title: Factors affecting perceptual thresholds in a suprachoroidal retinal prosthesis.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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]).
  
  
  CONCLUSIONS
  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.).
authors.affiliation: Bionics Institute, East Melbourne, Australia Department of Medical Bionics, University of Melbourne, Carlton, Australia.
authors.forename: Mohit N
authors.initials: MN
authors.lastname: Shivdasani
authors.affiliation: Bionics Institute, East Melbourne, Australia.
authors.forename: Nicholas C
authors.initials: NC
authors.lastname: Sinclair
authors.affiliation: Centre for Eye Research, University of Melbourne, Royal Victorian Eye and Ear Hospital, East Melbourne, Australia.
authors.forename: Peter N
authors.initials: PN
authors.lastname: Dimitrov
authors.affiliation: Centre for Eye Research, University of Melbourne, Royal Victorian Eye and Ear Hospital, East Melbourne, Australia.
authors.forename: Mary
authors.initials: M
authors.lastname: Varsamidis
authors.affiliation: Centre for Eye Research, University of Melbourne, Royal Victorian Eye and Ear Hospital, East Melbourne, Australia.
authors.forename: Lauren N
authors.initials: LN
authors.lastname: Ayton
authors.affiliation: Centre for Eye Research, University of Melbourne, Royal Victorian Eye and Ear Hospital, East Melbourne, Australia.
authors.forename: Chi D
authors.initials: CD
authors.lastname: Luu
authors.affiliation: Bionics Institute, East Melbourne, Australia.
authors.forename: Thushara
authors.initials: T
authors.lastname: Perera
authors.affiliation: Bionics Institute, East Melbourne, Australia Department of Medical Bionics, University of Melbourne, Carlton, Australia.
authors.forename: Hugh J
authors.initials: HJ
authors.lastname: McDermott
authors.affiliation: Bionics Institute, East Melbourne, Australia Department of Medical Bionics, University of Melbourne, Carlton, Australia.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Blamey
mesh_terms: D000328:Adult; D004558:Electric Stimulation; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008839:Microelectrodes; D008875:Middle Aged; D011474:Prosthesis Design; D012160:Retina / Q000503:physiopathology / Q000601:surgery*; D012174:Retinitis Pigmentosa / Q000503:physiopathology / Q000601:surgery*; D016896:Treatment Outcome; D014793:Visual Cortex / Q000503:physiopathology*; D014796:Visual Perception / Q000502:physiology*; D057488:Visual Prosthesis; D055815:Young Adult
keywords: retinal prosthesis; retinitis pigmentosa; suprachoroidal; threshold