3rd nerve palesey

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"oculomotor nerve palsy" AND etiology

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Third Nerve (Oculomotor) Palsy

Anatomy of the Oculomotor Nerve

The third cranial nerve (CN III) travels a long course from nucleus to orbit, and the level of the lesion determines the clinical picture.

Course of the oculomotor nerve from nucleus through cavernous sinus to orbit

Nuclear complex (midbrain, level of superior colliculus):

The levator subnucleus is unpaired and midline - a nuclear lesion causes bilateral ptosis
Each superior rectus subnucleus innervates the contralateral superior rectus - a nuclear lesion causes contralateral (not ipsilateral) superior rectus weakness
Medial rectus, inferior rectus, and inferior oblique subnuclei are ipsilateral

Fasciculus (through midbrain tegmentum): fibres pass through the red nucleus and cerebral peduncle, giving rise to classic brainstem syndromes (see below).

Subarachnoid/basilar course: nerve passes between the posterior cerebral artery and superior cerebellar artery, then runs parallel to the posterior communicating artery (PCoA) - making it vulnerable to aneurysmal compression here.

Cavernous sinus: CN III runs in the lateral wall, above CN IV. It divides into superior and inferior branches before entering the orbit through the superior orbital fissure.

Classic Clinical Features of a Complete Third Nerve Palsy

Finding	Cause
Profound ptosis	Weakness of levator palpebrae
Eye deviated "down and out"	Unopposed lateral rectus (CN VI) and superior oblique (CN IV)
Dilated, unreactive pupil (mydriasis)	Loss of parasympathetic (Edinger-Westphal nucleus) input
Cycloplegia (loss of accommodation)	Loss of ciliary muscle innervation
Diplopia	Misalignment of visual axes

A partial palsy may produce any combination of these findings. In primary position, the intact superior oblique also causes intorsion of the eye.

Pupil - The Most Clinically Critical Feature

Pupillary fibres lie dorsal and peripheral; vasa nervorum supply the core while pial vessels supply the surface

The parasympathetic pupillomotor fibres lie superficially in the superomedial nerve trunk, supplied by pial vessels. The main motor trunk is supplied by the vasa nervorum (intrinsic vasculature).

Surgical (compressive) lesions (aneurysm, uncal herniation, tumor):

Compress the pial vessels and superficial pupillary fibres first
Typically produce a pupil-involved (dilated, fixed) palsy
This is a medical emergency

Medical (ischaemic) lesions (diabetes, hypertension):

Infarct the vasa nervorum, damaging the central nerve trunk
Superficial pupillomotor fibres are spared - pupil-sparing palsy
Pain is often present (in ~2/3 of cases) even with ischaemia, so pain alone does not differentiate aetiology

Key caveat: Up to one-third of microvascular palsies may have mild relative pupil involvement (~0.8 mm anisocoria) with a reactive pupil. A completely fixed, dilated pupil with total ophthalmoplegia is the most specific sign of compression. Modern guidelines recommend emergent non-invasive vascular neuroimaging (CTA or MRA) for all third nerve palsies to exclude a PCoA aneurysm, regardless of pupil status.

Causes by Location

Nuclear (rare)

Vascular disease, primary tumors, metastases
Classic finding: bilateral ptosis + contralateral superior rectus weakness
WEBINO syndrome (wall-eyed bilateral internuclear ophthalmoplegia) if bilateral medial rectus subnuclei involved

Fascicular (midbrain) - Associated Brainstem Syndromes

Syndrome	Structures Involved	Features
Weber's	Cerebral peduncle	Ipsilateral CN III palsy + contralateral hemiparesis
Benedikt's	Red nucleus	Ipsilateral CN III palsy + contralateral tremor/chorea/athetosis
Nothnagel's	Superior cerebellar peduncle	Ipsilateral CN III palsy + cerebellar ataxia
Claude's	Red nucleus + superior cerebellar peduncle	Combination of Benedikt + Nothnagel

Subarachnoid/Basilar

PCoA aneurysm - most feared cause; acute, painful, pupil-involving. Compression only accounts for ~6% of all acute third nerve palsies but is immediately life-threatening
Uncal herniation - trauma, mass lesion; compresses nerve over tentorial edge; starts with miosis then progresses to mydriasis and complete palsy
Meningitis, subarachnoid haemorrhage, tumor, infarction

Cavernous Sinus

Typically involves multiple cranial nerves (CN III, IV, VI, V1)
Causes: carotid aneurysm, carotid-cavernous fistula, cavernous sinus thrombosis, pituitary adenoma, meningioma, Tolosa-Hunt syndrome, herpes zoster
Diabetes is a common cause here (microvascular ischaemia)

Orbital Apex

Involvement of CN III + IV + VI + V1 + optic nerve
Plus orbital signs: proptosis, chemosis, conjunctival injection
Causes: idiopathic orbital inflammation (IgG4), mucormycosis/aspergillosis (diabetics/immunosuppressed), tumor

Isolated Third Nerve Palsy - Clinical Approach

Pupil-involved + complete ophthalmoplegia → aneurysm until proven otherwise → emergent CTA/MRA
Pupil-sparing in an older patient with DM/HTN → microvascular ischaemia likely → CTA/MRA still recommended; ESR/CRP/CBC in elderly (exclude temporal arteritis)
Partial palsy → must image regardless; partial involvement does not exclude aneurysm
Pain → does NOT distinguish aneurysmal from ischaemic (present in ~2/3 of both)
Expected resolution for microvascular palsy: 8-12 weeks spontaneously. If no resolution or new findings appear, reconsider the diagnosis

Aberrant Regeneration

Aberrant regeneration occurs when regenerating axons misroute to the wrong target muscle or iris.

Classic signs:

Pseudo-von Graefe sign: eyelid elevation on downgaze or adduction (lid-gaze dyskinesis)
Pupil-gaze dyskinesis: pupil constricts on downgaze or adduction (not light)

Important rule:

After trauma or PCoA aneurysm compression - aberrant regeneration is expected
Primary aberrant regeneration (no prior acute palsy) = almost always indicates a slowly growing parasellar lesion (cavernous sinus meningioma or internal carotid aneurysm) - requires neuroimaging
Aberrant regeneration never follows microvascular ischaemia - if it appears in a presumed ischaemic palsy, imaging is mandatory

Key Differentials to Remember

Myasthenia gravis: variable ptosis and ophthalmoplegia, fluctuating; pupil never involved; Tensilon test/AChR antibodies
Thyroid ophthalmopathy: proptosis, lid retraction, restrictive pattern; pupil normal
Ophthalmoplegic migraine (rare): recurrent, resolves over weeks-months; diagnosed in children with multiple prior episodes; do not diagnose de novo in adults

Sources: Kanski's Clinical Ophthalmology 10th Ed. | Adams and Victor's Principles of Neurology, 12th Ed. | Harrison's Principles of Internal Medicine 22nd Ed. | Bradley and Daroff's Neurology in Clinical Practice | The Wills Eye Manual

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