Third, fourth and sixth cranial-nerve palsies

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

The third, fourth and sixth cranial nerves coordinate the extraocular muscles so that corresponding images reach the two retinas. A weakness in one component breaks alignment, producing two images whose separation changes with gaze. Establishing that the symptom is binocular is the first useful localisation step, but identifying a weak muscle is only the beginning of diagnosis: the lesion may lie in the brainstem, subarachnoid space, cavernous sinus or orbit.

Acute palsies can result from small-vessel ischaemia, aneurysmal compression, infarction, inflammation, trauma or raised intracranial pressure. Some older adults have an isolated microvascular palsy that resolves, but that conclusion requires an appropriate examination and investigation strategy. In particular, the historical division of third-nerve palsies into dangerous pupil-involving and safe pupil-sparing groups is inadequate for modern triage.

Key points

Binocular diplopia disappears when either eye is covered; persistent doubling in the uncovered eye suggests an optical or ocular cause.
Third-nerve palsy may combine ptosis with impaired adduction, elevation and depression; pupil findings do not reliably exclude aneurysm.
Fourth-nerve dysfunction usually produces vertical or tilted images, often troublesome when reading or descending stairs.
Sixth-nerve dysfunction causes impaired abduction and horizontal separation, usually worse at distance and towards the affected side.
Vascular risk factors support microvascular disease but do not establish that diagnosis or remove the need to investigate dangerous alternatives.
A confirmed microvascular palsy commonly improves over six to twelve weeks; progression or failure to recover requires reassessment.
Temporary occlusion or a fitted prism can reduce diplopia; patients should stop driving and follow medical and DVLA advice.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Vascular and compressive causes

Small-vessel injury can affect an ocular motor nerve, particularly with diabetes or hypertension. Aneurysms, tumours and other compressive lesions may produce similar initial deficits, making risk factors insufficient to establish the cause.

Inflammation, trauma and pressure

Brainstem inflammation or infarction, head injury, cavernous-sinus disease and raised intracranial pressure can disturb ocular motor function. Associated findings often provide more useful localisation than the direction of diplopia alone.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Third-nerve function
The oculomotor nerve supplies most extraocular muscles and the upper-lid elevator, alongside parasympathetic pupil fibres. Partial injury produces variable combinations of ptosis, movement limitation and pupil dysfunction rather than one obligatory complete syndrome.
2
Fourth-nerve function
The trochlear nerve supplies the superior oblique muscle. Weakness disrupts vertical and torsional alignment, especially in positions where that muscle contributes strongly, explaining difficulty with reading or walking down stairs.
3
Sixth-nerve function
The abducens nerve supplies the lateral rectus muscle. Weakness limits outward movement and increases horizontal misalignment when looking towards the affected side, while its long intracranial course creates vulnerability to pressure-related disturbance.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Confirm binocular separation

Ask the patient to describe horizontal, vertical or tilted images, then cover each eye separately. Binocular diplopia resolves with either eye covered. Record onset, progression, pain, trauma and whether the separation changes with distance, reading or a particular direction of gaze.

Third-nerve examination

Look for ptosis, an outward and downward resting position, and weakness of adduction, elevation and depression. Partial palsies may lack the complete pattern. Measure both pupils in light and darkness before dilating drops; a normal-sized reactive pupil does not exclude compression.

Fourth-nerve examination

Vertical and torsional diplopia may be most noticeable during downgaze tasks. A patient may tilt the head to compensate. Examine alignment in several gaze positions and during head tilt, while recognising that longstanding congenital misalignment and other muscle disorders can imitate this pattern.

Sixth-nerve examination

An abduction deficit creates horizontal diplopia that often increases at distance and in gaze towards the affected eye. Examine the optic discs and neurological system because sixth-nerve palsy may accompany raised intracranial pressure rather than identify a lesion at one precise site.

Look beyond the weak muscle

Document acuity, colour perception, fields, eyelids and other cranial nerves. Ataxia, weakness, altered sensation, proptosis or reduced corneal sensation changes the localisation. In an older adult, ask specifically about jaw claudication, scalp tenderness, new headache and transient visual loss.

Red flags requiring action

A new ocular motor palsy with severe headache, pupil enlargement or other neurological signs needs emergency hospital assessment.
Double vision with visual loss, proptosis, fever or several affected cranial nerves may indicate orbital or cavernous-sinus disease and requires immediate specialist assessment.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Urgent neurovascular imaging for an acute third-nerve palsyFirst step
Why
Exclude an aneurysm or other compressive lesion before assigning a microvascular explanation.
Interpretation and limitations
CTA or MRA is selected with the emergency and neuroradiology teams according to availability and clinical circumstances. All isolated third-nerve palsies should be imaged at presentation; the pupil examination cannot safely select patients who need no imaging.
02
Ophthalmic alignment and neurological assessment
Why
Define the deficit and identify additional ocular or neurological involvement.
Interpretation and limitations
Measure deviations in different gaze positions and assess ductions, pupils and optic discs. A changing pattern, fatigability or non-anatomical distribution should prompt reconsideration of a single nerve lesion rather than forced classification.
03
MRI directed by localisation and clinical course
Why
Investigate brainstem, cavernous-sinus, orbital or persistent unexplained palsies.
Interpretation and limitations
Additional neurological signs, several involved nerves, young age, progression or atypical recovery lower the threshold for imaging. A selected isolated fourth- or sixth-nerve palsy may be monitored after specialist assessment, with an explicit plan if improvement does not occur.
04
Blood pressure, glucose and selected inflammatory investigations
Why
Assess vascular contributors and identify arteritis when the history raises concern.
Interpretation and limitations
Diabetes or hypertension does not prove causation. In suspected giant-cell arteritis, obtain inflammatory markers and a full blood count promptly, but organise urgent specialist treatment without allowing testing to delay protection of vision.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Ocular myasthenia

Neuromuscular junction dysfunction can cause variable ptosis and diplopia that mimic several nerve patterns. Fatigability and changing alignment support consideration, while bulbar or respiratory weakness signals potentially generalised disease requiring urgent assessment.

