The optic nerve (CN II) is surrounded by meningeal sheaths (pia, arachnoid, dura) and a subarachnoid space continuous with the cranial subarachnoid space. This explains a clinical sign seen in raised intracranial pressure. What is this sign?

• A Pulsatile proptosis
• B Afferent pupillary defect
• C Arcus senilis
• D Papilloedema — swelling of the optic disc due to impeded axoplasmic flow from raised CSF pressure in the optic nerve sheath ✓

Explanation

The meningeal sheaths of the optic nerve form a subarachnoid space continuous with the intracranial subarachnoid space. When ICP rises, this elevated CSF pressure is transmitted along the optic nerve sheath and compresses the central retinal vein (which runs in the subarachnoid space of the nerve) and impedes orthograde axoplasmic flow at the lamina cribrosa, causing disc swelling (papilloedema). Papilloedema is bilateral (because both optic nerve sheaths connect to the same CSF space) and is a classic sign of raised ICP. Afferent pupillary defect indicates optic nerve damage, not simply raised ICP.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.