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Michaela Velikay-Parel, Domagoj Ivastinovic, Thomas Georgi, Gisbert Richard, Ralf Hornig; Perceptual Threshold And Neuronal Excitability As Long-term Safety Evaluation In Retinal Implants. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2590.
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As part of a clinical trial, an investigational epiretinal implant (IMI Intelligent Medical Implant) was implanted in a retinitis pigmentosa (RP) patient. The prosthesis was wirelessly controlled by a visual interface containing a micro camera, providing wireless infrared data and energy transmission. 49 electrodes were used for pattern recognition. This study examined the changes of perceptual threshold over time and its relation to long term stimulation. The goal of the study was to investigate stimulus induced depression of neuronal excitability (SIDNE) measurements as a method to gain further insight into the safety profile of epiretinal implants.
Stimulation threshold measurements were done using the Best-Pest method. The threshold was defined as the level of stimulation intensity at which the patient had a visual perception in a defined probability of 50%.SIDNE is measured by comparing the threshold changes before and after a stimulation session of two electrodes. One electrode is actively used during the stimulation, the other remains passive and serves as a control. Hypothetically safe stimulation would not lead to significant differences in threshold changes between active and control electrodes.
The initial threshold of the day remained stable over the observed period. However the threshold increased after the stimulation sessions. The relative threshold increase of the active electrodes never significantly exceeded that of the inactive electrode.
SIDNE measurement is feasible in epiretinal implants. The stimulation was performed within safety limits. After prolonged stimulation the thresholds may increase by more than a third due to the patients fading concentration even the stimulation remains within safety limits.
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