Birdshot Uveitis (Birdshot Chorioretinopathy) is a rare, chronic inflammatory eye disease affecting both eyes simultaneously. It is classified as a form of posterior uveitis, meaning the inflammation is located in the back portion of the eye. This inflammatory process primarily targets the retina, the light-sensitive tissue, and the choroid, the layer of blood vessels supplying the retina. The condition is progressive and can lead to irreversible visual impairment if the inflammation is not controlled through timely treatment.
Identifying the Condition
Patients often first notice subtle changes in their vision, typically beginning with floaters or a general blurring of central vision. These initial symptoms are caused by inflammation within the vitreous humor, the clear gel that fills the eye’s interior. Patients may also experience difficulty seeing in low light (nyctalopia), diminished contrast sensitivity, and changes in color perception.
The condition is named for the distinctive clinical finding seen upon examination of the eye’s fundus. Ophthalmologists observe multiple creamy-white or yellow lesions scattered across the choroid and retina, resembling the pattern left by a shotgun blast’s birdshot. These lesions are typically ovoid, 500 to 1500 micrometers in diameter, and often appear first near the optic nerve head. This physical appearance is a hallmark that helps distinguish this disease from other forms of uveitis.
These characteristic lesions represent areas of inflammation and dysfunction in the choroid and retina. The inflammation can also lead to cystoid macular edema, which is a swelling of the central part of the retina responsible for sharp, detailed vision. Macular edema is a primary cause of decreased visual acuity in people with this condition. The chronic inflammation, if left unchecked, causes damage to the tissue layers, contributing to the progressive nature of the vision loss.
The Role of Genetics in Development
Birdshot Uveitis exhibits a strong genetic association with the Human Leukocyte Antigen (HLA) system. The condition is overwhelmingly linked to the presence of the HLA-A29 gene allele. More than 95% of individuals diagnosed with the disease carry this specific genetic marker.
The HLA genes produce proteins that help the immune system distinguish between the body’s own cells and foreign invaders. The HLA-A29 allele is thought to predispose the immune system to misfire, potentially presenting a normal eye protein as a threat. This erroneous presentation leads to a T-cell-driven autoimmune attack on the eye’s tissues, resulting in chronic inflammation.
Despite this strong link, the HLA-A29 allele is present in about 7% to 9% of the general Caucasian population, yet the disease is rare. Inheriting the gene alone is not enough to cause the condition. Scientists theorize that a combination of the genetic predisposition and an unknown environmental trigger, such as a prior infection, is required for the disease to manifest. The presence of the gene acts as a susceptibility factor, but other factors are necessary to initiate the autoimmune process.
Confirming the Diagnosis
Diagnosing Birdshot Uveitis requires a specialized approach relying on clinical findings, specialized imaging, and genetic testing. The characteristic birdshot lesions must be numerous and scattered in both eyes, along with evidence of low-grade inflammation in the vitreous cavity.
Fluorescein Angiography (FA) involves injecting a fluorescent dye to visualize blood flow and detect leakage in the retinal vessels. FA is useful for identifying retinal vasculitis and cystoid macular edema. Indocyanine Green Angiography (ICG) uses a different dye to better visualize the choroid, the deep layer where the birdshot lesions originate. The lesions often appear as distinct hypofluorescent spots on ICG, revealing more lesions than are visible during a standard eye exam.
Optical Coherence Tomography (OCT) provides high-resolution cross-sectional images of the retina, allowing for the precise measurement of macular edema. This non-invasive test is invaluable for monitoring disease activity and the response to treatment. Electroretinography (ERG) is a functional test that measures the electrical responses of the light-sensitive cells in the retina. ERG is a sensitive tool for tracking progressive damage to retinal function. The diagnosis is further supported by a blood test confirming the presence of the HLA-A29 allele.
Strategies for Disease Management
Management focuses on controlling chronic inflammation using long-term, systemic medication rather than local eye drops. The initial strategy for managing an acute flare-up often involves high-dose corticosteroids.
Corticosteroids, administered orally or via local injections, quickly suppress the immune system and quell inflammation. Due to the serious long-term side effects of high-dose steroids, such as cataracts, glaucoma, and systemic issues, they are typically used only for a short period to gain rapid control. The goal is to transition the patient onto a steroid-sparing agent for long-term management.
Immunosuppressive therapy is the mainstay of treatment, necessary to maintain remission and avoid prolonged steroid use. These medications modulate the immune system to slow the autoimmune attack on the eye. Powerful immunosuppressants frequently used include:
- Methotrexate
- Azathioprine
- Mycophenolate mofetil
- Cyclosporine
- Tacrolimus
For patients whose inflammation remains active despite conventional therapy, or for those who cannot tolerate the side effects, biologic agents may be introduced. These targeted therapies, such as TNF-alpha inhibitors, block specific molecules in the inflammatory pathway, offering a more precise way to manage the disease. Consistent monitoring with specialized imaging and functional tests is required to ensure the inflammation remains suppressed and vision is preserved.