Fluorescein angiography
Fluorescein dye ranges from yellow to orange-red in color, depending on its concentration. Its peak excitation is 465–490 nm and its peak emission is 520–530 nm in physiologic environments. Fluorescein is approximately 80% protein-bound in circulation; the blood–retina barrier prevents it from diffusing into retinal tissue. However, leakage can show in areas with new vessel growth, which lack a blood–ocular barrier, or regions with blood–ocular barrier defects induced by inflammation or ischemia. Fluorescein readily leaks from the choriocapillaris, staining the surrounding tissue. This rapid leakage, as well as the light absorption and scattering by the pigment in the RPE and choroid, prevents widespread use of fluorescein in choroidal imaging.
Typically, 2–3 mL of a 25% sterile solution or 5 mL of a 10% sterile solution is injected in the antecubital vein. The dye is typically visible within 12 to 14 seconds, rapidly filling the arterial system (Fig 2-7). It is possible to detect dye in the choroidal circulation before the retinal arteries fill; in the eyes of young, healthy patients, the arteries fill in 1 or 2 seconds, and dye begins to appear in a laminar filling pattern in the retinal veins. The choroid may not fill uniformly. Any areas in the choroid that do not fill by the time the retinal circulation reaches the laminar flow stage are considered signs of abnormal choroidal filling. Once dye reaches the choriocapillaris, it leaks and stains Bruch membrane and the stroma, and details in the choroid are lost. If the dye injection was rapid, the bolus of dye will enter and leave the ocular circulation only to return a few seconds later; this is called the recirculation phase. During the fluorescein angiogram, the fovea appears darker than the surrounding areas because of the presence of macular pigment; the RPE cells under the macula are slightly taller and contain more melanin than peripheral RPE cells, and there are no retinal vessels. Over several minutes, the dye is removed from circulation and the intensity of the fluorescence decreases.
Abnormalities observed with FA can be grouped into 3 categories, each associated with one of the following types of fluorescence:
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autofluorescence
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hypofluorescence
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hyperfluorescence
Autofluorescence is fluorescence that appears with the excitation and barrier filters in place before the fluorescein dye is injected; it is caused by endogenously fluorescent constituents of tissue such as accumulation of outer segments, lipofuscin, or optic nerve head drusen. Hypofluorescence occurs when normal fluorescence is reduced or absent; it is present in 2 major patterns:
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vascular filling defects
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blocked fluorescence
Vascular filling defects are defects in which retinal or choroidal vessels fail to fill because of an intravascular obstruction that results in nonperfusion of an artery, vein, or capillary. These defects appear as either a delay in or complete absence of filling of the involved vessels. Blocked fluorescence occurs when the stimulation or visualization of the fluorescein is obstructed by fibrous tissue, pigment, or blood that blocks normal retinal or choroidal fluorescence in the area. The depth of a lesion can be easily determined by relating the level of the blocked fluorescence to details of the retinal circulation. For example, if lesions block the choroidal circulation, but retinal vessels are present on top of this blocking defect, then the lesions are located above the choroid and below the retinal vessels.
Hyperfluorescence occurs when the fluorescence is abnormally excessive, typically extending beyond the borders of recognized structures; this manifests in a few major patterns:
Leakage, which is a gradual, marked increase in fluorescence over the course of the study, results from the seepage of fluorescein molecules across the blood–retina barrier. When the outer blood–retina barrier is compromised, the dye traverses across the pigment epithelium into the subretinal space or neurosensory retina. When the inner blood–retina barrier is compromised, the dye leaks through vascular walls into the retinal parenchyma, fibrotic tissue, and cystoid spaces. Dye can also leak through the posterior blood–retina barrier (the RPE) and accumulate in the subretinal space, fibrotic tissue, or directly into the retina, if the external limiting membrane of the retina is compromised.
Staining refers to a pattern of hyperfluorescence in which the fluorescence increases in intensity through transit views, and persists in late views, but in which the borders remain intact throughout the study. Staining results from fluorescein entry into a solid tissue or material that retains the fluorescein, such as a scar, drusen, optic nerve tissue, or sclera.
Pooling refers to the accumulation of fluorescein in a fluid-filled space in the retina or choroid (Fig 2-8). As fluorescein leaks into the space, the margins of the space trap the fluorescein and appear distinct, for example, as seen in an RPE detachment in central serous chorioretinopathy.
