Retinal vasculitis in HLA-A29 birdshot retinochoroiditis, particularities and imaging narrative of an under-estimated and diagnostic component of the disease.

Background: HLA-A29 birdshot retinochoroiditis (BRC) is a primary stromal choroiditis (PSC), the hallmark being the choroidal rice-shaped hypopigmented fundus lesions ("birdshot lesions"). BRC is characterised by dual independent retinal vasculitis and choroiditis, the former often preceding manifest choroidal lesions. The purpose of this study was to analyse the type and severity of retinal vasculitis and determine whether it represented a diagnostic contribution. Medical records of patients with the diagnosis of BRC examined in the uveitis clinic of the Centre for Ophthalmic Specialised care (COS) in Lausanne from 1994 to 2020, were retrospectively reviewed. All patients had a complete ophthalmic examination, including visual field testing, optical coherence tomography (OCT), and fluorescein (FA) and indocyanine green (ICGA) angiography. Key retinal angiographic features were assessed. The study also established the angiographic score for retinal (FA) compared to choroidal involvement (ICGA). Among the 2102 newly diagnosed patients, 33 (1.57%) were diagnosed as BRC. Of the 21 patients with sufficient data included, all exhibited bilateral retinal vasculitis, of which 5 (24%) had no "birdshot lesions" at presentation with ICGA however always showing choroidal involvement. FA characteristics included (1) profuse retinal exudation in 17/21 cases (81%), (2) macular oedema in 17 patients (81%) with foveolar sparing for 14 of them (82%), (3) thick sheathing/staining of large posterior pole vessels in 13 patients (62%) and (4) profuse disc hyperfluorescence in all 21 patients. (5) A specific feature was the so-called pseudo arterio-venous circulatory delay in 17/21 cases (81%). The FA angiographic score at presentation was 14.49 ± 5.1 equivalent to the ICGA angiographic score of 14.29 ± 3.6, and higher than in other chorioretinitis entities. Both angiographic scores decreased similarly after treatment with a slower response of the retinal involvement.

Conclusions: Retinal vasculitis in BRC is often very pronounced and presents distinct angiographic features that help substantially in the diagnosis and understanding of the disease course. Retinal vasculitis can present initially as an isolated feature in absence of the characteristic "birdshot lesions". The presence of all or some of the specific FA features strongly orient towards BRC to seek confirmation by ICGA and the search for the HLA-A29 antigen.