A 62-year-old man with type 2 diabetes and family history of blindness presents for routine eye examination. He is asymptomatic with normal visual acuity. Intraocular pressure is 26 mmHg OD and 24 mmHg OS. Gonioscopy shows an open trabecular meshwork. On dilated fundoscopy, the right optic disc demonstrates the finding marked **A** in the diagram—a vertically elongated cup occupying more than 0.7 of the disc diameter. The inferotemporal neuroretinal rim is markedly thinned, and retinal vessels show bayoneting. Humphrey 24-2 perimetry reveals a dense superior arcuate scotoma with nasal step OD and early paracentral defect OS. Which of the following best describes the pathophysiological significance of the enlarged cup marked **A** in the context of this patient's diagnosis?

Question

Accepted Answer

A. It represents irreversible loss of axonal tissue and ganglion cells due to chronically elevated intraocular pressure exceeding the autoregulatory capacity of the optic nerve head. ## Why option 1 is right

The enlarged cup with cup-to-disc ratio >0.7 in primary open-angle glaucoma (POAG) represents irreversible excavation of the optic nerve head caused by chronic, progressive loss of retinal ganglion cells and their axons. This occurs when intraocular pressure chronically exceeds the autoregulatory capacity of the optic nerve head, leading to mechanical compression and ischaemic damage at the lamina cribrosa. The correlation between the enlarged cup (marked **A**), the thinned neuroretinal rim, the bayoneting of vessels, the RNFL wedge defect, and the corresponding visual field defect (superior arcuate scotoma and nasal step) confirms that this cup enlargement is a structural manifestation of glaucomatous optic neuropathy. According to the AAO Basic and Clinical Science Course Section 10, the vertical cup-to-disc ratio >0.7 is a hallmark sign of advanced glaucomatous cupping and indicates substantial axonal loss. This is the defining pathophysiological feature of POAG.

## Why each distractor is wrong

- **Option 2**: Acute ischaemic necrosis occurs with sudden, severe pressure elevation (typically >60 mmHg) and presents with acute vision loss and optic nerve head swelling—not a chronic, excavated cup. This patient's pressures are only mildly elevated (26 and 24 mmHg) and he is asymptomatic, ruling out acute ischaemia.
- **Option 3**: Papilloedema causes optic nerve head swelling with blurred margins and preserved or elevated cup margins—the opposite of the excavated cup seen here. Moreover, papilloedema is reversible if the underlying cause is treated, whereas glaucomatous cupping is irreversible.
- **Option 4**: Congenital optic nerve hypoplasia presents with a small, pale optic disc with reduced axon count from birth, not an acquired, progressive excavation in response to elevated intraocular pressure. The clinical context of elevated IOP, open angles, and progressive visual field loss excludes a congenital cause.

**High-Yield:** In POAG, the enlarged cup (cup-to-disc ratio >0.7) is an irreversible marker of chronic ganglion cell loss caused by IOP-induced ischaemia at the lamina cribrosa; it correlates with RNFL defects and visual field loss and mandates lifelong pressure-lowering therapy and monitoring.

[cite: AAO Basic and Clinical Science Course, Section 10: Glaucoma]

Answer

B. It indicates acute ischaemic necrosis of the optic nerve head secondary to sudden elevation of intraocular pressure above 60 mmHg

Answer

C. It demonstrates congenital hypoplasia of the optic nerve with primary absence of ganglion cells unrelated to intraocular pressure

Answer

D. It reflects reversible swelling of the optic nerve head caused by papilloedema from impaired axoplasmic flow

Explanation

Why option 1 is right

Why each distractor is wrong

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