Optical Coherence Tomography Angiography for Choroidal and Vitreoretinal Disorders - Part 1

 Optical coherence tomography (OCT) has proven to be an effective diagnostic technique for evaluating ocular structures, particularly for studying retinal layers and other areas of the posterior segment of the eye. The incorporation of strategies and algorithms that allow the observation of the retinal microvasculature and the flow of red blood cells currently represents important advances in the diagnosis and treatment of inflammatory, neural, and vascular retinal diseases. The advantage is that OCT is a non-invasive method that does not require the use of contrast dyes. For this reason, OCT combined with angiography (OCTA) is one of the most important techniques for the study of vitreoretinal disorders. Its optical principle, which is based on the Doppler technique, allows us to understand how OCTA equipment acquires and processes images to facilitate visualization and interpretation through their two- and three-dimensional reconstructions. In addition, OCTA allows the identification of signal alterations that could appear as artifacts on each tomography or angiographic scan. This chapter aims to explore the characteristics and further applications of OCTA in addition to its relevance in ophthalmological clinical practice.

Optical coherence tomography angiography (OCT-A) is an advanced noninvasive retinal blood flow imaging technique. It uses motion-contrast imaging to obtain high-resolution volumetric blood flow information to enhance the study of retinal and choroidal vascular pathologies. OCT-A can obtain detailed images of the radial peripapillary network, the deep capillary plexus (DCP), the superficial capillary plexus (SCP) and the choriocapillaris. In addition, compared to fluorescein angiography (FA), this technique does not require the use of injected dye. This chapter aims to present OCT-A technology and clarify its terminology and limitations. The discussion summarizes the potential application of the technology in different retinal and choroidal diseases.

Optical coherence tomography (OCT) is a practical and common imaging method for the study of diseases of the retina, choroid, and vitreoretinal interface. Software and technological advances have allowed us to observe changes in the retinal at resolutions less than 5 µm; the development of angiography with OCT (OCTA) allows us to three-dimensionally evaluate the existing perfusion in the analyzed retina and choroid non-invasively and without a specific dye, such as fluorescein or indocyanine green angiography. We can detect important clinical differences between OCT and OCTA, although these approaches are complementary. Diabetic retinopathy, vascular occlusions, and choroidal neo-vascularization secondary to age-related macular degeneration and other causes are among the conditions whose diagnosis, treatment, and follow-up benefit from applying these techniques. Leak quantification in cases of macular edema is a good candidate for future objective evaluation; currently, its existence is only demonstrable in structural OCT, although it can be indirectly inferred in OCTA by observing vascular displacements and deformity of the capillary walls. Using OCTA, it is possible to detect intravascular flow even in fibrous tissue, thereby allowing the evaluation of neo-vascular activity in vasoproliferative diseases. 

One of the most significant developments in ocular imaging in the last century was optical coherence tomography (OCT). OCT angiography (OCT-A), an extension of OCT technology, offers depth-resolved images of the blood flow in the choroid-retina that are much more detailed than those produced by earlier imaging techniques such as fluorescein angiography (FA). Due to its requirements of novel tools and processing methods, the prevailing imaging constraints, the rapid improvements in imaging technology, and our knowledge of the imaging and relevant pathology of the retina and choroid, this novel modality has been challenging to implement in daily clinical practice. Even those familiar with dye-based ocular angiography will find that mastering OCT-A technology requires a steep learning curve due to these issues. Potential applications of OCT-A include almost all diseases of the choroid and retina, as well as anterior segment diseases. Currently, the most common indications are age-related macular degeneration and ischemic retinopathies, including diabetic retinopathy and retinal occlusive vascular disorders. Incorporating OCT-A into multimodal imaging for the comprehensive assessment of retinal pathology is a fast-growing area, and it has expanded our knowledge of these complex diseases in terms of diagnosis and treatment. This review describes the current main indications of OCT-A in retinal and choroidal diseases.

