Retinal vein occlusion (RVO), a common retinal vascular disease, is frequently treated with anti-vascular endothelial growth factor (anti-VEGF) agents as first-line therapy. However, anti-VEGF monotherapy lacks neuroprotective effects, primarily targets vascular leakage and neovascularization, and requires frequent long-term injections that impose substantial economic burdens. Combined therapeutic strategies addressing both vascular pathology and neural damage are therefore being explored.

This article describes the protocol for a randomized, outcome-blinded, placebo-controlled clinical trial evaluating mecobalamin (a widely used neuroprotective drug) in combination with anti-VEGF for the treatment of macular edema (ME). A total of 120 eligible RVO patients will be enrolled from the First Affiliated Hospital of Chongqing Medical University. Participants will be randomly assigned (1:1) to an experimental group and a control group. The experimental group will receive conventional anti-VEGF therapy plus oral mecobalamin capsules for 6 months, while the control group will receive the same anti-VEGF treatment plus a placebo for 6 months. All patients will undergo one year of follow-up after initial treatment, with visits at 1, 3, 6, 9, and 12 months.

The primary outcome is the change in central subfield thickness (CST) from baseline to one year post-initial treatment. Secondary outcomes include:

• Change in best-corrected visual acuity (BCVA) from baseline over time,
• Capillary density,
• Cone photoreceptor distribution characteristics,
• Mean light sensitivity and fixation stability,
• Serum vitamin B12 levels,
• Number of anti-VEGF treatments,
• Injection frequency (times per year),
• Treatment interval,
• Incidence and severity of adverse events (AEs) and serious adverse events (SAEs).

This trial evaluates a novel "neuroprotection + vascular intervention" strategy combining mecobalamin with anti-VEGF therapy. The trial aims to provide high-level evidence for synergistic RVO treatment, with the potential to reduce recurrence rates and improve long-term visual function prognosis.

Retinal vein occlusion (RVO) is the second leading cause of vision loss resulting from retinal vascular disease, with a prevalence of approximately 0.6%-1.6%. Macular edema (ME) secondary to RVO is a common cause of vision loss. RVO induces hypoxia and ischemia, leading to neuronal loss and increased vascular permeability. Therefore, anti-vascular endothelial growth factor (anti-VEGF) injections are widely used as first-line treatment for RVO-ME, achieving remarkable therapeutic effects. Pivotal trials (BRAVO and CRUISE) demonstrated that anti-VEGF therapy significantly improves best-corrected visual acuity (BCVA) and reduces retinal swelling, enhancing BCVA by 12-15 ETDRS letters and decreasing central subfield thickness (CST) by about 200 μm.

However, therapies targeting VEGF alone may not sufficiently manage this condition. Although anti-VEGF agents improve edema and short-term visual acuity, long-term efficacy is limited by frequent injections and inadequate nerve repair. Additionally, anti-VEGF fails to reverse neurodegenerative changes such as photoreceptor apoptosis and Müller cell activation. Consequently, combined strategies synchronizing vascular leakage repair and neuroprotection have become an international research priority.

As a typical neurotrophic drug, mecobalamin repairs nerve tissue and accelerates axonal regeneration and myelination. Mecobalamin, a vitamin B12 derivative, is more readily absorbed by nerve cells than vitamin B12 itself. As a methyl donor, it facilitates homocysteine-to-methionine conversion, supporting nucleic acid, protein, and phospholipid synthesis critical for myelin formation. This process is essential for nerve myelin formation and repair. Mecobalamin accelerates lipid, protein, and nucleic acid metabolism in neurons, enhancing nerve function recovery and alleviating neurological symptoms. Previous studies found that oral vitamin B1 and mecobalamin improve corneal nerve fiber density. The neuroprotective drug was shown to attenuate oxidative stress in animal models, but clinical translation evidence remains insufficient.

Notably, VEGF exhibits dual roles as both a permeability factor and neuroprotectant. Excessive VEGF inhibition may exacerbate retinal ganglion cell apoptosis. Although mecobalamin promotes neural repair, its spatiotemporal synergy with anti-VEGF in neurovascular unit (NVU) recovery remains uncharacterized. This prospective, double-blind, randomized controlled trial evaluates the synergistic effects of mecobalamin combined with anti-VEGF therapy on NVU repair in RVO-ME, utilizing dual-targeted "vascular intervention + neuroprotection" to overcome current limitations.

Current ME efficacy assessment relies primarily on optical coherence tomography (OCT)-derived CST, with few tertiary centers employing OCT angiography (OCTA), multifocal electroretinography (mfERG), or adaptive optics (AO). Existing metrics inadequately characterize NVU function or anti-VEGF effects on non-vascular components (glia, photoreceptors). The absence of standardized NVU repair quantification impedes cross-study comparisons. To address this, the investigators propose a multimodal "OCT structural tomography + OCTA flow stratification + AO cellular morphology + microperimetry functional mapping + mfERG functional localization" assessment to establish a new therapeutic standard.

