Indocyanine Green Angiography

Kirby R. Miller
Jervey Eye Group
Greenville, S.C.

Indocyanine Green Chorioangiography (ICG) is a procedure which images a layer of the posterior part of the eye. This layer, the choroid, is deeper than the retina and normally obscured by pigmentation. The choroid contains a unique network of blood vessels. These blood vessels flow very fast and probably help to control the temperature of the eyeball in addition to carrying nutrients to the photoreceptors that convert light into vision. Unfortunately, these large, fast flowing vessels can be affected by disease and change as we age.

Recurrent Choroidal New Vessel
A. Late Phase Fluorescein Angiography
B. Mid Phase ICG Angiography

Indocyanine dye is very special. It glows or "fluoresces" in the infrared spectrum. This means it can't be seen with the naked eye as the test is done. It also means that when it is imaged, we can usually see through pigmentation, fluid, or blood in the back of the eye. This unique quality requires some special equipment. ICG imaging has been done many times on infrared film, usually in research applications. Digital imaging systems have proven to be much more appropriate in clinical practice. They allow the image to be viewed as the test is done. Digital cameras can be adjusted to high sensitivities which allow the dye to be captured even when it grows very dim.

Digital systems also feature electronic manipulation which can improve low contrast and dim images. For the most part, in the U. S., ICG dye is imaged on a digital imaging system. The scanning laser ophthalmoscope (SLO) is also used to image ICG. It offers a tremendous advantage in following the dynamic aspects of the dye transit, often recording at 30 frames per second versus one per second for a digital system attached to a fundus camera. The digital systems and SLO's are highly specialized and expensive.

A. Mid Phase Fluorescein Angiography
B. Late Phase ICG Angiography

An ICG test requires that the patient's head be placed in an apparatus in front of the fundus camera or SLO. This holds the head still and places the eye in appropriate range for focusing by the technician. The dye is injected and images are taken rapidly in the beginning. Images are usually taken at intervals at least up to thirty minutes, and perhaps as long as an hour after the injection.

ICG angiography is similar to fluorescein angiography. Most clinical ophthalmologists consider ICG angiography to be an adjunctive or secondary test which adds information to the clinical picture and a fluorescein angiogram. There is controversy in that some ophthalmologists feel the test can be extremely helpful in some situations, while others feel that the test adds little. ICG angiography is certainly not as widely practiced as fluorescein angiography. The recent interest in this procedure has been the result of two factors: the development of digital systems which let us see the choroid, and the interest of ophthalmologists in very carefully directed laser treatment in order to destroy the smallest amount of working retina.

A. Late Phase Fluorescein Angiography
B. Late Phase ICG Angiography

ICG angiography has application in research. Various disease processed have been investigated with this procedure. Researchers have looked at common eye diseases like diabetic retinopathy and discovered a "choroidal component". Exotic diseases which directly affect the choroidal layer are also being researched.

The most practical clinical application of the procedure has been in those patients with age related macular degeneration (ARMD). This is the leading cause of new blindness in patients over 55 years. ARMD has two forms. Most often, the loss of vision is due to slow, painless atrophy over the course of our life. The pigmentation breaks down and the visual receptors are slowly lost. This is called atrophic or dry ARMD. There is little in the way of positive treatment. About l5 - 20% of the time, patients' develop a new vessels which is growing from the rich choroidal vessels and up into and among the visual receptors. The vessels push the receptors away from their usual position and visual problems result. This condition is called exudative or wet ARMD. The specialist can use a special laser to close off these vessels and prevent further damage. These vessels can grow very fast. They leak fluid and can hemorrhage. When that happens, the doctor can't see them clearly. They are called "occult" new vessels, meaning that they are obscured by fluid or blood. A fluorescein angiogram might do a good job of showing where the laser should be pointed when there is nothing in the way. In occult situations, however, many ophthalmologists want to see an ICG study in addition, to confirm their suspicions or to provide a better idea of where the laser should be done. In wet ARMD patients, about l5 - 30% of the time, an ICG study might be helpful. This is a relatively small percentage of patients that have either form of ARMD.

Recurrent Choroidal New Vessel
A. Late Phase Fluorescein Angiography
B. Late Phase ICG Angiography

The ophthalmic photographer with experience in fundus photography and fluorescein angiography will need to make some adjustments when ICG angiography is added. It is a longer test. It requires some basic facility with computer equipment. ICG may be done as a stand alone procedure or in addition to sodium fluorescein. The dyes may be mixed together and given in one injection.