Retinal angiography allows the physician both to analyze intraretinal circulation and to define more completely the interrelationships between the various layers of the retina, RPE complex, and choroid. Angiography is not only documentary and supportive of our clinical differential diagnosis but also enables us to make reasoned therapeutic decisions, particularly in the treatment of ARMD and diabetic maculopathy. Although we may suspect a particular clinical diagnosis, angiography allows us to confirm the diagnosis and retain a "hard copy" for analysis throughout the course of the disease. Accurate analysis and meaningful diagnostic conclusions result from the recognition of several important phenomena that occur during the course of an angiographic study.
A standard protocol should be followed when interpreting a fluorescein angiogram. This section begins by describing the standard phases of a normal retinal angiogram, then continues with a suggested protocol for evaluating the test.
Sodium fluorescein is a fluorescent dye compound that can be administered intravenously or orally. When stimulated by the "exciting" light of the fundus camera, it emits yellow-green light, which is captured either with film or with digital imaging. In traditional fluorescein angiography, 5 ml of 10% sodium fluorescein is injected into the antecubital vein in a 3- to 7-second bolus. Mild, moderate, and severe reactions to the injection of sodium fluorescein have been described.13 The dye flows to the heart and lungs and back through the heart before entering the postequatorial choroidal circulation approximately 10-15 seconds later via the short posterior ciliary arteries. This initiates the transit phase of the angiogram. The transit phase (10-30 seconds postinjection) records the first passage of dye as it flows through the ocular blood vessels and into the ocular tissue.
The earliest stage of dye filling is called the prearterial phase (Figure 1-8). Dye diffuses throughout the choroid and into the choriocapillaris, where it appears as "choroidal flush" or "irregular background fluorescence." The RPE functions as a filter and screens out much of the choroidal fluorescence, especially in the macula, where RPE pigmentation is most concentrated. The choroidal pattern is also poorly visualized due to rapid diffusion of dye into the choriocapillaris. Macular xanthophyll pigment blocks part of the choroidal fluorescence, as doeschoroidal melanin. This may vary with the pigmentation of the patient. Cilioretinal arteries usually fill with dye concurrently with the choroid.
FIGURE 1-8. The characteristic phases in a normal fluorescein angiogram. Green filter (red-free) photographs document the clinical ophthalmoscopic appearance of the retina. A and B represent preinjection photographs; C–E are the transit (early) phase of the angiogram; and F–I are the recirculation ("mid" and "late") phase. A. (preinjection) With exciter and barrier in place, the featureless black control photograph reveals either the presence or absence of auto- and pseudofluorescence. B. (0 secs after injection) The tran-sit phase begins with choroidal filling. C. (10 secs) Described as patchy, the transit phase is simultaneous with the filling of cilio-retinal arteries. D. (12 secs) The retinal arteries are infused. E. (15 secs) The dye returns via the retinal veins. Note the laminar flow during the arteriovenous phase (inset). F. (about 30 secs) The angiogram is brightest and microvasculature most visible. G–I. (5 and 10 mins) As the fluorescein dye diffuses through the tissue, contrast decreases and the optic nerve head stains.
One second later the arterial phase begins as the retinal arteries fill. The veins are still devoid of dye, standing in black relief against the muted, diffusely fluorescent choroid.
During the venous stage, the veins completely fill with dye while the arteries begin to empty. As dye moves through the retinal vasculature, the arteriovenous phase shows laminar flow in the venous system. Laminar flow refers to the pattern of filling in the outer region of the blood column, within the major retinal veins. This pattern appears as fluorescein flows from the venules into the peripheral cuff of blood plasma near the edge of the veins, rather than directly into the central venous blood column. This makes the vessel appear more fluorescent at its margin than at the center. Most notable in the arteriovenous phase are the fine vascular details of the macular arterioles, the pattern of the foveal avascular zone, and the diffuse background fluorescence from the choroid.
As the blood leaves the venous system, it begins to recirculate through the retinal vasculature. Mid- (3-5 minutes) and late (10-15 minutes) recirculation phase photographs document the dye's diffusion into normal tissue and its leaking from, pooling within, or staining of abnormal tissue.
