Ocular Motor Cranial Nerve Palsies

Ophthalmology · Neuro-ophthalmology · lean revision notes

Ocular Motor Cranial Nerve Palsies

The three ocular motor cranial nerves — third (oculomotor), fourth (trochlear) and sixth (abducens) — drive every conjugate and disconjugate movement of the globe. Their isolated or combined palsies produce characteristic diplopia patterns, and the supranuclear/internuclear lesions (INO, one-and-a-half syndrome) are favourite NEET PG discriminators. This chapter builds the topic from anatomy → localisation → "surgical vs medical" decision-making.

The functional anatomy you must own

The six extraocular muscles and their innervation form the bedrock of every diplopia question.

Muscle	Innervation	Primary action	Secondary/tertiary
Lateral rectus (LR)	CN VI	Abduction	—
Medial rectus (MR)	CN III	Adduction	—
Superior rectus (SR)	CN III	Elevation (in abduction)	Intorsion, adduction
Inferior rectus (IR)	CN III	Depression (in abduction)	Extorsion, adduction
Superior oblique (SO)	CN IV	Intorsion (primary)	Depression (in adduction), abduction
Inferior oblique (IO)	CN III	Extorsion (primary)	Elevation (in adduction), abduction

Mnemonic for the muscles not supplied by CN III: "LR₆ SO₄" — Lateral Rectus = 6th, Superior Oblique = 4th; everything else (MR, SR, IR, IO + levator palpebrae superioris + pupillary sphincter via parasympathetics) = 3rd.

High-yield: The superior oblique depresses the eye maximally when the eye is adducted, because in adduction the SO's pull aligns with the visual axis. This is why a CN IV palsy patient gets worst diplopia on reading/downgaze toward the nose and on stairs.

The parasympathetic pupillomotor fibres travel on the superficial dorsomedial surface of the third nerve. This anatomy is the single most exam-tested fact in the chapter — see "pupil rule" below.

Third (oculomotor) nerve palsy

Clinical features

A complete CN III palsy gives the classic "down and out" eye:

Ptosis (levator palpebrae paralysis) — often complete.
Eye deviated down and out (unopposed LR abducts, unopposed SO depresses/intorts).
Mydriasis with loss of light and accommodation reflex if the pupil is involved.
Diplopia (horizontal + vertical) once the lid is lifted.

Causes by anatomical level

Brainstem (nucleus/fascicle): vascular infarct, demyelination, tumour. The CN III nucleus has a peculiar layout — a single midline caudal central subnucleus supplies both levators, and the superior rectus subnucleus is crossed. Therefore a nuclear lesion gives bilateral ptosis and contralateral SR weakness along with ipsilateral signs.

Subarachnoid space: posterior communicating artery (PCoA) aneurysm is the classic compressive cause — and it is the prototype pupil-involving palsy. Uncal herniation also compresses CN III here (Hutchinson pupil).

Cavernous sinus / superior orbital fissure: combined III, IV, VI, V1 (± V2) palsies; look for proptosis, chemosis.

Orbit: restricted, often with proptosis and optic nerve signs.

The pupil rule — surgical versus medical

This is the crux of the entire chapter.

High-yield: The pupillomotor fibres run superficially and derive their blood supply from the pia mater (vasa nervorum), whereas the central somatic fibres depend on the vasa nervorum within the nerve trunk.

→ Compressive lesions (aneurysm, tumour, uncal herniation) hit the superficial fibres first → pupil DILATED ("surgical" third).

→ Ischaemic / microvascular lesions (diabetes, hypertension) infarct the core but the pia-fed superficial pupil fibres are spared → pupil SPARED ("medical" third).

