07 OCULOMOTOR CN III

# 08 OCULOMOTOR CN III

The oculomotor nerve is the most heavily examined cranial nerve in MRCS Part A — partly because its anatomy explains so much pathology (head injury, aneurysm, diabetes, cavernous sinus disease), and partly because the pupil-sparing vs pupil-involving distinction is a classic SBA trap. Master the nerve, its course, and the order in which fibres fail, and a whole family of questions becomes easy.

Origin and central anatomy

CN III arises from two nuclei in the midbrain, at the level of the superior colliculus:

- ➡ Oculomotor nucleus — somatic motor fibres to four of the six extraocular muscles plus the levator palpebrae superioris.

- ➡ Edinger–Westphal nucleus — preganglionic parasympathetic fibres to the sphincter pupillae and ciliary muscle.

Fibres emerge from the interpeduncular fossa on the ventral surface of the midbrain, between the cerebral peduncles. This anatomical relationship matters: a lesion of the peduncle plus the exiting nerve gives Weber's syndrome — ipsilateral CN III palsy with contralateral hemiplegia.

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Course of the nerve

After leaving the midbrain, CN III passes between the posterior cerebral and superior cerebellar arteries — the reason a posterior communicating artery (PCom) aneurysm is the classic compressive cause of a third nerve palsy.

It then pierces the dura and runs forward in the lateral wall of the cavernous sinus, alongside CN IV, V1, V2 and the internal carotid (CN VI sits free in the middle of the sinus, not the wall — a high-yield distinction).

The nerve enters the orbit through the superior orbital fissure, inside the annulus of Zinn, where it divides into:

- Superior division ➡ superior rectus and levator palpebrae superioris.

- Inferior division ➡ medial rectus, inferior rectus, inferior oblique, and the parasympathetic branch to the ciliary ganglion.

Motor function — remember LR6 SO4 (3)

CN III supplies every extraocular muscle except the lateral rectus (CN VI) and the superior oblique (CN IV). It also supplies the levator palpebrae superioris, which elevates the upper eyelid.

Muscle	Action	Nerve
Medial, superior, inferior rectus	Adduction, elevation, depression	CN III
Inferior oblique	Elevation in adduction	CN III
Levator palpebrae superioris	Eyelid elevation	CN III
Lateral rectus	Abduction	CN VI
Superior oblique	Depression in adduction / intorsion	CN IV

👩‍⚕️ The superior tarsal muscle (Müller's) also helps hold the lid up but is sympathetic, not CN III — that is why Horner's gives only a partial ptosis.

Parasympathetic function

Preganglionic fibres from the Edinger–Westphal nucleus travel on the outer surface of the nerve and synapse in the ciliary ganglion. Postganglionic fibres travel via the short ciliary nerves to:

- Sphincter pupillae ➡ pupil constriction (miosis).

- Ciliary muscle ➡ lens accommodation for near vision.

That superficial location is the single most important fact in CN III pathology — it explains the pupil-sparing rule below.

Pupillary light reflex

1. Light hits the retina.

2. Afferent: optic nerve (CN II).

3. Bilateral pretectal nuclei in the midbrain.

4. Bilateral Edinger–Westphal nuclei.

5. Efferent: parasympathetic fibres of CN III.

6. Ciliary ganglion → short ciliary nerves.

7. Sphincter pupillae contracts → direct and consensual constriction.

Because of the bilateral pretectal crossover, light in one eye constricts both pupils. A unilateral CN III lesion abolishes both the direct response in the affected eye and the consensual response from the affected eye to the other side.

Third nerve palsy — the clinical picture

Classic triad:

- ➡ "Down and out" eye — unopposed lateral rectus (CN VI) and superior oblique (CN IV).

- ➡ Ptosis — paralysis of levator palpebrae superioris (often complete).

- ➡ Mydriasis — loss of parasympathetic tone leaves the sympathetic dilator unopposed; pupil is fixed and dilated, unreactive to light or accommodation.

Surgical vs medical third — the pupil rule

This is the single highest-yield CN III concept in MRCS Part A.

Feature	Surgical (compressive)	Medical (ischaemic)
Pupil	Involved — fixed, dilated	Spared — normal size and reactive
Mechanism	External compression hits superficial parasympathetic fibres first	Microvascular infarction of the central motor fibres; pial vessels supplying the outer parasympathetic fibres are preserved
Classic causes	PCom aneurysm, uncal herniation, tumour, cavernous sinus pathology	Diabetes mellitus, hypertension, vasculitis (GCA, SLE), MS
Onset	Sudden, often painful	Usually painless, recovers over weeks
Action	Urgent imaging (CT angiogram)	Investigate vascular risk factors

👩‍⚕️ Why the pupil rule works: parasympathetic fibres run on the outside of the nerve, so an external compressive lesion (aneurysm, herniating uncus) squashes them first. Microvascular ischaemia hits the core motor fibres but spares the surface — hence the pupil is preserved.

Causes worth memorising

- Uncal (transtentorial) herniation — classically from a rapidly expanding extradural haematoma. The medial temporal lobe pushes over the tentorial edge, crushing CN III against the sharp dural fold. Head injury + ipsilateral fixed dilated pupil = uncal herniation until proven otherwise.

- PCom artery aneurysm — painful third with pupil involvement; a surgical emergency.

- Weber's syndrome — midbrain stroke: ipsilateral CN III + contralateral hemiplegia.

- Cavernous sinus pathology — thrombosis, pituitary apoplexy, tumour: CN III rarely alone — look for IV, V1, V2, VI deficits too.

