Oculomotor nerve (III) !

The oculomotor nerve (cranial nerve III) moves most eye muscles, lifts the eyelid and narrows the pupil; if compressed, the lid droops and the pupil widens.

Official name: Oculomotor nerve

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What it is

The nerve from the midbrain to most eye muscles, the eyelid lifter and the pupil.

What it does

Moves the eye up, down and in, lifts the lid and narrows the pupil; when compressed the eye turns down and out with a wide pupil.

Key facts

Roots: Nuclei: oculomotor and Edinger-Westphal (midbrain); exits the interpeduncular fossa; superior orbital fissure
Moves: Superior division: SR, levator palpebrae; inferior division: MR, IR, IO and the parasympathetic root to the ciliary ganglion (from the nerve to IO): sphincter pupillae, ciliary muscle
Feeling from: None

Relations

Course: Leaves the midbrain in the interpeduncular fossa, passes between the posterior cerebral artery and the superior cerebellar artery, runs forward beside the posterior communicating artery, through the roof and lateral wall of the cavernous sinus and into the orbit through the superior orbital fissure, inside the common tendinous ring, where it splits into superior and inferior divisions
Above: It is the highest nerve in the lateral wall of the cavernous sinus, above the trochlear nerve (IV) and the V1 ophthalmic nerve
Lateral: At the tentorial notch, the uncus of the temporal lobe and the free edge of the tentorium; a swelling hemisphere pushes the uncus onto the nerve

How to find it

It cannot be felt; it is assessed through eye movements, the eyelid and the pupil. On MRI or in dissection it is found emerging between the cerebral peduncles and running forward between the posterior cerebral and superior cerebellar arteries.

Examination

Eye movements in an H pattern: weak adduction, elevation and depression point to CN III; in primary gaze the eye rests down and out
Pupillary light reflex: a dilated pupil that does not constrict to direct or consensual light on that side means the efferent limb (CN III) is affected
Near (accommodation) reflex: on the affected side the pupil does not constrict and the eye does not converge
Eyelid: ptosis from levator palpebrae weakness, often complete in a full palsy

On imaging

When the pupil is involved, CT or MR angiography looks for a posterior communicating artery aneurysm; MRI shows the midbrain nucleus, the cisternal segment and the cavernous sinus.

Common problems

Palsy: droopy lid, the eye turned down and out, double vision.
A palsy with a wide pupil can mean an aneurysm pressing on it: an emergency.
Diabetes can cause a palsy that usually spares the pupil.

How it is diagnosed

Eye and pupil exam; urgent CT/MR angiography when the pupil is involved.

Treatment

Treat the cause.
Securing an aneurysm (clip or coils).
Most diabetic palsies recover within months.

Clinical notes

Weber syndrome: a lesion of the midbrain gives a CN III palsy on the same side with weakness of the opposite side of the body
Cavernous sinus disease (thrombosis, tumour, carotid-cavernous fistula) can affect III together with IV, V1, V2 and VI
The ptosis of a CN III palsy is usually marked and the pupil large, unlike the mild ptosis with a small pupil of Horner syndrome

Memory aid

Eye muscle nerves: "LR6 SO4, the rest 3": lateral rectus by VI, superior oblique by IV, all the others by III.

Test yourself

Think of the answer, then tap the question to see it.

What is the eye position in a complete oculomotor nerve (III) palsy, and why?

Down and out, with marked ptosis and a fixed dilated pupil: the lateral rectus (VI) and superior oblique (IV) act unopposed.

In an oculomotor nerve palsy, what does pupil involvement suggest compared with pupil sparing?

Pupil involved: compression, classically a posterior communicating artery aneurysm (emergency). Spared: usually diabetic or hypertensive microvascular palsy.

Why does outside compression of the oculomotor nerve affect the pupil first?

The parasympathetic pupil fibres run on the surface of the nerve, while small-vessel ischaemia damages its core.

What early sign does uncal herniation produce through the oculomotor nerve?

A fixed, dilated pupil on the side of the mass, as the uncus presses the nerve against the tentorium.

What is Weber syndrome?

A midbrain lesion giving an oculomotor nerve (III) palsy on the same side with weakness of the opposite side of the body.

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

Its names

Official Latin (TA2): nervus oculomotorius TA2 6187
Official Spanish (DeCS): Nervio Oculomotor DeCS D009802
English (TA2): Oculomotor nerve
Also called: cranial nerve III

Sources: Terminologia Anatomica (TA2); DeCS; Neuroanatomy, Cranial Nerve 3 (Oculomotor).

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Representative model, for educational purposes only.

Oculomotor nerve (III) highlighted on the Durgical 3D model