Indocyanine green enhanced retinal vessel laser closure in rats: histologic and immunohistochemical observations

Feeder vessel photocoagulation using both thermal and indocyanine green (ICG) enhanced applications as a treatment for choroidal neovascularization is under investigation. While closure of feeder vessels is achievable, reperfusion of these vessels occurs. The purpose of the following study was to compare, contrast anatomic, and immunohistochemical findings in rat retinal arterioles following attempts at vessel closure using either the diode (810 nm) laser alone or in conjunction with intravascular ICG.

The retinal arterioles of adult Lewis or Brown Norway rats were treated with diode laser alone or immediately following intravenous injection with 75 mg ml(-1) ICG. Retinal vessel closure was determined by examination of retinal flatmounts following FITC-dextran or rhodamine-dextran perfusion. Anatomic changes were examined by electron microscopy and quantitative cellular changes were measured by perfusion with Hoechst 33342 nuclear staining. Recruited macrophages were detected by ED1 immunohistochemistry.

Treatment with diode laser alone resulted in partial retinal arteriolar closure seen only in pigmented animals. The use of adjuvant ICG achieved complete vessel closure in albino animals with reperfusion seen in all vessels by 7 days. Electron microscopy revealed an intraluminal clot only in ICG-enhanced diode laser treated animals, but with accompanying endothelial and perivascular cellular damage. Immunohistochemistry of the site of retinal arteriolar closure revealed a large increase in perivascular cellularity with an apparent influx of ED1 positive cells.

ICG-enhanced diode laser photocoagulation appears to be superior to diode treatment alone in achieving vessel closure, but is limited by clot resolution due to both excessive vascular damage and an accompanying inflammatory response. These results suggest that more durable feeder vessel closure rates may be achievable with either the use of accompanying anti-inflammatory therapies or with a less vascular damaging photoactivating dye.