The results revealed that all eyes with CSC showed, in addition to a thickened subfoveal choroid, a thinning of the inner choroidal layer and an enlargement of the underlying hyporeflective lumina in the ICGA hyperfluorescent regions. The diameter of the hyporeflective lumina was significantly (P < 0.001) associated with subfoveal choroidal thickness. A double-layer sign of the RPE/Bruch's membrane complex (seen in 51 [75%] eyes) was significantly (P = 0.025) more common in the chronic CSC group than that in the acute CSC group. The prevalence of an RPE microrip (8/68; 12 eyes) did not differ significantly between the chronic CSC group and the acute CSC group. A dome-shaped RPE detachment was present in 36 (53%) eyes.
The results of our study agree with previous investigations on an increased choroidal thickness in eyes with CSC and dilated choroidal vessels in the ICGA hyperfluorescent area.
1–3,6,12–15 The measurements in the previous studies (e.g., by Imamura et al.
3 : 505 ± 124 μm) yielded results similar to those in our study, 478 ± 114 μm (range, 232–695 μm). The finding of dilated choroidal vessels confirmed previous studies that applied indocyanine green angiography and reported on congested and dilated choroidal vessels close to the fovea in eyes with CSC.
1,2 The diameter of the hyporeflective choroidal lumina with a mean value of 330 ± 103 μm agreed with another recent study on patients with CSC, in which the diameter of the largest choroidal vessels was 305 ± 101 μm.
20 Besides the vessel dilatation as cause for the choroidal thickening, an interstitial choroidal edema may have played a role. Hyperpermeability of the choriocapillaris has been considered to be responsible for the focal choroidal hyperfluorescence observed on ICGA images in patients with CSC.
2 The reasons have remained unclear as to why the choroidal vessels showed a dilatation with secondary choroidal thickening in patients with CSC. Since choroidal vessels are innervated by the autonomic system and since ganglion cells have been found in the posterior choroid, the dilatation of the choroidal vessels may have been regulated by innervation.
21,22 Additionally, substances such as endothelin-1 or inflammatory cytokines may participate in the pathogenesis of the choroidal swelling in eyes with CSC, since endothelin-1 has been reported to be involved in the autoregulation of the choroidal vasculature.
23,24
An RPE detachment is a well-known feature of CSC. The prevalence of an RPE detachment of 53% (36/68) partially differs from the findings obtained in other investigations. Using a time-domain OCT and six radial scans, Montero and Ruiz-Monero
25 detected an RPE detachment in 8% of eyes with acute CSC. Applying en face OCT, van Velthoven and colleagues
26 found an RPE detachment in 52% of eyes with active CSC and in 100% of eyes with inactive CSC. Mitarai and associates
27 reported on the prevalence of 63% for an RPE detachment in eyes with acute CSC. Reasons for the discrepancies between the studies may be the difference in the composition of the study population (acute CSC versus chronic CSC), the relatively small size of the study samples, and differences in the methodology such as the type of OCT device and the type of scanning pattern used. In addition to an already described dome-shaped RPE detachment, we found in our study a so-called double-layer sign characterized by two highly reflective layers: an undulated RPE line and a hyperreflective straight line representing Bruch's membrane. A similar finding has been reported for eyes with polypoidal choroidal vasculopathy.
18 Our study showed that the double-layer sign was not pathognomonic for polypoidal choroidal vasculopathy, but that it could also be found in CSC. In five eyes with a double-layer sign, the latter became continuous with a dome-shaped RPE detachment in the ICGA hyperfluorescent region. It may lead to the assumption that both changes were the result of choroidal hypermeability, and that the double-layer sign was a special form of an RPE detachment. Examining eyes with polypoidal choroidal vasculopathy, Ojima and colleagues
28 noted a hyperreflective material between the two layers of the double-layer sign, and considered it as a sign of polypoidal lesions and branching vascular networks. In our study, however, the space between the RPE line and Bruch's membrane line in the region of the double-layer sign appeared to be hyporeflective in the majority (80%) of the eyes. One may postulate that the difference in the reflectivity between eyes with PCV and eyes with CSC may be due to differences between the two diseases in the hyperpermeability of choroidal vessels.
A tear or rip of the RPE in CSC has been described in CSC eyes of patients after a steroid exposure or of younger patients showing subretinal fibrin and a rapidly increasing fluid accumulation.
29 The patients included in our study had no history of steroid intake and were predominantly middle-aged. These differences may explain why the prevalence of RPE microrips was lower in our study population than that in the patients of previous investigations. An increased choroidal hydrostatic pressure due to the hypermeability of the choroidal vessels has been considered to be responsible for the RPE detachment.
1,2,30 This mechanism may lead to a breach or microrip of RPE by which the pressure beneath the RPE and above the RPE equalizes. It has remained unclear whether such an RPE microrip is a necessary condition for the fluid access from the sub-RPE space into the subretinal space in eyes with CSC. In that case, a prevalence of 100% instead of 12% as in our study should be expected. It has remained elusive whether limitations in the axial and lateral resolution of available OCT images and the scanning pattern may have been reasons for missing some microrips. The spectral domain OCT (Spectralis; Heidelberg Engineering Co.) has roughly a lateral resolution of 14 μm and an axial resolution of 7 μm. In contrast to a typical large RPE rip characterized by a torn RPE and a bared choroid, an RPE microrip is a small defect in the RPE layer, which may close spontaneously.
31
The present study in association with the investigations mentioned above showed that OCT characteristics of CSC were an increased subfoveal choroidal thickness with dilated hyporeflective lumina (vessels) beneath a thinned inner choroidal layer in the ICGA hyperfluorescent area, RPE detachments with hyporeflectivity beneath the detached RPE, and RPE microrips. These features may be useful to differentiate CSC from masquerading diseases affecting the macular region, such as Vogt-Koyanagi-Harada's syndrome, uveal effusion syndrome, polypoidal choroidal vasculopathy, posterior scleritis, pregnancy-induced choroidopathy, choroidal hemangioma, lymphoblastic leukemia, or other metastatic choroidal disease.
32,33
The internal reflectivity within the dilated choroidal lumina partially appeared to be similar to that of subretinal fluid. Since this was a study based on imaging techniques, one could not infer that the similar appearance of the fluid in the hyporeflective lumina and in the subretinal fluid could be taken as a hint that the biochemical compositions of the fluids were similar.
Potential limitations of our study should be mentioned. First, the enhanced depth imaging mode of the OCT technique is an indirect method to image and measure the RPE, Bruch's membrane, and the choroid. It has not yet been unequivocally proven which histologic structures correspond to which line in the EDI-OCT images. Second, due to limitations in the spatial resolution of the EDI-OCT and the scanning pattern, some RPE microrips may have been overlooked, leading to an underestimation of the prevalence of this abnormality. Third, since it is an imaging study, nothing could be said about the biochemical composition of the material beneath the RPE and in the dilated choroidal lumina.
In conclusion, acute and chronic CSCs are characterized by a thinning of the inner choroidal layer with an enlargement of the underlying large hyporeflective choroidal lumina found in all eyes, in addition to a dome-shaped RPE elevation, a double-layer sign of the RPE/Bruch's membrane complex, and RPE microrips in some eyes. EDI-OCT may be a valuable help in the diagnosis of CSC.