Understanding Proliferative Diabetic Retinopathy: A Progressive Threat to Vision

Proliferative Diabetic Retinopathy (PDR) represents the most advanced stage of diabetic eye disease and remains a leading cause of blindness among working-age adults worldwide. The condition develops when chronic hyperglycemia damages retinal blood vessels, triggering an ischemic response that stimulates the growth of abnormal, fragile new vessels—a process called neovascularization. These new vessels are structurally weak, prone to leakage, and can lead to vitreous hemorrhage, tractional retinal detachment, and severe vision loss if left untreated.

The complexity of PDR stems from its multifactorial pathophysiology: vascular endothelial growth factor (VEGF) is a key driver, but inflammatory cytokines, oxidative stress, and mechanical forces from scar tissue also contribute. Traditional monotherapies have historically been the standard of care—laser photocoagulation (panretinal photocoagulation, PRP) and anti-VEGF intravitreal injections—but each approach has limitations. PRP is effective at reducing new vessel formation but can cause peripheral vision loss, night blindness, and does not address macular edema in all cases. Anti-VEGF injections offer rapid regression of neovascularization and improvement in macular edema, yet they require frequent, long-term follow-up, and disease recurrence is common when treatments are delayed or spaced out.

Given these challenges, the paradigm of PDR management has shifted toward combination therapies—using two or more treatment modalities in a coordinated manner to achieve superior outcomes, reduce treatment burden, and address the heterogeneous nature of the disease. This article examines the scientific rationale, clinical evidence, and practical considerations behind combination therapy approaches for PDR.

Why Monotherapy Often Falls Short in PDR

The limitations of monotherapy in advanced PDR are well documented. Panretinal photocoagulation (PRP) has been the backbone of PDR treatment for decades, but it works by ablating ischemic retina to reduce VEGF production—a process that can take weeks to months to be fully effective. During this window, active bleeding may continue, and PRP itself can induce inflammation and exacerbate macular edema in some patients. In addition, PRP does not address the persistent pro-angiogenic milieu once ischemia is established; neovascularization can recur if the underlying stimulus is not completely suppressed.

Anti-VEGF monotherapy, on the other hand, provides a more targeted, reversible approach. Drugs such as ranibizumab, aflibercept, and bevacizumab quickly suppress VEGF-mediated signaling, leading to rapid regression of active new vessels. However, the effects are short-lived—typical dosing intervals range from four to eight weeks—and maintaining suppression over years of treatment is burdensome. Moreover, anti-VEGF therapy does not directly address structural damage (such as fibrovascular proliferation or traction), and some patients develop a form of resistance or tachyphylaxis over time. A meta-analysis published in Ophthalmology showed that while anti-VEGF agents effectively reduce disease progression, up to 30% of PDR eyes may still require PRP within two years of initiation, highlighting the need for complementary approaches.

Defining Combination Therapies: Mechanisms and Rationale

Combination therapy in PDR refers to the concurrent or sequential use of different treatment modalities to leverage their distinct mechanisms of action. The goal is not simply additive effect but often synergy: each agent addresses a different part of the disease cascade while potentially reducing the side-effects or limitations of individual treatments. Common combination regimens include:

  • Anti-VEGF plus PRP: This is the most widely studied combination. Anti-VEGF injections can be administered before, during, or after PRP to rapidly suppress active bleeding and reduce inflammation, allowing PRP to work more efficiently. Conversely, PRP provides long-term reduction of overall VEGF burden, potentially extending the interval between anti-VEGF injections.
  • Anti-VEGF plus corticosteroid implants: Corticosteroids such as dexamethasone intravitreal implant (Ozurdex) or fluocinolone acetonide (Iluvien) target inflammatory pathways and also inhibit VEGF to some extent. Combining an anti-VEGF agent with a steroid can offer broader anti-inflammatory, anti-angiogenic, and anti-edematous effects, particularly in eyes with concurrent diabetic macular edema (DME).
  • Anti-VEGF plus vitrectomy: In eyes with dense vitreous hemorrhage or tractional retinal detachment, surgical vitrectomy may be necessary to clear the media and relieve traction. Combining vitrectomy with either preoperative or intraoperative anti-VEGF injection reduces the risk of intraoperative bleeding and recurrence of neovascularization after surgery.
  • PRP plus focal/grid laser for DME: For patients with PDR and concomitant clinically significant macular edema, combining PRP with targeted macular laser has been a longstanding strategy to manage both neovascularization and edema simultaneously.

The rationale for each combination varies, but core principles include: reducing treatment frequency (by extending injection intervals via PRP's durable effect), improving anatomical outcomes (by addressing both VEGF and inflammation), enhancing safety (by lowering doses or frequency of individual agents), and personalizing therapy to a patient's specific disease phenotype (e.g., presence of hemorrhage, DME, or fibrovascular proliferation).

