inorganic iodine in ICG preparations would appear to be tenuous at best. On the other hand, patients with renal disease or liver dysfunction may truly have an increased risk for adverse reactions [39]. ICG has not been shown to cross the placental barrier, however formal studies on fetal toxicity have not been performed [38].

While fluorescein and ICG angiography are generally very safe in-office diagnostic procedures, adverse reactions can occur and may occasionally be severe. It is worthwhile to emphasize that patients should be screened carefully for potential risk factors and as with all intravenously administered agents, it is essential that adequate resuscitation facilities and properly trained personnel be available to manage these problems. Particularly for more remote office locations, it is prudent to establish a working protocol that addresses stabilization of a patient until paramedics can arrive. Although there is no official mandate to have written consent for fundus angiography, many ophthalmologistsprudentlyelecttomaintaindocumentation of the informed consent process. At a minimum, each patient should be made aware of the risks and benefits of fundus angiography.

Fluorescein Injection

Solutions containing fluorescein sodium are available in single-use ampules in one of two different volumes and concentrations that are functionally similar: 5 ml of a 10% solution and 2 ml of a 25% solution. The former is commonly used while the latter may be appropriate in cases when a highly sensitive imaging system such as an SLO is used. The fluorescein dye is drawn up into a syringe and connected to a 23-gauge butterfly needle via flexible clear tubing, which allows the individual injecting the dye to be certain that the needle is in the vein due to a small reflux or “flashback” of blood into the tubing.

Selecting an optimal site for injection requires experience and may need to be tailored to the patient’s anatomy. A favorable target for injection is the antecubital vein, which is commonly used due to its relatively large size and readily accessible location. In some patients, particularly those who are obese, the veins on the dorsal surface of

the hands are often easier to visualize and inject. However, injecting dye into these vessels may be slightly more painful, and rolling of the veins is more likely to occur. Localized ecchymosis following injection is also more readily noticeable. It is important to recognize that although arm-to-choroid and arm-to-retina circulation times are sometimes used to assess ocular perfusion, these measurements may be difficult to interpret since they are influenced by numerous factors including the injection location and the rate of injection.

Fluorescein Technique

Following fluorescein injection, the photographer waits expectantly while focusing on the fundus through the camera’s viewfinder. Appearance of the dye in younger individuals typically takes place within 12 s, though it may be mildly delayed in older patients. As soon as fluorescein dye is detected, the photographer begins recording images in rapid sequence. A digital fundus camera system permits the photographer to take as many photographs early after injection as needed, since it is easy and convenient to delete unwanted frames at a later time. Experienced photographers are adept at performing the necessary fine focus and illumination adjustments early on in the angiographic series. After the early phase, stereo pairs of the fundus can be taken of both eyes if desired. There are stereoviewers available for digital angiography, although stereophotography is now less frequently pursued in this era of digital imaging. If the patient has macular disease, the photographer should visually scan the retina peripheral to the vascular arcades in effort to document any abnormalities. Midphase photographs are then acquired 1–2 min later. At about 5 or 6 min after the injection, additional stereo pairs are taken of the macula and any pathologic areas. Many clinicians also request a series of late-phase photographs between 10 and 15 min after fluorescein dye injection.

Indocyanine Green Technique

Dye injection for ICG angiography is performed in a similar fashion to fluorescein angiography. However, the photographer should be poised to record images almost immediately after injection

since the choroidal vasculature starts to fill even before the retinal vasculature. Mid-phase photographs are routinely taken at 5 and 10 min after injection while late-phase photographs are taken at the 30 min time point. In general, SLOs record the filling phases with greater temporal resolution than fundus camera systems. However, latephase image quality is better with fundus camera-based systems.

Fluorescein Angiographic

Interpretation

Within the intravascular compartment, approximately 80% of the fluorescein sodium is bound to serum proteins while the other 20% remains unbound; binding alters the absorption and emission spectra only slightly. Unbound fluorescein is freely diffusible, yet diffusion is normally restricted by the blood-retinal barrier, which is part of the blood-ocular barrier and an amalgam of two separate anatomical boundaries that restrict the passage of large molecules into the retina. The retinal pigment epithelium maintains the outer portion of the blood-retinal barrier while the inner blood-retinal barrier is comprised of a single layer of nonfenestrated endothelial cells, which have intercellular tight junctions.

The filling sequence in fluorescein angiography reflects the anatomy of the vasculature supplying the globe, choroid, and retina. Details of retinal circulation are readily visible due to the contrast provided by the underlying RPE. In addition, it fills from a central point and its presence is more or less confined to two dimensions. Initially, the large choroidal vessels begin to fluoresce, approximately 1–2 s prior to the first appearance of dye in the retinal arterioles. This initial phase of fluorescein filling the arterioles is referred to as the “arterial phase” or “early phase”. Although the dye moves swiftly through the retinal vessels, an experienced photographer may be able to capture the dye front in one of the early frames as it moves through the retinal arterioles and approaches the capillary bed. Filling of the capillary network introduces an abrupt increase in the retinal fluorescence. In the absence of

media opacities, individual perifoveal capillaries are readily apparent. In contrast, it is more difficult to distinguish fine vessels within the lobular choriocapillaris layer. Nevertheless, the choriocapillaris is better visualized on fluorescein angiography than on ICG angiography because the plane of focus for the photographer is shallower during fluorescein angiography.

