Medicine – Sixth Nerve Palsy

Sixth nerve palsy is dysfunction of the abducens nerve, cranial nerve VI, which supplies the lateral rectus muscle. Because the lateral rectus abducts the eye, sixth nerve palsy causes failure of abduction and produces horizontal diplopia, especially when looking toward the affected side.

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. Function of the Sixth Cranial Nerve

The abducens nerve supplies the lateral rectus muscle.

The lateral rectus moves the eye outward, away from the nose.

Therefore:

CN VI → lateral rectus → abduction of the eye.

2. Sixth Nerve Nucleus

The abducens nucleus is located in the pons, close to the floor of the fourth ventricle.

The fibres of the facial nerve loop around the abducens nucleus, forming the facial colliculus.

This anatomical relationship is important because pontine lesions may produce combined sixth and seventh cranial nerve abnormalities.

3. Effect of Sixth Nerve Palsy

When the lateral rectus is paralysed, the affected eye cannot abduct normally.

The unopposed action of the medial rectus, supplied by cranial nerve III, pulls the eye inward.

Therefore:

CN VI palsy → lateral rectus weakness → affected eye deviates medially.

This inward deviation is called esotropia.

4. Diplopia

Sixth nerve palsy typically causes horizontal diplopia.

The double vision becomes worse when the patient looks toward the affected side, because this requires contraction of the weak lateral rectus.

For example:

Right CN VI palsy → right eye cannot abduct → diplopia worse on looking right.

5. Diplopia at Distance

Diplopia may be particularly noticeable when looking at distant objects.

This is because distance fixation requires relatively greater divergence of the eyes.

Patients may turn their head toward the affected side to reduce diplopia.

6. Examination

Ask the patient to follow a target through the six cardinal positions of gaze.

In sixth nerve palsy, the affected eye shows:

Reduced or absent abduction.

The eye may be medially deviated in the primary position if the palsy is significant.

7. Raised Intracranial Pressure*

Raised intracranial pressure is an important cause of sixth nerve palsy.

The abducens nerve has a relatively long intracranial course and is vulnerable to stretching or displacement when intracranial pressure rises.

For this reason, sixth nerve palsy can occur even when the underlying pathology is not anatomically close to the sixth nerve nucleus.

8. False Localising Sign

A sixth nerve palsy caused by raised intracranial pressure is classically described as a false localising sign.

This means that the site of the cranial nerve palsy does not necessarily indicate the location of the underlying intracranial lesion.

For example, a distant mass lesion causing raised intracranial pressure may produce CN VI palsy through nerve stretching.

Therefore:

Raised ICP + CN VI palsy does not necessarily mean a pontine lesion.

9. Microvascular Causes*

Small-vessel ischaemia is a common cause of isolated sixth nerve palsy, particularly in older adults.

Important vascular risk factors include:

Diabetes mellitus.

Hypertension.

Other vascular disease.

These palsies are often termed microvascular cranial neuropathies.

10. Diabetes Mellitus

Diabetes can cause ischaemic injury to the sixth cranial nerve.

The patient may develop sudden-onset horizontal diplopia with an isolated abduction deficit.

Many presumed microvascular palsies improve spontaneously over weeks to months, but the diagnosis depends on the clinical context and exclusion of concerning alternative causes.

11. Demyelination

Multiple sclerosis and other demyelinating disorders can affect the sixth nerve nucleus, fascicle, or related brainstem pathways.

In a younger patient with sixth nerve palsy and additional neurological symptoms, demyelination should be considered.

12. Trauma*

Head trauma can injure the abducens nerve because of its long intracranial course and its passage over the petrous temporal bone.

Traumatic sixth nerve palsy may be unilateral or bilateral.

13. Cavernous Sinus Disease

The abducens nerve passes through the cavernous sinus, making it vulnerable to lesions in this region.

Causes may include:

Cavernous sinus thrombosis.

Tumours.

Aneurysmal or other vascular lesions.

Inflammatory disease.

14. Why Cavernous Sinus Lesions Are Important

Within the cavernous sinus, several cranial nerves lie close together.

These include:

CN III.

CN IV.

CN V1.

CN V2.

CN VI.

Therefore, a cavernous sinus lesion may produce a combination of:

Ophthalmoplegia.

Facial sensory loss.

Ptosis.

Diplopia.

An isolated sixth nerve palsy is possible, but multiple cranial nerve abnormalities make cavernous sinus disease more likely.

15. Orbital Apex Disease

Lesions at the orbital apex can affect several structures entering the orbit.

These may include:

CN II.

CN III.

CN IV.

CN VI.

Branches of CN V1.

Orbital apex disease may therefore produce ophthalmoplegia together with visual loss or sensory abnormalities.

16. Pontine Lesions

A lesion involving the abducens nucleus or fascicle within the pons can cause sixth nerve dysfunction.

Because of nearby structures, there may be additional neurological findings such as:

Facial weakness.

Gaze abnormalities.

Long-tract motor or sensory signs.

A nuclear lesion may produce more complex horizontal gaze abnormalities than an isolated peripheral sixth nerve palsy.

17. Sixth Nerve Palsy – Note Form

Cranial nerve: VI, abducens nerve.

Muscle supplied: lateral rectus.

Action: abducts the eye.

Nucleus: pons.

Palsy: affected eye fails to abduct.

Eye position: deviates medially because medial rectus is unopposed.

Diplopia: horizontal.

Diplopia worst: looking toward the affected side.

*Raised intracranial pressure: ** important cause and classic false localising sign.

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

Other causes: demyelination, trauma, cavernous sinus disease and orbital apex disease.

18. Important Clinical Example

If the patient has a right sixth nerve palsy:

Right lateral rectus is weak.

Right eye cannot move fully to the right.

Right eye tends to deviate medially.

Horizontal diplopia becomes worse when looking to the right.

19. Localisation Clues

Isolated CN VI palsy in an older diabetic patient → consider microvascular ischaemia.

CN VI palsy + headache/papilloedema → consider raised intracranial pressure.

CN VI + III/IV/V abnormalities → consider cavernous sinus lesion.

CN VI + visual loss/multiple orbital nerve deficits → consider orbital apex disease.

CN VI + other brainstem signs → consider pontine lesion or demyelination.

Key Clinical Pattern

Remember sixth nerve palsy as:

CN VI → lateral rectus → abduction.

Therefore:

Sixth nerve palsy → eye cannot abduct → eye turns inward → horizontal diplopia worse on looking toward the affected side.

The most important causes to remember are:

Raised intracranial pressure + microvascular disease/diabetes + demyelination + trauma + cavernous sinus disease + orbital apex disease.

And the classic exam phrase is:

Raised ICP causing sixth nerve palsy = false localising sign.