Clinical Evidence: What the Studies Show

Anti-VEGF Plus PRP: A Synergistic Approach

The combination of anti-VEGF and PRP has been evaluated in several randomized clinical trials. The DRCR.net Protocol S initially compared ranibizumab monotherapy to PRP alone for PDR and found that ranibizumab was non-inferior to PRP for visual acuity outcomes and had significantly less visual field loss. However, patients in the ranibizumab group required frequent injections (median of 7 injections over 2 years) and still about one-third eventually needed rescue PRP. Subsequent analyses highlighted that combining both treatments could reduce the injection burden (NEJM, 2015).

More recent studies have specifically investigated upfront combination therapy. For instance, the CLARITY study (2017) compared a treat-and-extend regimen of ranibizumab with or without initial PRP and found that the combination group achieved faster regression of neovascularization and longer intervals between injections. A systematic review and network meta-analysis in JAMA Ophthalmology reported that combination therapy (anti-VEGF + PRP) was associated with a significantly lower risk of vitreous hemorrhage at 12 months compared to PRP alone, and also reduced the number of required injections compared to anti-VEGF alone (JAMA Ophthalmology, 2021).

Adding Corticosteroids: The Rise of Triple Therapy

For patients with PDR complicated by persistent DME or in whom anti-VEGF monotherapy is insufficient, the addition of corticosteroids has shown promise. A retrospective cohort study of 120 eyes with PDR and DME treated with ranibizumab plus a dexamethasone implant found superior improvement in central subfield thickness and visual acuity compared to ranibizumab alone at 12 months, with fewer injections overall. The corticosteroid helps suppress the inflammatory component that anti-VEGF alone may not fully control. Furthermore, a randomized trial (the BEVORDEX trial) compared bevacizumab plus dexamethasone implant versus aflibercept monotherapy for DME and found comparable visual outcomes but fewer injections in the combination group. While specifically targeting DME, these data indirectly support a role for combination steroid/anti-VEGF in PDR.

Preoperative Anti-VEGF with Vitrectomy

When vitrectomy is needed for active PDR (e.g., non-clearing vitreous hemorrhage or tractional detachment), administering an anti-VEGF injection 1–7 days before surgery significantly reduces intraoperative bleeding and facilitates dissection of fibrovascular membranes. A meta-analysis of 15 randomized trials involving 1,200 eyes showed that preoperative anti-VEGF decreased the risk of intraoperative hemorrhage by 50% and reduced the need for endophotocoagulation and silicone oil tamponade. This combination has become a standard practice in many vitreoretinal surgical centers.

Benefits of Combination Therapies: Expanding Beyond the Obvious

The advantages of combination therapy extend well beyond improved visual acuity. Below are key benefits that align with both patient-centered and system-level priorities.

  • Longer Durable Disease Control: By using multiple mechanisms, combination therapy can disrupt the feedback loops that drive neovascularization. For example, PRP reduces total retinal VEGF production, while anti-VEGF injections block circulating VEGF—together, VEGF levels can be suppressed more consistently and for longer periods. This translates to fewer disease reactivations and less frequent need for retreatment.
  • Reduced Treatment Burden and Improved Compliance: Frequent intravitreal injections are a major source of patient anxiety, missed work, and travel costs. Combination therapy—especially with PRP—often allows for longer injection intervals (e.g., every 3 months instead of monthly) without sacrificing disease control. In a real-world study, patients receiving combination therapy had a median injection frequency of 4 per year versus 7 per year with anti-VEGF alone, representing a 40% reduction.
  • Better Anatomical Outcomes for Macular Edema: PDR often coexists with DME. While anti-VEGF agents are effective for DME, adding PRP or corticosteroids can more completely resolve edema and prevent structural damage to the macula. The Diabetic Retinopathy Clinical Research Network reported that eyes receiving both PRP and anti-VEGF had lower rates of vision-threatening macular edema at 2 years than those receiving PRP alone.
  • Lower Risk of Vitreous Hemorrhage and Retinal Detachment: By rapidly regressing neovascularization and reducing traction, combination therapy decreases the likelihood of acute bleeding events that require emergency treatment. The CLARITY study showed a 60% reduction in the risk of vitreous hemorrhage recurrence in the combination group compared to anti-VEGF monotherapy.
  • Cost-Effectiveness: While drug costs may increase initially, the reduction in injection burden, emergency visits, and long-term surgical interventions can make combination therapy more cost-effective over a 2- to 5-year horizon. Health economic analyses are underway but early data suggest that combination strategies are favorable, especially for patients with high disease activity.

Risks, Side Effects, and Considerations

Despite these benefits, combination therapy is not without risks. The most common concerns include:

  • Increased intraocular inflammation: Combining multiple intravitreal agents may elevate the risk of endophthalmitis or sterile uveitis. However, large registry studies (e.g., IRIS) have not shown a clinically significant increase when agents are administered at separate visits or appropriately spaced.
  • Higher intraocular pressure (IOP): Corticosteroids are well known to cause steroid-induced glaucoma. The addition of a steroid implant to an anti-VEGF regimen increases the need for IOP monitoring and may require glaucoma medications or surgical intervention in susceptible patients.
  • Cataract progression: Corticosteroids also accelerate cataract formation. For phakic patients, combination therapy that includes a steroid should be weighed against future cataract surgery risk, especially in younger patients.
  • Greater number of procedures: Some combinations require separate visits for laser and injection, or multiple injections at a single visit. This can increase discomfort and the logistical burden for patients.
  • Uncertainty regarding optimal sequencing: There is no consensus on the ideal order or timing of treatments. For example, giving anti-VEGF too soon before PRP may reduce the inflammatory response that PRP relies on for its therapeutic effect, though evidence suggests this is not a significant issue.

Clinicians must carefully select patients who are likely to benefit most from combination therapy—those with high-risk PDR characteristics (e.g., active neovascularization, vitreous hemorrhage, recalcitrant DME) and good compliance for follow-up. Shared decision-making with patients about the expected benefits and risks is essential.

Personalizing Combination Therapy: The Future of PDR Management

No single combination fits every PDR patient. Managing this condition requires a personalized, dynamic approach that adapts to disease activity, patient preferences, and available resources. Key factors influencing the choice of combination include:

  • Disease activity: Eyes with florid neovascularization may need an initial anti-VEGF "load" followed by PRP, while eyes with predominantly fibrovascular proliferation and traction may benefit more from vitrectomy combined with intraoperative anti-VEGF.
  • Presence of DME: For patients with DME, combination therapy that includes a corticosteroid implant may be preferred over PRP alone, as PRP can sometimes worsen edema in the short term.
  • Patient age and lens status: Phakic patients who are good candidates for cataract surgery may be more accepting of corticosteroid-related cataract risk, while pseudophakic patients have no such concern.
  • Treatment adherence and access: Patients with limited access to frequent injections might receive initial PRP to reduce injection frequency, whereas those who can attend monthly visits could start with anti-VEGF alone and add laser only if needed.
  • Systemic comorbidities: Anti-VEGF agents are associated with rare systemic thromboembolic events; in patients with recent myocardial infarction or stroke, PRP or less frequent anti-VEGF may be safer.

The concept of a "treat-and-extend" regimen combined with PRP has gained traction. In this protocol, after initial intensive therapy (e.g., monthly anti-VEGF plus PRP), subsequent injections are spaced based on disease stability, with the goal of eventually maintaining control with infrequent injections or no treatment. Several centers now use a protocol where PRP is administered in the first session along with an anti-VEGF injection, then the patient returns at 4 weeks for assessment. If the neovascularization has regressed completely, the injection interval is extended by 2 weeks at a time, up to 12 weeks. If reactivation occurs, PRP can be supplemented, or a switch to a different anti-VEGF agent (e.g., from bevacizumab to aflibercept) can be considered.

Emerging Therapies and Future Directions

The landscape of PDR therapy is evolving rapidly. Several novel approaches are on the horizon that may further refine combination strategies:

  • Longer-acting anti-VEGF agents: Drugs such as faricimab (VEGF-A/Ang-2 bispecific), brolucizumab, and high-dose aflibercept have shown extended durability, with injection intervals of up to 16 weeks in some trials. Combining these longer-acting agents with PRP could potentially reduce injections to a few times per year.
  • Gene therapy for sustained anti-VEGF production: Early-phase trials are exploring intravitreal or subretinal delivery of vectors encoding anti-VEGF proteins. If successful, this could provide a "one-shot" treatment that eliminates the need for regular injections, and combination with PRP might offer even more comprehensive control.
  • Targeting non-VEGF pathways: Inflammation, oxidative stress, and the angiopoietin/Tie-2 axis are increasingly recognized as key players. Agents such as AKB-4924 (HIF inhibitor) or anti-inflammatory biologics (e.g., IL-6 inhibitors) may be used in conjunction with VEGF blockade and laser.
  • Artificial intelligence and personalized algorithms: Machine learning models that integrate imaging data (OCT, OCTA, widefield angiography) with clinical variables could help predict which combination therapy is best for a given patient and when to switch modalities.

Clinical trials such as the ongoing studies registered at ClinicalTrials.gov are actively testing these emerging concepts. The next five years will likely see a shift from empirical combinations to evidence-based, algorithm-driven therapy.

Conclusion: The Paradigm Shift Toward Integration

Combination therapies for proliferative diabetic retinopathy represent a maturation of our understanding of the disease. Rather than viewing treatments as competing alternatives, the current approach recognizes that PRP, anti-VEGF injections, corticosteroids, and vitrectomy each address different aspects of the disease process and can be synergistically combined to optimize outcomes. The evidence clearly supports that combination strategies lead to better visual outcomes, fewer complications, and reduced treatment burden compared to monotherapy in many clinical scenarios.

However, implementation requires careful patient selection, close monitoring, and flexibility to adjust therapy based on disease response. As research continues to refine the optimal sequencing, dosing, and new agents, combination therapy will become the new standard of care for PDR—moving away from a one-size-fits-all approach toward a personalized, integrated treatment plan that maximizes vision preservation and quality of life for patients with this devastating complication of diabetes.

For clinicians and patients alike, the message is clear: when managing PDR, the sum is often greater than the parts. Making combination therapy a deliberate, early consideration in the treatment plan can dramatically improve long-term outcomes and reduce the overall burden of care.