This prospective study was approved by the local ethics committee (EA4/055/16) and adhered to the tenets of the declaration of Helsinki. Patients with SC were included between April and November 2016 after informed consent was given, so that their data could be used in this study as well as for the ability to perform a clinical examination and imaging. Diagnosis of SC was confirmed after exclusion of infectious diseases such as tuberculous uveitis .
Clinical examination and imaging
Full ophthalmologic examination, including snellen visual acuity test, Goldmann contact tonometry, slit-lamp examination, and fundoscopy with a 90-diopter lens, was performed and all patients received the following imaging modalities: EDI-OCT, FA, ICGA, and FAF. Visual acuity results were converted into a logMAR scale, according to the visual acuity measurement standard from the international council of ophthalmology.
All imaging modalities were carried out by an experienced ophthalmic photographer with the Heidelberg SPECTRALIS OCT, Heidelberg Engineering, Heidelberg, Germany (v1.9.2014.0 - Acquisition Module v6.4). The technical aspects of the machine were standardized as follows: for the EDI-OCT - Scan angle of 20°, 512 A-Scans for 19 Sections, inter-section distance of 250 µm, scaling of 11.5 µm/pixel and an ART of 100 frames; for the FAF - Scan angle of 55°, 768 A-Scans, scaling of 20 µm/pixel, and an ART of 100 frames.
FA and ICGA were performed. The early, intermediate and late phases of both imaging modalities were examined and the activity state was determined.
Inactive SC was identified based on clinical findings of fundoscopy and on multimodal imaging.
Active SC can lead to worsening of the vision, metamorphopsia, inflammation signs at the border of the previous lesions and neovascularization at the borders of the lesion. In FAF, active SC is described by images showing a peripheral hypoautofluorescence area surrounding the hyperautofluorescent borders of the lesions. Old atrophic areas are (because of the loss of choriocapillaris) completely hypoautofluorescent [32-34]. In FA, active SC shows hypofluorescence surrounded by a hyperfluorescent border, with staining and leakage in the late phases. Inactive SC lesions are hypofluorescent at first, acquiring a hyperfluorescent edge later. ICGA shows hypofluorescent lesions throughout the entire examination, but usually of greater (and more reliable) extent in comparison to FA [35, 36].
OCTA images were acquired using a prototype SPECTRALIS OCT device (SPECTRALIS®, Heidelberg Engineering, Heidelberg, Germany) using a prototype software (188.8.131.52) that applied an OCTA acquisition algorithm on which the now commercially released version is based. Images were acquired with an A-scan rate of 70,000 per second and a 15°x10° scan angle protocol was used. A total of 261 B-scans resulting in images with an axial resolution of approximately 4 μm, within B-scan resolution of approximately 11 μm (6.99µmpixel), and between B-scan resolution of also approximately 11 μm. The OCTA C-scan derived from the B-scans allows for 3D visualization of the different retinal and choroidal vascular plexuses. The scanning frame dimensions were 4.2x2.8 mm, being centered at the macula.
A standardized reproduction of the superficial capillary plexus (SCP – at 30 µm ± 30 µm below the inner limiting membrane, ILM, representing the ganglion cell layer), deep capillary plexus (DCP - at 130 µm ± 12.5 µm below the ILM, representing the inner nuclear layer), RPE, Choriocapillaris (at 10 µm ± 0 µm below the Basement Membrane, BM), Sattler’s Layer (at 70 µm ± 10 µm below the BM,) and Haller’s Layer (at 140 µm ± 10 µm below the BM) were attempted based on the “Atlas OCT Angiography in AMD” . Each OCTA produced by the software’s algorithm was analyzed for the presence of the expected anatomy to be found at the respective depth of the retina, this being also based on the “Atlas OCT Angiography in AMD” . For this comparison, interobserver correlation was assessed. The identification and differentiation between the Sattler’s and Haller’s layer were done with recourse to the following criteria: the Sattler’s layer was identified by vessel-like entities in a hyper-intense grayish background, appearing below the choriocapillaris until reaching the Haller’s layer, an area of hypo- and hyper-intense signals corresponding to bigger vessels.
Furthermore, the images were then evaluated with respect to the presence of segmentation errors, which were defined by clear deviations of the segmentation line from the observable path of the anatomical structures, due to imprecise or altogether lacking identification of the true reference point. Afterwards, the images were optimized to improve the analysis. Segmentation boundaries were manually changed to best reproduce the expected anatomy (mostly in the retinal pigment epithelium and below).
Characterization of lesions
The atrophic area at the level of the choriocapillaris and RPE were measured manually (in mm2) after proper and individual segmentation by ML and SM of the corresponding OCTA slab. This was achieved using the linear caliper tool of the OCTA embedded in the Heidelberg Eye Explorer Software (Viewing Module 184.108.40.206), in order to compare both measurements and reliability of the technology across observers.
Statistical Analysis was performed using IBM SPSS Statistics v23. First, we analyzed the data with descriptive and frequency statistics. A t-Test-Paired Two-Sample Test assuming Equal Variances was employed to compare the area of atrophy at the choriocapillaris and RPE of each patient. A Mann-Whitney U Test was used to compare the best corrected visual acuity (BCVA) of patients with SE and patients without SE. A one-way ANOVA-Test was used to assess the relationship between the BCVA and the presence of SE on the OCTA’s. Interobserver agreement for qualitative data was studied using Cohen’s kappa and Lin’s concordance correlation coefficient for quantitative data. The results were regarded as statistically significant if p was below 0.05 .