Ratios in Ophthalmology

Last updated: October 2026

Kestenbaum's Index (corrected)

The Kestenbaum capillary number index is the number of small vessels/capillaries crossing the optic disc margin — counted starting at the 12 o'clock position, going around the full circumference:

• Normal count: approximately 10 (classically described as ~9 large vessels [4-5 veins + 4-5 arteries] plus ~10 small vessels, without pupil dilation)
• Optic atrophy: count reduces to fewer than 6
• Hyperemic disc: count exceeds 12

First described by Alfred Kestenbaum in 1947, this remains a simple, no-equipment bedside way to help quantify borderline optic atrophy.

Cup-Disc Ratio

Normal cup-disc ratio averages around 0.3, with wide physiological variation (up to 0.5-0.6 can be normal in an otherwise healthy, large disc).

• A large cup may be physiological (especially in large discs) or glaucomatous — asymmetry between the two eyes (>0.2 difference) is a more reliable red flag than an isolated large cup
• The cup enlarges/deepens with progressive glaucomatous damage
• The disc becomes "full" (cup obscured) in papilloedema and papillitis, since swelling fills in the physiological cup

Arden Ratio (EOG)

Arden ratio = (light peak / dark trough) × 100, from electro-oculography:

• 1.80 (180%) or more: normal (some laboratories use 1.85)
• 1.65–1.80: subnormal
• Below 1.65: clearly abnormal; Best disease is typically below 1.5

Severely reduced in Best vitelliform macular dystrophy — see our Electro-Oculography page for the full clinical significance of this test.

Arteriovenous (A:V) Ratio

Normal retinal arteriole:venule caliber ratio is roughly 2:3. Deviations are clinically meaningful:

• Arteriolar narrowing: hypertensive retinopathy, arteriosclerosis, central retinal artery occlusion
• Venous tortuosity: diabetes mellitus, central retinal vein occlusion, blood dyscrasias (e.g., polycythemia, hyperviscosity states)
• Vascular sheathing: periphlebitis retinae, hypertensive retinopathy

A few more high-yield ratios

• AC/A ratio (accommodative convergence to accommodation): normal is roughly 3-5 prism diopters of convergence per diopter of accommodation. A high AC/A ratio is central to the diagnosis and management of accommodative esotropia and convergence excess
• Rim-to-disc ratio (used in the ISNT rule): the healthy neuroretinal rim is normally thickest Inferiorly, then Superiorly, then Nasally, then Temporally (I > S > N > T). Violation of this pattern is an early, subtle sign of glaucomatous damage, often preceding a frankly enlarged cup-disc ratio
• Axial length to corneal diameter ratio — a supportive clue in congenital glaucoma workup, since disproportionate globe enlargement (buphthalmos) reflects sustained elevated IOP in the pre-fusion sclera

Key reference: Kestenbaum Index, EyeWiki (2025).