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Field-specific visual-evoked potentials

Identifying field defects in vigabatrin-treated children

From the Neurosciences Research Institute (Dr. Harding, E.L. Spencer, and M. Conway), Aston University, Birmingham, UK; Department of Optometry and Vision Sciences (Dr. Wild), Cardiff University, Cardiff, UK; and Aventis Pharma (Dr. Bohn), Bridgewater, NJ.

Address correspondence and reprint requests to Prof. Graham Harding, Neurosciences Research Institute, Aston University, Aston Triangle, Birmingham B4 7ET, UK; e-mail: g.f.a.harding{at}aston.ac.uk

Objective: To derive a visual-evoked potential (VEP) technique for identifying visual field defects in children with epilepsy treated with vigabatrin and unable to perform perimetry.

Background: Studies have linked vigabatrin to a specific pattern of visual field loss. Few studies have included the pediatric population because of difficulties in assessing the visual field by perimetry below a developmental age of 9 years.

Methods: A field-specific VEP was developed with a central (0° to 5° radius) and peripheral stimulus (30° to 60° radius). Stimuli consisted of black and white checks that increased in size with eccentricity. Checks reversed at different rates, allowing separate central and peripheral responses to be recorded. Five vigabatrin-treated young adults with field defects were identified using this stimulus. Electroretinograms (ERG) were recorded to examine the effects of vigabatrin on retinal function. Thirty-nine children aged 3 to 15 years were included in the study. Twelve patients were examined by both the field-specific stimulus test and perimetry. The diagnostic performance of the field-specific stimulus test was compared with that of perimetry.

Results: Thirty-five of 39 children complied with the field-specific stimulus, 26 of 39 complied with the ERG, and 12 of 39 complied with perimetry. Using the summed amplitude of the peripheral response from O₂ and O₁, responses below 10 µV were deemed abnormal. The field-specific stimulus identified 3 of 4 abnormal perimetry results and 7 of 8 normal perimetry results, giving a sensitivity of 75% and a specificity of 87.5%. When comparing perimetry results with the ERG parameters, only the 30-Hz flicker amplitude, with a cutoff below 70 µV, gave a useful indication of visual field loss.

Conclusion: Field-specific VEP are well tolerated by children older than 2 years of age and are sensitive and specific in identifying vigabatrin-associated peripheral field defects.