Restrictive orbital disease

Thyroid eye disease, orbital inflammation or trauma may mechanically restrict eye movement. Proptosis, lid changes, pain, swelling or an appropriate injury history suggest an orbital process rather than isolated nerve weakness.

Decompensated longstanding misalignment

A previously compensated deviation can become symptomatic with illness or age. Old photographs, longstanding head posture and prior intermittent symptoms help establish chronicity, but should not be used to dismiss genuinely new neurological findings.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01EmergencyNew third-nerve or complex palsyFirst stepAcute third-nerve dysfunction or diplopia with neurological, orbital or severe headache red flags.

1Arrange immediate hospital assessment when aneurysm, stroke, infection or another dangerous intracranial process is suspected.
2Document onset, pupils, acuity, eye movements and neurological findings without delaying emergency referral for a complete clinic work-up.
3Discuss urgent CTA or MRA and further imaging with the receiving team; communicate explicitly that pupil sparing does not exclude aneurysm.
4Treat the identified cause through the relevant neurological, neurosurgical or ophthalmic pathway rather than prescribing empiric treatment for presumed microvascular disease.

02AssessmentSelected isolated fourth or sixth palsyA stable isolated deficit has been assessed and no emergency feature has been found.

1Review vascular risks, trauma history, previous photographs and any longstanding head posture or intermittent childhood symptoms.
2Agree with the specialist whether imaging is needed now and document why a monitored microvascular diagnosis is reasonable if chosen.
3Arrange a defined follow-up appointment and explain that recovery commonly takes six to twelve weeks in confirmed microvascular palsy.
4Bring reassessment forward for worsening separation, new pain, pupil change, neurological symptoms or lack of the expected recovery.

03RecoveryControl diplopia and restore functionThe cause has been investigated and double vision remains troublesome during recovery.

1Offer practical symptomatic help through orthoptics, using temporary occlusion or an individually fitted prism when appropriate.
2Explain that a prism may improve straight-ahead viewing while leaving double vision in other gaze positions.
3Advise stopping driving and notifying DVLA in Great Britain; Northern Ireland licence holders contact DVA. Resumption requires appropriate control and licensing assessment, with stricter Group 2 rules and no acceptance of patching for Group 2 licensing.
4Consider longer-term prism or strabismus intervention only after specialist reassessment of a stable residual deviation.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Missed intracranial disease

Attributing a new palsy to vascular risk factors without adequate investigation may delay recognition of aneurysm, stroke or malignancy. The consequences depend on the underlying lesion and can extend far beyond persistent diplopia.

Functional visual disability

Double vision interferes with reading, mobility and work even when each eye has normal acuity. Temporary occlusion may help but reduces binocular depth perception, requiring practical advice about falls and everyday tasks.

Persistent ocular misalignment

Some patients retain a stable deviation after the underlying disease has settled. Specialist prism correction or selected eye-muscle surgery may improve function once the cause and degree of residual weakness are established.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

At follow-up, repeat the movement and alignment measurements rather than judging recovery solely from the patient's adaptation to symptoms.
Track vascular risk management with primary care, including blood pressure, diabetes control and smoking, without promising that these measures accelerate nerve recovery.
Revisit the diagnosis if a presumed microvascular palsy progresses, acquires additional signs or has not followed the expected six-to-twelve-week improvement pattern.
Assess reading, stairs, work and falls risk; a patient who closes one eye may still have impaired depth judgement and practical limitations.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

A painless pupil-sparing presentation still matters

Neither absence of pain nor a reactive pupil reliably excludes an aneurysm in an acute third-nerve palsy. The decision to image rests on the palsy itself and the complete presentation, not a single reassuring sign.

A sixth nerve can mislead localisation

Raised intracranial pressure may stretch the sixth nerve along its course. Headache, transient visual obscurations and disc swelling therefore require investigation of pressure and its cause, even when the eye-movement deficit appears isolated.

Fatigability suggests another mechanism

Variable ptosis and diplopia that change during the examination can reflect ocular myasthenia. Pupils are usually unaffected, but a normal pupil alone cannot distinguish myasthenia from a nerve palsy; the pattern and appropriate testing remain essential.

Recovery support has realistic limits

Occlusion removes the conflicting second image but does not repair the nerve. Prisms compensate for a particular deviation, and exercises have not been shown to speed recovery from a microvascular cranial nerve palsy.

11Common pitfallsFrequent interpretation and management errors.

01
Calling a third-nerve palsy microvascular because the patient has diabetes and the pupil is normal.
02
Ordering an unenhanced head CT and assuming that this necessarily excludes a causative aneurysm.
03
Treating an abduction deficit as an isolated peripheral nerve problem without examining the optic discs or neurological system.
04
Offering open-ended reassurance without a review date or instructions for progression and new symptoms.