# Diabetic Retinopathy: Classification, Screening, and Management

## Pathophysiology

Diabetic retinopathy is fundamentally a disease of the retinal microvasculature driven by chronic hyperglycemia. The earliest cellular insult is the loss of pericytes -- the contractile cells that wrap around retinal capillaries and help maintain vessel integrity. Without pericyte support, focal outpouchings of the capillary wall develop, forming microaneurysms, which represent the earliest clinically detectable sign of diabetic retinopathy. As endothelial cell damage and basement membrane thickening progress, the inner blood-retinal barrier breaks down, allowing plasma constituents to leak into the retinal tissue. This leakage produces retinal edema and the deposition of hard exudates, which are lipid and protein residues left behind as the fluid is reabsorbed.

As the disease advances, progressive capillary closure leads to areas of retinal non-perfusion. The resulting ischemia triggers upregulation of vascular endothelial growth factor (VEGF), which drives the formation of new, abnormal blood vessels -- the hallmark of proliferative disease. Four major biochemical pathways mediate hyperglycemic damage to the retinal vasculature: the polyol pathway, the accumulation of advanced glycation end-products (AGEs), activation of protein kinase C, and the hexosamine pathway. Each contributes to oxidative stress, inflammation, and vascular dysfunction.

## Classification (ETDRS-Based)

### Non-Proliferative Diabetic Retinopathy (NPDR)

#### Mild NPDR

Mild NPDR is defined by the presence of at least one microaneurysm in the absence of any other retinopathy findings. This is the earliest stage of clinically detectable disease.

#### Moderate NPDR

Moderate NPDR encompasses findings that are more than mild but do not meet the criteria for severe disease. Typical findings include microaneurysms, dot-blot hemorrhages, hard exudates, and cotton-wool spots. Venous beading may be present but is limited to fewer than two quadrants.

#### Severe NPDR (4-2-1 Rule -- any one of the following)

| Criterion | Finding Required |
|-----------|-----------------|
| "4" | Severe hemorrhages/microaneurysms in all **4** quadrants |
| "2" | Venous beading in **2** or more quadrants |
| "1" | IRMA in at least **1** quadrant |

Any ONE of the above qualifies as severe NPDR (~50% risk of PDR within 1 year).

Severe NPDR is classified using the 4-2-1 rule, which requires the presence of any one of three findings: severe hemorrhages and microaneurysms in all four quadrants, venous beading in two or more quadrants, or intraretinal microvascular abnormalities (IRMA) in at least one quadrant. This stage carries approximately a 50% risk of progression to proliferative diabetic retinopathy within one year, making it a critical threshold for escalating surveillance and considering intervention.

#### Very Severe NPDR

Very severe NPDR is diagnosed when two or more of the severe NPDR criteria are met simultaneously. The risk of progression to PDR within one year rises to approximately 75%.

### Proliferative Diabetic Retinopathy (PDR)

Proliferative diabetic retinopathy is defined by the presence of neovascularization, either at the optic disc (NVD) or elsewhere in the retina (NVE). The Diabetic Retinopathy Study established high-risk characteristics that identify eyes at greatest risk for severe vision loss: NVD greater than one-quarter to one-third disc area in size, any NVD accompanied by vitreous or preretinal hemorrhage, or NVE greater than one-half disc area with vitreous or preretinal hemorrhage. The complications of PDR are devastating and include vitreous hemorrhage from bleeding new vessels, tractional retinal detachment from contraction of the fibrovascular proliferative tissue, and neovascular glaucoma from anterior segment neovascularization driven by the same VEGF stimulus.

