Diabetic Macular Edema: A Dual‑Pathway Disease

Diabetic macular edema (DME) remains a leading cause of preventable vision loss among working‑age adults with diabetes. The pathophysiology originates from chronic hyperglycemia that damages retinal microvasculature: pericyte loss, endothelial dysfunction, and breakdown of the inner blood‑retinal barrier (BRB). This allows plasma constituents—fluid, lipids, and inflammatory mediators—to accumulate in the macula, leading to central retinal thickening that impairs high‑acuity vision. Without timely intervention, irreversible photoreceptor damage ensues.

Two principal drivers sustain BRB breakdown:

  • Vascular endothelial growth factor (VEGF) – upregulates vascular permeability and stimulates angiogenesis.
  • Inflammatory cytokines (interleukin‑6, tumor necrosis factor‑alpha, monocyte chemoattractant protein‑1) – propagate edema through leukostasis, capillary dropout, and further barrier disruption.

Conventional monotherapies target only one of these pathways. This limitation explains why a substantial proportion of patients experience persistent edema, require frequent retreatment, or develop suboptimal responses. Dual therapy—combining two agents with complementary mechanisms—has rationally emerged to improve visual outcomes, reduce injection burden, and address the multifaceted pathophysiology of DME. This article provides an evidence‑based overview of dual therapy approaches, supported by key clinical data and practical guidance for patient selection and monitoring.

Traditional Monotherapies and Their Limitations

Anti‑VEGF Agents

Intravitreal anti‑VEGF injections are the current first‑line standard for center‑involving DME. Ranibizumab (Lucentis), aflibercept (Eylea), and bevacizumab (Avastin, used off‑label) have proven efficacy in landmark trials such as DRCR.net Protocol T, Protocol I, and the VIVID/VISTA studies. These agents reduce central subfield thickness (CST) and improve best‑corrected visual acuity (BCVA). However, real‑world outcomes often fall short of trial results due to treatment burden: monthly or bimonthly injections are necessary for optimal effect, and up to 40% of patients show an incomplete response. Non‑responders may have a more prominent inflammatory component that anti‑VEGF alone cannot control. For example, in Protocol T, approximately 30% of eyes receiving ranibizumab or aflibercept still had central subfield thickness ≥250 μm at 1 year, indicating residual edema. This heterogeneity in response underscores the need for alternative or adjunctive strategies.

Corticosteroids

Corticosteroids (dexamethasone intravitreal implant [Ozurdex], fluocinolone acetonide implant [Iluvien]) potently suppress inflammation, reduce VEGF indirectly, and stabilize the BRB. They are especially useful in pseudophakic eyes, patients with chronic DME unresponsive to anti‑VEGF, or those unable to tolerate frequent injections. The MEAD study (dexamethasone) reported a mean improvement of 4.1 letters at 3 years with implant treatment (0.7 mg), with 22% gaining ≥15 letters. The FAME study (fluocinolone) similarly showed significant edema reduction and vision gains in chronic DME. However, corticosteroids carry well‑known side effects: intraocular pressure (IOP) elevation and cataract progression. IOP elevation requiring topical therapy occurs in 30–40% of eyes, and cataract surgery is eventually needed in a majority of phakic eyes. In the MEAD study, 68% of phakic eyes developed cataract and 42% required surgery. Consequently, corticosteroids are rarely used as first‑line monotherapy in phakic patients.

Laser Photocoagulation

Focal/grid laser photocoagulation was the historical standard before the anti‑VEGF era. It seals leaking microaneurysms and destroys oxygen‑deprived retina, thereby reducing local VEGF production. While laser monotherapy now yields inferior visual outcomes compared with anti‑VEGF, it remains useful as an adjunctive treatment—especially when combined with anti‑VEGF—to reduce injection frequency and manage non‑center‑involving edema. The ETDRS study originally established the benefit of laser for DME, though modern laser techniques such as pattern scanning (PASCAL) and micropulse subthreshold laser minimize collateral damage and expand the therapeutic window.