Over the next few seconds in a healthy individual, the dye reaches the posterior capillary venules and rapidly progresses to fill the larger veins. In elderly patients, however, this arteriovenous transit time may be slightly increased and still considered physiologic. Eventually, the initial bolus of fluorescein dye leaves the ocular circulation, which causes the total fundus fluorescence to fade to some degree. Once the bolus of dye has traveled through the systemic circulation, however, it reappears in the ocular circulation and produces a corresponding increase in fundus fluorescence. This stage of the angiogram is referred to as the “recirculation phase” and occurs within about a minute after the fluorescein injection. Recirculation is readily apparent to the photographer or when reviewing videography from a scanning laser ophthalmoscope, but is less evident when reviewing still frames in isolation. Photographs recorded from 1 to 3 min after fluorescein injection are commonly referred to as mid-phase images. As the dye is cleared from the blood stream by the renal system, the total fluorescence gradually diminishes. This stage of the angiogram is referred to as the “late phase” and applies to images taken after the 5 min time point. In these frames, the fundus appears darker than after the initial dye injection.

The Macula

In fundus imaging, the macula has a characteristic appearance and fluorescence pattern that can be attributed its unique histologic features. Due to an abundance of xanthophyll pigment, which absorbs shorter wavelengths within the visible light spectrum, the macula normally exhibits a yellowish hue on fundus photography. On fluorescein angiography, the macula appears darker than the surrounding areas

because RPE cells in the macular region are taller and are packed with a higher concentration of melanin granules than anywhere else in the fundus, which leads to reduced transmission of fluorescent light from the underlying choroidal vessels. Furthermore, there is a physiologic absence of central fluorescence within the retinal capillary network corresponding to the foveal avascular zone.

Deviations from Normal Angiographic Appearance

Hyperfluorescence refers to an abnormally bright area on the fluorescein angiogram, where the fluorescence is in excess of what would be expected on a physiologic angiogram (Table 4.1). One reason for hyperfluorescence is simply when fluorescence is more readily visible. Normally, melanin pigment in the choroid and RPE layers and xanthophyll pigment in the macula block underlying choroidal fluorescence. Focal or generalized attenuation or loss of either of these two layers permits increased transmission and visibility of choroidal fluorescence referred to as transmission or window defects, which appear in the early phase of the angiogram. Another reason for hyperfluorescence is the excess accumulation of fluorescein dye either within vascular abnormalities of the optic disc, retina, or choroid, or within the extravascular space, which is typically observed in the mid to late phases. As an example, CNV demonstrates a network of abnormal vessels as well as a leakage of dye into the surrounding extravascular compartment (Fig. 4.6).

Normally about 45–60 s after injection, fluorescence of the retinal and choroidal vessels begins to wane. By the 10–15 min time point, fluorescein is completely emptied from the retinal and choroidal circulation. With exception to fluorescence of the lamina cribosa or a scleral crescent, any other late-phase hyperfluorescence represents extravascular fluorescein and is referred to as leakage. By convention, fluorescein leakage is further categorized based on its appearance. Fluorescein leakage that flows into welldemarcated anatomic or pathologic spaces is referred to as pooling, while leakage of dye that has diffused into surrounding tissue is called

Table 4.1 Fluorescein hyperfluorescence associated with age-related macular degeneration

Table 4.2 Fluorescein hypofluorescence associated with age-related macular degeneration

staining. Finally, although not usually associated with AMD, there are two other forms of abnormal late fluorescence worth mentioning; one occurs when the dye enters the vitreous and the other when it leaks into the optic nerve head.

There are two main causes of hypofluorescence. Either there is less fluorescein present within the vasculature than expected in a particular fundus region or there is something impeding our view of retinal or choroidal fluorescence (Table 4.2).

Indocyanine Green Angiographic Interpretation

The phases of ICG angiography are similar to those of fluorescein angiography, with the obvious addition of choroidal filling phases. Choroidal filling occurs largely in parallel to retinal vascular filling, yet they are slightly out of phase since the choroidal vasculature starts to fill first in the early phase of the angiogram. Since there is sequential filling of the short posterior ciliary arteries, a “watershed” effect may be seen. The transit phase begins with filling of the medium

Fig. 4.6 Classic choroidal neovascularization. (a) Digital color fundus photograph from an eye with exudative agerelated macular degeneration. (b) Mid-phase digital fluorescein angiogram shows appearance of a well-demarcated “classic” CNV. Fluorescence is blocked in the area of subretinal hemorrhage. (c) Late-phase fluorescein angiogram shows leakage of dye from the CNV. (d) OCT image dem-

onstrates a partially-detached posterior hyaloid, intraretinal cystic fluid, subretinal fluid, a subfoveal lesion underlying the RPE, with high reflectivity and shadowing of underlying choroidal structure, and a broader moderately-reflective lesion overlying the RPE (arrow), likely representing the “classic” component of the CNV. (e) OCT thickness map. (f) ETDRS OCT thickness map

choroidal arteries; the choriocapillaris remains dark during this phase. At the peak of the transit phase, the retinal vessels are filled with ICG dye and demonstrate venous laminar flow. The transition to the venous phase is more rapid in the

choroid than in the retinal circulation. As the choroidal veins fill during this middle phase, it may become increasingly difficult to distinguish the choroidal arteries as a diffuse, homogenous choroidal fluorescence is observed. It is during