<image>Composite fundus photographs showing progression from mild NPDR (microaneurysms only) through moderate NPDR (hemorrhages, exudates) to severe NPDR (4-2-1 rule findings) and proliferative DR with neovascularization</image>

## Screening Guidelines

### Timing

Screening recommendations differ by diabetes type because of their distinct natural histories. For type 1 diabetes, the first dilated eye examination should be performed five years after diagnosis, typically after the onset of puberty, because clinically significant retinopathy rarely develops before that point. For type 2 diabetes, a screening examination should be performed at the time of diagnosis, since many patients have had years of undiagnosed hyperglycemia and may already have retinopathy at presentation. Pregnant women with pre-existing diabetes should be examined in the first trimester and monitored each trimester thereafter, as diabetic retinopathy can worsen significantly during pregnancy.

### Screening Intervals

Follow-up intervals are determined by the severity of retinopathy. Patients with no retinopathy can be screened every one to two years. Mild NPDR warrants re-examination every six to twelve months. Moderate NPDR requires evaluation every three to six months. Severe NPDR should be followed every two to four months, and panretinal photocoagulation should be considered at this stage. Proliferative diabetic retinopathy requires urgent treatment.

### Methods

The gold standard for screening remains the dilated fundus examination. Fundus photography, particularly through teleretinal screening programs, has expanded access to screening in underserved areas. FDA-cleared autonomous artificial intelligence systems, including IDx-DR and EyeArt, can now detect referable diabetic retinopathy from fundus photographs without requiring interpretation by an ophthalmologist. Optical coherence tomography is used as an adjunct to detect diabetic macular edema, which may not be apparent on clinical examination alone.

## Management

### Systemic Risk Factor Control

Systemic management is the foundation of diabetic retinopathy treatment. Tight glycemic control, targeting a hemoglobin A1c below 7%, has been shown by the DCCT/EDIC trial in type 1 diabetes and the UKPDS in type 2 diabetes to significantly reduce the risk of developing and progressing retinopathy. Blood pressure control provides additional benefit, as demonstrated by the ACCORD Eye Study. Lipid management with fenofibrate has been shown in the FIELD and ACCORD studies to reduce retinopathy progression, independent of its lipid-lowering effects. An important caveat is that rapid normalization of blood glucose in a patient with long-standing poor control can transiently worsen retinopathy -- a phenomenon sometimes called early worsening -- so glycemic improvement should be pursued gradually in patients with significant existing disease.

### Panretinal Photocoagulation (PRP)

Panretinal photocoagulation remains the standard of care for high-risk proliferative diabetic retinopathy. The technique involves applying 1200 to 1600 laser burns of 500 micrometer spot size to the peripheral retina, outside the vascular arcades. The rationale is straightforward: by destroying ischemic peripheral retina, PRP reduces the total retinal demand for oxygen and decreases VEGF production, leading to regression of neovascularization. The Diabetic Retinopathy Study demonstrated that PRP reduces the risk of severe vision loss by approximately 50%. However, PRP is not without cost -- side effects include peripheral visual field loss, decreased night vision, pain during treatment, exacerbation of macular edema, and rarely choroidal effusion.

### Anti-VEGF Therapy for PDR

The DRCR.net Protocol S trial compared intravitreal ranibizumab with PRP for the treatment of proliferative diabetic retinopathy. At two years, ranibizumab was non-inferior to PRP in visual acuity outcomes. The ranibizumab group demonstrated several advantages, including better preservation of peripheral visual fields, lower rates of diabetic macular edema development, and fewer vitrectomies. However, anti-VEGF therapy requires frequent injections and close follow-up, and patients who are lost to follow-up are at risk of rapid disease progression without the durable protection that PRP provides.

Anti-VEGF agents serve multiple roles in the management of PDR. They can be used as primary treatment when reliable follow-up can be ensured, as an adjunct to PRP by pre-treating to reduce neovascularization before laser is applied, and as bridge therapy before vitrectomy to reduce intraoperative bleeding and facilitate surgical dissection.

