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Mohit N. Shivdasani, Nicholas C. Sinclair, Peter N. Dimitrov, Mary Varsamidis, Lauren N. Ayton, Chi D. Luu, Thushara Perera, Hugh J. McDermott, Peter J. Blamey; Factors Affecting Perceptual Thresholds in a Suprachoroidal Retinal Prosthesis. Invest. Ophthalmol. Vis. Sci. 2014;55(10):6467-6481. doi: https://doi.org/10.1167/iovs.14-14396.
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The suprachoroidal location for a retinal prosthesis provides advantages over other locations in terms of a simplified surgical procedure and a potentially more stable electrode–neural interface. The aim of this study was to assess the factors affecting perceptual thresholds, and to optimize stimulus parameters to achieve the lowest thresholds in patients implanted with a suprachoroidal retinal prosthesis.
Three patients with profound vision loss from retinitis pigmentosa were implanted with a suprachoroidal array. Perceptual thresholds measured on individual electrodes were analyzed as a function of stimulus (return configuration, pulse polarity, pulse width, interphase gap, and rate), electrode (area and number of ganged electrodes), and clinical (retinal thickness and electrode–retina distance) parameters.
A total of 92.8% of 904 measurements made up to 680 days post implantation yielded thresholds (range, 44–436 nanocoulombs [nC]) below the safe charge limit. Thresholds were found to vary between individuals and to depend significantly on electrode–retina distance, negligibly on retinal thickness, and not on electrode area or the number of ganged electrodes. Lowest thresholds were achieved when using a monopolar return, anodic-first polarity, short pulse widths (100 μs) combined with long interphase gaps (500 μs), and high stimulation rates (≥400 pulses per second [pps]).
With suprachoroidal stimulation, anodic-first pulses with a monopolar return are most efficacious. To enable high rates, an appropriate combination of pulse width and interphase gap must be chosen to ensure low thresholds and electrode voltages. Electrode–retina distance needs to be monitored carefully owing to its influence on thresholds. These results inform implantable stimulator specifications for a suprachoroidal retinal prosthesis. (ClinicalTrials.gov number, NCT01603576.)
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