Foveal Eversion is Associated with High Persistence of Macular Edema and Visual Acuity Deterioration in Retinal Vein Occlusion

Retinal vein occlusion (RVO) represents the second most common cause of retinal vascular diseases after diabetic retinopathy and a major cause of vision deterioration in developed countries [1‐3]. A common complication of both central RVO (CRVO) and branch RVO (BRVO) is macular edema, leading to reduction of visual acuity. Macular edema is successfully managed by anti-vascular endothelial growth factor (anti-VEGF) injections and corticosteroid implants [4‐11], providing fundamental tools for inducing the resolution of edema and preserving visual function [12, 13]. The presence of ischemia (i-RVO) worsens the clinical picture of CRVO and BRVO, and laser treatment may be considered an additional procedure [14‐17]. However, long-term studies showed how heterogeneous the response to treatments and visual outcome may be [18, 19]. Similarly to what happens in diabetic retinopathy, a challenging and unpredictable condition is the presence of persistent macular edema, remarkably affecting the morpho-functional outcome. In a recent paper, we proposed foveal eversion (FE) as a structural optical coherence tomography (OCT) biomarker associated with high prevalence of macular edema and poor outcome in diabetic retinopathy [20]. Furthermore, the identification of more patterns of FE further improves the clinically meaningful categorization of diabetic patients [21], supporting the role of FE as a feasible and useful biomarker to be used in clinical practice.

Our main hypothesis is that FE might also be applied to macular edema secondary to CRVO and BRVO. Hence, the main aim of the present study was to investigate the role of FE in the RVO clinical setting, assessing the relationship to persistent macular edema and visual outcome.

The study was designed as a retrospective, observational case series. Patients affected by CRVO and BRVO were analyzed at the Department of Ophthalmology, IRCCS San Raffaele Scientific Institute, Milan, Italy. The study was approved by the Ethics Committee of San Raffaele Scientific Institute, Milan, Italy (protocol ID: MIRD), and conducted in compliance with Ethics Guidelines and in accordance with the Declaration of Helsinki. No identifiable data were used, and all patients provided informed consent. The diagnosis of CRVO or BRVO was made by structural OCT and fluorescein angiography examinations.

The inclusion criterion was the diagnosis of CRVO or BRVO complicated by a clinically significant macular edema, treated by anti-VEGF and/or corticosteroids, with a minimum follow-up of 12 months. Exclusion criteria were: high media opacities, other eye diseases potentially affecting the interpretation of the present findings, uncontrolled arterial hypertension or diabetes mellitus, and any ophthalmic surgery in the 6 months before the diagnosis of RVO. Furthermore, we excluded those eyes affected by combined RVO and retinal artery occlusion and eyes characterized by extensive involvement of the posterior pole with severe disruption of retinal structures and baseline visual acuity < + 1.0 LogMAR.

The collected data included LogMAR best corrected visual acuity (BCVA) evaluated by standard ETDRS charts, anterior and posterior segment slit-lamp evaluation and Goldmann applanation tonometry. Structural OCT examination (Spectralis HRA2 + OCT, Heidelberg Engineering, Heidelberg, Germany) was performed at each follow-up visit, with radial, raster and dense scans (ART > 25; enhanced depth imaging).

Structural OCT scans were used to extract central macular thickness (CMT), considering the central ETDRS subfield; fluorescein angiography was revised to assess the presence of peripheral capillary non-perfusion, intended as at least ten-disc areas that lacked fluorescein-filled capillaries.

Moreover, based on structural OCT images, we classified RVO eyes in two subgroups, namely with or without FE. FE was defined as the complete eversion of the foveal profile, as detected on a horizontal, foveal-centered OCT scan. We assessed the presence of FE at baseline or its occurrence during the follow-up. Furthermore, we classified FE eyes in three further subgroups or FE patterns: pattern 1a is defined as the presence of FE with intraretinal vertical columns developed in the context of the hyporeflective cystic space; pattern 1b is defined as the presence of FE with multiple intraretinal fan-like, vertical lines developed in the context of the hyporeflective cystic space; pattern 2 is defined as the presence of FE with only hyporeflective intraretinal cystic signal and no signs of hyperreflective vertical lines. To standardize the differentiation of pattern 1a and pattern 1b, we established FE pattern 1a as the presence of a major central column of at least 100 µm with the possible presence of thinner lines. The other cases were considered FE pattern 1b (Fig. 1). Eyes with FE were categorized accordingly with the first appearance of FE subtype (namely pattern 1a, 1b or 2).

All the eyes were treated using a loading dose of 3-monthly anti-VEGF injections following a treat-and-extend treatment regimen. Criteria for treatment switching to another anti-VEGF molecule of corticosteroid implant included BCVA deterioration > 1 ETDRS line and/or OCT-based evidence of CMT reduction < 25% after treatments, established by ophthalmologist discretion.

Furthermore, persistent macular edema was defined as the persistence of intraretinal fluid for > 6 months, whereas resolved macular edema was defined as complete resolution with recovery of the foveal depression for > 6 months. All the other cases were considered recurrent macular edema.

