Chronic Electrical Stimulation with a Suprachoroidal Retinal Prosthesis: A Preclinical Safety and Efficacy Study

<div><p>Purpose</p><p>To assess the safety and efficacy of chronic electrical stimulation of the retina with a suprachoroidal visual prosthesis.</p><p>Methods</p><p>Seven normally-sighted feline subjects were implanted for 96–143 days with a suprachoroidal electrode array and six were chronically stimulated for 70–105 days at levels that activated the visual cortex. Charge balanced, biphasic, current pulses were delivered to platinum electrodes in a monopolar stimulation mode. Retinal integrity/function and the mechanical stability of the implant were assessed monthly using electroretinography (ERG), optical coherence tomography (OCT) and fundus photography. Electrode impedances were measured weekly and electrically-evoked visual cortex potentials (eEVCPs) were measured monthly to verify that chronic stimuli were suprathreshold. At the end of the chronic stimulation period, thresholds were confirmed with multi-unit recordings from the visual cortex. Randomized, blinded histological assessments were performed by two pathologists to compare the stimulated and non-stimulated retina and adjacent tissue.</p><p>Results</p><p>All subjects tolerated the surgical and stimulation procedure with no evidence of discomfort or unexpected adverse outcomes. After an initial post-operative settling period, electrode arrays were mechanically stable. Mean electrode impedances were stable between 11–15 kΩ during the implantation period. Visually-evoked ERGs & OCT were normal, and mean eEVCP thresholds did not substantially differ over time. In 81 of 84 electrode-adjacent tissue samples examined, there were no discernible histopathological differences between stimulated and unstimulated tissue. In the remaining three tissue samples there were minor focal fibroblastic and acute inflammatory responses.</p><p>Conclusions</p><p>Chronic suprathreshold electrical stimulation of the retina using a suprachoroidal electrode array evoked a minimal tissue response and no adverse clinical or histological findings. Moreover, thresholds and electrode impedance remained stable for stimulation durations of up to 15 weeks. This study has demonstrated the safety and efficacy of suprachoroidal stimulation with charge balanced stimulus currents.</p></div

Combined rod-cone maximal full-field electroretinogram (ERG) responses at the completion of the chronic stimulation period.

<p>Examples of the ERG waveforms are shown in the inset with the a- and b-waves indicated. Dashed horizontal line indicates recording baseline. a-wave amplitude is taken from baseline to a-wave trough; b-wave amplitude is taken from a-wave trough to b-wave peak. The median and interquartile range of ERG responses from n = 7 subjects are presented in the box plots. Box plots show median (midline), 1^st and 3^rd quartiles (box edges), whiskers have a maximum length 1.5 times the interquartile range and open circles represent individual points. There was substantial overlap between the measured response amplitude of the a- and b-waves between the implanted and control eyes. The paired <i>t</i> procedures showed the mean difference (implanted minus non-implanted) for the a-wave was −14.3 µV (95% confidence interval: –50.5, 21.8, <i>p</i> = 0.37), and for the b-wave was −66.6 µV (95% confidence interval: –135.9, 2.8, <i>p</i> = 0.057). Minor differences between eyes can be attributed to electrode placement or normal biological variation.</p

Retinal Histology.

<p>(<b>A</b>) Photomicrograph of a representative retina following chronic electrical stimulation. The red and green arrows indicate histological dye used to mark the sclera at the site of individual electrodes within the implanted array. In this case, red dye was used to indicate non-stimulated electrodes and green dye to indicate stimulated electrodes. Boxes show magnified tissue regions adjacent to the non-stimulated (red box) and stimulated (green box) platinum (Pt) electrodes as well as 500 µm distal to a Pt electrode (blue box). Scale bar = 1 mm. Asterisk denotes the space occupied by the electrode array. (<b>B</b>) Representative example of retina from paired, non-implanted, control eye. The scale bar in panel B = 50 µm, and applies to all magnified boxed regions. In all cases, the inner and outer retina as well as the tapetum, choroid and sclera did not show any significant histomorphological abnormality. There were no observable differences in overall retinal histopathology between samples. Abbreviations: GCL, ganglion cell layer; INL, inner nuclear layer; ONL, outer nuclear layer; P, photoreceptors; T, tapetum, C: choroid. Subject ID: 502.</p

Summary of chronic stimulation cohort.

