Fig. 4.4 Drusen with pigment epithelial atrophy. (a) Digital color fundus photograph from an eye with drusen and central RPE atrophy. (b) A mid-phase SLO fluorescein angiogram shows hyperfluorescence of the drusen. (c) OCT image demonstrates a focal area of RPE atrophy and corresponding increased signal penetration into the

choriocapillaris. Focal elevations of the RPE represent drusen. Intraretinal hyper-reflective lesions likely represent intraretinal migration of RPE pigment (arrow). (d) Enhanced depth imaging OCT allows clearer visualization of the outer retinal layers and choriocapillaris

structures in the eye are no longer dark in this inverted image owing to a differential in the sensitivity. Clinical applications of EDI-OCT continue to expand and it may add additional information to the diagnosis and analysis of AMD.

Fundus Angiography

Fundus angiography encompasses the diagnostic application of two different dyes that are administered intravenously: fluorescein sodium, which

Fig. 4.5 Occult choroidal neovascularization. (a) Digital color fundus photograph from an eye with exudative agerelated macular degeneration. (b) OCT image shows an elevation of the sub-foveal RPE (arrow) by a fibrovascular membrane. There is a pocket of associated subretinal

fluid. (c) SLO mid-phase fluorescein angiogram shows mottled hyperfluorescence without the appearance of well-defined “classic” CNV. (d) SLO ICG angiogram shows a more-clearly demarcated area of hyperfluorescence representing CNV

enables a detailed assessment of the retinal vasculature, and indocyanine green (ICG), which due to its more highly protein-bound nature is more useful for studying the choroidal circulation [1, 2]. Fluorescein angiography preceded ICG angiography and accordingly, its correlation to histopathologic findings is better understood in comparison with ICGA angiography. In addition, the number of clinical indications for its use exceeds that of ICG angiography. Indeed, the clinical significance of many ICG angiographic findings remains either unclear or undefined. Nevertheless, ICG angiography may prove to be helpful when attempting to differentiate neovascular AMD from its various masqueraders, including polypoidal choroidal vasculopathy and

central serous chorioretinopathy. Furthermore, it has contributed greatly to our understanding about the pathophysiology of different types of CNV, particularly poorly defined or occult CNV, which may be imaged inadequately by fluorescein angiography (Fig. 4.5) [30].

Fluorescein Dye Characteristics

Fluorescein sodium is a highly water soluble dye with a molecular weight of 376 that is rapidly metabolized to fluorescein monoglucuronide and eliminated via renal excretion. The fluorescein molecule achieves a higher energy state following exposure to blue-green light that is in the range of

465–490 nm, which is the absorption peak of fluorescein excitation. Upon returning to the lower energy state, emission of a longer wavelength (520–530 nm) occurs for every quantum of stimulating light and corresponds to a yellowgreen color. Traditional fundus cameras possess two interference bandpass filters, which block out all light except for a specific wavelength. An excitation filter permits imaging blue-green light to pass through and a barrier filter that blocks most of the reflected blue light while allowing yellow-green wavelengths to return unimpeded to the fundus camera’s detection system, either 35 mm film or a digital CCD. Little extraneous light is able to reach the detection system because the excitation and barrier filters have minimal overlap with regard to wavelength transmission.

However, it is this small mismatch in the filters that is responsible for the phenomenon of pseudofluorescence, in which nonfluorescent light is imaged. Certain material in the fundus such as hard exudates reflect a large proportion of the excitation light. Transmission of some of this reflected light through the barrier filter is what generates this false fluorescence, which is now largely avoided due to the utilization of modern filters. Nevertheless, after several years of use, these filters will allow increasing degrees of pseudofluorescence as they wear thin. Other substances in the fundus such as lipofuscin and optic nerve head drusen actually fluoresce upon exposure to the excitation light. This distinct phenomenon, referred to as autofluorescence, is captured prior to the injection of any fluorescein dye.

Indocyanine Green Dye Characteristics

ICG, or tricarbocyanine, is a dye that binds tightly to serum proteins, confining it to the intravascular compartment until it is cleared by the liver. Only 60–80% of fluorescein is protein bound as compared with 98% of ICG. ICG is characterized by a peak spectral absorption in the near infrared range of the light spectrum of approximately 800 nm. Emission of fluorescent light in the range of 825–835 nm occurs during its decay following excitation. ICG angiography camera systems incorporate a barrier filter which

blocks reflected light shorter than 825 nm. Light absorption and fluorescence occur within the near infrared spectrum, which allows ICG to provide better visualization through the RPE, shallow hemorrhages, lipid, and serosanguinous fluid. The longer wavelengths involved in ICG angiography as compared with fluorescein angiography convey less extensive scattering and greater penetrance through overlying pigment. Collectively, these attributes permit ICG angiography to deliver enhanced visualization of the choroidal vasculature.