A transmission defect, or window defect, refers to a view of the normal choroidal fluorescence through a defect in the pigment of the RPE. In a transmission defect, hyperfluorescence occurs early, corresponding to filling of the choroidal circulation, and reaches its greatest intensity with the peak of choroidal filling. The fluorescence does not increase in intensity or shape and usually fades in the late phases as the choroidal fluorescence becomes diluted by blood that does not contain fluorescein. The fluorescein remains in the choroid and does not enter the retina.
Multiple defects may be present in a diseased eye. For example, in an elderly patient with CNV, the choroid will often show segmental filling delays; there will be hyperfluorescence in the fovea because of the proliferation of vessels that leak; and there will be late leakage from CNV (Fig 2-9).
Fluorescein angiography has traditionally been imaged with a fundus camera or an SLO that had a field of view up to about 50°. There are also wide-angle scanning laser systems that are able to image most of the fundus, including the periphery (Fig 2-10).
Adverse effects of fluorescein angiography All patients injected with fluorescein experience a temporary yellowing of the skin and conjunctiva that lasts 6–12 hours. The most common adverse effects include nausea and vomiting (in approximately 5% of injections) and the development of hives (also in approximately 5% of injections). The nausea will pass in a few seconds without treatment. Hives, unless very mild, are usually treated with diphenhydramine. More serious adverse effects such as hypotension, shock, laryngeal spasm, or even death have occurred, but only in rare instances. Prior urticarial reactions increase a patient’s risk of having a similar reaction after subsequent injections; however, premedicating the individual with antihistamines, corticosteroids, or both appears to decrease the risk. Extravasation of the dye into the skin during injection can be painful, requiring application of ice-cold compresses to the affected area for 5–10 minutes. Close follow-up of the patient over hours or days until the edema, pain, and redness resolve is advised. Although teratogenic effects have not been identified, many ophthalmologists avoid using FA in pregnant women in the first trimester unless absolutely necessary. In lactating women, fluorescein is transmitted to breast milk.
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Berkow JW, Flower RW, Orth DH, Kelley JS. Fluorescein and Indocyanine Green Angiography: Technique and Interpretation. 2nd ed. Ophthalmology Monograph 5. San Francisco: American Academy of Ophthalmology; 1997.
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Kwiterovich KA, Maguire MG, Murphy RP, et al. Frequency of adverse systemic reactions after fluorescein angiography. Results of a prospective study. Ophthalmology. 1991;98(7): 1139–1142.
Indocyanine green angiography
Indocyanine green (ICG) is a water-soluble, tricarbocyanine dye that is almost completely protein-bound (98%) after intravenous injection. Because the dye is protein-bound, diffusion through the small fenestrations of the choriocapillaris is limited. The intravascular retention of ICG, coupled with low permeability, makes ICG angiography ideal for imaging choroidal vessels. ICG is metabolized in the liver and excreted into the bile. Both the excitation (790–805 nm) and emission peak (825–835 nm) are in the near-infrared range. Because its fluorescence efficacy is low, practical ICG angiography can only be performed with digital sensors.
ICG angiography is used to image polypoidal choroidal vasculopathy, which is a common form of choroidal neovascularization (Fig 2-11), and to provide important information about the pathophysiology of type 3 neovascularization (also known as retinal angiomatous proliferation). With the use of ICG angiography, it was discovered that patients with drusen could have asymptomatic choroidal neovascularization. ICG angiography may also be used to help diagnose inflammatory diseases such as birdshot chorioretinopathy, multifocal choroiditis, and panuveitis. Eyes with central serous chorioretinopathy show multifocal areas of choroidal vascular hyperpermeability when visualized with ICG angiography. However, the use of ICG angiography to diagnose uveitis or central serous chorioretinopathy has been mostly supplanted by the use of a combination of autofluorescence imaging with enhanced depth imaging (EDI) OCT.
Adverse effects of indocyanine green angiography Mild adverse events occur in fewer than 1% of patients. ICG is dissolved in a 5% sodium iodide (an additive used in table salt) solution. There is no reason to suspect that a shellfish allergy should preclude the use of ICG. However, angiographic facilities should have emergency plans and establish protocols to manage complications associated with either fluorescein or ICG administration, including anaphylaxis. ICG may persist in the blood longer in patients with liver disease.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.