Medicine – Third Nerve Palsy

Third nerve palsy is dysfunction of the oculomotor nerve, cranial nerve III, which supplies most of the extraocular muscles, the levator palpebrae superioris, and parasympathetic fibres responsible for pupillary constriction.

A useful memory rule is:

LR6 SO4, all the rest 3

meaning:

Lateral rectus → CN VI

Superior oblique → CN IV

Most other extraocular muscles → CN III

1. Muscles Supplied by the Third Cranial Nerve

The oculomotor nerve supplies most muscles responsible for eye movement.

These include:

Medial rectus.

Superior rectus.

Inferior rectus.

Inferior oblique.

It also supplies:

Levator palpebrae superioris, which raises the upper eyelid.

The two major extraocular muscles not supplied by CN III are:

Superior oblique → CN IV.

Lateral rectus → CN VI.

2. Parasympathetic Fibres

The third cranial nerve also carries parasympathetic fibres to the eye.

These fibres ultimately supply the:

Sphincter pupillae → constricts the pupil.

Ciliary muscle → allows accommodation for near vision.

Therefore, a complete third nerve palsy may cause:

Dilated pupil.

Poor or absent pupillary reaction to light.

Loss of accommodation.

3. Location of the Third Nerve Nucleus

An important correction to the original note is that the oculomotor nucleus is located in the midbrain, not the pons.

It lies at approximately the level of the superior colliculus, close to the cerebral aqueduct.

Therefore:

CN III nucleus → midbrain.

CN VI nucleus → pons.

4. Main Features of Third Nerve Palsy

A complete third nerve palsy can produce a characteristic combination of:

Ptosis.

Dilated poorly reactive or unreactive pupil.

Impaired adduction.

Impaired elevation.

Impaired depression.

Eye resting in a “down and out” position.

These findings result from paralysis of most muscles supplied by CN III.

5. Ptosis

Ptosis occurs because the third nerve supplies the levator palpebrae superioris.

When this muscle is paralysed, the upper eyelid droops.

Therefore:

CN III palsy → levator weakness → ptosis.

In a complete palsy, the ptosis may be marked and may partially hide the abnormal position of the eye.

6. “Down and Out” Eye Position

In a complete third nerve palsy, most extraocular muscles are paralysed.

However, two muscles remain functional:

Lateral rectus → CN VI.

Superior oblique → CN IV.

The lateral rectus pulls the eye outward, while the superior oblique contributes to downward movement.

The result is the classic:

“Down and out” position of the affected eye.

7. Impaired Eye Movements

Because CN III supplies the medial, superior and inferior recti plus the inferior oblique, the affected eye has difficulty moving:

Medially.

Upward.

Downward.

Abduction is relatively preserved because the lateral rectus is supplied by CN VI.

8. Diplopia

Third nerve palsy usually causes diplopia because the two eyes are no longer aligned.

The exact direction in which diplopia is worst depends on the degree of weakness and which branches are affected.

In a complete palsy, the abnormal resting position is often obvious.

9. Pupillary Dilatation

The parasympathetic fibres responsible for pupillary constriction run superficially in the third nerve.

If these fibres are damaged, the pupil becomes:

Dilated.

Poorly reactive or unreactive to light.

A dilated pupil in the setting of a new third nerve palsy is particularly important because it may suggest a compressive lesion, especially an aneurysm.

10. Pupil-Involving Third Nerve Palsy

A painful third nerve palsy with a dilated pupil is a neurological emergency until a compressive aneurysm has been excluded.

The classic concern is a:

Posterior communicating artery aneurysm.

The aneurysm may compress the superficial parasympathetic fibres of CN III, producing early pupillary involvement.

Therefore:

Painful CN III palsy + dilated pupil → urgently exclude posterior communicating artery aneurysm.

11. Posterior Communicating Artery Aneurysm*

A posterior communicating artery aneurysm is one of the most important causes of third nerve palsy.

Typical findings may include:

Sudden severe headache.

Pain around the eye.

Ptosis.

Down-and-out eye.

Dilated poorly reactive pupil.

This presentation requires urgent vascular imaging and specialist assessment.

12. Microvascular Third Nerve Palsy*

Microvascular ischaemia is another common acquired cause, particularly in older adults with vascular risk factors.

Important associations include:

Diabetes mellitus.

Hypertension.

Other small-vessel disease.

Because the central motor fibres may be affected more than the superficial parasympathetic fibres, a microvascular palsy may be pupil-sparing.

13. Pupil-Sparing Third Nerve Palsy

A classic teaching pattern is:

Microvascular/diabetic CN III palsy → pupil often spared.

This is because the pupillary parasympathetic fibres lie superficially around the nerve and may escape central ischaemic damage.

However, pupil sparing is not absolute, and clinical assessment should not rely on this sign alone.

14. Demyelination

Demyelinating disease, including multiple sclerosis, can affect the oculomotor fascicles or associated brainstem pathways.

