3.1 Diagnostic/Technology Overview
Indocyanine green (ICG) is a dye used for visualizing the choroidal circulation. During testing, 25 mg of ICG dye in 5 mL of water is injected intravenously with photographs taken starting shortly after injection, and late photographs taken 10 to 40 minutes after injection. An excitation filter (640–780 nm) should be used, along with a barrier filter (820–900 nm). Due to the transmission peak being near the infrared range, ICG angiography (ICGA, commonly referred to as simply ICG) has better penetration through hemorrhage, nuclear sclerotic cataracts, retinal pigment epithelium, cloudy vitreous fluid, and choroidal melanin pigment than fluorescein angiography (FA; ▶ Fig. 3.1). ICG dye is cleared quickly from the bloodstream, with only 4% remaining in plasma 20 minutes after injection. It is then excreted via the hepatic system.
Fig. 3.1 (a) Normal ultra-widefield indocyanine green at approximately 30 seconds. (b) Normal ultra-widefield ICG at 7 minutes.
Adverse reactions to ICG are uncommon. Nausea, vomiting, and itching occur in 0.2% of the patients. Rarely, ICG can result in severe reactions such as urticaria and syncope in 0.2% of the patients and anaphylaxis in 0.05% of the patients. Seafood allergies and liver disease are contraindications to ICG administration. In addition, dialysis patients have an increased risk of ICG-related complications.
3.2 Key Applications
3.2.1 Central Serous Chorioretinopathy
Central serous chorioretinopathy (CSR) will demonstrate areas of choroidal hyperpermeability that are more prominent in middle and late phases of the study (▶ Fig. 3.2). Dilated choroidal vessels with choroidal leakage can also be seen. These regions of choroidal hyperpermeability can remain in chronic and severe CSR cases. Occult serous pigment epithelial detachments, delays in choroidal arterial filling, and venous congestion may also be visible. Of note, ICG is useful in distinguishing CSR from occult choroidal neovascularization (CNV) in macular degeneration as well as polypoidal choroidal vasculopathy (PCV).
Fig. 3.2 Ultra-widefield indocyanine green in central serous retinopathy with multiple areas of hypercyanescence in the macula and near periphery with increased choroidal hyperpermeability. This image was taken at 5 minutes.
3.2.2 Polypoidal Choroidal Vasculopathy
PCV is characterized by an abnormal choroidal vascular network ending in an outpouching in the choroidal circulation that may be visible as a spherical polyp configuration. PCV can cause multiple serous and retinal pigment epithelial detachments from leakage of these vascular abnormalities. ICG can be used to identify the polyps seen in PCV, allowing for the potential of therapeutic treatments. Polyps often begin to appear approximately 6 minutes after injection of ICG. Choroidal vessels can be seen in a branching pattern with nodules that can correspond to orange subretinal nodules on exam. Early phase ICG will show a network of vessels with a hypercyanescent nodule that will gradually expand and leak slowly (▶ Fig. 3.3). Late phase ICG can reveal hypocyanescence of the lesion core with surrounding ring-shaped staining of the polyps. A pulsatile nodule on dynamic ICG is distinct to PCV.
Fig. 3.3 Indocyanine green in polypoidal choroidal vasculopathy near the superotemporal margin of the optic disc with a polyp shaped area of hypercyanescence. This corresponded to an area of hyperfluorescence on fluorescein angiography. This image was taken at 5 minutes.
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