Ocular Motor Cranial Nerve Palsies
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Direct answer
Ocular motor palsies are paralyses of the third, fourth or sixth cranial nerves, presenting as binocular diplopia with a distinctive pattern for each. A third nerve palsy gives ptosis, a dilated pupil (when the parasympathetic fibres are compressed) and an eye deviated down and out; a fourth nerve palsy gives vertical diplopia and a head tilt away from the affected side, worse on downgaze and reading, confirmed by the Bielschowsky test; a sixth nerve palsy gives esotropia with failure of abduction and diplopia worst at distance and toward the affected side — the classic false localising sign of raised intracranial pressure. The pupil is the third nerve's danger gauge — involvement means compression (aneurysm, tumour, herniation) until imaging proves otherwise, while pupil-sparing painful palsy in an older vasculopath is typically microvascular. Management is cause first, then observation for recovery, with prisms, occlusion and — after six months of stability — surgery.
What you must remember
- Third nerve (oculomotor) palsy: ptosis, divergent strabismus with the eye "down and out" (unopposed lateral rectus and superior oblique), dilated sluggish pupil, loss of accommodation, diplopia hidden under the lid, and later aberrant regeneration with lid-gaze synkinesis.
- The pupil rule to quote: a pupil-involving third nerve palsy is surgical — posterior communicating aneurysm, cavernous mass or uncal herniation — and warrants urgent angiography; a pupil-sparing painful palsy in diabetes or hypertension is usually microvascular, imaged if recovery does not begin within three months.
- Fourth nerve (trochlear) palsy: superior oblique weakness with vertical or torsional diplopia, worst on downgaze toward the opposite side (reading, stairs), head tilt away from the weak side and a positive Bielschowsky test; the commonest congenital palsy, decompensating in adulthood.
- Sixth nerve (abducens) palsy: esotropia worse at distance, abduction deficit, diplopia maximal on gaze toward the lesion; causes span raised intracranial pressure (false localising sign, long course over the petrous ridge), microvascular disease, trauma and Gradenigo syndrome; congenital sixth palsy is often Duane syndrome.
- Combined palsies localise by company: III, IV, VI with V1/V2 and sympathetic involvement points to the cavernous sinus (Tolosa-Hunt if painfully idiopathic and steroid-responsive); third nerve palsy with proptosis and chemosis suggests orbital apex or cavernous disease.
- Myasthenia masquerade: any painless, variable, pupil-sparing ophthalmoplegia deserves the ice pack test before anatomical blame is assigned.
- Management ladder: treat the cause; observe (most microvascular palsies recover by three months); prisms or patching meanwhile; botulinum toxin to the antagonist in recent sixth palsy; surgery once the deviation is stable.
How to work through three patterns of double vision
Three patients, three nerves. A 66-year-old diabetic wakes with the worst headache of his life and a droopy right lid; beneath it the eye is down and out with a fully dilated pupil — the aneurysm scenario: CT angiography the same day, because posterior communicating aneurysms bleed and kill; A painless pupil-sparing version in a similar hypertensive is the microvascular mirror-image, observed medically. The second is a 30-year-old whose childhood photographs show a constant left head tilt, now with vertical diplopia after a road accident: decompensated congenital fourth nerve palsy — the three-step test (hypertropia, worse on opposite gaze, worse on ipsilateral tilt) confirms the left superior oblique, and the old photographs excuse the trauma from blame. The third, a 40-year-old with weeks of morning headache, shows limited right abduction in an otherwise normal eye — a false-localising sixth nerve palsy until proven otherwise: image for raised intracranial pressure, because the abducens fails early when the brain is squeezed.
Where students slip
The head tilt direction is the classic reversal: the patient tilts away from the weak side, and the Bielschowsky test increases the deviation on tilting toward it — swapping the two prescribes the wrong surgery. The second slip is treating every sixth nerve palsy as microvascular: in a child, or with headache and papilloedema, it is a pressure sign requiring imaging, and reassurance is the dangerous option. Third, the pupil rule is quoted backwards under stress — pupil involvement equals compression and urgency, not a benign course; and always lift the ptotic lid to check the pupil before calling a palsy "pupil-sparing".
Frequently asked questions
Why is the eye "down and out" in a third nerve palsy?
Paralysis of the superior, inferior and medial recti and inferior oblique leaves the unopposed lateral rectus (abduction) and superior oblique (depression and intorsion) to position the globe.
What does pupil involvement in a third nerve palsy signify?
Compression of the superficial parasympathetic fibres — aneurysm, tumour or herniation — mandating urgent imaging, unlike the pupil-sparing microvascular pattern.
What is the Bielschowsky head tilt test?
Tilting toward the side of a paretic superior oblique increases the vertical deviation, and the patient habitually tilts away — the confirmatory sign of fourth nerve palsy.
Why is the sixth nerve a "false localising sign"?
Its long intracranial course makes it vulnerable to raised pressure of any cause, so a sixth nerve palsy can localise disease elsewhere than its own nucleus.
How is recent-onset diplopia from ocular motor palsy managed while awaiting recovery?
Treat the cause, patch or use Fresnel prisms for comfort, monitor for spontaneous recovery over three to six months, and reserve surgery — or botulinum toxin for sixth nerve palsies — for stable residual deviations.