Feature	Surgical (compressive)	Medical (ischaemic)
Classic cause	PCoA aneurysm	Diabetes mellitus, HTN
Pupil	Dilated, fixed	Spared (reactive)
Pain	Often severe, sudden	Mild peri-orbital ache
Age	Any	Usually > 50 yrs, vasculopath
Recovery	Needs intervention	Spontaneous in 3 months
Imaging	CT/MR angiography urgently	Observe, control glucose/BP

Stepwise approach to an acute isolated third nerve palsy:

Is the pupil involved? Examine carefully (size, light reaction).
Pupil involved OR painful OR < 50 yrs → treat as aneurysm until proven otherwise → emergent CTA/MRA (± DSA).
Pupil spared, vasculopath, > 50 yrs → likely microvascular → observe, optimise risk factors, expect recovery in ~3 months.
No recovery by 3 months, or aberrant regeneration, or progression → image.

High-yield: A "pupil-sparing complete external ophthalmoplegia of CN III in a diabetic" is the textbook microvascular palsy. But beware: an early or partial aneurysmal compression can transiently spare the pupil — hence many examiners now favour neuroimaging in nearly all acute CN III palsies.

Aberrant regeneration (lid elevation on downgaze = pseudo-Graefe sign, or pupil constriction on adduction) indicates a compressive/traumatic aetiology and never occurs after pure microvascular palsy — its presence mandates imaging.

Fourth (trochlear) nerve palsy

CN IV is the longest and thinnest intracranial nerve, the only one to exit dorsally from the brainstem, and it decussates — so the nucleus supplies the contralateral SO. These quirks make it the commonest nerve injured in closed head trauma.

Clinical features — the vertical diplopia triad

The SO depresses and intorts; its palsy gives:

Vertical (and torsional) diplopia, worse on downgaze and on looking toward the opposite side / reading.
Hypertropia of the affected eye (the higher eye is the palsied one).
Compensatory head tilt to the OPPOSITE shoulder (to relieve diplopia) — characteristic ocular torticollis.

Park's three-step test

The definitive bedside localiser for a vertical/cyclo-deviation, isolating the paretic muscle:

Which eye is hypertropic in primary gaze? (e.g., right hypertropia → narrows to weak RSO or RIR, LSR or LIO).
Does the hyper increase on left or right gaze? (Right hyper worse on left gaze → RSO or LIR).
Does the hyper increase on left or right head tilt? (Right hyper worse on right (ipsilateral) head tilt = Bielschowsky head-tilt test positive → Right superior oblique palsy).

High-yield: Bielschowsky head-tilt test — tilting the head to the side of the affected eye worsens the hypertropia in a SO palsy. A positive test toward the higher eye clinches CN IV palsy.

Bilateral CN IV palsy is suggested by a large excyclotorsion (> 10°), V-pattern esotropia, and right hyper on left gaze + left hyper on right gaze (reversing hypertropia) — think head trauma.

Eponym: the "3-step test" is also called the Parks–Bielschowsky test. Long-standing congenital CN IV palsy presents with a large vertical fusional amplitude and is often picked up on old photographs showing a head tilt.

Sixth (abducens) nerve palsy

Clinical features

LR paralysis → failure of abduction, with:

Horizontal, uncrossed diplopia, maximal on gaze toward the side of the lesion and at distance.
Esotropia in primary gaze (eye pulled in by unopposed MR).
Compensatory face turn toward the side of the palsy.

Why it is the "false localising sign"

CN VI has the longest intracranial subarachnoid course and bends sharply over the petrous apex (Dorello's canal, under the petroclinoid/Gruber ligament). Raised intracranial pressure stretches the nerve here regardless of where the pathology is.

High-yield: Sixth nerve palsy is the classic false localising sign of raised ICP — it indicates high pressure but not the location of the lesion. Look for papilloedema.

Named syndromes around CN VI

Gradenigo syndrome — petrous apicitis (otitis media complication): CN VI palsy + ipsilateral facial pain (V1) + otorrhoea/deafness.
Foville syndrome (dorsal pons) — ipsilateral VI + VII + horizontal gaze palsy + contralateral hemiparesis.
Millard–Gubler syndrome (ventral pons) — ipsilateral VI + VII palsy + contralateral hemiparesis.
Duane retraction syndrome — congenital miswiring (CN VI nucleus hypoplasia, MR co-innervation): globe retraction and palpebral narrowing on adduction; a key differential, not a true palsy.

