Blood Supply of the Intraorbital Part of the Optic Nerve

1. Periaxial System (Extrinsic / Peripheral)

a) Ophthalmic Artery — the dominant contributor

• Arises from the internal carotid artery and runs inferolateral to the optic nerve as it enters the orbit, then crosses superomedially
• Gives off multiple direct pial branches along its course that supply the peripheral portions of the intraorbital nerve
• These branches enter the pia mater and form the pial arterial plexus (pia plexus), which encircles the nerve throughout its orbital course

b) Short Posterior Ciliary Arteries (SPCAs)

• 10–20 small branches arising from the ophthalmic artery
• Primarily supply the optic nerve head (prelaminar and laminar regions), but some branches also contribute to the proximal intraorbital segment via the pial plexus

c) Long Posterior Ciliary Arteries (LPCAs)

• Also contribute to the pial plexus supplying the intraorbital optic nerve

d) Other Branches

• Muscular branches and recurrent choroidal branches can contribute collaterally

The pial plexus is the key conduit — it supplies the peripheral portions (outer ~2/3) of the intraorbital nerve via centripetal branches penetrating inward through septal vessels.

2. Axial System (Intrinsic / Central)

Central Retinal Artery (CRA)

• The CRA is the first major branch of the ophthalmic artery
• It enters the optic nerve 8–12 mm behind the globe, piercing the inferior surface of the nerve
• As it travels anteriorly through the nerve to supply the retina, it gives off centrifugal branches that supply the central axial core of the intraorbital optic nerve
• These branches supply the inner ~1/3 of the nerve (axial zone)

Summary Table

Clinical Relevance

• Posterior Ischemic Optic Neuropathy (PION): Ischemia of the retrolaminar intraorbital optic nerve, caused by compromise of the surrounding pial capillary plexus (supplied by small pial branches of the ophthalmic artery). Seen in major spine/cardiac surgeries with hypotension and blood loss. The CT scan may reveal enlargement of the intraorbital optic nerve. — Kanski's Clinical Ophthalmology, p. 778; Barash's Clinical Anesthesia
• The central retinal artery enters the nerve 8–12 mm posterior to the globe; optic nerve sheath procedures or any mass posterior to this point can affect both the CRA (axial supply) and pial supply simultaneously.
• Because the two systems (periaxial and axial) supply different zones, isolated CRA occlusion typically spares the outer nerve fibers.

Pial Plexus of the Optic Nerve — Formed By:

Retrolaminar (Proximal Intraorbital) — Anterior Segment

1. Peripapillary choroidal vessels
2. Circle of Zinn–Haller (intrascleral arterial circle formed by SPCAs)
3. Central retinal artery (pial branches given off before it enters the nerve)
4. Ophthalmic artery (direct pial branches)
5. Other orbital arteries (collateral pial branches)

Posterior Intraorbital — Posterior Segment

1. Ophthalmic artery — multiple small collateral pial branches arising directly from the ophthalmic artery (the dominant and most consistent contributor)
2. Other orbital arteries — less frequently

In ~10% of optic nerves, there is also an axial centrifugal system extending backward via intraneural branches of the CRA.

Summary

Key point (Hayreh): The ophthalmic artery is the single most important contributor to the pial plexus of the intraorbital optic nerve. Compromise of these small pial collaterals — as in perioperative hypotension or orbital compression — underlies Posterior Ischemic Optic Neuropathy (PION).