The Value of Optical Coherence Tomography Angiography (OCT-A) in Neurological Diseases

Abstract

Optical coherence tomography angiography (OCT-A) was commercially introduced in 2014. OCT-A allows a fast, non-invasive, three-dimensional analysis of the retinal vasculature from the vitreoretinal interface to the choriocapillaris. The results can be evaluated separately in automated or custom-defined retinal layers. Since its introduction, OCT-A has also been used in patients with neurological diseases in order to find and characterize retinal biomarkers. Many neurological diseases have retinal manifestations, often preceding the key symptoms of the neurological disease. Anatomically and developmentally, the retina is a part of the brain. In contrast to the brain, the retina is easily accessible for imaging methods; moreover, retinal imaging is more cost-effective than brain imaging. In this review, the current knowledge about OCT-A findings and possible OCT-A biomarkers in neurological diseases is summarized and discussed regarding the value of OCT-A as a diagnostic tool in neurological diseases.

Keywords: biomarker; central nervous system; microvasculature; neurological disease; optical coherence tomography angiography; retina.

Figure 1

Segmented OCT-A 3 × 3 mm volume scan (304 × 304 A-lines) of a healthy central retina (macula). (a) OCT-A scan of the normal superficial capillary plexus located in the ganglion cell layer. The foveal avascular zone (FAZ) is spared by the ramifying vessels. (b) OCT-a scan of the normal deep capillary plexus located between the inner nuclear layer and the outer plexiform layer. The vessels form a complex, regular network, and the FAZ is spared. (c) OCT-A scan of the normal outer retina. The outer retina is avascular in the healthy eye. (d) OCT-A scan of the normal choriocapillaris showing a fine, granular texture. (Scans obtained with Optovue Angio Vue with OCT scanning speed of 70.000 A-scans per second).

Figure 2

Example of color-coded images of vessel length density and vessel perfusion density of a 3 × 3 mm OCT-A scan of a left eye performed with Zeiss Cirrus 6000 (100.000 scans per second). (A) Corresponding fundus image with the location of 3 × 3 mm scan in the center of the macula (red outlined square). Note that the subject has vitreous floaters (blue arrows) inside the scan area that negatively influence the OCT-A signal quality and analysis in the areas below the floaters (shadow artifact). (B) Color-coded image of the vessel length density [mm⁻¹] (Zeiss definition: total length of vessels per unit area; large and small vessels have equal influence on the result). Dark blue areas as the FAZ are completely avascular, red areas have a high vessel length density. Due to low signal quality in the areas below the vitreous floaters, vessel length density is falsely reduced in these areas due to the shadow artifact. Numbers in the sectors of the superimposed EDTRS-grid show the mean sector vessel length density. (C) Color-coded image of the vessel perfusion density [%] (Zeiss definition: total vessel area per unit area; large vessels have greater influence than small vessels). Dark blue areas, such as the FAZ, are avascular, and yellow and red areas have higher vessel perfusion density. Due to low signal quality in the areas below the vitreous floaters, vessel perfusion density is falsely reduced in these areas due to the shadow artifact. Numbers in the sectors of the superimposed EDTRS-grid show the mean sector vessel perfusion density. FAZ = foveal avascular zone.

Figure 3

Macular OCT-A scans sized 3x3mm (68.000 A-scans per second) of the superficial capillary plexus of a healthy subject (A–C) and of a patient with AD (D–F). The vessel density (B,E) and the perfusion density (C,F) is reduced in the AD patient. (reprinted with permission from ref. [15]. Copyright 2019 Yoon et al.; scans obtained with Zeiss Cirrus 5000).

Figure 4

OCT-A scans of superficial (SVP) and deep (DVP) vascular plexus of a healthy control (left column) and of an MS patient (right column). The FAZ is enlarged in the retina of the MS patient. Scale bar: 1mm. Scans are 3 × 3 mm and obtained with the Heidelberg Engineering OCT2 system with 85.000 scanning speed. FAZ = foveal avascular zone, MS = multiple sclerosis. (reprinted with permission from ref. [26]. Copyright 2020 Kleerekooper et al.).