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An 18 year old male who had been taking vigabatrin for complex partial seizure was referred for visual field testing. The cumulative vigabatrin dose was about 6.0 kg over the seven and a half year period. There were bilateral symmetric visual field constrictions with greater loss of the nasal field (fig 1). On the retinal nerve fibre layer (RNFL) photographs discernible RNFL losses in the peripheral retina with a relative preservation of papillomacular bundles were observed bilaterally (fig 2). RNFL thickness around the optic disc, as measured by optical coherence tomography, was markedly reduced in all quadrants except for the temporal quadrant in both eyes and the inferior quadrant in the right eye (fig 3).
Vigabatrin inhibits irreversibly γ aminobutyric acid (GABA) transaminase, resulting in an increase in GABA in the brain and in the retina.1 As a result, irreversible visual field constriction may be developed.2 The mechanism of visual dysfunction is poorly understood but an altered inner retinal function with ganglion cell loss may be a possible cause.2,3 Pathological study at postmortem supported the idea that the primary site of injury lies within the ganglion cells of the retina.4
In the present case, RNFL loss was demonstrated by RNFL photography. This finding correlated with the visual field defect and quantitative analysis of RNFL thickness. Further study is necessary in patients taking vigabatrin to determine whether RNFL photography identify RNFL loss before visual field defect development. If it does, as it does in glaucoma,5 RNFL photography will be a valuable tool for deciding whether to continue with vigabatrin administration.