Reference values of optical coherence tomography angiography metrics vary according to the device used to measure them and even based on the software on the same device. There might exist measurement differences between different maps within the same device: Variables such as age, gender, and signal strength might induce changes in the measurement outputs.

This chapter deals with the values of vessel length and vessel area densities, and foveal avascular zone values of healthy emmetropic people via the 3 × 3 mm map used in the most common equipment that are available commercially. The text includes metrics of the parafovea and fovea at the superficial, intermediate, and deep capillary plexuses. These measurements corresponded to the adult non-diabetic population and were distributed as center (foveal), inner (parafoveal) and full (whole map) densities, depending on the evaluated region, according to densities in the foveal, parafoveal, and whole map measurements. Metrics of the parafovea by subfield were also included. We also report current cut-off values that have been proposed as normality references in some variables. Values for the remaining metrics and devices can later be proposed. We dedicate a special section to non-diabetic patients with high myopia without pathology, which includes the same metrics as in emmetropic patients. The evaluation of perfusion indices benefits from the simultaneous measurement of metrics as well as regional evaluation. The signal strength is a key variable to consider.

Diabetes damages retinal capillaries before clinical changes appear. Optical coherence tomography can quantify changes in vessel length density and vessel area density in diabetics without retinopathy and can lead to a reduction of these metrics in different capillary plexuses. The mean values of vessel densities vary according to the device used. Here, we review the values of vessel length density, vessel area density, and foveal avascular zone metrics in diabetics without retinopathy in a 3 x 3 mm map of the most used commercially available devices. We included measurements for the superficial, intermediate, and deep capillary plexuses in the parafoveal region. The information refers to adult type 2 diabetic people according to densities in the foveal, parafoveal, and whole map measurements. We also included parafoveal distribution by field as well. There are references to the foveal avascular zone—a common variable measured to detect ischemia in patients with diabetic retinopathy—and we report them for both superficial and deep capillary plexuses. We also include the proposed cut-off values for normality for metrics of the superficial capillary plexus and propose an explanation for the differences that exist between measurements with the same device as related to diabetes duration. 

Myopia is a global public health problem leading to visual impairment and blinding complications. Myopic foveoschisis (MF)/foveoretinal detachment (FRD) might be responsible for progressive visual loss in eyes with macular traction maculopathy (MTM). An assessment of the macular microcirculation might identify defects that are potentially implicated in subsequent pathological changes. In the present chapter, macular perfusion in normal eyes was compared with that in highly myopic eyes with MF/FRD. Vessel density (VD) differed between the groups, and the superficial area of the foveal avascular zone (FAZ) was significantly larger in the control groups. Better final visual acuity results were significantly correlated with fewer structural findings and greater VD (p < 0.05). The central subfoveal thickness was significantly larger in the control groups and significantly smaller in the surgery group. These findings suggested a higher incidence of macular perfusional VD deficiencies and abnormalities in the FAZ area in the highly myopic eyes. 

Rhegmatogenous retinal detachment (RRD) is fairly common and one of the main causes of blindness if left untreated. In spite of the high anatomical success rate for retinal detachment, visual recovery is lagging. Microvascular changes in the macular area might play a role in determining poor visual outcomes. Methods: Optical coherence tomography (OCT) and OCT angiography (OCT-A) technologies have been used to determine the relationship between microvascular macular changes and visual acuity. Results: RRD seems to alter microcirculatory anatomy in the macular area by increasing the foveal avascular zone (FAZ) and diminishing the vascular density (VD) of the superficial, deep and choroidal capillary plexuses. More so if the macula is detached, these changes appear to recover with time and might be correlated with postoperative visual acuity, but apparently do not entirely explain the sometimes-unexpected poor visual results. 

