously developed a new method for estimating objective visual acuity by means of pattern visual evoked potentials (PVEP). In this study, this method was applied to the diagnosis of malingering.

Materials and Methods 

Six patients ranging in age from 40 to 54 years (mean 47 years) with suspected malingering were evaluated by means of the visual evoked potential test, optokinetic nystagmus (OKN) inhibition test, and the visual field test. In the PVEP study, the stimulus consisted of black and white checkerboards (39´, 26´, 15´, and 9´) with a visual angle of 8°, contrast level of 15%, and a frequency of 0.7 Hz. One hundred PVEP responses were averaged per session.

Results 

Routine ophthalmic examinations were normal in all patients. Five patients had a tubularly constricted visual field, and the remaining patient had a normal visual field. The objective visual acuities of the six patients estimated from PVEP were better than their subjective visual acuities estimated with Landolt rings.

Conclusions 

Among a variety of psychophysical and electrophysiologic ancillary tests, we consider our PVEP method a useful method for objectively determining visual acuity in a patient with signs of ocular malingering.

Diagnosis of ocular malingering is rarely difficult, but objectively determining what the symptomatic patient sees can be challenging. Ohm (¹) and Tsutsui et al. (^2–4) reported on objective evaluation of visual acuity using the optokinetic nystagmus (OKN) inhibition test. Recently Gräf et al. (⁵) investigated assessment of visual acuity using OKN with monitoring eye of movement. Whether it is possible to determine visual acuity (^6–18) with pattern visual evoked potentials (PVEP) has been controversial. We have made an attempt to develop a new method of PVEP testing in which predominantly the retinal X-ganglion cells are stimulated (¹⁹). With this method, we have been able to measure visual acuity objectively (²⁰). In this study, our PVEP recording method was applied in patients with ocular malingering who were claiming to have traumatically impaired visual acuity.

PATIENTS AND METHODS

Patients

Six patients ranging in age from 40 to 54 years (mean 47 years) had clinical signs of ocular malingering. They were referred from other ophthalmologists or courts of law. Fifteen eyes of 14 age-matched healthy adults were used as controls (mean 46 years). Examinations of patients and subjects included visual acuities with Landolt rings, tests of pupillary reaction to light, visual fields tests looking for signs of tubular constriction, ophthalmoscopic examination, OKN inhibition tests, and our PVEP recordings. PVEPs were always performed with appropriate refractive correction. This investigation was performed according to the guidelines of the Helsinki Declaration after informed consent was obtained from all subjects.

Stimuli

For PVEP recording, each subject viewed a white and black checkerboard pattern on a television monitor. The checkerboard stimulus subtended a visual angle of 8°, and the contrast was 15%. The check sizes were 39´, 26´, 15´, and 9´. The checks were reversed at 0.7 Hz. The computer analysis time of the PVEP was 512 milliseconds, measured by a Signal Processor DP1200 (NEC Sanei, Tokyo, Japan).

Recording methods

One experimenter monitored the patients' ocular fixation, which was directed toward the TV screen in a shielded room as a monocular PVEP was recorded. Recording scalp electrodes were placed on 16 sites according to the international 10 to 20 method. The reference electrodes were attached at A1 and A2, and the forehead was grounded. Signals were amplified with a preamplifier. The bandpass was 1.59 to 70 Hz, and the artifact reject was 100 μV peak-to-peak.

DATA ANALYSIS

The P100 component of Oz was used to estimate objective visual acuity. The PVEP recorded from healthy adults showed a close correlation between the relative amplitude of the PVEP to different check sizes and subjective visual acuity (²⁰). Visual acuity of 0.1 corresponded to the 39´ pattern, 0.2 to the 26´ pattern, 0.5 to the 15´ pattern, and 1.0 to the 9´ pattern. Responses to the 39´ pattern but not to the 26´ pattern were considered nearly equivalent to a visual acuity of 0.1. Statistics for the patients and controls were evaluated by the unpaired t-test.

RESULTS

The clinical characteristics of our patients, including sex, age, visual acuity with Landolt rings, PVEP visual acuity, and visual fields, are summarized in Table 1. Four patients had histories of trauma in traffic accidents (patients 1, 2, 3, and 5). Two patients had documented abnormalities in the fellow eye (patients 4 and 6). Patient 4 had optic atrophy in the fellow eye resulting from head trauma, and patient 6 had retinal detachment in the fellow eye resulting from ocular trauma. Tubularly constricted visual fields were found in patients 1, 2, 3, 5, and 6, whereas patient 4 had a normal visual field. Ophthalmic examination results, including pupillary light reactions, were normal. Computed topographic scanning results of the brain and orbits were normal in patients 1, 2, 3, 5, and 6. The OKN inhibition test was performed on two patients. In patient 2, visual acuity with the OKN inhibition test was 1.5 OD, whereas in patient 6, visual acuity with this test was 0.5 OS.

TABLE 1: Clinical characteristics of patients with malingering

The PVEP visual acuities of all the patients were better than their subjective visual acuities with optical correction. The latencies of the P100 components tended to be shorter in the patients than in the controls (Table 2), but the unpaired t-test showed that the difference in the latencies of the P100 component between the patients and controls was significant with only the 9´ pattern (p < 0.05). The patients' P100-component amplitudes were nearly equal to those of the controls' and were not statistically significant (Table 3).

