Scotopic threshold response changes after vigabatrin therapy in a child without visual field defects: a new electroretinographic marker of early damage?

Vigabatrin (VGB) has been widely used in patients affected by drug-resistant epilepsy and West syndrome. Following reports of visual field loss associated with vigabatrin therapy, some authors have investigated retinal electrophysiologic variables to identify early electrophysiologic markers and pathogenetic mechanisms of retinal damage. There are no previous reports of a scotopic threshold response (STR) reduction associated with vigabatrin therapy. A 13-year-old male child was submitted to a complete electroretinographic study before and after the start of vigabatrin therapy. Of the electroretinographic responses analyzed, only the scotopic threshold response was altered. The scotopic threshold response is a corneal-negative wave in the electroretinogram (ERG) of a fully dark-adapted eye. In cat, this response has been shown to be mediated by K+ spatial buffer currents that flow from proximal to distal retina in retinal glia as a result of elevated concentration of K+ in proximal retina following depolarization of local neurons in response to light onset. The prospective nature of the study in a previously untreated patient on vigabatrin monotherapy allows us to speculate on the underlying pathogenetic mechanisms and level of action of vigabatrin therapy-related retinal damage. If the predictive value of the scotopic threshold response changes is documented, this ERG response could be used to perform a preliminary evaluation of drugs, which modify gamma-aminobutyric acid (GABA) receptors and/or GABA levels.