Diabetic macular edema

What it is

Diabetic macular edema (DME) is the most common cause of vision loss in people with diabetes, affecting approximately 10% of all diabetic patients — a figure that rises sharply with disease duration and poor glycaemic control. Also called diabetic macular oedema, DME occurs when chronically elevated blood sugar damages the small blood vessels of the retina, causing them to leak fluid into the macula: the small central zone of the retina responsible for sharp, detailed vision used in reading, driving, and recognizing faces. As fluid accumulates, the macula swells and central vision becomes blurred or distorted. Peripheral vision is typically preserved.

In Canada, where Diabetes Canada estimates that over 11 million people are living with diabetes or prediabetes, DME represents a significant and preventable cause of working-age vision impairment. Provincial eye-screening programs and annual dilated fundus exams — recommended at diagnosis for type 2 diabetes and within 5 years of diagnosis for type 1 — are the primary tools for catching DME before irreversible damage occurs.

DME develops as a consequence of diabetic retinopathy (DR), the broader condition of diabetes-related retinal vessel damage. Not everyone with DR develops DME, but the two conditions frequently coexist, and the presence of more advanced retinopathy substantially increases DME risk.

Causes and mechanism

The central driver is chronic hyperglycaemia. Persistently elevated blood glucose weakens the tight junctions between retinal endothelial cells, breaking down the blood-retinal barrier and allowing fluid and proteins to seep into surrounding tissue. Hyperglycaemia also stimulates overproduction of vascular endothelial growth factor (VEGF), a protein that promotes the growth of new but structurally fragile blood vessels and increases vascular permeability. Excess VEGF is the dominant molecular mechanism behind fluid accumulation in DME. Oxidative stress and low-grade retinal inflammation amplify vessel damage further.

Key modifiable and non-modifiable risk factors:

Risk factor	Direction of risk
Diabetes duration	Risk increases with every additional year
HbA1c above target	Strongest modifiable risk factor; each 1% reduction cuts retinopathy progression ~35%
Hypertension	Independently damages retinal vasculature
Elevated LDL cholesterol	Promotes lipid (hard exudate) deposits in the retina
Diabetic kidney disease	Presence significantly increases DME likelihood
Pregnancy	Can accelerate diabetic eye changes

Symptoms and diagnosis

Early DME is often asymptomatic, which is why screening — not symptom-driven visits — catches most cases at a treatable stage.

As fluid accumulates, patients may notice:

• Blurred or hazy central vision
• Difficulty reading fine print or recognizing faces
• Metamorphopsia: straight lines appear wavy or bent
• Washed-out or faded colours
• A dark or empty patch in the centre of the visual field in advanced cases

Diagnosis relies on four complementary tools:

1. Dilated fundus examination — the ophthalmologist dilates the pupil and directly inspects the retina for haemorrhages, exudates, and swelling.
2. Optical coherence tomography (OCT) — the gold-standard test; provides high-resolution cross-sectional images of the retina, precisely measuring thickness and mapping fluid location. Available at most Canadian ophthalmology and optometry practices, and through LifeLabs and Dynacare referral networks in some provinces.
3. Fluorescein angiography — intravenous dye highlights leaking vessels and areas of non-perfusion.
4. Visual acuity testing — quantifies the degree of central vision loss, typically using the Early Treatment Diabetic Retinopathy Study (ETDRS) letter chart.

Treatment options

Treatment of DME has been transformed over the past decade. The goal is to reduce fluid, preserve existing vision, and — in many cases — recover lost acuity.

Anti-VEGF intravitreal injections are the current first-line standard of care. Agents including ranibizumab, bevacizumab, aflibercept, and the newer bispecific antibody faricimab are injected directly into the vitreous cavity to block VEGF signalling and reduce vascular leakage. The DRCR Retina Network Protocol T trial (NEJM, 2015) demonstrated that aflibercept, bevacizumab, and ranibizumab all improved vision significantly, with aflibercept showing an advantage in eyes with worse baseline acuity. Most patients require monthly injections for the first several months, followed by gradual interval extension as the retina stabilizes. Treat-and-extend protocols aim to reduce clinic burden while maintaining disease control.

Corticosteroid implants and injections — including the sustained-release dexamethasone implant (Ozurdex) and intravitreal triamcinolone — are used when anti-VEGF therapy is insufficient or contraindicated (for example, in pseudophakic patients or those who cannot attend frequent injection visits). Corticosteroids carry a meaningful risk of elevated intraocular pressure and cataract formation and are used selectively.

Focal laser photocoagulation was the standard of care for decades and remains useful in select cases — particularly for focal leaking microaneurysms away from the foveal centre — but has largely been replaced by anti-VEGF therapy as the primary modality.

Systemic risk factor management underpins all other treatments. Optimizing HbA1c, blood pressure, and lipids slows DME progression and improves the response to ocular therapy. A 1% reduction in HbA1c is associated with approximately a 35% reduction in retinopathy progression risk.

When to see a clinician in Canada

Diabetes Canada and the Canadian Ophthalmological Society recommend:

• Type 1 diabetes: first dilated eye exam within 5 years of diagnosis, then annually
• Type 2 diabetes: dilated eye exam at the time of diagnosis, then at least annually

Seek prompt ophthalmology assessment — do not wait for the next scheduled exam — if you notice sudden changes in central vision, new blurring or distortion, difficulty reading that has worsened over days to weeks, or a shadow appearing in the centre of your visual field. DME can progress quickly, and treatment started before structural macular damage is established produces substantially better outcomes than treatment initiated late.

Referrals in Canada are typically made through a family physician or optometrist to a retinal specialist or general ophthalmologist. Wait times vary by province; patients in rural or remote areas may access teleophthalmology screening programs offered through several provincial health authorities.

Limitations and open questions

Research is still emerging on the optimal long-term treatment interval for anti-VEGF therapy — specifically, how to identify patients who can safely extend or discontinue injections without risking recurrence. The comparative effectiveness of newer agents such as faricimab versus established options in real-world Canadian populations has not yet been fully characterized in prospective Canadian data.

Health Canada has not yet issued specific guidance on DME screening intervals for patients with gestational diabetes or those transitioning from pediatric to adult diabetes care, leaving clinicians to extrapolate from general guidelines. The role of emerging therapies — including sustained-release anti-VEGF implants and gene therapy approaches — remains investigational, and no Canadian public drug plan currently covers these modalities. Evidence on whether GLP-1 receptor agonists (now widely used for type 2 diabetes management) independently reduce DME risk beyond their glycaemic effects is promising but not yet definitive.