Rationale for Dual Therapy

DME is a disease of both excess VEGF and inflammation. Dual therapy attacks both drivers synergistically, often achieving faster and more robust edema resolution. The primary goals are:

  • Improved anatomical and functional outcomes – greater reduction in CST and a higher proportion of patients gaining ≥15 letters on the ETDRS chart.
  • Reduced treatment burden – fewer injections per year, improving patient compliance and quality of life.
  • Better management of refractory DME – patients who plateau on a single agent often respond to the addition of a second modality.

Additionally, dual therapy may allow the use of lower doses of each agent, potentially reducing side effects, though this remains theoretical for most combinations. Economic analyses suggest that reducing injection frequency also lowers direct medical costs and indirect costs related to travel and missed work.

Evidence‑Based Dual Therapy Strategies

Anti‑VEGF plus Corticosteroids

This combination has been the most extensively studied dual approach. The DRCR.net Protocol U trial randomized patients with persistent DME despite ≥3 anti‑VEGF injections to continued ranibizumab alone versus ranibizumab plus dexamethasone implant. The combination group achieved faster CST reduction, but visual acuity differences were not statistically significant at 24 weeks. Subgroup analyses suggested that pseudophakic patients and those with more chronic edema derived the greatest benefit. Another important study, the BEVORDEX trial, compared alternating bevacizumab and dexamethasone implants with bevacizumab monotherapy over 2 years. The combination reduced injection frequency by approximately 40% while maintaining vision gains. A 2023 meta‑analysis of 12 randomized controlled trials confirmed that anti‑VEGF plus corticosteroid is superior to anti‑VEGF alone in reducing CST, particularly in eyes with an inflammatory phenotype (DRCR.net Protocol U). Real‑world retrospective studies have also reported that adding a dexamethasone implant to an anti‑VEGF regimen can salvage eyes that have failed multiple anti‑VEGF injections, with up to 60% of such eyes achieving resolution of cystoid edema within 1–3 months.

Key considerations:

  • Lens status – Pseudophakic eyes tolerate corticosteroids much better than phakic eyes, which are prone to rapidly progressive cataract. In phakic patients, steroids should be used short‑term with close monitoring, or the combination of anti‑VEGF plus laser may be preferred.
  • IOP response – Patients with a history of ocular hypertension, glaucoma, or steroid‑induced IOP elevation are poor candidates for corticosteroid‑containing dual therapy. Baseline IOP should be measured, and post‑injection IOP should be monitored at each visit. The risk of IOP elevation is dose‑dependent and often manageable with topical medications.
  • Cost and coverage – In many healthcare systems, dexamethasone and fluocinolone implants are approved for chronic DME after a trial of anti‑VEGF. Off‑label use may require prior authorization. Bevacizumab plus preservative‑free triamcinolone (off‑label) is a low‑cost alternative explored in some settings.

Anti‑VEGF plus Laser Therapy

Combining anti‑VEGF with focal/grid laser was widely studied in the early 2010s. DRCR.net Protocol I compared ranibizumab plus prompt laser, ranibizumab plus deferred laser, laser alone, and triamcinolone plus laser. At two years, the ranibizumab‑plus‑laser groups had similar visual acuity gains as ranibizumab alone, but the combination reduced the number of injections needed in the first year (median 8 vs. 10). The READ‑3 study found that combining ranibizumab with more intensive laser (every 4 months) did not improve outcomes compared with ranibizumab alone. More recent studies using pattern‑scanning laser (PASCAL) suggest that subthreshold laser can reduce edema without the scarring associated with conventional laser. The 2022 AAO Preferred Practice Pattern for Diabetic Retinopathy notes that laser can be considered as an adjunct to anti‑VEGF, particularly in eyes with non‑center‑involving DME or to reduce injection frequency in compliant patients (AAO PPP 2022). In clinical practice, many retina specialists reserve laser for persistent focal leaks after anti‑VEGF therapy rather than using it upfront. Micropulse laser, delivering subvisible burns, is increasingly popular because it spares the neurosensory retina while targeting the retinal pigment epithelium.