This is less common than microvascular or compressive causes but should be considered in younger patients or when other neurological symptoms are present.

15. Trauma

Head or orbital trauma may damage the third cranial nerve directly or indirectly.

Traumatic CN III palsy may be associated with:

Ptosis.

Ophthalmoplegia.

Pupillary abnormalities.

Other cranial nerve injuries.

The presence of multiple neurological deficits suggests more extensive injury.

16. Cavernous Sinus Disease

CN III passes through the lateral wall of the cavernous sinus.

Disease in this region can therefore affect the third nerve together with other cranial nerves.

Important neighbouring nerves include:

CN IV.

CN V1.

CN V2.

CN VI.

Therefore, a cavernous sinus lesion often causes multiple cranial neuropathies, not an isolated third nerve palsy.

17. Cavernous Sinus Thrombosis

Cavernous sinus thrombosis can cause:

Painful ophthalmoplegia.

Ptosis.

Proptosis.

Chemosis.

Sensory loss in V1/V2 distribution.

Fever or systemic infection signs, depending on cause.

It is an emergency requiring urgent treatment.

18. Orbital Tumour

An orbital tumour can impair ocular movements by:

Compressing cranial nerves.

Restricting extraocular muscles mechanically.

Causing proptosis.

Therefore, an orbital mass may produce ophthalmoplegia, but the pattern may not correspond neatly to a single cranial nerve palsy.

19. Thyroid Eye Disease

Thyroid eye disease can cause diplopia and restricted eye movement, but it is important to distinguish it from a true third nerve palsy.

In thyroid eye disease, the main problem is usually extraocular muscle enlargement and mechanical restriction, not direct CN III damage.

The inferior rectus and medial rectus are commonly affected.

Typical features may include:

Proptosis.

Lid retraction.

Restricted elevation.

Diplopia.

Exposure symptoms.

So thyroid eye disease is better considered a mimic of ocular motor nerve palsy rather than a classic cause of isolated CN III palsy.

20. Midbrain Lesions

Because the third nerve nucleus and fascicles are in the midbrain, lesions here may cause third nerve palsy together with other neurological abnormalities.

Possible causes include:

Stroke.

Tumour.

Demyelination.

Trauma.

Additional long-tract or cerebellar signs may help localise the lesion to the brainstem.

21. Third Nerve Palsy – Note Form

Cranial nerve: III, oculomotor nerve.

Nucleus: midbrain, not pons.

Muscles supplied: medial rectus, superior rectus, inferior rectus, inferior oblique and levator palpebrae superioris.

Muscles not supplied by CN III: lateral rectus and superior oblique.

Parasympathetic function: constricts pupil and assists accommodation.

Ptosis: caused by levator palpebrae weakness.

Eye position: down and out.

Eye movements impaired: upward, downward and medial movement.

Pupil: may be dilated and poorly reactive if parasympathetic fibres are involved.

*Microvascular causes: ** diabetes, hypertension and other vascular disease.

*Compressive cause: ** posterior communicating artery aneurysm.

Other causes: demyelination, trauma, cavernous sinus disease and midbrain lesions.

Orbital tumours: can cause ophthalmoplegia through compression or mechanical restriction.

Thyroid eye disease: usually a restrictive extraocular muscle disorder rather than a true isolated CN III palsy.

22. Important Clinical Example

If the patient has a right complete third nerve palsy:

Right eyelid is ptotic.

Right eye lies down and out.

Right eye cannot adduct normally.

Elevation and depression are impaired.

Right pupil may be dilated and unreactive if parasympathetic fibres are involved.

23. Pupil-Involving versus Pupil-Sparing – Note Form

Pupil-involving CN III palsy:

Think particularly about compressive pathology, especially a posterior communicating artery aneurysm.

Pupil-sparing CN III palsy:

Think particularly about microvascular ischaemia, such as diabetes.

However, this distinction is a useful clinical clue rather than an absolute rule.

24. Third, Fourth and Sixth Nerves – Quick Note Form

CN III: most extraocular muscles + levator + parasympathetic pupil fibres.

CN IV: superior oblique.

CN VI: lateral rectus.

CN III palsy: ptosis + down-and-out eye ± dilated pupil.

CN IV palsy: vertical diplopia worse looking down and in.

CN VI palsy: horizontal diplopia with impaired abduction.

Key Clinical Pattern

Remember third nerve palsy as:

CN III → most eye movements + eyelid elevation + pupil constriction.

Therefore:

Third nerve palsy → PTOSIS + DOWN-AND-OUT EYE + impaired adduction/elevation/depression ± DILATED PUPIL.

The most important emergency pattern is:

Painful third nerve palsy + dilated pupil → urgently exclude posterior communicating artery aneurysm.

And the important correction is:

The third nerve nucleus is in the MIDBRAIN, not the pons.