Abducens Nerve (CN VI)

The abducens nerve is the sixth cranial nerve responsible for eye abduction (lateral movement). It has the longest intracranial subarachnoid course of any cranial nerve, making it vulnerable to increased intracranial pressure. It innervates only the lateral rectus muscle.

Origin

Abducens nucleus in caudal pons (at floor of fourth ventricle)

Foramina

Superior orbital fissure

Course

Exits pontomedullary junction → ascends clivus in subarachnoid space → passes through Dorello's canal under petroclinoid ligament → traverses cavernous sinus → enters orbit via superior orbital fissure

Functions

•Eye abduction (lateral gaze)

Branches

None - single nerve to lateral rectus

Structures Innervated

→Lateral rectus muscle

Clinical Testing

Test lateral gaze (abduction) of each eye. Look for esotropia (inward eye deviation) at rest. Check for diplopia on lateral gaze.

Clinical Relevance

CN VI palsy causes horizontal diplopia worse at distance and inability to abduct the eye. It is a classic "false localizing sign" of increased intracranial pressure because its long course makes it susceptible to stretching without a focal lesion.

Study Tips

✓LR6 - Lateral Rectus, CN VI (6)
✓Longest subarachnoid course = vulnerable to raised ICP
✓False localizing sign: CN VI palsy doesn't necessarily localize the lesion
✓Dorello's canal: CN VI trapped between petrous apex and clivus

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Abducens Nerve (CN VI) FAQs

Common questions about this cranial nerve

CN VI has the longest subarachnoid course. Increased ICP causes downward brain displacement, stretching CN VI over the petrous apex. This palsy doesn't indicate where the causative lesion is located.

Dorello's canal is the bony passage where CN VI runs beneath the petroclinoid ligament between the petrous apex and clivus. This fixed point makes the nerve vulnerable to compression from petrous apex lesions or inflammation (Gradenigo syndrome).

CN VI palsy prevents abduction of one eye. Internuclear ophthalmoplegia (MLF lesion) prevents adduction of one eye during conjugate gaze while convergence remains intact. Both cause horizontal diplopia but with different mechanisms.