Choroideremia - OCT angiography offers anatomic insights

A - Output from the general linear model, where brighter values represent higher likelihood of intact choriocapillaris; 
B - En face OCTA of choroidal flow with outlines of intact choriocapillaris as determined by the grader (yellow) and model (cyan); C - The en face structural OCT image of the ellipsoid band showing an island of intact photoreceptors with extending outer retinal tubulations. The outlines of intact choriocapillaris from panel B are also shown. Photoreceptors were preserved when overlying 
intact choriocapillaris. Choriocapillaris was absent beneath outer retinal tubulations. Courtesy of Casey Eye Institute, Oregon Health & Science University, Portland, Oregon, USA Researchers at the Casey Eye Institute, Oregon Health & Science University, Portland, USA, have developed a technique for identifying the choriocapillaris using optical coherence tomography angiography (OCTA). Speaking at the 2016 annual meeting of the Association for Research in Vision and Ophthalmology (ARVO) in Seattle, USA, Simon Gao PhD presented the method and reported on its use to assess eyes with choroideremia. The research was supported by the Choroideremia Research Foundation and National Eye Institute (R01 EY024544).

INHERITED BLINDING DISEASE

Dr Gao, a postdoctoral research fellow, noted that the accomplishment opens the door to better understanding the pathophysiology of the inherited blinding disease and ultimately to new treatments. Progressive vision loss in eyes with choroideremia occurs because of atrophy of the photoreceptors, retinal pigment epithelium (RPE), choriocapillaris, and choroid, but the sequence in which these different layers degenerate is unclear, in part because of difficulty visualising the choriocapillaris, he explained. “Using the flow signal, local standard deviation of the flow signal, and other image processing tools, we have been able to use OCTA to differentiate intact choriocapillaris from choroidal vessels and projection artefacts in eyes with choroideremia. Application of this approach should enable insight as to the primary sites and progression of cellular degeneration that is needed as a foundation to develop novel therapies,” he noted.

PROCESSING FILTERS

OCTA was performed using a spectral domain system (RTVue XR, Optovue) with the split-spectrum amplitude-decorrelation angiography (SSADA) algorithm to detect blood flow. Two types of image processing filters – a local standard deviation filter and a Gabor directional filter – were used to overcome shadowing and flow projection artefacts that create false signals, confounding interpretation of OCTA and the ability to identify intact choriocapillaris. A general linear model for detecting intact choriocapillaris was developed by fitting the image processing output to manual interpretation from an expert grader, using a training set of images from six eyes of four subjects with choroideremia. Its performance was validated using images from an independent cohort of 27 eyes of 15 patients with choroideremia. Subsequently, the technique was used in conjunction with en face OCT to study the extent of structural alterations of the choriocapillaris, RPE, and photoreceptors in eyes of patients with choroideremia. As was reported in their published paper (Jain N, et al. JAMA Ophthalmol. 2016;134(6):697-702), photoreceptors were seen to be preserved at sites overlying intact choriocapillaris. RPE and choriocapillaris were absent beneath outer retinal tubulations, which were present in all eyes with choroideremia, but longer and more numerous in more severely affected eyes. Simon Gao: gasi@ohsu.edu

Tags: choriocapillaris, optical coherence tomography