The recirculation phase photographs provide a less distinct picture of the retinal vascular pattern due to the staining of the retinal and choroidal vasculature and the diffusion of dye through the retinal tissues. In general, the first complete transit of fluorescein through the eye reveals the most distinct definition of retinal pathology.
The intensity of the dye as it moves through the retinal vasculature defines the baseline against which the terms hypofluorescence and hyperfluorescence are evaluated. Pathologic lesions seen on fluorescein angiography can be defined as hypofluorescent if they appear less brilliant on angiography than the normal background fluorescence of the fundus, and hyperfluorescent if they appear brighter than background structures. (Note: This nomenclature describes a "positive" angiogram in which the dye is visualized as white against an unilluminated [black] background. If original photographic negatives are evaluated, fluorescein dye appears black against a light background.)
Hypofluorescent lesions may be associated with decreased fluorescence due to blocking or obscuration of the fluorescent pattern, as with subretinal blood or scar tissue. Alternatively, hypofluorescence may be the result of poor filling of an area of retinal vasculature. For example, in the capillary nonperfusion associated with diabetic retinopathy or retinal vein occlusion, a decreased amount of fluorescein dye is present for providing the baseline level of fluorescence.
Variations in the patterns of retinal and choroidal disease become more visible when photographed angiographically. In particular, breakdown of the blood—inner retina barrier and blood—outer retina barrier may be diagnostic in a variety of disease states. If leakage from a breach in the tight junctions of the retinal vessel endothelium is observed, then there is damage to the vessel wall and a corresponding loss of physiologic balance in the osmotic gradient between the intra- and extravascular tissue compartments. Defects in the RPE barrier will be seen as leakage points or focal areas of hyperfluorescence and imply that fluid from the choriocapillaris and choroid may have entered the subretinal space. Choroidal vessels in an abnormal location (i.e., in the subretinal space anterior to the RPE) allow dye to leak from the choroidal intravascular space, causing relative hyperfluorescence. Abnormal vessels with non—tight junction endothelial cell morphology, such as neovascular blood vessels associated with diabetes or venous occlusion, leak dye or hyperfluorescence into the vitreous cavity.
As dye circulates, a variety of fundus lesions may absorb and retain it. Drusen or fibrovascular scar tissue will gradually stain, as will vessel walls that have been damaged or have lost a portion of their tight junction endothelial surfaces. Tissue staining occurs as the components of the vessel wall retain dye to which they would not be exposed with an intact vascular channel lining. Fluorescein dye may also exhibit confluence or pooling in areas of separation of potential tissue planes, as in focal detachments of the RPE, or of real tissue planes, as in detachment of the sensory retina.
Fluorescence before injection of dye may be termed autofluorescence or pseudofluorescence. Autofluorescence describes the natural fluorescent "glow" from structures internal to the eye, which occurs with both the exciter and barrier filters inserted but in the absence of sodium fluorescein dye. Autofluorescence is associated with optic nerve head drusen and astrocytic hamartomata. Pseudo-fluorescence describes lightened images from an inefficient filter system or an unbalanced exposure/ development system. It is also seen when the exciter and barrier filters areinserted (again, before dye) and may distort tonal information by increasing the intensity of the highlights in the final photograph.13 It may be seen at the margin of bright scar tissue and is particularly notable in large disciform fibrovascular scars associated with ARMD.
So-called window defects represent transmission of fluorescence from the choroidal and choriocapillaris ves-sels through the RPE. The RPE functions as a screen or filter for background fluorescence that is emitted from the choroidal and choriocapillaris vasculature. If this screen is damaged or lost in a specific area, more dye will appear to be fluorescing in the zone of pigment loss, which will seem slightly hyperfluorescent relative to the rest of the fundus.
Successful interpretation of ocular angiography depends on a knowledge of the typical patterns of dye hyper- and hypofluorescence that may be expected in each angiographic phase (Figure 1-9).
