Optic Disc & RNFL Structural Assessment

Cup-to-Disc Ratio & the ISNT Rule

The vertical cup-to-disc ratio (CDR, the vertical cup diameter divided by the vertical disc diameter) is the standard clinical measure: vertical rim loss occurs earliest and most characteristically in glaucoma. A CDR above 0.5, or an interocular asymmetry greater than 0.2, is suspicious for glaucoma. But disc size matters. A large disc can carry a proportionally larger cup-to-disc ratio, even 0.6, with a completely healthy, uniform rim and no pathology at all, since cup size simply scales with overall disc size. CDR read in isolation, without accounting for disc size, is a trap.

The ISNT rule describes the normal neuroretinal rim thickness order: Inferior > Superior > Nasal > Temporal. Glaucomatous damage typically begins inferiorly, then superiorly, which lines up with the superior and inferior arcuate patterns seen on visual field testing. Vertical cup elongation reflects exactly this preferential rim loss at the two poles.

Vascular & Structural Signs of Advanced Damage

Several exam signs mark deeper, more advanced excavation:

• Bayoneting sign: vessels bend sharply at the disc margin and again at the floor of the cup, reflecting posterior bowing of the lamina cribrosa.
• Laminar dots sign: visible perforations of the lamina cribrosa exposed as rim tissue is lost, indicating deep, advanced excavation.
• Disc (Drance) hemorrhage: a splinter-shaped hemorrhage at the disc margin, strongly predictive of progressive, localized RNFL loss. It marks active injury, not a stable finding.
• Beta-zone peripapillary atrophy: complete absence of RPE and choriocapillaris, making underlying sclera and choroidal vessels directly visible; its area correlates with glaucoma severity, unlike alpha-zone atrophy (irregular pigmentation of intact RPE), which doesn't.

One important negative sign: when disc pallor exceeds cupping, that pattern should raise suspicion for a non-glaucomatous optic neuropathy (compressive, ischemic, or inflammatory) rather than glaucoma, since glaucoma typically produces cupping out of proportion to pallor: the residual rim retains good perfusion. A "disc at risk," small and crowded with minimal physiologic cup, is a different entity. It predisposes to non-arteritic anterior ischemic optic neuropathy (NAION), not typical glaucomatous cupping.

OCT RNFL Measurement & the "Red Disease" Trap

Spectral-domain OCT is the standard quantitative tool for measuring circumpapillary RNFL thickness against age-matched normative databases. Early thinning typically occurs first in the superior and inferior quadrants, the same poles most vulnerable on disc exam, matching the normal RNFL profile's "double-hump" pattern. Red-free (green light) fundus photography enhances this by filtering red wavelengths, so RNFL striations and wedge-shaped defects appear as dark bands against a lighter background.

The opposite failure is the more dangerous one: a scan reads entirely within normal limits while real glaucomatous damage is already present, a pattern sometimes called "green disease." High myopia drives this too. An eye with naturally atypical nerve fiber layer anatomy can land inside the green range of a normative database built mostly from non-myopic eyes, so a focal defect gets diluted into an otherwise reassuring global average. A clean, all-green printout doesn't replace a direct look at the disc and the field.