We examined nine patients with a serous macular detachment secondary to an extramacular BRVO. Fluorescein angiography, performed in six of the nine patients, revealed dye leakage in the extramacular area but not in the macular area. OCT revealed serous retinal detachment in the macula, as well as outer retinal swelling. Importantly, our results demonstrated that retinal swelling extended from the extramacular BRVO toward the fovea.
Retinal swelling and cystoid macular edema are usually caused by leakage from retinal vessels, such as in retinal vascular disease and uveitis. Subretinal fluid can also cause structural changes in the detached neurosensory retina.13
OCT analysis of rhegmatogenous retinal detachment showed intraretinal cyst formation, intraretinal separation, and undulation of the outer detached retina.13
In the acute phase of central serous chorioretinopathy, the neurosensory retina is thickened within the area of serous retinal detachment.15
As such, it was previously thought that intraretinal edema was a secondary change in serous retinal detachment caused by distant BRVO, and that the subretinal fluid in the macula originated from distant retinal vascular leakage through the subretinal space.10
However, in the present study, OCT revealed that outer retinal swelling was present not only in the detached retina but also in the attached retina, extending from extramacular permeable lesions toward the fovea. These findings suggest the possibility that distant retinal vascular leakage diffuses through the outer retina to the macula and accumulates there. These accumulations may exude from the intraretinal tissue to the subretinal space at the fovea.
Wise and Wangvivat proposed that the macula has an exaggerated pathological response to some generalized retinal disorders, such as retinal vein occlusion, angioid streaks, uveitis, diabetic retinopathy, and Coats’ disease.16
We found that although outer retinal swelling was identified between extramacular lesions and the fovea, it was seldom observed on the other side of the fovea. This indicates that intraretinal leakage tends to accumulate toward the macula, which may constitute an exaggerated macular response. Yamaguchi et al reported that the incidence of serous retinal detachment was higher in a group with major BRVO (defined as BRVO involving a large area outside the vascular arcade in addition to the macular area) than in a group with macular BRVO (defined as BRVO in which retinal hemorrhage is confined within the major vascular arcade) and that foveal thickness in individuals with major BRVO (mean of 610 μm) was significantly greater than that in individuals with macular BRVO (mean of 500 μm).6
This also indicates that vascular leakage from congested retinal veins outside the macular area is related to retinal swelling and subretinal fluid accumulation in the macula. Although the reason for this exaggerated macular response is unclear, Naumann and Apple proposed the following factors as partially responsible for the occurrence of this phenomenon: increased thickness of the sensory retina in the perifoveal region; special orientation of the fibers of the outer plexiform layer in this region, forming Henle’s fiber layer; high content of melanin within retinal pigment epithelial cells in this region; and high concentration of retinal pigment epithelial cells at the posterior pole.17
An increased vascular endothelial growth factor level may also cause serous retinal detachment in the macula and outer retinal swelling, because fluorescein angiography showed widespread nonperfusion of the retinal capillary bed in three of six eyes examined. In addition, the fact that serous retinal detachment and outer retinal swelling resolved after laser photocoagulation or intravitreal injection of bevacizumab suggests involvement of vascular endothelial growth factor. However, it is unlikely that macular vessels were directly affected by vascular endothelial growth factor because fluorescein angiography showed no macular leakage. In conclusion, distant retinal vascular leakage seems to diffuse through the outer retina to the macula and permeate into the subretinal space.