Practical points:

  • Laser is most effective when applied to discrete microaneurysms that are actively leaking on fluorescein angiography.
  • Combination with anti‑VEGF can reduce the total number of laser spots needed, minimizing retinal damage.
  • Subthreshold laser modalities (e.g., micropulse) are gaining popularity because they provide therapeutic effect with minimal scarring and less post‑laser pain.
  • Laser can also be used to treat areas of capillary non‑perfusion that contribute to VEGF production.

Emerging and Future Combinations

Bispecific antibodies: Faricimab (Vabysmo) simultaneously inhibits VEGF‑A and angiopoietin‑2 (Ang‑2), addressing both angiogenic and inflammatory pathways. In the YOSEMITE and RHINE phase 3 trials, faricimab achieved comparable or superior anatomical outcomes with extended dosing intervals (up to 16 weeks) compared with aflibercept (Faricimab phase 3 trials). While not a combination of two distinct therapies, faricimab represents a pharmacologic dual strategy that simplifies treatment regimens and reduces injection burden. The safety profile of faricimab is similar to other anti‑VEGF agents, with no unexpected IOP elevations or cataract issues.

Triple therapy (anti‑VEGF + steroid + laser): This is rarely used in routine care due to additive risk and complexity. However, small case series and retrospective analyses suggest it may be effective for extremely refractory DME, particularly in eyes with chronic cystoid changes that have failed multiple prior treatments. A 2022 study found that in eyes with persistent DME after >6 injections, triple therapy (ranibizumab + dexamethasone implant + micropulse laser) achieved CST reduction of >150 μm at 6 months. The additive IOP and cataract risks must be carefully weighed against the potential benefits. Triple therapy is best reserved for pseudophakic eyes with adequate IOP control.

Sustained‑release devices: The Port Delivery System (PDS) with ranibizumab, already approved for neovascular age‑related macular degeneration, is under investigation for DME. Combining PDS with a long‑acting corticosteroid could theoretically provide extended control with minimal intervention. The PDS implant is refillable every 6 months. Suprachoroidal delivery platforms (e.g., CLS‑AX, a microparticle formulation of axitinib, a tyrosine kinase inhibitor) are also in development and may allow for safer combination with topical agents or sustained‑release steroids. Axitinib inhibits multiple angiogenic receptors, providing broader anti‑permeability effects.

Patient Selection and Individualized Treatment

Not every patient with DME is a candidate for dual therapy. The decision should be based on:

  • Lens status: Pseudophakic eyes tolerate corticosteroids well; phakic eyes require careful discussion of cataract risk. If a phakic patient has a visually significant cataract that is likely to be removed soon, temporizing with steroid implant may be acceptable.
  • IOP profile: Baseline IOP, cup‑to‑disc ratio, and history of glaucoma or ocular hypertension should be assessed. Steroid‑containing dual therapy is relatively contraindicated in eyes with poorly controlled glaucoma. A baseline IOP >21 mmHg or known steroid response should prompt avoidance of corticosteroids.
  • Prior treatment response: Patients with incomplete anatomical response after 3–6 monthly anti‑VEGF injections (e.g., persistent cystoid edema on OCT) are prime candidates for adding a steroid. Those with a strong initial response but requiring high‑frequency maintenance may benefit from the extended durability of a steroid implant.
  • Injection burden and compliance: Patients who struggle with monthly visits may benefit from combination with a sustained‑release implant (dexamethasone implant lasts 3–6 months; fluocinolone implant lasts up to 3 years). Social and psychological factors should be considered.
  • Biomarkers: Baseline OCT features such as subretinal fluid, hyperreflective foci, and disorganization of retinal inner layers may predict a more inflammatory phenotype that responds better to steroid‑containing dual therapy. A 2021 review on DME biomarkers discusses how OCT characteristics can guide therapy selection (DME biomarkers review). For example, eyes with a hyperreflective foci count >20 at baseline show better response to dexamethasone implant.
  • Vitrectomy status: Prior vitrectomy can accelerate clearance of intravitreal drugs, potentially reducing treatment durability. In these eyes, combination therapy with a longer‑acting steroid may be advantageous.