Microvascular (diabetic) CN VI palsy in adults > 50 is common and recovers in ~3 months; in children, an isolated CN VI palsy is pontine glioma until excluded.

Supranuclear & internuclear disorders (NEET discriminators)

These are the high-difficulty items the blurb flags.

Internuclear ophthalmoplegia (INO)

Lesion of the medial longitudinal fasciculus (MLF), which connects the contralateral CN VI nucleus to the ipsilateral CN III (MR) subnucleus.

Signs (named by side of the LESION = side of adduction failure):

Failure/slowing of adduction of the eye ipsilateral to the MLF lesion.
Abducting nystagmus in the contralateral (abducting) eye.
Convergence preserved (if the lesion is purely in the MLF and CN III nucleus is intact).

Cause of INO	Clue
Bilateral INO in a young adult	Multiple sclerosis (most likely)
Unilateral INO in an elderly patient	Brainstem (pontine) infarct

High-yield: Bilateral INO + young woman = multiple sclerosis until proven otherwise. WEBINO (Wall-Eyed Bilateral INO) = bilateral INO with exotropia in primary gaze.

One-and-a-half syndrome

Lesion of the paramedian pontine reticular formation (PPRF) or CN VI nucleus PLUS the adjacent MLF on the same side.

→ Ipsilateral conjugate gaze palsy ("the one": neither eye moves toward the lesion) plus an INO on attempted gaze away ("the half": the ipsilateral eye cannot adduct).

Result: the only horizontal movement preserved is abduction of the contralateral eye → "paralytic pontine exotropia." Causes: pontine infarct (elderly), MS (young), tumour.

Memory line: "one-and-a-half = (gaze palsy to one side) + (INO = half of the other side's gaze)."

Quick localiser

Horizontal gaze: frontal eye field → contralateral PPRF (pons) → ipsilateral CN VI nucleus → (LR same side) + via MLF → contralateral CN III (MR). A PPRF/VI-nucleus lesion gives an ipsilateral conjugate gaze palsy; a frontal lobe (cortical) lesion makes the eyes deviate toward the lesion ("look at the lesion") in stroke.

Combined / multiple palsies — localisation by company kept

When more than one ocular motor nerve is involved, the company localises the lesion:

Region	Nerves involved	Extra clues
Cavernous sinus	III, IV, VI, V1 (±V2), sympathetics	Horner + III = cavernous sinus; proptosis, chemosis
Superior orbital fissure	III, IV, VI, V1	No V2 involvement (Rochon-Duvigneaud syndrome)
Orbital apex	III, IV, VI, V1, + II (optic)	Vision loss distinguishes from SOF
Petrous apex	VI + V (± VII)	Gradenigo

High-yield: A painful ophthalmoplegia with multiple cranial nerves and a granulomatous cavernous sinus lesion that responds dramatically to steroids = Tolosa–Hunt syndrome (diagnosis of exclusion, MRI + steroid response).

Investigations — overview

Investigation of choice depends on the suspected mechanism:

Acute pupil-involving CN III → CT/MR angiography (CTA/MRA); DSA is gold standard for aneurysm.
Suspected brainstem/INO/one-and-a-half/MS → MRI brain with contrast (DWI for infarct; demyelinating plaques).
Children with CN VI / multiple palsies → MRI to exclude pontine glioma/raised ICP.
Vasculopathic isolated palsy > 50 → blood sugar, HbA1c, BP, lipids, ESR/CRP (exclude giant cell arteritis in CN VI palsy of the elderly).
Myasthenia gravis (the great mimic) → ice-pack test, AChR antibodies, repetitive nerve stimulation/SFEMG — suspect when ptosis/diplopia is fatigable, variable and pupil is spared.

Management & drug-of-choice principles

Treat the cause: aneurysm → coiling/clipping; tumour → excision; raised ICP → address; GCA → high-dose corticosteroids immediately; Tolosa–Hunt → corticosteroids.
Microvascular palsy: observation + tight glycaemic/BP control; expect recovery by 3 months.
Symptomatic diplopia: occlusion (patching/Bangerter foil) or Fresnel prisms in the acute/recovering phase.
Stable deviation > 6 months with no recovery: strabismus surgery (e.g., recession–resection, or transposition such as Hummelsheim/Jensen procedure for total CN VI palsy); Botulinum toxin to the antagonist (MR in CN VI palsy) can be a temporising measure.

