Overview

Diameter≈1.5–1.8 mm (vertically oval)

Position≈3–4 mm nasal to the fovea

Blind spot≈15° temporal to fixation, ≈5° × 7°

Normal cup-to-disc ratio≈0.3 (≤0.5)

The optic disc is slightly vertically oval (≈1.8 mm high × 1.7 mm wide). Axons of the nerve-fibre layer turn 90° here to pass backwards through the lamina cribrosa of the sclera; behind it they acquire myelin (from oligodendrocytes), so the nerve widens from ≈1.5 mm to ≈3 mm. The central retinal artery and vein enter and leave through the centre of the disc.

On fundoscopy the disc is pink-orange with a central pale depression, the physiological cup, devoid of axons; the pink rim around it (the neuroretinal rim) contains the axons. A normal cup-to-disc ratio is ≈0.3 (≤0.5); the rim is usually thickest inferiorly, then superiorly, nasally and temporally (ISNT rule). Spontaneous venous pulsation is seen at the disc in ≈80–90% of normal people and is lost when intracranial pressure rises.

Because there are no photoreceptors, the disc produces a blind spot in the visual field ≈15° temporal to fixation and slightly below the horizontal — not noticed in daily life because the other eye covers it and the brain "fills in" the gap.

Location: Retina, ≈3–4 mm nasal and slightly superior to the fovea; corresponds to the blind spot ≈15° temporal to fixation.

Anatomy

Arterial supply: Prelaminar and laminar regions: short posterior ciliary arteries (circle of Zinn–Haller); surface nerve-fibre layer: central retinal artery branches.

Function

Exit point of retinal ganglion cell axons (optic nerve)
Entry and exit of the central retinal artery and vein

Clinical significance

Papilloedema

Bilateral optic disc swelling due to raised intracranial pressure (tumour, idiopathic intracranial hypertension, venous sinus thrombosis, malignant hypertension). The subarachnoid space extends along the optic nerve sheath, so raised CSF pressure impairs axoplasmic flow at the lamina cribrosa. Signs: blurred elevated disc margins, loss of the cup, loss of spontaneous venous pulsation (early), haemorrhages and cotton-wool spots, enlarged blind spot. Visual acuity is preserved until late (unlike optic neuritis). Never perform a lumbar puncture before excluding a mass lesion.

Glaucomatous cupping

Loss of rim tissue enlarges the cup (C:D >0.6, asymmetry >0.2 between eyes, notching especially inferiorly and superiorly, bayoneting of vessels), corresponding to arcuate field defects.

Optic atrophy and optic neuritis

Optic atrophy: pale disc after optic nerve damage (compression, ischaemia, glaucoma, MS). Optic neuritis (often MS): painful eye movements, reduced acuity and colour vision (red desaturation), central scotoma and RAPD; the disc looks normal in two-thirds (retrobulbar neuritis) — "the patient sees nothing and the doctor sees nothing".

Test yourself

Bilateral optic disc swelling with loss of spontaneous venous pulsation, enlarged blind spots and preserved acuity most suggests:

Optic neuritis
Central retinal artery occlusion
Papilloedema from raised intracranial pressure
Glaucomatous optic neuropathy

Answer: Papilloedema from raised intracranial pressure

Papilloedema is disc swelling caused by raised ICP transmitted along the optic nerve sheath; acuity is preserved until late. Optic neuritis is usually unilateral with early loss of acuity and colour vision.