Challenges and Considerations

Dual therapy is not without trade‑offs. The most significant risks are those associated with corticosteroids: cataract formation and IOP elevation. In the MEAD study, 68% of phakic eyes treated with dexamethasone implant developed cataract, and 42% required surgery. IOP elevation ≥10 mmHg from baseline occurred in 32% of eyes, with 1–2% requiring glaucoma surgery. For fluocinolone implant, the FAME study reported IOP elevation >30 mmHg in 37% of eyes, necessitating topical therapy in most. These risks must be discussed with patients and managed proactively with topical antiglaucoma medications or implant removal if necessary. Injection frequency can be reduced, but patients still require regular monitoring for IOP and retinal status.

Additionally, the cost of dual therapy—particularly branded anti‑VEGF agents plus a branded steroid implant—can be high. In resource‑limited settings, the choice may default to bevacizumab monotherapy, though even adding a generic steroid (e.g., preservative‑free triamcinolone acetonide, used off‑label) may be cost‑effective. Insurance coverage for combination therapy varies; prior authorization is often required. A cost‑effectiveness analysis published in 2023 suggested that anti‑VEGF plus dexamethasone implant for refractory DME was cost‑effective compared with continuing monthly anti‑VEGF monotherapy.

Monitoring protocols should include:

  • OCT imaging at each visit to assess CST and qualitative fluid status (intraretinal, subretinal).
  • IOP measurement at every visit, with a low threshold for starting topical therapy if IOP rises above 25 mmHg or increases >10 mmHg from baseline.
  • Slit‑lamp examination to monitor lens clarity in phakic eyes; consider earlier cataract surgery if vision is compromised.
  • Visual acuity testing using ETDRS or Snellen charts at each visit.
  • Anterior chamber reaction in the first week after steroid implant to rule out sterile hypopyon or endophthalmitis (rare).

Long‑term data from the FAME study (3‑year follow‑up) indicate that the benefits of fluocinolone implant outweigh risks when careful monitoring and management of IOP are employed.

Future Directions

Ongoing research is refining dual therapy through sustained‑delivery technologies and novel agents. The Port Delivery System with ranibizumab is being studied for DME; combining PDS with a long‑acting corticosteroid could provide near‑elimination of injections. Suprachoroidal delivery of tyrosine kinase inhibitors (e.g., axitinib) may allow for safer combination with topical agents. Biomarker‑driven algorithms are being developed to identify which patients will benefit most from a dual approach. Artificial intelligence analysis of OCT images may soon predict inflammatory phenotypes. Finally, oral or topical anti‑VEGF and anti‑inflammatory agents are in preclinical development, which could further transform the paradigm and make dual therapy more accessible. For example, oral pan‑VEGF receptor tyrosine kinase inhibitors like pazopanib and axitinib, while still early in ocular development, may offer systemic adjunctive therapy for DME. A recent review of emerging DME therapies highlights many of these innovations (emerging therapies review). Additionally, the DRCR.net continues to evaluate new combination strategies, including faricimab plus laser (DRCR.net clinical trials).

Conclusion

Dual therapy strategies for diabetic macular edema represent a rational evolution from monotherapy, addressing the disease's dual drivers: VEGF‑mediated vascular leakage and inflammation. The strongest evidence supports combining anti‑VEGF agents with corticosteroids in pseudophakic eyes or patients with persistent edema, while anti‑VEGF plus laser retains a role in select cases to reduce injection burden. Emerging bispecific antibodies such as faricimab and sustained‑delivery devices promise to simplify dual therapy without sacrificing efficacy. As with any advanced treatment paradigm, patient selection, shared decision‑making, and vigilant monitoring are essential to maximize benefits and minimize risks. With a growing armamentarium, clinicians can now tailor therapy to the individual patient's anatomy, lens status, and lifestyle, offering the best chance for preserving vision in this challenging disease.