High-yield: Do not operate for diplopia until the deviation has been stable for at least 6 months — spontaneous recovery of microvascular palsies is the rule.

Complications

Persistent diplopia and abnormal head posture (torticollis, secondary musculoskeletal strain).
Aberrant regeneration of CN III (pseudo-Graefe lid sign, pupil-gaze dyskinesis) after compressive/traumatic injury.
Amblyopia in children if a palsy is congenital/longstanding and uncorrected.
Exposure keratopathy with associated CN VII/V involvement.
Missed aneurysm → catastrophic subarachnoid haemorrhage (the reason the pupil rule is sacred).

Key differentials

Myasthenia gravis — fatigable, variable, pupil always spared, can mimic any pattern including pseudo-INO.
Thyroid eye disease (Graves) — restrictive; forced duction test positive; IR and MR most affected; proptosis.
Orbital myositis / blow-out fracture — restriction, positive forced ductions, trauma history.
Duane syndrome — congenital, globe retraction on adduction (mimics CN VI palsy).
Skew deviation — vertical misalignment from posterior fossa/utricular lesion; differentiate CN IV palsy from skew with the upright–supine test (skew deviation decreases > 50% when supine).
Progressive supranuclear palsy / CPEO — slow bilateral, symmetrical, often no diplopia.

Recently asked / exam angle

"Pupil-sparing third nerve palsy in a diabetic" → microvascular; cause = ischaemia of central fibres, pial supply spares the pupil.
"Down and out eye with ptosis and dilated pupil after sudden headache" → PCoA aneurysm → investigation = CTA/CT angiography.
"Vertical diplopia, head tilt to the opposite side, worse on reading/downgaze" → CN IV (SO) palsy; test = Bielschowsky head-tilt / Parks 3-step.
"Failure of adduction of left eye with nystagmus of abducting right eye, convergence intact, young female" → left INO from MS.
"Only the right eye abducts; nothing else moves horizontally" → left one-and-a-half syndrome (pontine lesion).
"Isolated CN VI palsy with papilloedema" → false localising sign of raised ICP.
"Painful ophthalmoplegia responding to steroids" → Tolosa–Hunt.
"Commonest cranial nerve injured in head trauma" → CN IV (longest, thinnest, dorsal exit).
"Nerve with the longest intracranial course / passes through Dorello's canal" → CN VI.

Rapid revision

LR₆SO₄ — everything else (MR, SR, IR, IO, levator, pupil sphincter) is CN III.
CN III pupil fibres run superficially, pia-fed → compression dilates, ischaemia spares.
Surgical third = pupil involved + pain → image for PCoA aneurysm (CTA/MRA).
Medical third = pupil spared, diabetic > 50 → observe, recovers in ~3 months.
Aberrant regeneration of CN III never follows pure microvascular palsy → mandates imaging.
CN IV palsy = vertical/torsional diplopia, hypertropia of affected eye, head tilt to the opposite shoulder, Bielschowsky positive.
CN IV is the commonest cranial nerve injured in closed head trauma; bilateral palsy → large excyclotorsion + V-pattern eso.
CN VI palsy = horizontal uncrossed diplopia, esotropia, false localising sign of raised ICP; Dorello's canal/petrous apex.
Isolated CN VI in a child = rule out pontine glioma; in the elderly = exclude giant cell arteritis.
Bilateral INO in a young woman = multiple sclerosis; unilateral in elderly = pontine infarct; convergence preserved.
One-and-a-half syndrome = ipsilateral gaze palsy + INO → only contralateral eye abduction survives (PPRF/VI nucleus + MLF).
No diplopia surgery until the deviation is stable ≥ 6 months; prisms/occlusion meanwhile; MG is the great pupil-sparing mimic.

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