Optic nerve (CN II)
Optic nerve · Monocular loss RAPD
About 1.2 million ganglion-cell axons that leave the eye through the lamina cribrosa and run roughly 50 mm through the orbit, optic canal and cranial cavity to the chiasm. It is a central nervous system tract: myelinated by oligodendrocytes and sheathed in dura, arachnoid and pia with a CSF-filled subarachnoid space.
Traced from the start
- Temporal hemiretinaNasal hemiretinaMacula and papillomacular bundle
- Optic nerve (CN II)
Detail
- Segments
- Intraocular 1 mm, intraorbital about 25 mm, intracanalicular 4-10 mm, intracranial about 10 mm
- Optic canal
- In the lesser wing of the sphenoid, shared with the ophthalmic artery
- Myelin
- Oligodendrocytes, so it is a target of multiple sclerosis; axons are unmyelinated until the lamina cribrosa
- Blood supply
- Pial plexus and short posterior ciliary branches of the ophthalmic artery; the central retinal artery runs inside the nerve
- Carries
- Whole field of one eye, plus the afferent limb of the pupillary light reflex
When it goes wrong
Complete lesion of one optic nerve (trauma, compression, severe neuritis)
Blindness of that eye with an absent direct but preserved consensual light reflex; partial lesions give a relative afferent pupillary defect on the swinging-torch test
Demyelinating optic neuritis (multiple sclerosis)
Subacute monocular loss of vision over days with pain on eye movement, reduced colour vision, central scotoma and RAPD; the disc looks normal when the lesion is retrobulbar; recovery over weeks
Anterior ischaemic optic neuropathy, arteritic (giant cell arteritis) or non-arteritic
Sudden painless monocular loss with an altitudinal field defect and RAPD; in a patient over 50 check ESR/CRP, jaw claudication and temporal artery tenderness
Slow compression (sphenoid wing meningioma, optic nerve glioma, thyroid eye disease)
Gradual monocular loss of acuity and colour with RAPD, progressing to optic atrophy
Practise this structure
4 questions in the bank tagged Cranial Nerves.
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