- Diabetic mononeuropathy — the classic pupil-sparing third.

CN III palsy vs Horner's syndrome

Feature	CN III palsy	Horner's syndrome
Ptosis	Severe (levator)	Mild (superior tarsal)
Pupil	Mydriasis (dilated)	Miosis (constricted)
Eye position	Down and out	Normal
Sweating	Normal	Anhidrosis of face
Lesion	CN III (parasympathetic + motor)	Sympathetic chain

👩‍⚕️ Mnemonics:

- CN III = Ptosis + Mydriasis + Lateral squint.

- Horner's = Ptosis + Miosis + Anhidrosis.


Test yourself

After a head injury, a patient develops a fixed, dilated pupil. What is the cause?

• ((Unopposed sympathetic fibres::☑️ Loss of CN III parasympathetic input leaves sympathetic dilation unopposed → fixed dilated pupil.))
• ((Unopposed parasympathetic fibres::Would cause miosis, not mydriasis.))
• ((Direct iris sphincter spasm::Would produce a constricted pupil.))
• ((Horner syndrome::Causes miosis with mild ptosis and anhidrosis — not a blown pupil.))

👩‍⚕️ Compressive CN III lesions hit superficial parasympathetic fibres first — the pupil blows before the eye moves.

A patient develops a "down-and-out" eye after neck surgery. Which feature accompanies this?

• ((Exophthalmos::Feature of Graves' orbitopathy, not isolated CN III palsy.))
• ((Mydriasis::☑️ CN III parasympathetic loss prevents sphincter pupillae constriction → dilated pupil.))
• ((Nystagmus::Suggests vestibular or cerebellar pathology, not CN III.))
• ((Loss of corneal reflex::Corneal reflex is V1 (afferent) and VII (efferent), not CN III.))

👩‍⚕️ Classic triad: down-and-out eye, ptosis, mydriasis.

CT shows bilateral extradural haematomas. The anaesthetist notes one dilated pupil. Which side is decompressed first?

• ((Ipsilateral side::☑️ Dilated pupil marks ipsilateral uncal herniation compressing CN III — decompress that side first.))
• ((Contralateral side::Would not relieve the mass effect causing herniation.))
• ((Vertex::Not a standard site for EDH decompression.))
• ((Midline frontal::Used for ventricular access, not haematoma evacuation.))

👩‍⚕️ The blown pupil localises the herniating side — that is the surgical priority.

A patient with initial GCS 15 deteriorates with a blown pupil. Diagnosis?

• ((Hydrocephalus::Raised ICP usually gives bilateral signs, not an isolated blown pupil.))
• ((Transtentorial (uncal) herniation::☑️ Medial temporal lobe herniates over the tentorium, compressing CN III.))
• ((Diffuse axonal injury::Reduces consciousness but does not cause unilateral mydriasis.))
• ((Subdural haematoma without herniation::A blown pupil implies herniation has occurred.))

👩‍⚕️ The "lucid interval" followed by deterioration and a blown pupil is the textbook extradural picture.

A patient sustains head trauma, is conscious in A&E with a right fixed dilated pupil, then deteriorates and dies. Likely cause?

• ((Left extradural haematoma::Would compress the left CN III, giving a left dilated pupil.))
• ((Right extradural haematoma::☑️ Right-sided expansion → right uncal herniation → right CN III compression → right blown pupil.))
• ((Acute subdural haematoma::Bridging vein bleed; usually gradual decline, not classic lucid interval.))
• ((Subarachnoid haemorrhage::Thunderclap headache with meningism, not a lucid interval.))

👩‍⚕️ The dilated pupil is ipsilateral to the haematoma — parasympathetic fibres on the nerve's surface fail first.

A 65-year-old diabetic develops sudden ptosis and a down-and-out eye, with a normal-sized reactive pupil. Most likely cause?

• ((PCom artery aneurysm::Would cause pupil involvement (compressive) and is typically painful.))
• ((Uncal herniation::Surgical third; pupil would be fixed and dilated.))
• ((Diabetic mononeuropathy::☑️ Microvascular infarction of central motor fibres; superficial parasympathetics spared → pupil-sparing third.))
• ((Cavernous sinus thrombosis::Usually combines with CN IV, V1, V2, VI deficits and proptosis.))

👩‍⚕️ Pupil-sparing third in a vasculopath = medical. Pupil-involving = surgical until imaged.

Revision summary

- Nuclei: oculomotor + Edinger–Westphal (parasympathetic), in the midbrain at superior colliculus level.

- Course: interpeduncular fossa → between PCA and SCA → lateral wall of cavernous sinus → superior orbital fissure (within annulus of Zinn) → superior and inferior divisions.

- Motor: all extraocular muscles except lateral rectus (VI) and superior oblique (IV); plus levator palpebrae superioris. LR6 SO4 (3).

- Parasympathetic: Edinger–Westphal → ciliary ganglion → short ciliary nerves → sphincter pupillae (miosis) + ciliary muscle (accommodation).

- Palsy triad: down-and-out eye + ptosis + mydriasis.

- Pupil rule: parasympathetic fibres run on the outside of the nerve.

- Surgical (compressive) ➡ pupil involved: PCom aneurysm, uncal herniation.

- Medical (ischaemic) ➡ pupil spared: diabetes, hypertension, vasculitis.

- Head injury + ipsilateral blown pupil = uncal herniation from extradural haematoma — decompress that side first.

- Weber's syndrome: ipsilateral CN III + contralateral hemiplegia (midbrain stroke).

- CN III vs Horner's: mydriasis + severe ptosis vs miosis + mild ptosis + anhidrosis.