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In this article the authors aim to give an overview of the current literature concerning the application of OCT-A in geographic atrophy (GA). GA is a disease characterised by loss of outer retinal layers including photoreceptors, degeneration of the retinal pigment epithelium (RPE) and rarefication of the choriocapillaris. OCTA may be a valuable tool in detecting choriocapillaris impairment surrounding the GA. In areas affected with GA, an increased choriocapillaris rarefication up to total loss with the presence of larger choroidal vessels directly adjacent to the Bruch’s membrane has been described. The reduced choriocapillaris flow signals not only precede the disease but also exceed the boundaries of GA. Moreover, the sections of the GA margin with greater underlying choriocapillaris flow signal impairment were reported to expand significantly faster compared to the remaining sections of the GA margin. OCTA is also reported to improve visualisation of quiescent (also called “silent,” “inactive,” or “non-exudative”) choroidal neovascularisation (CNV). E flow signal deficits also help to differentiate GA from mimicking disease entities like late-onset Stargardt disease, with 65.0% sensitivity and 92.3% specificity. Stargardt disease revealed significantly fewer flow signal deficits outside the area of atrophy, and more pronounced signal attenuation within the area of atrophy in late disease. OCT-A also detected that retinal flow signal found in superficial, intermediate and deep capillary plexus of retina were significantly altered in GA. The reduced flow signal of the superficial and intermediate capillary plexus has been further described to extend into the GA rim region (defined as an area of 500µm thickness around the GA) suggesting an important role of the inner retina in the development and progression of GA. Limitations: OCTA technology is prone to segmentation errors, projection and shadow artefacts. However, most modern OCT-A devices now use long-wavelength (>1,050 nm) laser light with increased tissue penetration and reduced signal attenuation from overlying cells and extracellular material, and this has improved its repeatability and reliability. Despite this, the inter-device comparison of images might still not be possible due to differences in technique and algorithms. Furthermore, as only a flow signal is assessed by OCT-A, a very fast or slow flow may lead to false impression of flow voids. Also the technology is dependent on patient co-operation (avoidance of blinking artefacts) and stable fixation to obtain high-quality imaging. Future therapeutic implications of OCTA in GA: Choriocapillaris flow and structure might be potential therapeutic targets and / or susceptible early surrogate markers, possibly before the point of no return. Since inner retinal vascular alterations have distinct association with GA, the preliminary disease stages using OCT-A will be useful in the context of pathophysiologic understanding and development of therapies like retinal prosthesis or stem cell therapy. Since OCT-A analysis have revealed that quiescent CNV might have a protective effect on GA progression having a potential nutritive effect on the RPE and overlying neurosensory retina in the context of choriocapillaris attenuation and age-associated Bruch’s membrane changes, it opens the question of the beneficial effect of early or complete treatment of quiescent CNV in this context.

Optical coherence tomography angiography in geographic atrophy.
Müller PL, Pfaua M, Schmitz-Valckenberga S, et al.
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Sofia Rokerya

MBBS MRCOphth FRCSI, King's College University Hospital, UK.

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