Skeletal system · Head and neck

Optic canal !

The optic canal is a short bony canal at the back of the eye socket; it carries the optic nerve and the ophthalmic artery to the eye.

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What it is

A short canal at the back of the eye socket.

What it does

Carries the optic nerve and the ophthalmic artery.

Key facts

Where

Lesser wing of the sphenoid

Function

Transmits the optic nerve (II) and ophthalmic artery

Relations

Contents

Optic nerve (II) with its dura, arachnoid and pia, so the subarachnoid space reaches the back of the eye; the ophthalmic artery with its sympathetic fibres

Course

Runs forward and laterally through the lesser wing of the sphenoid bone, from the middle cranial fossa beside the optic chiasm to the apex of the orbit, where it opens inside the common tendinous ring

Medial

Body of the sphenoid with the sphenoid sinus; sometimes a posterior ethmoid (Onodi) cell

Lateral

Anterior clinoid process; below and lateral, the optic strut, a bar of bone that separates it from the superior orbital fissure

Above

Upper root of the lesser wing and, at its inner end, a fold of dura over the nerve; the frontal lobe above

How to find it

It cannot be felt. It is the hole at the very apex of the orbit, above and medial to the superior orbital fissure. From inside the skull it opens just in front of the anterior clinoid process, beside the tuberculum sellae; surgeons find it endoscopically as a bulge in the upper lateral wall of the sphenoid sinus.

Examination

• Swinging-flashlight test after a blow to the brow: both pupils widening as the light moves onto the injured eye (relative afferent pupillary defect) points to traumatic optic neuropathy, even when the eye itself looks normal
• Visual acuity and colour vision, one eye at a time: lower acuity and red objects looking washed out (red desaturation) suggest an optic nerve lesion; they are recorded early and repeated

On imaging

Thin-slice CT with bone windows, axial and coronal, shows canal fractures, bone fragments pressing on the nerve and how thin the wall with the sphenoid sinus is. MRI with fat suppression and contrast shows the nerve, its sheath and tumours that narrow the canal.

Clinical notes

• Optic canal decompression, now usually endoscopic through the sphenoid sinus, is used for selected compressive lesions; its benefit in traumatic optic neuropathy is still debated
• Masses that narrow the canal include optic nerve sheath meningioma, meningiomas of the planum or clinoid, fibrous dysplasia and optic pathway glioma (in children, linked to neurofibromatosis type 1)
• Endoscopic sinus surgery can injure the nerve where the sinus wall is thin or an Onodi cell wraps around it

Anatomical variants

• Onodi cell: a posterior ethmoid cell extending above or beside the sphenoid sinus, next to the canal; reported frequency differs widely between studies
• The canal may bulge into the sphenoid sinus or have gaps in its wall (dehiscence), leaving the nerve covered only by mucosa
• Rarely the ophthalmic artery runs in its own separate small canal

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2)

canalis opticus TA2 605

Official Spanish (DeCS)

Conducto óptico

English (TA2)

Optic canal

Also called

optic foramen · foramen opticum · canalis fasciculi optici

Related

• Optic nerve

Sources: Terminologia Anatomica (TA2); Anatomy, Head and Neck, Optic Canal; Anatomy, Head and Neck, Skull Foramen.

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Representative model, for educational purposes only.