Oculomotor (third) nerve palsy !

Damage to the third cranial nerve makes the eyelid droop and the eye turn down and out, causing double vision. A blown pupil can signal an aneurysm.

Updated 2026-11-01

Structures involved

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Why it happens

What it is

Weakness or paralysis of the muscles supplied by the oculomotor nerve (cranial nerve III), which lifts the eyelid, moves the eye up, down and inward, and constricts the pupil. It may be complete or partial and may spare or involve the pupil.

Who gets it

One of the most frequent acquired cranial nerve palsies in adults. In older adults with diabetes or hypertension, microvascular ischaemia is the leading cause; in Colombia and Latin America, where diabetes and hypertension are common, this pattern is often seen in clinics. Compressive causes, above all aneurysms of the posterior communicating artery, are less common but are the ones that must not be missed.

Causes

Microvascular ischaemia of the nerve (diabetes, hypertension, dyslipidaemia), usually pupil-sparing
Compression by an aneurysm, classically at the junction of the internal carotid and posterior communicating arteries
Uncal (transtentorial) herniation from a mass or bleed raising intracranial pressure
Midbrain stroke, tumours, demyelination or trauma
Cavernous sinus or orbital apex lesions (thrombosis, tumours, pituitary apoplexy), often with other cranial nerves involved
Inflammatory causes such as giant cell arteritis, and rarely ophthalmoplegic migraine in children

Risk factors

Diabetes mellitus
Arterial hypertension and dyslipidaemia
Age over 50 years
Known or familial intracranial aneurysm, polycystic kidney disease or connective tissue disorders
Head trauma

Pathophysiology

The pupillomotor parasympathetic fibres run on the outer (superficial) part of the nerve and get their blood from pial vessels, while the motor fibres lie deeper. An external mass such as an aneurysm compresses the outer fibres first, so the pupil dilates early. Microvascular ischaemia damages the core of the nerve and typically leaves the pupil spared. Loss of the medial, superior and inferior rectus and inferior oblique leaves the lateral rectus (VI) and superior oblique (IV) unopposed, so the eye rests down and out.

Classification

Complete vs partial palsy
Pupil-involving vs pupil-sparing
By site: nuclear (midbrain), fascicular, subarachnoid, cavernous sinus, orbital
Isolated vs with other cranial nerve or long-tract signs

Symptoms and signs

Drooping of the upper eyelid (ptosis), often complete
Eye turned down and out at rest
Double vision, mainly horizontal or oblique, that disappears when the eyelid covers the eye
Inability to move the eye inward, upward or downward
Dilated pupil that reacts poorly to light (in compressive causes)
Pain around the eye or forehead, which can occur in both ischaemic and aneurysmal cases

Diagnosis

Clinical: Inspection of the eyelid, eye position and pupils in light and dark, versions in all directions of gaze, and checking of trochlear function (intorsion on attempted downgaze) and of the other cranial nerves. The key bedside question is whether the pupil is involved.
Tests: Glucose, HbA1c, lipids and blood pressure for vascular risk; ESR and CRP in older adults to look for giant cell arteritis; lumbar puncture when subarachnoid haemorrhage, infection or malignancy is suspected and imaging is negative.
Imaging: Urgent CT angiography or MR angiography when the pupil is involved, the palsy is partial or there are other worrying features; catheter angiography if non-invasive imaging is inconclusive. MRI of the brain and orbits looks for midbrain, cavernous sinus and orbital lesions.

Differential diagnosis

Myasthenia gravis (fluctuating ptosis and ophthalmoplegia with a normal pupil)
Horner syndrome (mild ptosis with a small pupil)
Internuclear ophthalmoplegia
Thyroid eye disease and orbital disease
Fourth or sixth nerve palsy and skew deviation
Pharmacological mydriasis

Goal of treatment

Find and treat the cause promptly, above all an aneurysm or herniation, relieve double vision and protect the eye while the nerve recovers.

Treatment

Acute: A pupil-involving or otherwise suspicious palsy is a neurosurgical emergency: an aneurysm is treated by endovascular coiling or surgical clipping, and raised intracranial pressure is managed urgently.
Medical: For microvascular palsy, control of glucose, blood pressure and lipids, with antiplatelet therapy in selected patients; corticosteroids when giant cell arteritis or another inflammatory cause is confirmed.
Conservative: Occlusion of one eye (patch or occlusive lens) or temporary prisms to relieve double vision while recovery takes place.
Follow-up: Close follow-up of the pupil and eye movements in the first days and weeks; any new pupil involvement or other neurological sign leads to urgent vascular imaging.
Surgical: Strabismus surgery or eyelid surgery for stable deficits that persist after six to twelve months.

Complications

Rupture of an underlying aneurysm with subarachnoid haemorrhage
Permanent ptosis or strabismus with lasting double vision
Aberrant regeneration (for example, eyelid elevation on attempted downgaze or adduction), which suggests compression or trauma rather than ischaemia
Amblyopia in young children

Prognosis

Microvascular palsies usually recover partly or completely within about three to six months. Outcomes in compressive and traumatic cases depend on the cause and on how quickly it is treated; aneurysmal palsies may improve after the aneurysm is secured.

Test yourself

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

What is the classic eye position in a complete oculomotor nerve palsy?

Down and out, with ptosis, because the lateral rectus and superior oblique act unopposed.

Why does an aneurysm usually dilate the pupil while a diabetic palsy spares it?

Pupillary fibres run superficially and are compressed first by an external mass, whereas ischaemia damages the core of the nerve.

Which aneurysm classically causes a painful pupil-involving third nerve palsy?

An aneurysm of the posterior communicating artery at its junction with the internal carotid artery.

What is the first imaging step for a pupil-involving third nerve palsy?

Urgent CT angiography or MR angiography of the intracranial vessels.

A drowsy patient with a fixed dilated pupil after a head injury most likely has what?

Uncal (transtentorial) herniation compressing the oculomotor nerve, a neurosurgical emergency.

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

Sources: Cranial mononeuropathy III - diabetic type: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2025).

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Representative model, for educational purposes only.

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