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FLUORESCEIN ANGIOGRAPHY
Properties
Fluorescein angiography was first attempted by MacLean and Maumenee in 1960 (10), but it was not until the advent of the electronic flash that Novotny and Alvis were able to perform the first successful fluorescein angiogram (11). This procedure has since been instrumental to our knowledge and treatment of chorioretinal diseases.
Sodium fluorescein is a yellow–red dye with a molecular weight of 376.67 kDa, a spectrum of absorption at 465–490 nm (blue), and excitation at 520–530 nm (yellow– green) (12). The dye, either 2–3 ml of 25% concentration or 5 ml of 10% concentration, is injected as a bolus into a peripheral vein. Once injected, 80% of the dye binds with plasma proteins, particularly albumin. It is metabolized by the liver and kidney within 24–36 h and is eliminated in the urine. Under normal conditions, fluorescein is retained within the capillary walls due to the tight blood–retinal barrier. Conditions leading to the breakdown of the blood–retinal barrier lead to the leakage of fluorescein into the retina and vitreous.
Side Effects
The most common side effect of fluorescein is the temporary yellowing of the skin and conjunctiva lasting up to 12 h after injection, as well as an orange–yellow discoloration of the urine that lasts from 24 to 36 h (13, 14). Other side effects include nausea, vomiting, or vasovagal reaction, which occurs in approximately 10% of patients. Severe vasovagal reactions resulting in bradycardia and hypotension are rare. Dye extravasation may cause pain, local tissue necrosis, subcutaneous granuloma, or toxic neuritis, although these are rare. Urticarial reactions occur in about 1% of cases, and can be avoided by premedicating the patient with antihistamines and/or corticosteroids. True anaphylaxis occurs in less than 1 in 100,000 cases. Although no teratogenic effects have been identified, the use of fluorescein in pregnant or lactating women in general should be avoided unless absolutely necessary.
Normal Fluorescein Angiography
A choroidal flush and optic nerve head fluorescence appear in 10–15 s after injection of dye (arm-to-eye circulation time) (Fig. 1a) (15). In 10–15% of patients, a cilioretinal artery stemming from the choroidal circulation is present and will fluoresce simultaneously with the choroid. Since choroidal vessels are fenestrated, fluorescein molecules diffuse out of the choriocapillaris, giving the appearance of generalized choroidal fluorescence, which may be mottled or patchy due to the overlying retinal pigment epithelium.
Unlike choroidal vessels, normal retinal vessels and capillaries are impermeable to fluorescein due to endothelial tight junctions. The path of the dye as it travels through the retinal vasculature is therefore quite demarcated. Fluorescein filling of retinal arteries begins approximately 1 s after choroidal fluorescence (Fig. 1a). The arteriovenous phase is characterized by complete filling of the arteries and capillaries, with laminar filling of the veins (Fig. 1b). This has been attributed to the faster flow of blood as well
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Fig. 1. Normal fluorescein angiography. (a) Choroidal filling is followed by arterial filling. (b) The arteriovenous phase is characterized by appearance of dye in a laminar pattern in the retinal veins. (c) The recirculation phase demonstrates declining fluorescence. (d) Late frames show staining of the disc, choroid, and Bruch’s membrane.
as a higher concentration of erythrocytes in the central venous lumen. By 30 s, the first pass, or transit phase, of fluorescein through the retinal and choroidal vasculature is complete (Fig. 1c). This is followed by recirculation phases where there is intermittent mild fluorescence. At 10 min, both circulations are generally devoid of fluorescein. The late angiogram is characterized by staining of Bruch’s membrane, the choroid, sclera, and margins of the optic nerve head (Fig. 1d).
A dark background in the macula is created by blockage of choroidal fluorescence by xanthophyll pigment and a high density of retinal pigment epithelial cells. The normal capillary free zone or foveal avascular zone (FAZ) is 300–500 m.
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Terminology
Several terms are commonly used to describe fluorescence abnormalities that aid in clinical correlation (15, 16). Angiographic lesions may be hypofluorescent or hyperfluorescent. Hypofluorescence can be categorized into blockage (masking of fluorescence) such as with blood, or vascular filling defect due to deficient circulation, as in macular ischemia. Hyperfluorescence is caused by an increase in normal fluorescence or presence of abnormal fluorescence. Autofluorescence is seen in preinjection photographs and is caused by highly reflective substances such as optic disc drusen. Transmission window defects occur due to a decrease or absence of the retinal pigment epithelium, and appear as sharply defined hyperfluorescence that appears early and does not change through the angiograms. Leakage refers to the gradual increase in fluorescence throughout the angiogram due to fluorescein diffusing through the RPE into the subretinal space or neurosensory retina, out of retinal vessels into the retinal interstitium, or from retinal neovascularization into the vitreous. The borders of hyperfluorescence become increasingly blurred, and the greatest intensity occurs in the late frames of the angiogram. Staining results from fluorescein entry into a solid tissue that retains the dye, and appears as fluorescence that gradually increases in intensity through transit views and persists in late views, but its borders remain fixed throughout the angiogram. Pooling refers to the accumulation of fluorescein in a fluid-filled space in the retina or choroid with distinct margins.
Fluorescein Angiography in the Evaluation of Diabetic Retinopathy
FA has provided great understanding of the microvascular changes caused by diabetes. In diabetic retinopathy, endothelial tight junctions are destroyed, so that fluorescein can diffuse out of retinal vessels. The development of microaneurysms and increased capillary permeability are the earliest detectable changes (Fig. 2) (17–19). These can often be
Fig. 2. Fluorescein angiography of background diabetic retinopathy is characterized by blocking defects appearing as local hypofluorescence and corresponding to intraretinal blood, and by small, round, or fusiform areas of hyperfluorescent corresponding to microaneurysms.
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visualized on FA prior to being detected by funduscopic examination. The microaneurysms are predominantly on the venous side of the capillary bed. Microaneurysms may be round or fusiform, and scattered in the macular and perimacular regions, with no particular relationship to the distribution of the major retinal vessels. The dot and blot hemorrhages characteristic of DR block out fluorescence locally. Extensive macular microaneurysms may be seen without significant loss of visual acuity.
Focal areas of capillary closure may develop within the capillary bed affected by marked aneurysmal formation (20, 21). Capillary closure occurs much more frequently and to a greater extent initially in the midperipheral fundus and generally increases toward the periphery (Fig. 3) (22). Extensive midperipheral and peripheral capillary closure may not be apparent ophthalmoscopically. When nonperfusion results in deformation of the outline of the FAZ, it is termed macular ischemia (Fig. 4). Some enlargement of the FAZ occurs commonly in diabetes, but is usually not associated with visual loss until the FAZ approaches 1,000 m in diameter (17, 23–26). Dilated, tortuous, shunt capillaries may be evident in the ischemic peripheral retina. There is typically no angiographic evidence of choroidal vascular disease.
Neovascular proliferation is characterized by dye leakage into the vitreous (Fig. 5). Retinal neovascularization is often first seen at the junction of nonischemic and ischemic retina (18, 27). Optic disc neovascularization is a reflection of widespread capillary nonperfusion. The new blood vessels on the optic disc tend to fill before the normal retinal arteries, suggesting that the choroid may be the source of blood for new vessels.
Fig. 3. Peripheral capillary nonperfusion appearing as hypofluorescence due to vascular filling defects. Adjacent to the zone of ischemia are areas of hyperfluorescence representing microaneurysms and leaking vessels.