Primary objectives:

1. Evaluate mecobalamin combined with anti-VEGF therapy on CST and BCVA improvement in RVO-ME patients
2. Validate mecobalamin's synergistic contribution to NVU repair using multimodal imaging

This prospective, double-blind, randomized controlled intervention study will enroll RVO patients at the First Affiliated Hospital of Chongqing Medical University from 1 August 2025 to 31 July 2027. The study aims to evaluate mecobalamin combined with conbercept on retinal NVU repair in RVO-ME patients. The Institutional Review Board approved the protocol (No. 2025-387-01) on 26 May 2025. The study adheres to the Declaration of Helsinki and Ethical Guidelines for Medical and Biological Research Involving Human Subjects. All participants will provide written informed consent; an opt-out approach will be implemented where applicable.

Primary outcomes will be assessed at 12 months post-initial treatment. Eligible patients will be identified during initial ophthalmology visits. Recruitment will be performed by ophthalmologists. After eligibility confirmation and informed consent, standard treatment commences immediately. Reasons for non-participation will be documented. Enrolled eyes will receive alphanumeric ID codes and be randomly assigned to treatment groups two days before intervention using block randomization (Statistical Package R, v3.6).

Treatment groups (1:1 randomization):

Experimental: Intravitreal conbercept (0.5 mg monthly × 3 months, then pro re nata) + oral mecobalamin (0.5 mg TID × 6 months) Control: Identical anti-VEGF regimen + placebo capsules (TID × 6 months)

Recurrence criteria:

• CST increase ≥50 μm from minimum recorded value
• BCVA decrease ≥5 ETDRS letters
• New/increased intraretinal/subretinal fluid

Assessments occur at baseline, initial treatment day, and 1, 3, 6, 9, and 12 months post-treatment. Follow-up requirements are detailed in supplementary materials. Study exit occurs at 1-year completion or if:

1. Three consecutive missed visits
2. Poor compliance/withdrawal request
3. New systemic disease affecting outcomes

Protocol violators or cases with essential missing data will be excluded from per-protocol analysis. Clinical trial staff will collect data under investigator supervision. Researchers will ensure data accuracy, completeness, and timeliness. The schedule for enrollment, interventions, and assessments is outlined in supplementary materials. Clinical/ophthalmic data will be entered into an electronic data capture system. All data will remain confidential per regulatory requirements.

Inclusion Criteria:

Diagnosis of RVO meeting the international diagnostic criteria 4,12, age 18-80 years, gender not limited Treatment-naive RVO-ME (no prior anti-VEGF, glucocorticoid, or laser therapy) The CST is confirmed to be ≥300μm by OCT The baseline BCVA (ETDRS letter count) was 20/400 to 20/40 (34-78 letters) Signed informed consent and ability to comply with follow-up.

Exclusion Criteria:

Combined with other eye diseases that cause ME (such as diabetic retinopathy, uveitis) Media opacities affecting imaging (such as severe cataract, vitreous hemorrhage) Prior anti-VEGF, steroid, or macular laser therapy Systemic use of glucocorticoids or immunosuppressants within 3 months Uncontrolled systemic disease (hypertension, diabetes, hepatic/renal dysfunction); pregnant or lactating women Allergy to mecobalamin or conbercept Unable to cooperate with examinations or follow-up, or participate in other clinical trials within one year

This study commits to sharing fully anonymized individual participant data that underpin the conclusions of the primary results paper (evaluating the efficacy and safety of mecobalamin combined with anti-VEGF therapy for RVO-ME).

De-identified individual participant data (including e.g., Baseline Characteristics & Demographics, Primary and Secondary Outcome Measures) and the study protocol/statistical plan will be shared after the primary results are published. Access: Available to researchers with a sound methodological proposal for approved scientific purposes. Requests should be directed to the Corresponding Author (dengwenli@hospital.cqmu.edu.cn) and require a signed Data Access Agreement.

Data will become available after the publication of the primary trial results and will be accessible for a reasonable period.

Data are available to researchers for legitimate scientific purposes. Accessible information includes de-identified participant data and supporting documents.Access is contingent upon submission of a research proposal and signing a Data Access Agreement.

All study participants will be blinded to their intervention assignment. The drug and placebo were identical in packaging, appearance, and smell. Mecobalamin and placebo were assigned by an independent pharmacy according to random codes. Evaluators were blinded to group allocation. All study medications (active and placebo) will be identically labeled to ensure adequate blinding of study physicians and patients. Patients, principal investigators, assessor physicians, imaging technicians, central reading center graders, study coordinators, and BCVA examiners will be blinded to treatment assignment.