**4. Swept-source optical coherence tomography (SS-OCT), SS-OCT angiography (SS-OCTA), and OCTARA algorithm**

SS-OCTA incorporates a blood flow detection algorithm; OCTARA (Optical Coherence Tomography Angiography Ratio Analysis); Topcon Corporation, Tokyo, Japan. OCTARA uses decorrelation motion contrast between rapidly repeated SS-OCT B-scans to detect moving erythrocytes in relation to static tissue [20]. SS-OCTA is integrated in the SS-OCT technology which incorporates a long-wavelength (1050-nm) scanning light, reduced sensitivity roll-off feature, and ultrahigh-speed image acquisition. These implements enable deeper penetration with minimal light scattering, hence superior axial resolution and segmentation of different retinal layers. The result is the generation of ultrahigh-definition images of the retinal microstructure, retinal vascular plexuses, and the choriocapillaris, while obviating the need for dye injection. It is worth noting that OCTARA algorithm generates SS-OCTA images by registering B-scan repetition at each scan location, thereby computing a ratio-based result between corresponding image pixels. This method preserves the integrity of the OCT spectrum and does not result in compromised axial resolution, an inherent disadvantage of other OCTA technologies [20–25]. In addition, SS-OCTA software generates color-coded flow density maps of the retinal vascular plexuses and the choriocapillaris, each layer separately. In this map, vessel density in a given area is inferred from the decorrelation motion contrast signal provided by SS-OCTA, where high flow is represented by an increased vessel density and vice versa. Different vessel densities are then given color codes and numeric percentage values that reflect the percentage area occupied by blood vessels, where bright red color represents areas of highest density and hence a high numeric percentage, whereas dark blue represents areas of no detectable vessels and hence low or zero numeric percentage. Intermediate color shades represent variable grades of vessel density [26, 27].

fovea [13]. On the other hand, OCTA imaging reveals generalized attenuation of the vascular layers of the ocular fundus, though the most significant vascular changes are located in the choriocapillaris, which exhibit generalized loss of the normal homogeneous hyperintense texture with the development of vascular rarefaction and flow-void areas. These morphological changes are attributed to the patchy loss of choriocapillaris or to the masking effect of the pisciform flecks [15, 29]. In some cases, there is enhanced visualization of the underlying Sattler's layer [16]. In terms of retinal capillary plexuses, vascular alterations of the SCP layer include rarefaction of the peri-foveal arcade with the enlargement of the foveal avascular zone (FAZ), and generalized reduction of vessel density in the SCP and the DCP layers, though

Novel Insight into Morphological Features and Vascular Profile of Selected Macular Dystrophies…

http://dx.doi.org/10.5772/intechopen.78679

7

Best's disease is an autosomal-dominant disorder whose primary target tissue is the RPE cell layer. The condition is caused by mutation in the BEST1 gene which causes the production of abnormal bestrophin protein. Normally located in the RPE plasma membrane, bestrophin acts as a calcium-dependent chloride channel and is responsible for normal ionic conduction across the RPE cell. Abnormal bestrophin formation causes the disruption of the ionic conduction within the RPE and interferes with the normal calcium metabolism that is essential for adhesiveness between interphotoreceptor matrix and the RPE layer. The end result is the deposition of abnormal amorphous vitelliform or egg yolk-like material in the photoreceptors' outer segments, within the RPE and sub-RPE. Though considered the hallmark of Best's disease, the origin of the vitelliform material remains uncertain. One plausible theory is that it is derived from the accumulation of lipofuscin material within over-shed photoreceptors' outer segments due to abnormal phagocytosis by RPE cell layer. Clinically, the disease is characterized by a solitary sub-retinal vitelliform lesion occupying the macular area. Less commonly, lesions may be multiple or eccentric in location. With time, the vitelliform lesion undergoes degeneration and may even get resorbed completely, with ensuing atrophic changes and scarring. Vision remains unaffected in early stages with most individuals maintaining reading and driving vision well into adult life until the atrophic or cicatricial stages develop. The abnormal ionic conduction in Best's disease is responsible for the characteristic loss of light

response and abnormal Arden ratio on electro-oculogram (EOG) examination [1, 2].

SD-OCT in early stages of the disease shows a sub-retinal smooth dome-shaped amorphous optically opaque material. As the disease progresses, degenerative changes ensue on the vitelliform structure causing it to break down. The corresponding SD-OCT features consist of irregular optically—opaque amorphous deposits alternating with optically—translucent areas that correspond to the resorbed vitelliform material. In addition, RPE irregularities could be detected including irregular thickened RPE layer, and solitary or multiple RPE detachment(s) (PED). The atrophic stage of the disease is marked by diffuse disruption of the outer retinal layers including the external limiting membrane (ELM), inner segment/outer segment (IS/OS) photoreceptor junction, and marked atrophy or even disappearance of RPE cell layer [30, 31]. OCTA in Best's disease demonstrates generalized rarefaction of the SCP and the DCP layers along with a reduced vessel density. The mechanism of vascular rarefaction in Best's

these changes are most pronounced in the DCP layer [15, 28, 29].

**6.2. Best's disease (vitelliform macular dystrophy)**
