Introduction: The Growing Challenge of Diabetic Eye Disease

Diabetic eye disease, primarily diabetic retinopathy (DR) and diabetic macular edema (DME), remains the leading cause of preventable blindness among working-age adults worldwide. With the global diabetes epidemic accelerating, the burden of vision-threatening retinopathy continues to rise at an alarming rate. The International Diabetes Federation projects that over 700 million adults will have diabetes by 2045, placing unprecedented pressure on healthcare systems to manage retinal complications. Effective treatment strategies are essential to halt disease progression and preserve visual function, yet the optimal therapeutic approach remains a subject of active debate. Among the most consequential clinical decisions clinicians face is whether to treat with a single modality, known as monotherapy, or to combine two therapeutic approaches, termed dual therapy. This expanded analysis examines the evidence, practical considerations, patient selection criteria, and future directions for both strategies, equipping clinicians and patients with the knowledge needed to make informed, individualized treatment decisions.

Pathophysiology and Clinical Spectrum of Diabetic Retinopathy

Diabetic retinopathy results from chronic hyperglycemia-induced damage to the retinal microvasculature, a process that unfolds over years and involves multiple interconnected pathways. Key mechanisms include pericyte loss, which weakens capillary walls and leads to microaneurysm formation; capillary occlusion caused by endothelial dysfunction and leukostasis; retinal ischemia resulting from progressive capillary nonperfusion; and upregulation of vascular endothelial growth factor (VEGF) driven by hypoxia. VEGF drives abnormal blood vessel growth, known as proliferative DR, and increases vascular permeability, leading to DME. Inflammatory pathways also contribute significantly, particularly through the release of cytokines such as interleukin-6 and tumor necrosis factor-alpha, as well as leukocyte adhesion and infiltration. Understanding these interdependent processes explains why targeting a single pathway may be insufficient in advanced disease, forming the strong rationale for dual therapy approaches that address both angiogenic and inflammatory components simultaneously.

The disease is classified into distinct stages based on clinical examination and imaging findings. Non-proliferative DR (NPDR) is graded as mild, moderate, or severe depending on the extent of retinal hemorrhages, microaneurysms, venous beading, and intraretinal microvascular abnormalities. Proliferative DR (PDR) is characterized by the development of neovascularization on the optic disc or elsewhere in the retina, often accompanied by vitreous hemorrhage or fibrous traction. DME, defined as retinal thickening involving the macula, can occur at any stage of DR and is the most common cause of vision loss in diabetic patients. Treatment decisions depend on severity, the presence of center-involving macular edema, and patient-specific factors such as glycemic control, blood pressure, and renal function. This complexity underscores the need for a nuanced approach to therapy selection.

Monotherapy: Established Approaches and Their Strengths

Anti-VEGF Injections as Monotherapy

Anti-VEGF agents, including ranibizumab, aflibercept, and bevacizumab, have revolutionized the management of both DME and PDR over the past two decades. As monotherapy, these drugs achieve significant reduction in central macular thickness and clinically meaningful improvement in best-corrected visual acuity. The landmark clinical trials provide robust evidence: the DRCR.net Protocol T demonstrated that aflibercept provides superior visual gains in eyes with worse baseline acuity, while all three agents show comparable efficacy in eyes with better starting vision. Monotherapy is favored for its targeted action on the VEGF pathway, a predictable safety profile, and a lower burden of side effects compared to combination regimens that involve corticosteroids or laser. However, anti-VEGF monotherapy often requires frequent injections, typically monthly or bimonthly during the first year, placing a significant burden on patients and healthcare systems. Importantly, a subset of patients experience incomplete or inadequate response due to the contribution of non-VEGF-mediated inflammatory pathways, which highlights the limitations of single-agent approaches in complex disease.

Laser Photocoagulation as Monotherapy

Focal and grid laser photocoagulation for DME and panretinal photocoagulation (PRP) for PDR were the standard of care before the advent of anti-VEGF therapy and remain important tools in the modern armamentarium. Laser monotherapy reduces the risk of severe vision loss by ablating ischemic retina, which decreases VEGF production, and by sealing leaking microaneurysms and abnormal blood vessels. It is particularly effective for non-center-involving DME and in clinical settings where intravitreal injections are not feasible due to cost, access, or patient preference. In many resource-limited regions, laser remains the primary treatment modality for PDR. However, laser therapy can cause permanent scotomas, reduced night vision, and progressive retinal damage over time due to the destruction of retinal tissue. The advent of pattern scanning laser and micropulse laser technologies has reduced some of these adverse effects by delivering more targeted, less destructive energy. In contemporary practice, laser is often reserved for adjunctive or second-line roles, though PRP monotherapy remains a highly effective and durable option for PDR in patients who cannot receive or decline anti-VEGF therapy.

