What is diabetic macular edema and its mechanism?

• A. Detachment of macula due to traction
• B. Occlusion of central retinal artery
• C. Thickening of macula due to breakdown of blood-retinal barrier with fluid accumulation
• D. Inflammation of optic nerve

Answer: (C)

Diabetic macular edema is thickening of the macula caused by fluid leaking from retinal vessels when the blood-retinal barrier is compromised. In diabetes, prolonged high blood sugar damages retinal capillaries: pericytes are lost, the basement membrane thickens, microaneurysms form, and inflammatory factors rise. This damage increases the permeability of the inner blood-retinal barrier, and VEGF driven by retinal ischemia further promotes leakage of fluid into the retinal layers. The fluid collects mainly in the macula, creating intraretinal cysts or subretinal fluid, which distorts central vision. The macula is especially vulnerable because of its high metabolic demand and dense capillary network.

Other choices describe different problems: detachment due to traction involves pulling forces on the retina; central retinal artery occlusion causes sudden, severe vision loss with retinal whitening; optic nerve inflammation affects vision through optic neuropathy, not macular edema.