TABLE 2: Latency of the P100 component

TABLE 3: Amplitude of the P100 component

Figure 1 shows a control's PVEP OD. She was a 47-year-old Japanese woman. Because the P100 component of 01, 02, and Oz were recorded, the PVEP visual acuity OD was judged to be 1.0. The P100 components of Oz were obtained from 13 of 15 eyes in the controls. The accuracy for visual acuity of the controls was 86.7% for 0.1, 0.2, 0.5, and 1.0.

FIG. 1.: Pattern visual evoked potentials records for the 39´, 26´, 15´, and 9´ patterns from one healthy adult. The P100 components were always present.

Patient 1 was referred to our hospital in February 1995 because of unaccountable visual loss after a truck collision in September 1993. On ophthalmic examination, corrected visual acuity was 0.3 OD and 0.4 OS. Critical flicker frequency (CFF) OD was 32, 31, and 36 Hz, and those values OS were 25, 31, and 34 Hz. He had normal pupillary reflexes, normal anterior segments, normal ocular media, and normal fundi. The visual fields OU were tubularly constricted.

Figure 2 shows the patient's PVEP. In the PVEP with the 39´ pattern, there were P100 components of O1, O2, and Oz. With the 26´ pattern, there were also P100 components of O1, O2, and Oz. With the 15´ and 9´ patterns, the same findings were noted. PVEP visual acuity was 1.0 OU.

FIG. 2.: Pattern visual evoked potentials records for the 39´, 26´, 15´, and 9´ patterns from one patient with suspected malingering. The P100 components were always present.

DISCUSSION

Objective assessment of visual acuity was performed on patients with clinical signs of ocular malingering using PVEP. Tyler et al. (^15,16) and Nelson et al. (^17,18) were using the sweep technique to determine visual acuity. In this study, the PVEP was recorded with low-contrast stimulation with a small angle and low frequency, and the transient technique was also used. Generally, stimulation has been set with high contrast in the PVEP (^6–8,12,13), but high contrast activates motion perception (^21,22), whereas low contrast tends to activate form sensation. Accordingly, we decided to use a low-contrast rate of 15% in this study.

Regan (²³) reported that 5 to 9 cycles per second evoked the greatest responses for small patterns of about 10´, whereas larger patterns of 40´ resulted in the greatest responses at temporal frequencies of 10 to 20 cycles per second. In this study, the 9´ pattern was used to measure visual acuity of 1.0 using the PVEP. The frequency of 0.7 Hz was then determined.

First, we investigated the PVEP with low contrast and low frequency. De Keyser (¹⁴) reported that there was a linear relation between log subjective visual acuity and log check size of the PVEP. In reference to that finding, the check sizes of the patterns used were 39´, 20´, 13´, and 9´. However, the 20´ and 13´ patterns with a visual acuity of 0.2, and the 13´ and 9´ patterns with a visual acuity of 0.5 elicited the P100 components. Visual acuities of 0.2 and 0.5 could not be determined by this method. Having met with failure in our first study, we revised our method to deal with a number of its problems.

One problem was considered to be the angle of the 35 x 26-cm television screen that was used. Because the retinal X-ganglion cells are suggested to be the ones predominantly stimulated (^19,20,24), the angle of the stimulus needed to be smaller. Therefore, the visual angle of 8° was chosen for this study. A second problem was that the graphic correlation differed between the 20´ and 13´ patterns and the visual acuity of 0.2 and 0.5. Therefore, the checkerboard stimuli were changed from 20´ and 13´ patterns to 26´ and 15´ patterns. When the PVEP was recorded with these stimuli, the accuracy for visual acuity was 76.9% for 0.1, 71.4% for 0.2, 70.0% for 0.5, and 58.3% for 1.0 in 18 healthy eyes (mean 22 years) (¹⁹). This result was not enough to measure visual acuity of 1.0. Therefore, in this study, PVEP of controls in the fifth decade was initially recorded using the procedure described above. The accuracy for visual acuity of 1.0 was 85.6%. Thus, this method was thought to be applicable to estimation of visual acuity.

There are two problem in measuring PVEP with this method. One problem is fixation. If the patient cannot stably fixate, PVEP visual acuity cannot be assessed. In this study, visual acuity could be measured in six patients with good fixation, but another patient with suspected malingering could not be assessed using the PVEP. He complained of left ptosis and visual loss after left ocular trauma. When it was stressed that he should open his left eye, his eyes deviated in a right downward direction. Then the PVEP could not be performed with this patient.

A second problem is that this method is time consuming. It requires 1 hour to measure the visual acuity OU. Therefore, this method needs to be improved to shorten the examination time.

CONCLUSION

We evaluated objective visual acuity using the PVEP in patients with ocular malingering. PVEP can be useful in objectively establishing the true acuity in an eye that a patient is spuriously claiming to be impaired.

Acknowledgment:

The authors thank William F. Hoyt, MD, of the University of California, San Francisco, for his critical reading of the manuscript. This study was presented in part at the 12th International Neuro-Ophthalmology Symposium, Dublin, Ireland, July 19–22, 1998.