Corticosteroid Monotherapy

Intravitreal corticosteroids, including triamcinolone acetonide, the dexamethasone implant, and the fluocinolone acetonide implant, act on multiple inflammatory cytokines and reduce VEGF expression indirectly through their broad anti-inflammatory effects. These agents are particularly useful for patients with persistent DME despite anti-VEGF therapy, those who have contraindications to anti-VEGF agents such as recent thromboembolic events, or for pseudophakic eyes where cataract risk is less relevant. As monotherapy, steroids can effectively reduce macular edema, but their use is limited by a well-characterized side effect profile. Efficacy in DME is supported by pivotal trials such as the MEAD study for the dexamethasone implant and the FAME study for the fluocinolone acetonide implant, both of which demonstrated sustained improvements in visual acuity and macular thickness. However, corticosteroids carry significant risks, including cataract progression in phakic eyes and elevated intraocular pressure, which can require glaucoma medications or even surgical intervention. These safety concerns often necessitate close monitoring and may make dual therapy with anti-VEGF agents attractive, as lower steroid doses can be used while maintaining efficacy.

Dual Therapy: Rationale and Common Combinations

Dual therapy aims to simultaneously block VEGF and inflammatory pathways, thereby addressing the multifactorial nature of diabetic retinopathy more comprehensively than any single agent alone. The theoretical advantage is a synergistic effect: anti-VEGF agents reduce vascular permeability and neovascularization rapidly, while corticosteroids suppress inflammatory cytokine release and stabilize the blood-retinal barrier over a longer duration. Additionally, combining a corticosteroid may reduce the frequency of anti-VEGF injections needed, easing the treatment burden on patients and reducing the cumulative risks associated with repeated intravitreal injections. Common combination strategies used in clinical practice include:

  • Anti-VEGF plus corticosteroid, such as ranibizumab or aflibercept combined with the dexamethasone implant or triamcinolone acetonide
  • Anti-VEGF plus laser photocoagulation, such as aflibercept combined with focal or grid laser for DME, or ranibizumab combined with PRP for PDR
  • Corticosteroid plus laser, a less commonly employed combination due to overlapping risks, particularly ocular hypertension and cataract

The choice of combination depends on disease phenotype, prior treatment response, and patient characteristics. For patients with prominent inflammatory features on OCT, such as cystoid spaces or subretinal fluid, an anti-VEGF plus corticosteroid approach may be particularly advantageous.

Evidence from Comparative Clinical Studies

Several high-quality clinical trials have compared monotherapy versus dual therapy in DME and PDR, providing evidence to guide clinical decision-making. The DRCR.net Protocol I originally tested ranibizumab with prompt versus deferred laser and found that both anti-VEGF plus laser and anti-VEGF alone improved vision, with fewer injections required in the combination arm, though visual outcomes were similar at three years. This finding suggests that combination therapy can reduce treatment burden without compromising visual results. For PDR, the Protocol S study demonstrated that ranibizumab monotherapy was non-inferior to PRP for visual acuity at two years, but combination therapy involving ranibizumab plus PRP reduced the need for subsequent vitrectomy, indicating a potential structural benefit. A comprehensive meta-analysis published in Ophthalmology in 2019 concluded that anti-VEGF plus corticosteroid therapy leads to greater reduction in central subfield thickness than anti-VEGF monotherapy in DME, though this anatomic advantage did not consistently translate into greater visual gain and was accompanied by a higher rate of intraocular pressure elevation. Thus, dual therapy may be most beneficial for patients with refractory DME, persistent intraretinal fluid, or high-risk PDR, where anatomic improvements are critical for long-term outcomes.

A notable study from the Diabetic Retinopathy Clinical Research Network found that in eyes with persistent DME after six monthly anti-VEGF injections, switching to a dexamethasone implant in combination with continued anti-VEGF therapy improved anatomic outcomes, including central subfield thickness and OCT fluid resolution. Similarly, the VIVID and VISTA trials for aflibercept allowed rescue laser, showing additional benefit in cases with suboptimal response. Across studies, dual therapy appears superior for anatomic response but not always for visual acuity endpoints, and the risk-benefit ratio must be individualized based on the specific clinical scenario. These data underscore that dual therapy should not be employed routinely but reserved for situations where monotherapy is insufficient.

Risks and Burdens of Dual Therapy

Combining treatments increases the potential for adverse events and adds complexity to patient management. Key considerations include the following:

  • Elevated intraocular pressure: Corticosteroids, especially when used in dual therapy, commonly raise intraocular pressure, often within weeks to months of initiation. This requires regular monitoring and may necessitate glaucoma medications or, in refractory cases, surgical intervention. Patients with pre-existing ocular hypertension or glaucoma are particularly vulnerable.
  • Cataract progression: Long-term steroid use accelerates cataract formation, particularly in phakic patients. Cataract surgery may eventually be required, which adds additional risk and cost, especially in patients with diabetic eye disease who may have compromised corneal endothelium or poor wound healing.
  • Endophthalmitis and sterile inflammation: More injections, or the combination of injections and laser procedures, increase the cumulative risk of ocular infection and sterile inflammatory reactions. While rates remain low, the consequences of endophthalmitis can be devastating.
  • Systemic risks: Anti-VEGF agents carry small but real risks of cardiovascular and thromboembolic events, particularly in diabetic patients with multiple comorbidities such as hypertension, hyperlipidemia, and prior stroke or myocardial infarction. Dual therapy does not appear to significantly amplify this risk but warrants caution in high-risk populations.
  • Cost and access barriers: Dual therapy increases direct medical costs due to the use of multiple drugs and procedures, requires more frequent clinic visits for monitoring and administration, and may not be reimbursed by insurance or available in resource-limited settings. This can lead to inequities in access to optimal care.

