gms | German Medical Science

Purpose: Restoration of letter reading and stripe pattern recognition via subretinal electronic implants in blind RP patients.

Methods: Subretinal implants were placed transchoroidally near the macula, consisting of two arrays: 4 x 4 electrodes (100 x 100 µm), spaced 280 µm, controlled retroauricularly via a subdermal line for direct stimulation (“DS array”) and a "chip" (3 x 3 x 0,1 mm) with 1500 electrodes (50 x 50 µm) of the same kind,each electrode being activated by light falling onto a neighboring micro-photodiode that controls the output of its subretinal amplifier (Details see presentation of Dr. Wrobel). Letters were presented to 3 patients either by stimulating retinal cells in 10ms steps via individual electrodes in a sequence patients had learned to write such letters or – via the light sensitive chip – by individual letters or stripe patterns steadily presented at a screen.

Results: On the DS array patients reported uniformly for each electrode that the sensation evoked by each individual pulse (0.5–4 ms, 0.1 V above threshold) consisted of a whitish round dot, clearly separated from its neighbor. Patterns consisting of such 4 x 4 dots correspond to letters of approximately 5 cm diameter presented at 60 cm distance.

Pat. 1 correctly (20/24) recognized the direction of the letter “U”, presented with the opening in four different directions in in a 4 alternative forced choice (4AFC) mode.

Pat. 2 correctly (12/12) differentiated letters (e.g. C, O, I, L, Z, V) within few seconds, presented via DSelectrodes in random order (4AFC). With the light sensitive subretinal chip, he also correctly (22/24) differentiated without head movements letters (e.g. L,I,T,Z; 8,5 cm high, 1.7 cm line width) steadily presented on a screen at 62 cm distance with a red light (630nm cutoff) of 3.4 cd/m². Pat.3 recognized (15/20 correct, 4AFC) the direction of lines or stripe patterns with the chip, as did Pat.1 (11/14, 2AFC) and Pat.2 (11/12 4AFC) up to 0.35 cycles/deg. Regular ophthalmological tests with Landolt-C rings(4AFC) revealed a visual acuity of >20/1000 in Pat. 2 exceeding the limits of blindness according to German definitions.

Conclusions: Active subretinal multielectrode implants with currents close to recognition threshold (10 to 27 nC/electrode) produce retinotopically correct patterns that allow for the first time recognition of individual letters (8cm high, viewed in appr. 62 cm distance) even at low luminance levels. Stripe patterns of moderate luminance can be resolved up to 0.35 cycles/deg via the subretinal chip with visual acuity >20/1000 (Landolt-C ring). This clearly supports the feasibility of light sensitive subretinal multi-electrode devices for restoration of useful visual percepts in blind patients.