FIGURE 1-9. Patterns in angiography. Common patterns of dye distribution and/or leakage seen angio-graphically correspond in general to those observed ophthalmoscopically, although they may demonstrate both subtle and striking variations. A. Focal macular leakage (i.e., choroidal neovascular membrane). B. Bean-shaped blister, small size (i.e., retinal pig-ment epithelium detachment). C. Circumscribed blis-ter, moderate size (i.e., central serous retinopathy). D. Macular, diffuse, irregular border (i.e., clinically significant macular edema). E. Petaloid, circumscribed (i.e., cystoid macular edema). F. Multifocal, small macular (i.e., microaneurysm, drusen). G. Wedge, nerve fiber layer, vascular bed (i.e., branch vein occlusion). H. Quadrantic from optic nerve, vascular bed (i.e., branch artery occlusion). I. Hemispheric (i.e., hemivein occlusion). J. Posterior pole, geographic (i.e., serpiginous choriditis). K. Mid-peripheral, geographic, moderate to large (i.e., acute retinal necrosis). L. Neovascular fronds (neo-vascularization of the disc [NVD], neovascularization elsewhere [NVE]), (i.e., diabetic NVD/NVE, Eales NVE, sickle cell NVE). M. Perivascular (i.e., uveitis). N. Midperipheral, geographic, small (i.e., acute posterior multifocal placoid pigment epitheliopathy). 0. Bullous, half-spherical (i.e., tumor, retinal detachment, choroidal detachment). P. Small, multifocal, postpole and periphery (i.e., presumed ocular histoplamosis syndrome). Q. Peripapillary, concentric (i.e., choroidal rupture). R. "Watershed" effect (ICG pattern of sequential normal choroidal vascular filling).
The dynamic process of fluorescein angiography provides information about vascular perfusion of the optic nerve and retina, as well as the integrity of the individual tissues. Observation of temporal changes in dye flow are very important in the formulation of diagnostic conclusions (Figure 1-10).
FIGURE 1-10. Temporal patterns in angiogra-phy. Recognizing the temporal pattern of pro-gressive dye leakage, pooling, or staining is essential for accurate diagnostic conclusions. A. Represents rapidly increasing fluorescence of a small circumscribed lesion, most typical of choroidal neovascularization without an over-lying retinal pigment epithelium (RPE) or sensory retinal detachment. B. Represents increasing fluorescence visible in a circumscribed lesion that appears to grow larger through the course of the study. Central serous retinopathy is the most common example of this pattern, with dye eventually diffusing into the blister-like sensory retinal detachment overlying the inciting RPE leak(s). C. Multifocal small macular staining lesions, which do not brightly fluoresce, are typical of drusen in age-related macular degeneration. D. The accumulation of a petaloid, muted dye pattern slowly becomes prominent in the recirculation phases of a macular angiogram pathognomonic of cystoid macular edema. E. Neovascular, frond-like, preretinal vessels have incompetent endothelial cell junctions and hence leak fluorescein dye exuberantly, beginning in the early to mid-transit phase, and becoming increasingly hyperfluorescent throughout the study. They resemble fuzzy seaweed or lightbulbs in configuration, as opposed to intraretinal micro-vascular abnormalities, which do not leak fluorescein dye and hence do not hypofluoresce.
Angiographic interpretation should be performed with original black and white negatives; high-quality, first-generation film contact prints; or the large, high-resolution screen images of digital angiograms.
The standard interpretive protocol is initiated by reviewing a series of color photographs. Stereo photography documents the configuration of the optic cup and allows an estimate of the cup-to-disc ratio. The papillo-macular bundle, macula, retinal vessels, and mid- and far periphery are scanned in sequence. The person scanning should be alert in particular for possible macular anomalies, vascular caliber, configuration abnormalities such as arteriovenous crossing changes, and common abnormalities of the peripheral retina such as lattice degeneration, retinal holes or tears, and retinoschisis.
The fluorescein angiogram is then reviewed by noting the green (red-free) filter photographs of both eyes. The eyes are compared for symmetry; the cup-to-disc ratio is noted; evidence of any NFL defects is recorded; and the gross structure of the macula is reviewed, as well as the retinal vascular tree. The green photograph can also represent a "quick" color photograph, as color photographs frequently are not available for review immediately after the angiogram. The green photograph documents patterns of lipid deposition and superficial or intraretinal hemorrhage, which may not be visible on the fluorescein angiogram. The green photograph also provides baseline documentation of the pattern of the retinal vasculature in the macula. The retinal vasculature may be obscured by dye leakage during the course of the fluorescein study; if so, the green filter photograph provides an excellent image for mapping the precise location of CNV relative to the retinal vasculature when a transit-phase fluorescein angiogram frame defining the CNVM is superimposed on it electronically.