<p><i>Stimulation mode was monopolar. Stimulation was delivered using biphasic charge balanced rectangular current pulses (phase width: 400 µs; interphase gap: 20 µs). The maximum current denotes the stimulation level at the culmination of the ramp-up period (refer <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097182#pone-0097182-g001" target="_blank">Figure 1</a>). Charge density per phase was calculated by using the stimulation phase width of 400 µs per phase and the geometric electrode surface area of 0.2827 mm². Electrode utilisation, expressed as “X|Y|Z” denotes the number of electrodes that were stimulated maximally during the stimulation period (X); the number of electrodes that were partially stimulated, either at lower levels or for an incomplete subset of the stimulation duration (Y); and the number of unstimulated controls on the array (Z). The lead system was damaged during subject 505’s implantation; therefore it was only stimulated for 18 days (asterisk) before being disconnected from the stimulator.</i></p

Long-term mechanical stability of the suprachoridal electrode array taken from longitudinal fundus images.

<p>Digitised fundus images recorded immediately following surgery, and over the 15 week chronic implantation period, illustrating the mechanical stability of the implant with respect to the optic nerve and retinal vasculature. Each line depicts the edge of the suprachoroidal electrode array visualised through the fundus image (supplementary <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0097182#pone.0097182.s001" target="_blank">Figure S1</a>). An initial settling of the implant was observed during the first weeks with a displacement of 1–2 optic disc diameters (optic disc diameter is approximately 1.5 mm) in the implant outline. The implant location remained stable after 8 weeks of implantation. Note that data from all time points were not always available.</p

Longitudinal changes in electrode impedance.

<p>Electrode impedance (kΩ) across all subjects recorded periodically over the duration of the chronic implant period. Box plots show median (midline), 1^st and 3^rd quartiles (box edges), whiskers have a maximum length 1.5 times the interquartile range and open circles denote outliers. Pre-implantation, post-explantation and post-cleaning impedance measurements were performed in normal saline. The number of individual electrode measurements that comprise each box plot ranged from 21 to 67 (maximum possible = 72; subject 505 was excluded due to a damaged lead).</p

Longitudinal stability of electrically-evoked visual cortex potentials (eEVCP).

<p>(<b>A</b>) Average cortical responses to increasing current are shown on the left. The stimulus is repeated twice at each current level and both response traces are displayed. The amplitude (mV) of the evoked response (within the region indicated by the blue shaded region) as a function of current (mA) is plotted on the right. In this case, evoked cortical potential threshold was 100 µA (as denoted by “T”). (<b>B</b>) eEVCP thresholds for each subject, recorded monthly, starting immediately before the initiation of chronic stimulation (0 months). Box plots show median (midline), 1^st and 3^rd quartiles (box edges), whiskers have a maximum length 1.5 times the interquartile range and open circles represent individual points. No eEVCP data was available for subject 505 due to a damaged lead. (<b>C</b>) The change in threshold, on a per-electrode basis. The changes were calculated for each electrode separately, in monthly increments, and these data were combined. Box plots show median (midline), 1^st and 3^rd quartiles (box edges), whiskers have a maximum length 1.5 times the interquartile range and open circles denote outliers. There was little variation in median change in threshold over the three time points. The 95% confidence interval comparing switch on to three months was −119.5 to 383.9 µA; this shows a range of plausible values for the overall change over time, including zero.</p

Mean differences in eEVCP thresholds in adjacent months and overall.

<p>Mean differences in eEVCP thresholds in adjacent months and overall.</p