Rapid Glycemic Correction and the Paradox of Retinopathy Progression

The triumph of modern diabetes management lies in the ability to normalize glycemia both rapidly and durably, yet for the retina this achievement can be a double-edged sword. In the Semaglutide Unabated Sustainability in Treatment of Type 2 Diabetes (SUSTAIN-6) cardiovascular outcomes trial, the incidence of diabetic retinopathy complications—including the need for photocoagulation or intravitreal therapy, vitreous hemorrhage, or diabetes-related blindness—was significantly higher in the semaglutide group compared with placebo (hazard ratio [HR], 1.76; 95% confidence interval [CI], 1.11 to 2.78), despite substantial cardiovascular benefit and superior glucose reduction [1]. This paradox is not new. More than three decades earlier, the Diabetes Control and Complications Trial (DCCT) demonstrated that intensive insulin therapy in type 1 diabetes reduced the long-term risk of diabetic retinopathy progression by 76% over 6.5 years, yet 13.1% of intensively treated participants experienced an early three-step or greater worsening on the Early Treatment Diabetic Retinopathy Study (ETDRS) scale within the first 6–12 months, compared with 7.6% in the conventional therapy group [2]. Collectively, these landmark findings establish early worsening of diabetic retinopathy as a genuine treatment-related phenomenon associated with rapid glycemic correction [3].

Mechanistic insights have gradually emerged. A sharp decline in plasma glucose reduces intravascular osmotic pressure, facilitating water influx into retinal tissue and producing transient edema [4]. Experimentally, intensive insulin therapy has been shown to increase retinal expression of hypoxia-inducible factor-1α and vascular endothelial growth factor (VEGF), thereby disrupting the blood–retinal barrier and inducing neovascular changes [5]. In newly diagnosed patients who developed blurred vision after starting insulin, optical coherence tomography (OCT) revealed a transient increase in macular thickness, accompanied by reduced circulating soluble VEGF receptor (sFlt-1), consistent with diminished VEGF inhibition [6]. Insulin may also synergize with VEGF to enhance reactive oxygen species generation and promote angiogenesis [7]. Abrupt glucose normalization thus amplifies hypoxia-driven VEGF signaling in an already ischemic retina.

Clinical observations reinforce these mechanistic explanations. Rapid reductions in glycosylated hemoglobin (HbA1c) of ≥2% over 3–6 months substantially increase the short-term risk of diabetic retinopathy [8]. This phenomenon is not limited to insulin or glucagon-like peptide 1 receptor agonists; it has also been reported after bariatric surgery [9] and during pregnancy when glycemia is normalized quickly [10]. A comparable pattern occurs in treatment-induced neuropathy of diabetes, which manifests as acute painful neuropathy and autonomic dysfunction within weeks of a rapid HbA1c decline [11]. These converging lines of evidence suggest that the rate of metabolic improvement, superimposed on pre-existing microvascular vulnerability, is the primary driver rather than drug-specific toxicity [12].

Over time, however, the paradox resolves. In the DCCT, eyes that experienced early worsening ultimately achieved the same or better long-term outcomes as those without early worsening, provided intensive glycemic control was maintained. In the Epidemiology of Diabetes Interventions and Complications follow-up, the initial 6.5-year period of intensive therapy conferred durable protection, persisting for decades even after glycemic convergence [13]. Similar legacy effects were observed in the UK Prospective Diabetes Study (UKPDS) [14].

The semaglutide signal in SUSTAIN-6 should be interpreted within this framework. Post hoc analyses indicated that the excess risk clustered among participants with pre-existing diabetic retinopathy who experienced the largest and most rapid HbA1c reductions during the first 16 weeks [15]. However, the strength of this conclusion is limited by trial design features. One issue was the reliance on a dichotomous secondary endpoint that combined therapeutic procedures—such as the need for photocoagulation or intravitreal injection—with diagnostic findings, such as new vitreous hemorrhage or documented blindness, without differentiating treatment decisions from objective disease progression. Another was the lack of standardized baseline ETDRS grading, routine visual acuity testing, or OCT imaging, which together prevented precise characterization of subtle or progressive changes. To address these uncertainties, the FOCUS trial—a large randomized placebo-controlled study incorporating stratified baseline ETDRS grading and rigorous ophthalmic endpoints—is underway, with results expected in 2027 to clarify semaglutide’s long-term effects on diabetic retinopathy.