Clinicians must weigh these risks against the potential benefits of enhanced anatomic control when considering dual therapy, and they should engage patients in shared decision-making about the trade-offs involved.

Patient Selection: Tailoring the Approach to the Individual

Choosing between monotherapy and dual therapy requires careful consideration of several patient-specific factors, and no single approach is appropriate for all patients. The following sections outline the key determinants that guide this decision.

Disease Severity and Chronicity

Patients with center-involving DME and poor baseline vision, defined as 20/50 or worse, often benefit from initial anti-VEGF monotherapy using a high-dose agent such as aflibercept. If after three to six months of monthly loading doses there is insufficient anatomic improvement, defined as less than 20 percent reduction in central subfield thickness or persistent intraretinal fluid on OCT, then adding a corticosteroid or laser as dual therapy may be indicated. For patients with PDR and high-risk characteristics such as vitreous hemorrhage, neovascularization of the disc, or neovascularization elsewhere with vitreous hemorrhage, anti-VEGF monotherapy is effective, but adding PRP as dual therapy can reduce the need for frequent injections and provide long-term stability, particularly in patients who may have difficulty adhering to a strict injection schedule.

Patient Ocular and Systemic Health

Patients with steroid-responsive intraocular pressure elevation, advanced glaucoma, or ocular hypertension should avoid corticosteroid-based dual therapy, as the risks of pressure elevation outweigh potential benefits. Conversely, pseudophakic patients who have already undergone cataract surgery may tolerate steroids better, making anti-VEGF plus corticosteroid a more viable option. Patients with a history of cardiovascular or thromboembolic events may prefer anti-VEGF monotherapy with careful systemic monitoring, or may require consultation with their primary care physician before initiating therapy. Renal function is also relevant, as some anti-VEGF agents are cleared renally and dose adjustment may be needed in patients with chronic kidney disease.

Access, Adherence, and Socioeconomic Factors

In settings where frequent injections are challenging, such as rural areas with limited transportation, patients with poor health literacy, or those with adherence difficulties, dual therapy that reduces injection frequency may improve compliance and outcomes. For example, an anti-VEGF agent combined with the dexamethasone implant, which provides effect for three to six months, can significantly extend the interval between treatments. However, laser as monotherapy still plays an important role in such contexts, particularly when intravitreal injections are not available or affordable. Clinicians must consider the patient's social support, financial resources, and ability to attend follow-up appointments when selecting the therapeutic strategy.

Future Directions and Emerging Therapies

Research continues to refine diabetic retinopathy treatment, with several promising avenues on the horizon. Novel agents targeting the angiopoietin-2 pathway, such as faricimab, are showing impressive results as monotherapy by concurrently blocking both VEGF and Ang-2, two pathways that drive vascular instability and leakage. These bispecific antibodies may reduce injection frequency to every 12 to 16 weeks in many patients, potentially making dual therapy unnecessary for a larger proportion of cases. If these agents address multiple disease pathways without the side effects of corticosteroids, they could shift the treatment paradigm away from combination approaches. Additionally, sustained-release drug delivery systems are being developed, including port delivery systems, injectable hydrogels, and biodegradable implants that release anti-VEGF agents over months to years, making monotherapy more durable and convenient.

The role of combination therapy with nanotechnology or gene therapy is still experimental but holds considerable promise. Gene therapy approaches that induce sustained intraocular production of anti-VEGF proteins could eliminate the need for repeated injections entirely. Meanwhile, artificial intelligence and advanced imaging biomarkers, such as OCT-based predictors of treatment response and machine learning algorithms that analyze fluid location and morphology, are being developed to identify which patients will benefit most from dual therapy from the outset, avoiding trial-and-error and enabling truly personalized treatment plans. As these tools mature, they will become standard in clinical practice, allowing clinicians to match each patient with the optimal regimen from day one.

Conclusion

Both monotherapy and dual therapy have established and complementary roles in the management of diabetic eye disease. Monotherapy, particularly with anti-VEGF agents, remains the backbone of treatment for most patients due to its simplicity, well-documented safety profile, and cost-effectiveness when applied appropriately. Dual therapy offers meaningful advantages in carefully selected cases of refractory DME, eyes with persistent fluid despite adequate monotherapy, and high-risk PDR where anatomic control is paramount for preventing vision loss from complications such as vitreous hemorrhage or tractional retinal detachment. The choice between these approaches must be individualized based on disease severity, ocular and systemic health, patient preferences, and resource availability. Clinicians should regularly reassess treatment response using objective measures such as OCT imaging and visual acuity, and adjust therapy accordingly. With ongoing innovation in drug development, delivery systems, and personalized medicine, the future promises even more effective, durable, and patient-centered regimens that will continue to reduce the global burden of diabetic vision loss.

For further reading and evidence-based guidelines, consult the following resources: