Pericyte loss, microaneurysm formation, breakdown of the blood–retinal barrier, and capillary nonperfusion impair the nutrition of the neuroglial tissues in the retinal parenchyma, and the resultant hypoxia increases expression of vascular endothelial growth factor (VEGF), which promotes both angiogenic responses and vascular permeability and causes ischemic maculopathy, proliferative diabetic retinopathy (PDR), and diabetic macular edema (DME).4
In addition, the diabetic choroid has vascular-related changes similar to those in the diabetic retina. Consequently, systematic evaluations of retinal and choroidal capillaries are essential.
Fluorescein angiography (FA) was introduced in the 1960s as a method for visualizing the retinal and choroidal vessels and rapidly became the gold standard exam for identifying and classifying several of retinal vascular diseases.5 The method is based on intravenous injection of a fluorescein dye to evaluate retinal vascular capillary network.
Leakage, capillary nonperfusion, vascular structural abnormalities, neovascularization of the disc (NVD), and neovascularization elsewhere (NVE) are among the most common features observed in this technique in a patient with DR. However, this technique is time-consuming, invasive, and while considered harmless, the dyes pose risks ranging from nausea to allergic reactions, including anaphylaxis and, in rare instances, death.6,7
7.2 Optical Coherence Tomography Angiography Technique
Optical coherence tomography (OCT) has revolutionized the way we diagnose and treat the structural changes of DR, including macular edema. It provides a three-dimensional cross-sectional view of the retina with micrometer scale-depth resolution.
Optical coherence tomography angiography (OCTA) is a new noninvasive imaging technique that uses motion-contrast imaging by comparing the decorrelation signal between sequential OCT B-scans acquired at the exact same cross-sectional image to generate a blood flow angiogram.
Split-spectrum amplitude decorrelation angiography (SSADA) algorithm is the AngioVue software of the RTVue XR Avanti spectral-domain OCT (SD-OCT; Optovue, Inc., Fremont, CA). It obtains volumetric scans of 304 × 304 A-scans at 70,000 A-scans per second in approximately 3.0 seconds.8
Automated segmentation of superficial and deep inner retinal vascular plexuses, outer retina, and choriocapillaris can be observed in an automated software options of 2 × 2, 3 × 3, 6 × 6, and 8 × 8 mm OCT angiograms.
7.3 Nonproliferative Diabetic Retinopathy
DR is a microvasculopathy that features increased vascular permeability, microvasculature leaks, and capillaries that are lost early in the disease. Hyperglycemia and mitochondrial and extracellular reactive oxygen species (ROS) are toxic to endothelial cells (ECs), pericytes, and neurons, resulting in their death early in DR.9
One of the greatest advantages of OCTA is the ability of scroll through segmented en face slabs across the retinal and choroidal vasculature and therefore may help us to understand the pathophysiology in the course of the disease.
A software preset encompasses four en face zones: a superficial capillary plexus, at the level of the ganglion cell layer; a deep plexus, a network of capillaries between the outer boundary of the inner plexiform layer and the midpoint of the outer plexiform layer (total thickness, 55 μ); the outer retina (photoreceptors), which does not have vessels; and the choriocapillaris (choroid).
The first changes seen in OCTA in patients with nonproliferative DR are vascular remodeling bordering the foveal avascular zone (FAZ), followed by vascular tortuosity, narrowing of capillary lumens, and dilation of its ends. These changes are best seen at the level of the superficial capillary plexus ( ▶ Fig. 7.1).
Fig. 7.1 (a,b) Nonproliferative diabetic retinopathy: OCTA shows vascular remodelling bordering the FAZ, capillary tortuosity, narrowing of capillary lumens, and dilation of its terminals adjacent to FAZ at the superficial vascular plexus. (c) These changes cannot be seen in the same manner in fluorescein angiography. FAZ, foveal avascular zone; OCTA, optical coherence tomography angiography.
Changes in the deep capillary plexus are more difficult to observe due to size and morphology of the capillaries, but with the development of the disease, these modifications may be also appreciated.
This mechanism is explained by EC death from hyperglycemia or leukocyte oxidative burst and subsequent increased vascular permeability appears to occur before pericyte dropout occurs.10
OCTA has the disadvantage that it cannot visualize this vascular permeability, whereas FA shows dye leakage from abnormal retinal capillaries.
7.3.1 Microaneurysms
Visualization of microaneurysms is also well delineated with smaller angiograms, but not all microaneurysms are observed in both superficial and deep capillary network, most probably because OCTA is limited by the principle of slowest detectable flow ( ▶ Fig. 7.2).
Fig. 7.2 Visualization of microaneurysms is also well delineated with smaller angiograms, but not all microaneurysms are perceived in both superficial and deep capillary networks, most probably because OCTA is limited by the principle of slowest detectable flow. OCTA, optical coherence tomography angiography.
The most common microaneurysm morphologic patterns observed in OCTA images are fusiform, saccular, curved, and coiled. Some may have no erythrocytes or blood cells with less motility, which could not be visualized in the OCTA images, although the movement of erythrocytes is not continuous in the microaneurysms.11
Some inconsistency in microaneurysms imaging may be found when comparing FA and OCTA images. The explanation may be related to fluorescein dye filling without blood cells or staining of the vascular walls in the microaneurysms, which could be independent of the movement of the erythrocytes.12
7.3.2 Macular Edema
Increased permeability of fluid and protein can result in DME. DME is the common cause of visual function loss in both nonproliferative and proliferative DR. These changes depict vascular loops in the presence of cysts in both superficial and deep vessels. En face OCT is the best technique to outline cystic changes in DME, and the inner plexiform layer appears to be the best location to appreciate fine details ( ▶ Fig. 7.3).
Fig. 7.3 (a,b) Patient with diabetic macular edema. OCTA shows vascular loops and cysts in both superficial and deep plexus. (c,d) En face OCT is the best technique to outline cystic changes in DME. IPL appears to be the best location to appreciate (d) fine details and correlate it with (e) B-Scan OCT images. DME, diabetic macular edema; IPL, inner plexiform layer; OCTA, optical coherence tomography angiography.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