The clinical imperative, therefore, is not to avoid intensive therapy but to implement it with vigilance. Before initiating potent glucose-lowering regimens, patients should undergo a comprehensive retinal evaluation. Those with moderate to severe diabetic retinopathy require particularly close ophthalmologic monitoring—every 3 to 6 months early in therapy—to detect and treat proliferative changes promptly. Gradual titration of therapy may be appropriate when feasible, although the ultimate goal remains sustained glycemic control.

Current evidence highlights fenofibrate, a peroxisome proliferator-activated receptor-α agonist originally developed for hypertriglyceridemia, as a systemic therapy with direct protective effects on the diabetic retina. In the Action to Control Cardiovascular Risk in Diabetes (ACCORD) Eye study, fenofibrate added to simvastatin reduced retinopathy progression by approximately one-third over four years, independent of lipid or glycemic effects [16]. Similarly, in the Fenofibrate Intervention and Event Lowering in Diabetes (FIELD) trial, fenofibrate reduced the need for first laser photocoagulation by 31% and decreased retinopathy progression in participants with baseline disease [17]. More recently, the Lowering Events in Non-proliferative Retinopathy in Scotland (LENS) trial reported a 27% reduction in the risk of progression or treatment for diabetic retinopathy or maculopathy with fenofibrate compared with placebo (HR, 0.74; 95% CI, 0.61 to 0.90) [18]. Experimental studies suggest mechanisms beyond lipid lowering, including suppression of retinal VEGF signaling, anti-inflammatory activity, and stabilization of the blood-retinal barrier. Ongoing large trials such as FAME 1 Eye and Protocol AF will provide further confirmation in both type 1 and type 2 diabetes. Despite robust evidence and a favorable safety profile, fenofibrate remains underutilized. Australia is currently the only country where it is formally approved for the treatment of diabetic retinopathy, and broader regulatory adoption could help mitigate the transient hazard of rapid glycemic correction while offering durable protection against vision-threatening disease.

In summary, early worsening of diabetic retinopathy illustrates the complexities of intensive diabetes management (Fig. 1). From semaglutide in SUSTAIN-6 to the landmark DCCT, the pattern is consistent: rapid and substantial HbA1c reduction can transiently accelerate retinal pathology, especially in eyes with advanced baseline disease. The solution is not therapeutic inertia but anticipatory care—comprehensive retinal assessment, patient education, and timely ophthalmologic intervention—augmented by adjunctive measures such as fenofibrate. By integrating these strategies, clinicians can maximize the long-term vascular benefits of intensive glycemic control while minimizing its short-term retinal risks.

Fig. 1.

Lessons from Semaglutide Unabated Sustainability in Treatment of Type 2 Diabetes (SUSTAIN-6): rapid glycosylated hemoglobin (HbA1c) reduction and the risk of early worsening of diabetic retinopathy. Kaplan–Meier curves from SUSTAIN-6 show that diabetic retinopathy complications occurred predominantly in participants with pre-existing retinopathy who experienced large and rapid reductions in HbA1c after starting semaglutide, whereas the risk remained low in those without baseline disease. The trial enrolled a long-standing, highrisk type 2 diabetes population, and the diabetic retinopathy endpoint was based on predefined treatment or diagnostic events rather than standardized grading of disease progression. Clinically, careful baseline retinal assessment, frequent ophthalmic follow-up, and consideration of adjunctive therapy such as fenofibrate are key strategies to mitigate early worsening while maintaining long-term glycemic benefits [15]. HR, hazard ratio; CI, confidence interval.