Swelling of this patch, seen through the ophthalmoscope, with blurred margins, in a patient with severe headache and raised pressure inside the skull
Papilloedema — swelling of the optic disc from raised intracranial pressure
Anatomy · Eye, ear & senses
Optic disc
Where the optic nerve leaves the eye
Eye · retina, nasal to the macula
The round or slightly oval pale-pink patch on the retina, about 1.5 mm across, where about 1.2 million ganglion-cell axons converge and leave the eye as the optic nerve, and where the central retinal artery and vein enter and leave. It contains no photoreceptors, so it is the physiological blind spot. It lies to the nasal side of the macula, and it is what a doctor examines for signs of glaucoma and raised intracranial pressure.
How to spot them
- ▸A pale, round patch on the fundus with retinal vessels radiating from its centre
- ▸Lies on the nasal side of the macula, roughly 3–4 mm from the fovea
- ▸Its central pale dip is the physiological cup
- ▸The one place on the retina where you cannot see
What to know
The pale central dip enlarges and deepens, so that the cup-to-disc ratio rises above about 0.5, as nerve fibres are lost
Glaucomatous cupping of the optic disc
The gateway where the central retinal artery and vein pass into and out of the eye, in the head of the nerve
The optic disc — the entry and exit point for the retinal vessels
About 1.2 million fibres pass through a sieve-like plate of connective tissue here, the lamina cribrosa
The optic disc and the lamina cribrosa
Lies nasal to the fovea, so its blind spot falls about 15 degrees to the temporal side of the visual field
The optic disc and its blind spot
Octopus and squid have no such patch, because their photoreceptors face the light and their nerve fibres run behind
The optic disc, a consequence of the vertebrate retina being wired back to front
Key ideas
- It is the only place in the body where the fibres of the central nervous system can be seen directly, so a look at the optic disc gives an indirect look at the brain's pressure.
- The brain fills in the blind spot from the surrounding pattern, which is why you never notice a hole in your vision.
Quick-fire facts
- •About 1.5 mm across (1.5 × 1.75 mm), with the long axis vertical, and 3–4 mm nasal to the fovea
- •The optic nerve fibres are unmyelinated within the retina and gain their myelin just behind the lamina cribrosa
- •The physiological cup normally occupies about a third of the disc diameter
- •Papilloedema strictly means disc swelling caused by raised intracranial pressure; other causes of swelling are simply 'optic disc oedema'
- •The blind spot was described by Edme Mariotte in the 1660s
- •Cup-to-disc ratio and rim thinning are checked at every routine optometrist's eye test for glaucoma
- •Optic neuritis, common in multiple sclerosis, may make the disc look swollen or normal (retrobulbar)
- •Central retinal vein occlusion presents with a 'blood and thunder' fundus with disc swelling
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Sources
- TeachMeAnatomy — The Eyeball — Structure, Vasculature
- National Eye Institute (NIH) — Glaucoma