A 44 channel suprachoroidal retinal prosthesis: initial psychophysical results

Abstract

Purpose : The success of our clinical trial of a prototype suprachoroidal retinal prosthesis (NCT01603576) in 2014 led us to develop a 44 channel device, with the aim of providing visual information to patients with profound vision loss due to retinitis pigmentosa (RP).

Methods : A 44-channel electrode array was implanted in the suprachoroidal space in eligible patients with advanced retinal dystrophy. After recovery from the surgery they commenced fitting (‘switch-on’) and training with the device. Thresholds for visual perception of phosphenes were assessed using a 2-down 1-up modified staircase procedure. Discriminability of individual phosphenes was assessed using a 3-interval forced choice (3-IFC) ‘odd one out’ discrimination task across pairs of phosphenes selected from retinotopically central and peripheral locations. Functional vision, when using the device with a head-mounted camera (1:1 zoom), was assessed in a square localization task using 10-degree wide high-contrast squares and a 4-alternative forced choice (4-AFC) direction-of-motion task using 7 degree/second moving bars, on a 40-inch touchscreen at arm’s length.

Results : Four patients (age 39-66 years) with rod-cone dystrophy (RP) and with only perception of light vision were recruited. All participants were able to perceive phosphenes. Average phosphene thresholds of all 4 subjects in the first 30 days post switch-on were 182 ± 96 nC, (mean ± S.D., range 38 to 454 nC). Discriminability scores varied according to phosphene selection: pairs involving retinotopically central versus peripheral locations scored 67 to 100% across participants; pairs involving only central locations scored 33 to 100% across participants. Touch precision during the square localization task was significantly better with Device On versus Device Off for the two participants who had been in the study long enough to reach this evaluation time-point at 20 weeks post switch-on. The loci of touch points (mean ± S.D.) spanned 12.7 ± 8.0 degrees (Device On) vs. 33.0 ± 18.6 degrees (Device Off) for P1 and 9.8 ± 5.1 degrees (Device On) vs. 20.8 ± 10.6 degrees (Device Off) for P2 indicating greater precision for Device On. Detection of motion scores ranged from 35 to 100% with Device On, versus 25% with Device Off.

Conclusions : A 44-channel suprachoroidal retinal prosthesis is able to produce discriminable phosphenes and enable functional vision in screen-based tasks.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.