CN3 palsy

Question 1

ANATOMY of CN3

Nuclear complex

• In midbrain, at level of _____________
• Lesions confined to the nuclear complex are relatively uncommon. The most frequent causes are _______________

Fasciculus

• Consists of efferent fibres that pass from CN3 nucleus through the red nucleus and the medial aspect of the cerebral peduncle, which then emerge from midbrain into the ______________
• Causes are similar to nuclear complex, except that ____________ can affect fasciculus

Basilar part

• Here, CN3 passes between _____________ and ____________, running lateral to and parallel with the ___________________
• Because CN3 traverses the base of the skull along its _____________________ unaccompanied by any other CN, isolated CN3 palsy is commonly basilar

Intracavernous: CN3 runs in the lateral wall above CN4

Intra-orbital part

• Enters orbit via ____________
• Superior division 🡪 ____________
• Inferior division 🡪 ________________

Answer 1

superior colliculus;

vascular disease, primary tumours, and metastases;

interpeduncular space;

demyelination;

posterior cerebral artery (PCA) and superior cerebellar artery (SCA);

posterior communicating artery (PCOM artery);

subarachnoid space;

superior orbital fissure (SOF);

levator palpebrae superioris (LPS), superior rectus (SR);

medial rectus (MR), inferior rectus (IR), inferior oblique (IO), preganglionic parasympathetic to sphincter pupillae and ciliary muscle

Question 2

What are important basilar cause of CN3 palsy?

Answer 2

• PCOM aneurysm at its junction with ICA which typically presents acutely with pupil-involving painful CN3 palsy
• Head trauma (and resultant epidural or subdural haematoma), causing tentorial pressure cone and downward herniation of temporal lobe, compressing CN3 as it passes over the tentorial edge

Question 3

how does aberrant regeneration of CN3 present?

Answer 3

• May follow surgical (traumatic, compressive) CN3 palsy, but not medical (vascular) causes
• Occurs because endoneural nerve sheaths are possibly breached in traumatic or compressive lesions, but always remain intact in medical (vascular) lesions
• Misdirected regenerating axons 🡪 reinnervate incorrect extraocular muscles 🡪 defects in ocular motility:
• Lid: Elevation of upper eyelid on attempted adduction/depression
• Pupil: pupil constriction on attempted adduction/depression

Question 4

what are causes of isolated CN 3 palsy?

Answer 4

Microvascular disease, a/w systemic CVRF (HTN, DM, HLD)
- Most common cause of CN3 palsy
- In DM-related, EOM dysfunction is typically profound
PCOM aneurysm at junction with ICA

Very important cause of isolated pupil-involving CN3 palsy

• Pain is often present
• Extent of pupillary involvement > severity of EOM dysfunction/ophthalmoplegia
• Aneurysm of ICA within cavernous sinus tends to involve other CN

Trauma

• Direct, or secondary to subdural haematoma with uncal herniation
• Development of CN3 palsy following relatively trivial head trauma should alert the clinician to the possibility of underlying aneurysm or tumour

Episodic

• Brief episodes + spontaneous recovery
• Myasthenia gravis may mimic intermittent pupil-sparing (medical) CN3 palsy

Question 5

what are the investigations for isolated CN3?

Answer 5

If pupil sparing 🡪 vascular risk factor assessment

• As for retinal arterial disease (HTN, DM, HLD, etc): BP, fasting glucose, lipids
• If no microvascular ischaemic RF, consider neuroimaging

If pupil involving 🡪 STAT MR/CT angiography (neuroimaging)
- Obtain neuroimaging urgently to r/o PCOM aneurysm, if clinical features are suspicious for expanding aneurysm, e.g. marked pupillary involvement + milder EOM dysfunction + partial ptosis

Question 6

What is the management of medical CN3 palsy?

Answer 6

Manage ischaemic cardiovascular risk factors (CVRF)

Observation is appropriate for presumed microvascular causes, majority resolve over weeks-months
- Monitor closely in first week to ensure no pupillary involvement

For diplopia and strabismus

• Fresnel stick-on prisms (useful if angle of deviation is small)
• Eye occlusion (of affected eye), e.g. eye patch, Bangerter occlusion foil
• Monitor with Hess chart, binocular single vision (BSV) testing
• Consider surgery if no improvement, with aim to preserve primary gaze (straight) and downgaze (for reading)

Consider MR/CT angiogram if:

• Diplopia worsening
• Symptoms persists > 3 months w/o improvement