After review of the green filter photographs, and with the knowledge that the fluorescein dye has been injected into an antecubital vein at time zero, one expects to see dye perfusion of the choroid within 10-15 seconds. This is roughly analogous to the arm-to-tongue circulation time. At approximately 10 seconds, the choroid begins to fill with dye. Rather patchy, irregular filling of various choroidal zones becomes apparent, concurrent with filling of cil-ioretinal arteries at the temporal margin of the optic nerve. Cilioretinal arteries may be single or double and are seen to fill in 20% of the population.14 The pattern of the arterial vasculature is easily seen at this stage, and recognition of anomalous branching patterns or abnormalities of the arteriovenous crossing points is facilitated. Arteriovenous crossing points should be reviewed approximately 1 DD from the optic nerve for evidence of hypertensive or arteriosclerotic vasculopathy. Additionally, the initial fuzzy, acute hyperfluorescence of preretinal neovascularization and NVE may first become apparent at this stage.
The transit phase of the fluorescein angiogram occurs between 10 seconds and approximately 30 seconds. This is the most critical phase for evaluation of the acute hyperfluorescence associated with CNV. The peripapillary and central macular areas are scanned for evidence of fluorescence increasing much more carefully than surrounding tissues. The acute hyperfluorescence of a CNVM appears more intensely white on positive imagery and deeper gray on negative imagery, especially when compared to drusen, which show a faint early fluorescence at this stage. Recognition of this difference in intensity is the most important diagnostic skill to be acquired in interpreting fluorescein angiography, as ARMD is currently the primary cause of central visual loss in patients over age 60 in the developed world. Evaluation of post-treatment photocoagulation margins requires recognition of the degree of dye accumulation or late staining that is acceptable in pronouncing a lesion adequately or inadequately treated. Typically, a small to moderate amount of central pooling of dye within the treated lesion is acceptable in the late recirculation phases, but experience is required to make this judgment.
The macula is screened in the transit phase for capillary nonperfusion in the presence of diabetic retinopathy. Poor perfusion of the border of the foveal avascular zone may indicate against focal macular laser treatment, if perfusion is so limited that central vision has been permanently damaged. Focal laser photocoagulation of diabetic maculopathy is contraindicated when profound loss of juxtafoveal, parafoveal, or perifoveal capillary network is evident.
Another important diagnostic skill is the recognition of the lightbulb -like exuberant focal hyperfluorescence associated with retinal neovascularization in diabetic retinopathy. Incompetent neovascular blood vessels willleak dye shortly after the initial perfusion of the arterial system if these blood vessels emanate from the capillary arteriolar bed. Early in the recirculation phase, the ophthalmic photographer screens the retinal periphery in all four quadrants in both eyes, scanning for obvious pathologies such as peripheral NVE in diabetic retinopathy. A good photographer will also document other fundus anomalies, such as choroidal nevi, mass lesions, peripheral retinal pathology such as lattice degeneration, and nonspecific-appearing idiopathic scars.
Recirculation photographs (typically 1-, 3-, 5-, and 10-minute postinjection photographs) of each macula are then scanned to review the leakage pattern evident at dif-ferent points in time. For example, CNV typically shows profound late recirculation leakage from the area of an active CNVM. Treated CNV may show a residual puddle of dye accumulation in the center of the treated area but should not reveal a hyperfluorescent margin in an adequately treated lesion. Diabetic neovascularization, which may have leaked in a sluggish fashion early in the study, show definite, gross hyperfluorescence during the recirculation phase. Diffuse diabetic macular edema becomes much more prominent in later photographs, as does post-cataract extraction cystoid macular edema.
Fellow eyes should always be photographed, as many disease processes are bilateral. Additionally, the differential diagnoses of acquired retinal diseases versus inherited dystrophic disorders may be more confidently made when comparing one eye to the other.
Finally, the entire angiogram is screened again and reviewed for the presence of treatable retinal vascular pathology, as defined by treatment criteria of a variety of multicenter clinical trials. For example, an active, well-defined CNVM outside the margin of the foveal avascular zone would be judged to be a treatable lesion, whereas a poorly defined area of hyperfluorescence that underlies a dense RPE detachment obscuring the foveal avascular zone would not.