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Separating the retinal electrophysiologic effects of vigabatrin

Treatment versus field loss

From the Neurosciences Research Institute (Drs. Harding and Wild, and K.A. Robertson), School of Life and Health Sciences, Aston University, Birmingham, UK; Clinical Pharmacology (Dr. Rietbrock), Institute of Pharmacology, University of Cologne, Germany; and Global Pharmacoepidemiology (Dr. Martinez), Aventis Pharmaceuticals, Bridgewater, NJ.

Address correspondence and reprint requests to Professor G.F.A. Harding, Clinical Neurophysiology Unit, Neurosciences Research Institute, School of Life and Health Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, UK; e-mail: g.f.a.harding{at}aston.ac.uk

OBJECTIVE: To separate the retinal electrophysiologic markers associated with vigabatrin-attributed visual field loss (VGB-VFL) from those associated with current vigabatrin therapy.

METHODS: A nonrandomly selected cohort of 8 previous and 18 current vigabatrin users and a reference cohort of 8 never vigabatrin-treated patients with epilepsy receiving other antiepilepsy drugs (AED) underwent electro-oculography (EOG), electroretinography (ERG), and automated static threshold perimetry. A cohort of 22 normal subjects underwent ERG. The validity of the retinal electrophysiologic variables to detect the presence and severity of VGB-VFL was assessed using receiver operator characteristic curves.

RESULTS: Of 26 patients exposed to vigabatrin, 18 exhibited VGB-VFL. No patients receiving alternative AED showed this type of visual field abnormality. The presence and severity of VGB-VFL was significantly associated with the latency (implicit time) and amplitude of the ERG cone function. The amplitude of the cone flicker response was the strongest predictor of VGB-VFL and revealed a sensitivity of 100% at a specificity of 75%. The EOG, the photopic and scotopic ERG, and the latency of the ERG second oscillatory potential (OP2) were not significantly related to the presence of VGB-VFL. Vigabatrin therapy was significantly associated with the photopic amplitude, the scotopic a-wave latency, and the latency of OP2.

CONCLUSION: In patients who cannot perform reliable perimetry, the cone-specific ERG flicker amplitude provides the best screening method for detecting VGB-VFL.

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