What ocular phenomenon explains how macular edema can lead to metamorphopsia?

• A. Decreased optic nerve perfusion
• B. Corneal edema causing haze
• C. Retinal detachment at the macula
• D. Displacement of photoreceptors due to retinal edema causing distorted vision; detectable with an Amsler grid

Answer: (D)

Macular edema disturbs the precise arrangement of photoreceptors in the central retina by causing swelling and distortion of the retinal layers at the fovea. This disruption shifts the photoreceptors slightly from their normal alignment, so the image projected onto the retina is mapped onto a distorted mosaic. Because vision relies on that orderly photoreceptor layout, this displacement leads to metamorphopsia—straight lines look bent or wavy. The Amsler grid is a practical test for this phenomenon: when a patient fixes on the center dot, any distortion, bending, or gaps in the grid lines reveals metamorphopsia associated with macular edema.

Other choices don’t fit the mechanism: reduced optic nerve perfusion affects the visual pathways and field, not the photoreceptor arrangement at the macula; corneal edema causes diffuse haziness rather than targeted distortion of the central retina; retinal detachment at the macula can cause metamorphopsia but through detachment of the retina rather than edema-induced photoreceptor displacement. The described phenomenon directly links edema to distortion of the photoreceptor mosaic, producing metamorphopsia detectable with an Amsler grid.