Purpose: The absorption and emission characteristics of indocyanine green are associated with better penetration through ocular pigments, including melanin and blood, in comparison with fluorescein. Therefore, it has been assumed that indocyanine green angiography (ICG-A) allows better delineation of fluorescent structures including choroidal neovascularization in the presence of haemorrhages. The degree and frequency of blockage by haemorrhages during ICG-A and fluorescein angiography (Fl-A) were compared and absorption characteristics by blood were experimentally determined.
Methods: Simultaneous confocal scanning laser ophthalmoscopy was performed in patients with intraretinal or sub-retinal haemorrhages associated with various retinal diseases including neovascular age-related macular degeneration. Areas of blocked choroidal fluorescence were compared in Fl-A and ICG-A using a standardized classification system by two independent readers. Experimental absorption measurements were performed using blood-filled quartz cuvettes and laser light with 488 and 790 nm, respectively.
Results: Sixty eyes of 59 patients were analysed. Twelve eyes (20%) showed blockage in Fl-A only corresponding with funduscopically visible blood. In 35 eyes (58%) the extent of absorption was greater in Fl-A compared with ICG-A. An identical area of blockage in both Fl-A and ICG-A was noted in 13 eyes (22%). The coefficient of absorption was 18.4 mm(-1) for Fl-A (488 nm) and 5.4 mm(-1) for ICG-A (790 nm).
Conclusions: In contrast to previous assumptions, the findings indicate that clinically intraretinal or subretinal haemorrhages are frequently associated with blockage not only in Fl-A but also in ICG-A. This is in accordance with the experimentally determined coefficient of absorption. Apparently, haemorrhages occurring in association with retinal and choroidal diseases commonly have a thickness sufficient enough to induce relevant absorption during ICG-A, and thus impair delineation of fluorescent structures in planes posterior to the haemorrhage. Therefore, the diagnostic value of ICG-A in presence of subretinal or intra-retinal bleedings is limited.