Cameras and Angiography

Cameras that are used in fluorescein angiography are based on either one of two fundamentally different design principles: the scanning laser ophthalmoscope or the fundus camera. As mentioned earlier, SLOs image the fundus by sweeping focused 488 nm laser light across the fundus in a raster pattern and are capable of reconstructing a three-dimensional structure from the acquired raster images. A key feature of the SLO pertains to its application of confocal imaging, which means that only light from a conjugate plane of interest is detected by the image sensor. More specifically, use of point illumination and a spatial pinhole act to eliminate out-of-focus light from tissue planes that are outside the singular plane of interest [31]. Implementation of this optical imaging technique enables SLOs to deliver enhanced optical resolution and the contrast of vascular structures in particular. Another significant advantage of scanning laser systems is the ability to capture images at a high frame rate and in real time. In patients with neovascular AMD, this capability provides a detailed assessment of the microvasculature that feeds and drains a focus of neovascularization during the filling phase, with potential therapeutic implications. Indeed, the selective ablation of feeder vessels of choroidal neovascular membranes with ICG angiography-guided laser photocoagulation appears to be a viable treatment strategy in those patients with extrafoveal feeder vessels [32].

The alternative to the SLO is a fundus camerabased system. Modern fundus cameras utilize a

xenon flash to discharge a bank of capacitors and a digital imaging sensor to record the resultant photographs. Images acquired by digital fundus camera systems typically have a higher pixel density and less noise than those obtained with the SLO. However, a digital fundus camera system has some limitations. In particular, the frame rate is limited by the speed at which the system reads and resets the CCD sensor, recharges its capacitors, and writes data to the internal memory cache. As a result, most digital fundus cameras are unable to capture images at more than one or two frames per second. In addition, unlike SLOs, fundus cameras do not have confocal imaging, which means that every source of fluorescence within the optical pathway is imaged. This phenomenon may adversely impact image contrast in comparison with SLO-based systems.

Patient Consent and Instruction

Fluorescein angiography is one of the few invasive diagnostic tests that is commonly performed in the ophthalmologist’s office. Accordingly, physicians are obliged to inform patients of the potential risks of fluorescein angiography. In addition, it behooves the physician to review the medical history prior to ordering the test, to identify certain medical conditions such as asthma or uncontrolled hypertension that can increase the likelihood of an adverse reaction. Since fluorescein is excreted via the renal system, it is critical for ophthalmologists to elicit any history of renal disease that would limit a patient’s ability to adequately eliminate fluorescein. In such cases, it is prudent to communicate with their nephrologist or primary-care physician prior to proceeding with the test. Regarding women of child-bearing age, it is important to recognize that fluorescein sodium falls into pregnancy category C; adequate animal studies have not been conducted and it is unknown whether it has teratogenic effects. Therefore, it is sensible to avoid fluorescein angiography in a pregnant woman if at all possible.

The most common side effect of fluorescein angiography is that of a generalized yellowish hue to the skin. In addition, patients should be warned that their urine will appear a bright yellow-green color for roughly 24 h. Other

common side effects include nausea (about 5%), emesis, and the development of hives (also about 5%) [33, 34]. Fortunately, in a majority of patients the sensation of nausea is transient and resolves over a matter of 10–20 s without treatment. While hives are often mild and typically responsive to treatment with oral diphenhydramine or a similar antihistamine, these patients should be monitored to rule out the development of additional anaphylactoid symptoms. It is important to recognize that patients who experience such side effects are prone to a similar response with repeat fluorescein angiography [33]. On occasion there may be some infiltration of dye into subcutaneous tissue during the injection, which leads to pain and localized skin discoloration; ice is often therapeutic in this situation. In contrast, the constellation of lifethreatening adverse effects associated with anaphylactic shock is fortunately very uncommon. The incidence of death after fluorescein injection is estimated to be 1 in 222,000 [34].

Indocyanine green dye was first approved for human use in 1956, for the purpose of studying the hepatic and cardiac systems [35].Therefore, nearly 55 years of data regarding its side effect profile is available. In general, the more common side effects associated with fluorescein angiography occur less frequently with ICG angiography, perhaps because ICG is more highly protein bound and thus less likely to stimulate the chemoreceptor trigger zone. Mild gastrointestinal disturbance, itching, or hives are uncommon with ICG [36–38].Extravasation of injected ICG is painful and can cause a dark green spot that may last for several days. The incidence of death after ICG angiography has been estimated at 1 per 333,000 [34].

ICG contains approximately 5% iodine by weight. More specifically, it contains inorganic iodine, and the risks of administration in patients with allergies to organic iodine is unknown. While patients who are allergic to shellfish are often cautioned against receiving iodine-containing radiocontrast agents, there is little factual evidence to support such a recommendation. Since these patients typically have no difficulty consuming iodized salt, the rationale behind linking radiocontrast agents and ICG or shellfish and the