Birdshot uveitis is a rare, chronic, bilateral, posterior uveitis characterized by distinctive, multiple, hypopigmented choroidal and retinal lesions. The pathogenesis is unknown, but HLA-A29 positivity appears to confer a predisposition, and retinal autoimmunity seems to play a role. Most investigators recognize the presence of the HLA-A29 allele as a necessary criterion for birdshot uveitis diagnosis. HLA-A29 is present in as many as 7% of Caucasians and is subdivided into more than 20 subtypes.
Birdshot Uveitis (also known as Birdshot chorioretinopathy) is a well-characterized form of autoimmune uveitis (inflammation of the uveal layer of the eye) mostly known for its ovoid light lesions, which appear ‘shotgun pattern’-like distributed along the vascular arcades in the back of the eye (i.e., the ‘fundus’ of the eye where these lesions are visible by photography). Inflammation and extensive depigmentation of the choroid, macular edema, peripheral ischemia, degeneration of the retina, and the progressive formation of a thin layer of scar tissue on the retina (“epiretinal membrane”), progressively impair vision in a substantial proportion of patients. BU is unusual in the young and typically affects patients over 50 years of age of Western-European ancestry, with more women than men affected. Long-term systemic corticosteroid-sparing immunomodulatory therapy is the mainstay of treatment, but a fraction of patients may exhibit a more benign disease course that does not require systemic therapy.
The genetic association with HLA-A29
HLA-A29-positive testing is now widely considered critical to diagnosis and led key opinion leaders in the field propose to renaming the condition to “HLA-A29 uveitis”. HLA-A*29 is one of the hundreds of variants of the HLA-A gene.
Because all patients with BU carry a copy of the HLA-A29 allele, it is considered to be critically involved in the unidentified disease mechanisms. How exactly HLA-A29 contributes to eye inflammation is unknown.