Sweep-VEP Acuity as a Clinical Tool in the Diagnosis of Functional Visual Loss

Abstract

Purpose: Visual complaints without evidence of contributory ocular or non-ocular pathology are not uncommon presentations to the general physician, the neurologist, or the ophthalmologist. Visual evoked potential is an established method of assessing visual pathway and therefore remains an important tool in the assessment of unexplained visual loss. This study aimed to assess the clinical usefulness of grating acuity measured by sweep visual evoked potentials (sweep-VEP) in the diagnosis of psychogenic visual impairment and ocular malingering.

Methods: Patients with suspicion of functional visual loss were referred for electrophysiological testing. Only those with no ocular disease (except refractive errors) were included. Objective determination of grating acuity was performed by sweep-VEP using the PowerDiva system (phase reversal sine wave gratings; mean luminance; 80% contrast). All participants had previous transient pattern reversal VEP (reversal rate = 1.9Hz; checkerboard stimuli 15’ and 60’; 100% contrast) within normal limits for both amplitude and latency. Best corrected visual acuity was measured in each eye for distance with a retro-illuminated ETDRS chart. Comparison of optotype subjectively measured VA and grating acuity was performed by Wilcoxon rank test. Significance was set at P<0.05.

Results: A consecutive group of 12 patients (7 males - 53%) with ages ranging from 21 years to 56 years (mean=44.6±10.9, median= 51.6) was tested. Material gain was the motivation for 8 patients (7 males, 1 female) while the remaining 4 females had conversion reaction. Visual acuity loss was bilateral in 7 (58%) and unilateral in 5 (42%) subjects (a total of 19 eyes). Optotype acuity ranged from 0.5 logMAR to no light perception (mean=1.35±0.87 logMAR). Grating acuity ranged from -0.01 logMAR to 0.18 logMAR (mean=0.11±0.06 logMAR). Significantly better VA scores were obtained with grating acuity measured by sweep-VEP (P≤0.001) when compared with optotype acuity.

Conclusions: The sweep-VEP technique was a useful ancillary test for the diagnosis of malingering and psychogenic visual loss in adults. Although this electrophysiological technique had been designed for assessment of grating acuity in infants and babies, its clinical importance for legal decisions in ophthalmology cannot be disregarded. Sweep-VEP might be a helpful alternative for objective measurement of visual acuity in characterizing functional visual loss.

Keywords: 754 visual acuity • 612 neuro-ophthalmology: diagnosis • 507 electrophysiology: clinical