**3. Optical coherence tomography angiography (OCTA): a new imaging frontier**

**1. Introduction**

4 OCT - Applications in Ophthalmology

using low-vision aids [1, 2].

**dystrophies**

atrophy and loss [16].

Macular dystrophy is a unifying term used to describe a group of hereditary fundus disorders that exhibits Mendelian inheritance pattern and has varying degrees of expressivity and penetrance. These disorders share common criteria in that they are isolated, that is, confined to the eye with no systemic association, limited to the anatomic macula, exhibit bilateral involvement with striking symmetry, and have characteristic biomicroscopic features that manifest universally along with visual symptoms or often discovered on routine examination before symptoms develop. The classic tools for the diagnosis and follow-up of macular dystrophies were largely based on clinical appearance, electrophysiologic findings, and fundus fluorescein angiography (FFA). More recently, fundus autofluorescence (FAF) increasingly became an invaluable noninvasive tool in the follow-up scheme of these patients [1–7]. FAF captures the stimulated emission of light from lipofuscin molecules that accumulate excessively in cases of retinal pigment epithelium (RPE) dysfunction and depicts specific autofluorescence patterns that are characteristic for each disease [8–11]. To date, there is no known effective treatment for macular dystrophies other than the management of complications, for example, choroidal neovascularization (CNV) secondary to Best's disease and visual rehabilitation

**2. Theories of pathogenesis of vascular changes in macular** 

Histopathological studies in humans affected with macular dystrophies and in animal models of retinal degeneration demonstrated a wide-spread loss of the photoreceptors and the RPE, in addition to extensive vascular remodeling of the retinal vascular plexuses and the choriocapillaris [1, 2, 12]. These findings purported a cause–effect relationship between the morphological changes seen in retinal microstructure and the status of vascular nourishment, in the sense that one pathology is a consequence of the other, though the exact mechanism remains debatable [13–15]. One theory proposes that the progressive demise of photoreceptors and RPE causes a thinning out of the retina with subsequent progressive atrophy of retinal vasculature and choriocapillaris as part of a downregulation process due to a reduced vascular demand [16]. Another proposed mechanism is that retinal thinning due to the loss of photoreceptors and RPE allows more oxygen influx into the inner retinal layers from the choroidal circulation. The ensuing retinal hyperoxic state induces vasoconstriction and vascular rarefaction [12]. Another plausible theory is that progressive RPE loss results in a decreased release of vascular endothelial growth factor (VEGF) and other signaling factors that are essential for the viability of the choriocapillaris, hence precipitating choriocapillaris atrophy [17–19]. Finally, some researches propose that mechanical compression by the lipofuscin-laden RPE and accumulation of hyaline deposits between the RPE and Bruch's membrane exerts mechanical compression on the choriocapillaris with subsequent The prospect of an abnormal vascular profile going on in tandem with the morphological changes in retinal microstructure in macular dystrophies turned our attention to the diagnostic and therapeutic potentials of the early detection of abnormal changes in the retinal vascular plexuses and the choriocapillaris. Currently, the available diagnostic modalities rely on inference extrapolated from indirect evidence to determine disease stage and progression. For instance, FFA depicts disease patterns based on varying degrees of fluorescence; FAF identifies diseased or dead RPE cells based on varying intensities of lipofuscin autofluorescence, whereas electrophysiology records the amplitude and latency of electric transduction in retinal layers to identify different dystrophies [1–3, 8–11]. The common factor among these diagnostic tools is that they reveal useful information only when retinal function due to a given dystrophy has already been compromised. In comparison, OCTA offers direct noninvasive visualization of the vascular profile in macular dystrophies and thus has the clear advantages of screening vulnerable population and detecting the disease process in its nascence. Though no therapeutic line is currently available for macular dystrophies, the identification of early disease phases based on the integrity of the vascular profile helps selecting patients who would be best candidates for on-going research trials on gene therapy and pluripotent stem cell transplantation. On the other hand, identifying patients with severely compromised vascular profile, and in whom favorable outcome of these therapies is unlikely, will help avoid biased results and reduce the economic burden on health-care institutions.
