Involvement of illumination in indocyanine green toxicity after its washout in the ex vivo rat retina

Purpose: To elucidate the involvement of illumination in indocyanine green (ICG) retinal toxicity. METHODS: We incubated isolated rat retinas with or without illumination after exposure to 0.5% ICG. We also examined whether a time lag following ICG exposure before illumination altered the damage. Toxicity was evaluated by histologic and biochemical assays, including measurement of lactate dehydrogenase release. RESULTS: Retinas fixed immediately after ICG exposure showed minimal morphologic changes. However, illumination for 3 hours at 34°C starting after washout of ICG selectively damaged the outer nuclear layer. Retinas incubated for 3 hours under the same condition in the dark showed preserved morphology but were damaged by subsequent illumination. When retinas were illuminated after washout of ICG at a lower temperature (30°C), the damage was attenuated. Results obtained using lactate dehydrogenase release were consistent with these morphologic changes. CONCLUSIONS: Incubating retinas in the dark and cooling after ICG exposure significantly inhibited retinal damage, suggesting that ICG interacts with illumination to induce retinal damage.