A 58-year-old Indian male with a 5-year history of primary open-angle glaucoma presents for routine follow-up. Fundoscopy reveals progressive optic disc cupping with a cup-to-disc ratio of 0.75. The structure marked **C** in the diagram (nasal displacement of vessels) is noted. Which of the following best explains the mechanism behind this finding in chronic glaucoma?

Explanation

## Why option 1 is right Nasal displacement of vessels (marked **C**) is a hallmark sign of chronic glaucomatous optic neuropathy. As the neuroretinal rim progressively erodes—particularly in the temporal region (following the ISNT rule violation pattern)—the optic cup deepens and widens. The retinal vessels, which normally cross the disc margin, appear displaced nasally because the temporal rim has been preferentially thinned by glaucomatous damage. This is a structural consequence of progressive rim loss, not a primary vascular phenomenon. Per Khurana Ophthalmology 7e, nasal displacement of vessels is a characteristic optic disc change in chronic glaucoma reflecting the underlying structural damage to the neuroretinal rim. ## Why each distractor is wrong - **Option 2**: While raised IOP is a major risk factor in glaucoma, the nasal displacement of vessels is NOT a direct mechanical compression effect. The vessels shift because the rim erodes, not because they are squeezed. Additionally, normal-tension glaucoma (IOP < 21 mmHg) exhibits identical disc changes including nasal vessel displacement, proving that IOP alone does not mechanically displace vessels. - **Option 3**: Congenital misdirection would be present from birth and would not progress over 5 years in an adult. Nasal displacement is an acquired, progressive finding that develops as glaucomatous damage accumulates, not a developmental anomaly. - **Option 4**: Vasospasm is not the mechanism of nasal vessel displacement in glaucoma. Vasospasm might contribute to disc hemorrhages or ischemic events, but the anatomical shift of vessels nasally is a structural consequence of rim erosion, not a functional vascular response. **High-Yield:** Nasal displacement of vessels in glaucoma = progressive temporal rim erosion causing vessels to appear shifted nasally as the cup deepens; this is a STRUCTURAL change reflecting neuroretinal rim loss, not a primary vascular or mechanical compression phenomenon. [cite: AK Khurana Ophthalmology 7e — Optic Disc Changes in Chronic Glaucoma: Nasal Displacement of Vessels]