### Vitrectomy Indications

Pars plana vitrectomy is indicated in several clinical scenarios. Non-clearing vitreous hemorrhage -- generally defined as hemorrhage persisting beyond one month in type 1 diabetes or three months in type 2 diabetes -- is a common indication. Tractional retinal detachment involving or threatening the macula requires surgical intervention, as does a combined tractional-rhegmatogenous retinal detachment. Progressive fibrovascular proliferation that continues to advance despite adequate PRP also warrants vitrectomy. The Diabetic Retinopathy Vitrectomy Study demonstrated that early vitrectomy is particularly beneficial in type 1 diabetic patients with severe vitreous hemorrhage, as it allows earlier visual rehabilitation and permits the application of necessary laser treatment.

<image>Fluorescein angiogram of proliferative diabetic retinopathy showing neovascularization of the disc with profuse leakage, areas of capillary non-perfusion, and microaneurysm hyperfluorescence</image>

## Diabetic Macular Edema (Overview)

Diabetic macular edema can occur at any stage of diabetic retinopathy, from mild NPDR through PDR. The critical distinction on OCT is between center-involving and non-center-involving edema, which determines the treatment approach. This topic is covered in detail in Topic 30.

## Special Considerations

### Diabetic Retinopathy in Pregnancy

Diabetic retinopathy can worsen during pregnancy, particularly in women who already have preproliferative disease at conception. The mechanisms driving this progression include hormonal changes, hemodynamic shifts associated with pregnancy, and the rapid glycemic tightening that is often pursued to protect the fetus. Close monitoring each trimester is essential. PRP can be safely performed during pregnancy if progression to high-risk PDR occurs. Reassuringly, pregnancy-related worsening of retinopathy typically regresses in the postpartum period.

### Diabetic Retinopathy and Cataract Surgery

Diabetic retinopathy may progress after cataract surgery due to the inflammatory response and disruption of the blood-retinal barrier that accompanies intraocular surgery. For this reason, diabetic retinopathy status should be optimized before elective cataract surgery. Any existing diabetic macular edema should be treated and stable prior to the procedure. In patients with active PDR, perioperative anti-VEGF injection should be considered to reduce the risk of postoperative progression.

<image>Ultra-widefield fundus photograph showing panretinal photocoagulation scars in a patient with treated proliferative diabetic retinopathy, with regression of neovascularization</image>

## Clinical Pearls

The 4-2-1 rule is the cornerstone of classifying severe NPDR and must be committed to memory. Distinguishing IRMA from NVE is a common clinical and examination challenge: IRMA consists of dilated, tortuous vessels that remain within the retina, do not cross over major retinal vessels, and show only mild leakage on fluorescein angiography, whereas NVE grows on the retinal surface, freely crosses major vessels, and leaks profusely. Venous beading is the single strongest predictor of progression from NPDR to PDR. When choosing between anti-VEGF and PRP for proliferative disease, patient reliability is paramount -- PRP provides durable treatment that does not depend on follow-up compliance, making it the safer choice for patients who may not return for regular injections. Before dilating any diabetic patient, the iris should be examined at the slit lamp for neovascularization (rubeosis iridis), which signals neovascular glaucoma risk. Neovascularization characteristically develops at the border between perfused and non-perfused retina, where the ischemic gradient and VEGF concentration are highest. Because new vessels lack the tight junctions of normal retinal vasculature, they leak fluorescein profusely on angiography -- a feature that helps confirm their presence and distinguish them from IRMA.

## References

- Early Treatment Diabetic Retinopathy Study Research Group. Grading diabetic retinopathy from stereoscopic color fundus photographs. Ophthalmology. 1991;98(5 Suppl):786-806.
- Diabetic Retinopathy Study Research Group. Photocoagulation treatment of proliferative diabetic retinopathy: the second report of DRS findings. Ophthalmology. 1978;85(1):82-106.
- Gross JG, et al. Panretinal photocoagulation vs intravitreous ranibizumab for proliferative diabetic retinopathy (Protocol S). JAMA Ophthalmol. 2018;136(10):1138-1148.
- DCCT/EDIC Research Group. Intensive diabetes therapy and ocular surgery in type 1 diabetes. N Engl J Med. 2015;372(18):1722-1733.
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 12: Retina and Vitreous.