Influence of Infrared Irradiation on the retinal area overlying subretinal implants

Abstract

Abstract: : Purpose:Infrared (IR) Irradiation is a possible power source for subretinal implants. We further investigated (ARVO 2003, #5077) the temperature increase in the subretinal space as well as the cooling effect resulting from ocular circulation by transpupillary irradiation of a newly developed measurement device that was transchoroidally implanted into the subretinal space allowing simultaneous in situ measurements of both IR irradiation intensity and temperature increase. Additionally, histological examinations were performed. Methods:Polyimide foil strips (48.0 x 3.2 x 0.05 mm) carrying an optical sensor (silicon solar cell, 1.7 x 1.85 x 0.07 mm) and a thermal sensor (nickel nickel–chromium wire, d=25µm) were used. The thermal sensor positioned underneath the optical sensor was implanted subretinally at the posterior pole of 6 rabbit eyes. The area of the thermal sensor was continuously irradiated with IR laser (830 nm up to 12.7 mW/mm²) on the device. The sensor provided real–time measurements of absolute temperature and irradiation density. Thereby the temperature increase induced by different intensities of IR irradiation onto the chip was recorded directly. Possible IR–induced damage of the retina was examined in histological sections. In addition the influence of ocular blood flow on local subretinal temperature in rabbit eyes (including the choroidal cooling effect) was analyzed. Results:Temperature increased exponentially over a wide range (up to 12.7 mW/mm²) when plotted against IR irradiation power; only in the lower range (up to 3.2 mW/mm²) a linear increase by 0.95 °C per 1mW/mm² was found. Applying a steady IR irradiation power of 3.2 – 12.7 mW/mm², the temperature increase stabilized at approximately 4 °C. The temperature increase versus IR irradiation density post mortem (without ocular blood flow) indicated a linear correlation over the whole range with 1.15 °C per 1mW/mm². Therefore the cooling effect due to ocular blood flow varied depending on IR irradiance density. No IR–induced damage of the retina was detected. Conclusions:A temperature increase of approximately 3.2 °C in the living rabbit eye is expected when powering a subretinal implant by 4.8 mW/mm² IR power density, to be utilized as external power supply for an active implant with 1.500 electrodes. This appears to be a tolerable increase for eye tissues in the living rabbit

Keywords: retina • laser • vitreoretinal surgery