Sweep Visual Evoked Potentials In Patients With Suspected Functional Visual Acuity Loss

Abstract

Purpose: Early and accurate identification of organic vision loss is very important in patients with suspected functional visual loss (FVL). Visual evoked potential (VEP) offers a means of objectively evaluating visual function, and may assist with early and accurate diagnosis. This study's aim is to assess the clinical usability of the sweep VEP in patients with suspected FVL.

Methods: A retrospective chart-review was conducted. All adult patients with unexplained vision loss evaluated between August 12th of 2013 and December 5^th of 2014 with inconsistencies on examination suggestive of FVL were included. All patients underwent both pattern VEP (PVEP) and sweep VEP(SVEP) testing as part of their evaluation. Visual acuity(VA) was converted to decimal notation. VEP results were interpreted by an experienced electro-physiologist, masked from the patient clinical findings. Abnormal PVEP was defined by abnormal P-100 latency and/or amplitude, while SVEP values of <0.5 were considered abnormal. Two-sample t-test was used to compare the findings in the normal and abnormal PVEP groups.

Results: Twenty four patients were included (13 women and 11 men) with mean age of 41 years +/- 14.2 (range, 19-61 years of age). Visual acuity at presentation was 0.35+/-0.32 in each eye. Color vision measured 5.4+/-5.7 out of 13 Ishihara plates in each eye. Pattern VEP was normal (Group A) in 29 eyes (both eyes of 13 patients and one eye of 3 patients) and abnormal (Group B) in 7 eyes (both eyes of 2 patients and one eye of 3 patients). PVEP was Inconsistent or unobtainable due to lack of patient co-operation in 12 eyes (6 patients). SVEP estimated acuity in group A was 0.84 +/-0.21 compared to 0.47+/-0.11 in group B (p=0.00013). In Group A, the SVEP acuity was 0.26+/-0.17 better than the presenting acuity, while in Group B, the SVEP acuity was 0.37+/-0.51 better than the presenting acuity (p=0.59). Visual acuity retested using polarizing glasses was only 0.03+/-0.07 better than the presenting VA in Group A compared to 0.19+/-0.34 in Group B (p=0.014). SVEP was able to detect organic vision loss with sensitivity of 71.4% and 100% sensitivity with a positive predictive value of 100% and negative predictive value of 93.5%.

Conclusions: Sweep VEP is both a sensitive and specific quantitative and non-invasive test that can be used in evaluation of suspected FVL.