Visual Acuity Determined by Landolt C Test in a Blind Patient Provided with a Subretinal Electronic Implant

Abstract

Purpose: : To determine visual acuity (VA) in a blind person with end-stage Retinitis Pigmentosa after implantation of the Electronic Tübingen Subretinal Implant.

Methods: : The implant was introduced subretinally in a transchoroidal approach and placed under the macula. It consists of an active, light sensitive chip (3x3x0.1mm) comprising 1500 pixels, each consisting of one photodiode and one stimulation electrode (TiN, 50x50µm, 70µm grid spacing) and an additional passive, 4x4 electrode array, for light independent stimulation experiments only. Electrodes on the chip were driven with rectangular voltage pulses (0.5-6ms pulse duration, monophasic anodic, amplitude max 2.0V, scaled automatically to incident light). Landolt C's of different size were presented to one patient on a screen using a LCD projector (viewing distance 62cm). Neutral density filters were used to attenuate luminance, and a red filter (630nm cutoff) to shift the light spectrum towards lower retinal sensitivity without affecting the sensitivity of the chip.

Results: : Operating settings of the implant and presentation parameters of optotypes were optimized using a subjective routine, yielding a stimulation pulse duration of 0.5ms, at 7.5Hz repetition frequency and an optotype luminance of 3.4cd/m². Refraction (+7.0sph -1.50cyl 120°) was corrected according to objective and subjective measurements. The large spherical correction was due to silicon oil tamponade. Landolt C's subtending a diameter of 118 to 34mm with a respective gap size of 25 to 8mm were presented 8 times each, using 4 different orientations. The number of correct responses was plotted against the visual angle subtended by the gap. VA at threshold (62.5% correct) of 20/1108 (logMAR 1.74) was calculated.

Clinical Trial: : www.clinicaltrials.gov NCT00515814 0815

Keywords: retinal degenerations: hereditary • visual acuity • perception