## Most Common Structural Change in POAG **Key Point:** The most characteristic and earliest structural change in primary open-angle glaucoma is progressive neuroretinal rim thinning, which typically begins at the **inferotemporal pole** of the optic disc. ### Pathophysiology of Optic Nerve Damage 1. **Mechanical compression** from elevated IOP causes axonal loss in the retinal nerve fiber layer (RNFL) 2. **Vascular insufficiency** leads to ischemia of the optic nerve head 3. Progressive loss of ganglion cells → thinning of the neuroretinal rim 4. Enlargement of the optic cup as the rim thins ### Sequence of Optic Disc Changes in POAG | Stage | Finding | Location | |-------|---------|----------| | Early | Rim thinning | Inferotemporal pole (most common) | | Early | RNFL defect | Arcuate pattern | | Progressive | Generalized cup enlargement | All quadrants | | Advanced | Complete rim loss | Temporal pallor | | End-stage | Absolute cupping | Entire disc | **High-Yield:** The **inferotemporal pole** is the most vulnerable region because: - Thinnest neuroretinal rim normally - Poorest vascular supply - Maximum stress concentration from IOP **Clinical Pearl:** Optic disc cupping is NOT synonymous with glaucoma — it must be accompanied by: - Progressive RNFL loss on OCT - Visual field defects (arcuate scotomas, nasal step) - Elevated IOP or risk factors ### Why This Matters in POAG Unlike secondary glaucomas where angle structures are abnormal, POAG has: - Open angles on gonioscopy (normal-appearing trabecular meshwork) - Presumed defect in trabecular outflow facility - Progressive optic nerve damage as the hallmark **Mnemonic: RNFL-I** — **R**etinal **N**erve **F**iber **L**ayer loss at **I**nferotemporal pole (earliest sign).
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