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 ). 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 [13
]. 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 [20
]. 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 [23
]. 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.