Disability diagnosis is a part of the social security system. Determining eligibility for disability diagnosis is difficult because the process of evaluation is dependent upon pension insurance and private disability insurance benefits. Measurement of visual acuity, visual fields, and extra ocular movement are fundamental primary tests for disability determinations. However, there are no sharp inflections in performance abilities corresponding to specific visual acuity.

The cortical visual evoked potential (VEP) is an established method of assessing visual pathway function. Pattern VEP is generally used for the detection of visual function disability (demyelization disease, optic neuritis, ischemic optic neuropathy, compressive optic neuropathy, and amblyopia) and objective visual acuity assessment (visual function for children, malingering, and psychogenic visual disability) 12.

Although previous studies have used pattern VEP for the objective assessment of visual acuity, there is little consensus regarding the interpretation of visual acuity (VA) assessments 345678. Because there are large differences in VEP among individuals, previous studies involved comparison of the results for two eyes in the same individual rather than using absolute values of amplitude or latency period 9. Thus, it is difficult to assess visual function with VEP in cases in which both eyes have visual pathway disturbance or functional visual loss combined with visual pathway disability. The clinical usefulness of pattern visual evoked potentials (PVEP) to determine visual acuity is controversial 6101112. We determined a correlation between the PVEP data and visual acuity. The method was applied to patients with optic neuritis to confirm this correlation. Use of PVEP testing to predict objective visual acuity of disability patients was assessed. The present study was performed to obtain reference numbers for visual acuity testing in disability assessment with visual pathway lesions.

Retrospective clinical data from 20 normal subjects and 18 amblyopic subjects were obtained (Table 1). The average logMAR Snellen acuity of 20 normal subjects (20 normal right eyes) and 18 amblyopic subjects (18 amblyopic eyes) was 0.47 ± 0.57 logMAR. The average pattern amplitude of 20 normal subjects (20 normal right eye) and 18 amblyopic subjects (18 amblyopic eyes) was 10.52 ± 5.96 μV.

BCVA : best corrected visual acuity.

There was a significant correlation between amplitude and visual acuity in normal and amblyopic subjects (r = − 0.7649, p < 0.0001; Figure 1). Linear regression analysis of pattern amplitude and visual acuity in normal and amblyopic patients indicated an relationship of y = − 0.072x + 1.22 (−0.072) (x: amplitude, y: logMAR VA), which was statistically significant.

There was no significant correlation between latency period and visual acuity in normal subjects or amblyopic subjects (r = 0.191, p = 0.25) Therefore, linear regression analysis of pattern latency was not performed.

This study was performed to examine the usefulness of visual acuity prediction using absolute values, with no comparison between the two eyes of the same subject in one tertiary clinic, in order to diagnose the presence or absence of visual disability. Objective visual acuity is necessary to grade the disability patient. The relationship between amplitude and logMAR acuity is linear even though the VA and amplitude of the data we collected from 2007 to 2009 in amblyopic groups were much lower and not evenly distributed. Thus, a causal distribution of the data was in two clusters (Figure 1). However, the results indicated a statistically significant functional correlation between pattern VEP amplitude and visual acuity in normal vs amblyopic subjects.

We applied the obtained relationship to the optic neuritis patients to see if it is possible in disability patients to differentiate malingering. The pattern VEP- estimated VA was not different from the actually measured subjective visual acuity in optic neuritis cases. The slopes are not significantly different by comparison of linear regression between normal/amblyopia and optic neuritis, so this correlation can be used for evaluating the presence or absence of visual disability.

We had difficulties in evaluating the patients with relatively healthy optic discs in order to measure objective visual acuity for the evaluating the presence or absence of visual disability. An evaluation reference at the clinic is needed. We analyzed this relationship (y = −0.072x + 1.22) to find a reliable cutoff that could be applicable to estimate VA by the ROC curve (area 0.7169, p < 0.03011). Above 5.77 μV amplitude it would be reasonable to predict visual acuity by regression relationship. It is not possible to predict pattern VEP -estimated VA below 5.77 μV (corresponding to 0.8 logMAR: 20/120 Snellen acuity using function y = −0.072x +1.22) because of a very low amplitude.

However, patients with VEP amplitude below 5.77 μV would be compatible with legal blindness [visual acuity below 20/200 (1logMAR)] that is required for disability registration. Visual acuity below 20/200 (1logMAR) is defined as 100% visual acuity loss in visual disability assessment. Suspicious malingering with no obvious pale disc appearance can be ruled out if the amplitude is below 5.77 μV. Our results demonstrated that pattern VEP- estimated VA was useful in confirming subjective visual acuity in disability evaluation at given cutoff amplitude.

Patients with visual disability may exaggerate their decrease in visual acuity of the injured eye to maximize compensation. Clinicians have difficulty confirming visual acuity with visual pathway lesion without apparent pale optic disc. Such patients are particularly difficult to ascertain, and an objective measure of the VA will provide an important contribution to the evaluation of such cases. Electrophysiological testing can be used to evaluate the level of underlying organic dysfunction in patients with nonorganic overlay superimposed upon real dysfunction 13151617. Pattern VEP may be a useful tool for determining the level of visual acuity 18. Accordingly, cases where visual disability is accompanied by a history of trauma or in patients with amblyopia of definite etiology or bilateral visual loss, the correlation derived from VEP can be used as a reference value for visual acuity. The VEP has been used primarily for objective evaluations of visual acuity and refractive error, but it is impossible to accurately quantify visual function in amblyopia and cases of organic pathology with visual dysfunction.

Odom et al 19 compared subjective and VEP acuity of adults. Linear regression analyses gave good agreement between their subjective data and VEP visual acuity. Our study also showed a significant relationship between VEP amplitude and visual acuity.

This study had some limitations that should be taken into consideration. First, a relatively small number of variables were used, and further studies are needed for verification. However, in our study the range of VEP amplitude was 8-22 μV with 16’ check size in the normal group with 20/20, similar to previous reports 18. The further studies are needed for verification. Second, VEP amplitude will be affected by other causes of visual acuity loss because the pattern reversal visual evoked response (PVER) mainly represents the function of the macula and optic nerve 202122. However, we tried to exclude patients with disability registry with retinal disease by reviewing the results of multifocal ERG and FAG. Third, there has been controversy about the relationship between amplitude of VER and age 2324. The unilateral amblyopia group in our study consisted of patients requiring exam for entering the army. Therefore, they were all about 20 years of age. We should consider age differences affecting VEP in future studies. Fourth, the statistical result of optic neuritis in the difference between actually measured visual acuity and function value was marginal (p = 0.07). This non-significant difference was 0.18 logMAR, which is approaching the clinically significant level of 2 lines of letters on a logMAR chart, might be due to a lack of statistical power and/or high levels of variability in their data.

In conclusion, estimation of visual acuity in visual disability assessment through correlation of absolute amplitude values in pattern VEP might be useful for giving reference visual acuity associated with malingering vs. real disability in some ranges (>5.77 μV). Further studies are required to assess the reliability and to perform statistical verification with additional variables to provide support for use of this method in the assessment of cases of visual disability with visual pathway dysfunction.

The authors declare that they have no competing interest.

JHJ performed the statistical analysis and drafted the manuscript. SEK examined and evaluated the patients. SEK and SYO wrote the final manuscript. All authors read and approved the final manuscript.

This work was supported by the research fund of Dankook University in 2010.