Rhegmatogenous retinal detachment (RRD) associated with giant retinal tears (GRTs) can cause significant visual impairment due to structural or perfusional macular sequelae. This condition is an acute-onset incident that leads to a full-thickness circumferential retinal tear of at least 90°. Limited data are available concerning the patients´ long-term perfusional status after successful surgery for GRTs with maculaoff RRD. This chapter examines the long-term outcomes of eyes treated with varying degrees of GRT-associated RRD extensions and compares them with those of two control groups. The surgical group was subdivided according to GRT-associated RRD extension as follows: eyes with extension of <180° and eyes with extension > of >180°. The eyes were further classified according to whether complementary 360° scleral buckle (SB) placement was required. Postoperative optical coherence tomography (OCT) demonstrated that 33.3% of the eyes had abnormal foveal contours, 39.4% had ellipsoid zone (EZ) disruption, 2 had dissociated optic nerve fiber layer (DONFL) defects, and 45.4% had external limiting membrane (ELM) line discontinuities. OCT angiography (OCT-A) revealed abnormal perfusion indices in surgically treated eyes (p<0.0001). Postsurgical best-corrected visual acuity (BCVA) was negatively correlated with the superficial foveal avascular zone area, superficial parafoveal vessel density, and central subfoveal thickness but positively correlated with the choriocapillaris flow area (CFA). Moreover, eyes treated surgically for GRT-associated RRD had multiple structural alterations reflected by spectral-domain OCT biomarkers and OCT-A perfusional findings correlated with visual outcomes. Despite successful retinal reattachment without proliferation, management of GRT-associated RRD remains challenging.

Diabetic macular edema uses structural features as biomarkers and predictors of treatment response. Optical coherence tomography angiography (OCTA) metrics found a correlation between many structural biomarkers and reduced vessel density. We present recent references of vessel length density, vessel area density, and foveal avascular zone metrics in eyes with diabetic macular edema and comment on the associations found between them and structural biomarkers. Diabetic macular edema can change the level at which the capillary plexuses are located, with retinal cysts altering the strength signal. Though image evaluation requires adjustment, intra-subject comparison before and after treatment can be a useful tool to note changes in vessel perfusion, combined with structural changes, to assess treatment outcomes. Macular ischemia is a variable that can be identified reliably with OCTA and can be detected in different capillary plexuses. For eyes with retinal thickening, OCTA evaluation requires consistency to avoid inter-device variability. It is recommended to use the same device, the same scanning protocol, and preferably the same software, to obtain more reproducible measurements. 

Current imaging techniques based on optical coherence tomography (OCT) angiography are useful for observing different retinal microcirculation patterns. The primary purpose of this chapter was to describe the functional, structural, and serial perfusion postoperative outcomes of successfully reattached macula-off tractional retinal detachment (TRD). Patients who underwent a successful diabetic vitrectomy were analyzed. The mean differences between the preoperative best-corrected visual acuity (BCVA), 3-month BCVA, and final postoperative BCVA were statistically significant (p < 0.05). The duration of vision loss before surgery was 11.6 ± 2.3 weeks (mean ± standard deviation (SD)).

The mean duration (± SD) of the resolution of macular detachment was 3.6 ± 1.7 weeks in the pure macular TRD group and 1.8 ± 0.8 weeks in the combined tractional and rhegmatogenous macular detachment (p < 0.05) group. The mean follow-up duration of all patients was 11.4 ± 5.7 months (mean ± SD). Longitudinal multimodal imaging tests revealed abnormal superficial and deep microcirculation patterns with multiple microabnormalities in the foveal avascular zone and different but distinct areas of the non-perfused macula in different OCT angiography slabs. Additionally, disorganization of the retinal inner layers and chronic ischemic macular edema were observed in 82% of eyes examined using the spectral domain (SD) OCT. Therefore, these data suggest that despite the successful anatomical reattachment of the macula, long-term postoperative microcirculatory abnormalities were detected in both groups; however, these abnormalities were predominantly accompanied by severe persistent ischemia in the recurrent TRD group due to the presence of multiple microcirculatory defects.