Vigabatrin-associated retinal cone system dysfunction

Reply from the Authors: We appreciate Dr. Beck's comments about our article. In his letter, he voices concern regarding three main issues: 1) the accuracy of the visual field defects, 2) the significance of the electrophysiologic findings, and 3) the significance of the funduscopic findings. We would like to respond to all three issues. Dr. Beck appropriately questions the accuracy of visual field defects, because the quality of information obtained from perimetry and visual field testing depends on a patient's cooperation and ability to respond in a reliable fashion. Patients with refractory epilepsy tend to have more difficulty performing tests of visual function than do normal subjects. Thus, it is important to have some type of assessment of patient reliability and consistence to evaluate the significance of visual field information properly. With manual perimetry, the patient's fixation behavior can be monitored directly using a telescopic viewer to observe the patient's eye during testing. Catch trials can be used to determine the patient's response reliability. False-positive errors (responses when no stimulus is presented) and false-negative errors (failure to respond to a stimulus presented in a region previously determined to be able to detect equal or less detectable targets) can be monitored throughout the test procedure. Automated test procedures not only have the capability of monitoring false-positive errors, false-negative errors, and fixation behavior, but can also obtain an assessment of short-term response fluctuation by retesting a sample of visual field locations. …