Pathophysiology of Geographic Atrophy: RPE or Choroid as the Primary Culprit?

Age-related macular degeneration (AMD) remains the leading cause of irreversible, severe visual loss among elderly populations in industrialized nations. The clinical spectrum ranges from early forms, characterized by the accumulation of extracellular deposits known as drusen, to advanced, vision-threatening late stages. While anti-vascular endothelial growth factor (anti-VEGF) therapy for neovascular AMD is the standard of care, geographic atrophy has remained a progressive condition. Geographic atrophy is a chronic neurodegenerative disease defined pathologically by the concurrent, progressive loss of the retinal pigment epithelium (RPE), the overlying photoreceptors, and the underlying choriocapillaris. Historically, the diagnosis and monitoring of GA relied on color fundus photography, which identified the lesions as sharply demarcated, well-circumscribed round or oval areas of depigmentation through which underlying choroidal vessels became clinically visible. However, the advent of high-resolution multimodal imaging—particularly fundus autofluorescence (FAF) and optical coherence tomography (OCT)—has necessitated a modernization of this terminology. Despite the imaging consensus, the fundamental pathophysiology of GA remains the subject of intense academic and clinical debate. The controversy focuses on identifying the primary cellular culprit responsible for initiating the cascade of degeneration. The “top-down” hypothesis suggests that intracellular metabolic failure, oxidative stress, and senescence within the RPE trigger the secondary dysfunction and death of the adjacent photoreceptors and choriocapillaris. Conversely, the “bottom-up” vascular hypothesis argues that age-related and complement activation–driven hemodynamic insufficiency, ischemic dropout, and structural involution of the choriocapillaris precede RPE dysfunction, creating a hypoxic microenvironment that drives the subsequent epithelial collapse. This narrative review scrutinizes the exact molecular, genetic, and structural mechanisms governing the expansion of geographic atrophy. By dissecting the evidence supporting both the RPE and the choriocapillaris as primary instigators, this analysis explores the paradigm that GA represents a synergistic failure of the outer blood–retinal barrier—a symbiotic trophic unit highly dependent on precise vascular and epithelial crosstalk, tightly regulated complement cascades, and extracellular matrix integrity.