Laboratory Studies
Blood testing for HLA-A29 helps to support the diagnosis, but not all patients with birdshot chorioretinopathy are HLA-A29 positive. One must note that false-negative results with HLA-A29 testing may occur, and repeat blood testing is warranted in situations where clinical suspicion is high.
Other blood testing is not diagnostically helpful for patients with suspected birdshot chorioretinopathy. Fuerst and colleagues performed serologic testing of various markers of immune system activity and found elevated C4 levels; alpha-1-antitrypsin; C-reactive protein; rheumatoid factor; serum immunoglobulin G (IgG), immunoglobulin M (IgM), and immunoglobulin A (IgA); properdin factor B; and C3 were in the reference range. [7] However, their series of patients was small (ie, 9 patients). [7]
Testing for baseline renal function is necessary in those patients most likely to need cyclosporine therapy.
Purified protein derivative (PPD) may be performed.
Imaging Studies
Fluorescein angiography
Early cream-colored lesions, which are active sometimes, may present as isofluorescence, and this occurs when the lesion is deep or when the retinal pigment epithelium (RPE) and the choriocapillaris are intact. If there is disruption to any one of them, the lesion will be fluorescent, especially in the late phase. Late focal depigmentation or an atrophic lesion presents as hypofluorescence in the early phase and as diffuse hyperfluorescence in the late phase.
Retinal vascular system and cystoid macular edema: Delayed in the filling time and prolongation of the arteriovenous transient phase; hyperfluorescence of the optic disc and the macula that form cystoid macular edema
Indocyanine green angiography
Indocyanine green angiography (ICG) provides the additional dimensions of the choroidal lesion analysis in birdshot chorioretinopathy.
ICG reveals well-delineated hypofluorescence choroidal spots in the mid phase of the study. These hypofluorescent spots not only correspond to the location of the birdshot chorioretinopathy lesions but also are far more numerous than those seen on either fluorescein angiography or clinically. These choroidal lesions assume a vasotropic distribution bordered by medium- to large-sized choroidal blood vessels.
Ultra-high resolution optical coherence tomography
Ultra-high resolution optical coherence tomography (OCT) showed photoreceptor atrophy in several areas of both eyes. RPE degeneration was present underneath the areas of photoreceptor involvement. The inner retinal layers were hard to delineate because of the anatomical disorganization. [8]
Ultra-high resolution OCT imaging may help in understanding and following the progression of macular involvement in birdshot chorioretinopathy.
Chest radiography
Chest radiography may be performed.
Other Tests
Electrophysiologic testing may aid in determining the reason for complaints of problems with color perception or night vision. Both electro-oculograms (EOG) and electroretinograms (ERG) are affected. The presence of an abnormal electrophysiologic test may help distinguish it from other entities with similar funduscopic appearances. Currently, the use of serial ERGs as a tool to assist in monitoring birdshot chorioretinopathy activity and response to therapy is being investigated.
The ERG evolves into a negative pattern ERG, characterized by a decrease in b-wave amplitude, with no effect on a-wave amplitude. This occurs in diseases in which the retinal neural network function, corresponding to the b wave, is involved with progressive disease, but the photoreceptor function, represented by the a wave, initially is uninvolved.
In advanced birdshot chorioretinopathy, both a-wave and b-wave amplitudes are decreased, suggesting dysfunction of all retinal layers, including the photoreceptors.
EOG testing also was decreased in patients, representing RPE dysfunction.
Pattern evoked cortical potential (PECP) showed reduced amplitude and delayed response.
Dark adaptation abnormalities suggested that the rod system was more affected than cones. However, the case series was small, and more supportive data are needed to confirm these findings.
Histologic Findings
Only three histopathology studies have been obtained on the eyes of patients affected by birdshot chorioretinopathy.
Nussenblatt and coauthors described the histopathological findings of a single, phthisical eye enucleated from a patient with birdshot chorioretinopathy who exhibited a positive in vitro lymphocyte proliferative response to retinal S-Ag. [5] The histopathology revealed a mild lymphocytic response, whereas the retina was involved with a diffuse, chronic granulomatous inflammation.
Gaudio and coauthors described the histopathology of a blind, phthisical eye of a patient positive for the HLA-A29 gene and diagnosed with birdshot chorioretinopathy. [9] This study found aggregation of the lymphocytes with their foci in the deep choroid, with additional foci in the optic nerve head and along the retinal vasculature. These histopathological findings were noted to have a vasotropic distribution.
Pulido and coauthors treated a woman with both birdshot chorioretinopathy and ciliochoroidal melanoma who underwent enucleation. They concluded that birdshot chorioretinopathy is a nongranulomatous nodular infiltration of the choroid. [10]