The main outcome measure was the assessment of the relationship between FE and the clinical course of RVO, considering macular edema persistence, recurrence or resolution, and the visual outcome. The secondary outcome was the assessment of the relationship between each FE pattern (1a, 1b and 2) and both visual outcome and the presence of persistent macular edema. We considered the parameters measured at baseline after 1 year and at the last follow-up visit. Two independent blinded graders performed all the measurements (AA, AR). Interclass correlation coefficient was measured to assess the agreement between the two graders through a two-way random-effects model. For the statistical analyses we included the following fixed variables: age, gender, systemic arterial hypertension, and type and duration of diabetes mellitus. Additional variables were type and number of treatments (anti-VEGF, corticosteroids) and baseline features (BCVA, CMT, i-RVO). All the statistical analyses were performed using the SPSS software package (SPSS, Chicago, IL, USA). In our statistical models, we evaluated the normality distribution of each variable with frequency histograms and quantile-quantile plots. Descriptive statistics of continuous variables were reported as mean ± standard deviation, whereas frequency and proportions were described as categorical variables. Continuous variables were statistically evaluated using the two-tailed t-test. One-way ANOVA analysis assessed the differences between FE eyes and eyes without FE. Tau-Kendall correlation analysis was adopted to assess the relationship between the collected parameters. Statistical significance was set at p < 0.05.

In the present study we assessed the diagnostic role of FE in the clinical setting of CRVO and BRVO. The presence of FE was associated with worse clinical course and outcome compared with eyes without FE. CRVO and BRVO eyes may disclose three different patterns of FE. The clinical course of FE pattern 1a and FE pattern 1b was overall worse than in eyes without FE, although these patients showed similar BCVA at the end of the follow-up. Conversely, for CRVO and BRVO eyes, FE pattern 2 subgroup was characterized by remarkably high prevalence of persistent macular edema (> 50% of cases) and significantly worse visual outcome than all the other RVO eyes. FE characterized the minority of CRVO and BRVO eyes at baseline, whereas it may occur over the follow-up. In addition, the presence of peripheral capillary non-perfusion was slightly associated with worse clinical outcome and with the onset of FE.

Our data confirmed the fact that the number of intravitreal treatments may be associated with visual function stability, but not a significant improvement [18, 19]. Furthermore, we found no significant results related to laser treatments, further confirming its minor role in the management of RVO.

The most interesting result of the present study regards the perspective of making a new RVO classification based on the presence and type of FE. FE has been previously interpreted as a structural OCT sign of Müller cell impairment [20, 21]. Müller cells are responsible for the entire functional and structural homeostasis of the human retina, playing a major role in the intra-extracellular fluid balance and structural integrity of the foveal region [22, 23]. Although Müller cells are known to play a major role in the pathogenesis of diabetic retinopathy and diabetic macular edema [23‐25], in this study we hypothesized their potential role also in the pathogenesis and clinical course of macular edema secondary to RVO. Although representing a relatively novel research field, some support for our hypothesis can already be found in the literature. Indeed, a previous study highlighted how the assessment of macular edema features, including the extension and distribution of cysts, is clinically relevant for collecting information about RVO outcome [26]. Mouse models of BRVO highlighted the importance of Müller cells in governing the reabsorption of intraretinal fluid [27]. Müller cells showed a rearrangement of genetic expression after BRVO, with downregulation of key anti-edema factors such as Kir4.1 and aquaporins, osmotic swelling of Müller cells and their damage [28]. A likely mechanism leading to the damage of Müller cells is the hypoxic status induced by vascular stasis and capillary closure characterizing both CRVO and BRVO [29]. As a consequence of Müller cells impairment, it might be possible to observe upregulation of permeability and pro-edema factors and the loss of intraretinal fluid homeostasis [29, 30]. In this scenario, we might assume that the greater the vascular stasis and hypoxia are, the higher the probability to observe Müller cell damage and, as an indirect structural OCT sign, the onset of FE and its patterns. Our pathogenic hypothesis regarding the progression of FE patterns and the central role of Müller cells is shown in Fig. 5, where we used a case of BRVO with 5-year follow-up, showing the early sign of Müller cell gliosis following a progressive worsening of retinal morphology and evolution toward FE pattern 2. This should be considered only a humble attempt to visually represent our pathogenic hypothesis. The timing and all the microstructural considerations should be carefully evaluated since this is not supported by big data. Further studies involving more eyes and providing histological support are warranted to establish the pathogenesis of FE and involvement of Müller cells.

We are aware our study is potentially affected by several limitations, first of all related to the retrospective design of this investigation. This study setting made the assessment of FE progression toward different patterns impossible. Indeed, the retrospective nature of the present investigation carried a tangible risk of losing useful information to assess the evolution of FE over the follow-up. For this reason, further prospective studies are warranted to reliably define the evolution and progression of FE patterns over time. Since our study was mainly focused on the assessment of the intraretinal status in RVO, we did not perform specific assessments of outer retinal status, including the clinical impact of subretinal fluid. We are aware this represents an important biomarker in RVO [26]; however, we may assume a completely different pathogenesis as well as different retinal cytotypes involved regarding intraretinal fluid and FE. However, we believe that the clinical interaction among subretinal fluid, intraretinal fluid and FE should be investigated in the future. Moreover, the real-life setting of the study, with no standardized decisional process regarding the treatment strategy, and the variable follow-up remarkably affected the possibility to explore the effect of treatments, considering both intravitreal molecules and laser, on the onset and course of macular edema and FE. However, a strong point of the present study is the collection of data from a large cohort of eyes affected by CRVO and BRVO. Moreover, although all imaging-based approaches are potentially affected by artifacts [31], in this case their influence is poor because of the dichotomic nature of FE (present/absent) and the high feasibility of this kind of investigation. Furthermore, all our hypotheses represent mere speculations, making the present study a first exploratory investigation. Further prospective studies are warranted to draw definite conclusions regarding the role of FE evaluation in clinical practice.

Our study highlighted how FE, detected on structural OCT, can categorize more subtypes of CRVO and BRVO, characterized by different morphologies and functional outcomes. From the perspective of customized treatment strategies and the adoption of artificial intelligence-based diagnostic approaches, FE might represent a useful metric to be included in the diagnostic workup of RVO.