Anatomy Atlas

Abducens nerve (CN VI)

Nervus abducens
Also: CN VI · Cranial nerve VI · Sixth nerve palsy
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Supplies the lateral rectus. Its long course along the clivus and through the cavernous sinus makes it the cranial nerve most often affected by raised intracranial pressure, causing horizontal diplopia.

Core anatomy

Origin

Abducens nucleus in the floor of the fourth ventricle in the caudal pons; facial nerve fibers loop around it, forming the facial colliculus. The nerve emerges at the pontomedullary junction.

Course

Ascends along the clivus, crosses the petrous apex under the petrosphenoid ligament (Dorello canal), runs inside the cavernous sinus beside the internal carotid artery (the only nerve within the sinus itself — the others are in its wall; see dural venous sinuses), and enters the orbit through the superior orbital fissure within the common tendinous ring.

Motor

Lateral rectus only — abduction of the eye (see extraocular muscles). Mnemonic LR6 SO4.

Relations

The abducens nucleus also contains interneurons that project through the medial longitudinal fasciculus (MLF) to the contralateral medial rectus subnucleus, coordinating horizontal gaze.

Muscles innervated

  • Extraocular muscles — LR6 SO4, rest 3: lateral rectus — abducens nerve (CN VI); superior oblique — trochlear nerve (CN IV); superior, inferior and medial recti and inferior oblique — oculomotor nerve (CN III, which also supplies levator palpebrae superioris).

Found in

Mentioned in

Generated automatically from links in other entries.

Where the cranial nerves leave the brainstem (ventral view)The front of the brainstem. The optic nerves meet at the chiasm above. The oculomotor nerve emerges from the interpeduncular fossa between the cerebral peduncles of the midbrain. The trochlear nerve is the only one to leave from the back, just below the inferior colliculus, and winds forward around the peduncle. The trigeminal nerve leaves the lateral pons. The abducens leaves at the pontomedullary junction near the midline; the facial and vestibulocochlear nerves leave at the cerebellopontine angle. The glossopharyngeal and vagus nerves leave the postolivary sulcus of the medulla, the hypoglossal leaves the preolivary sulcus between the pyramid and the olive, and the accessory nerve rises from the upper cervical cord.Optic (II)Oculomotor (III)Trochlear (IV) — from the dorsal sideTrigeminal (V)Abducens (VI)Facial (VII)Vestibulocochlear (VIII)Glossopharyngeal (IX)Vagus (X)Accessory (XI) — from C1–C5Hypoglossal (XII)Midbrain (cerebral peduncle)PonsPyramidOliveSpinal cordMedullaVentral view · dashed IV leaves from the back

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Where the cranial nerves leave the brainstem. Ventral view. III leaves the interpeduncular fossa of the midbrain; IV is the only nerve to leave from the back (dashed); V leaves the lateral pons; VI the pontomedullary junction near the midline; VII and VIII the cerebellopontine angle; IX and X the postolivary sulcus; XII the preolivary sulcus between pyramid and olive; and XI rises from the upper cervical cord. Nuclei follow the same levels: III–IV midbrain, V–VIII pons, IX–XII medulla.
Original schematic, created for this atlas.
Section of the cavernous sinus showing the internal carotid artery and abducent nerve within the sinus, and the oculomotor, trochlear, ophthalmic and maxillary nerves in its lateral wall, beside the sphenoidal sinus
The abducent nerve inside the cavernous sinus beside the internal carotid artery.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 786. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))
Lateral view of the orbit showing the oculomotor nerve divisions, the ciliary ganglion, the abducent nerve to the lateral rectus and the trigeminal ganglion
The abducent nerve entering the lateral rectus.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 777. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Sixth nerve palsy PANCE

Mechanism / lesion

Raised intracranial pressure (a "false localizing sign" — the nerve is stretched along the clivus; e.g. idiopathic intracranial hypertension, tumors), microvascular ischemia (diabetes, hypertension), cavernous sinus lesions (thrombosis, carotid aneurysm, carotid–cavernous fistula), trauma, and after lumbar puncture.

Classic presentation

Horizontal diplopia worse at distance and on looking toward the affected side; the eye is turned in (esotropia) and cannot abduct; patients may turn the head toward the affected side.

Abducens nucleus and MLF lesions PANCE

Mechanism / lesion

Pontine stroke or demyelination (MS).

Classic presentation

Nucleus lesion → ipsilateral horizontal gaze palsy (neither eye looks to that side), often with an ipsilateral facial palsy (facial colliculus). MLF lesion → internuclear ophthalmoplegia (INO): the ipsilateral eye fails to adduct on lateral gaze while the other eye abducts with nystagmus; convergence is preserved. Bilateral INO in a young adult → MS.

High-yield PANCE pearls

  • Eye can't abduct + horizontal diplopia worse at distance → CN VI palsy.
  • CN VI palsy + headache + papilledema in an obese young woman → idiopathic intracranial hypertension.
  • CN VI is the nerve inside the cavernous sinus → first affected by cavernous sinus thrombosis.
  • Bilateral INO → multiple sclerosis (MLF).

Imaging anatomy

CT

CT head urgently if raised ICP is suspected; CT venography for venous sinus thrombosis.

MRI

MRI brain (with MR venography in suspected IIH or venous thrombosis) for any CN VI palsy that is not clearly microvascular, or in younger patients.

Physical exam

Strength / function testing

Test abduction in the H-pattern; cover test for esotropia. Check fundi for papilledema, which points to raised ICP.

Self-quiz

Q1.

A 26-year-old woman with obesity has daily headaches, transient visual blurring and horizontal double vision when looking to the left. Her left eye cannot abduct, and both optic discs are swollen. Which condition is most likely?

Show answer

B

Papilledema with a CN VI palsy as a false localizing sign in a young woman with obesity suggests idiopathic intracranial hypertension. Image (MRI with MRV) to exclude a mass or venous thrombosis, then lumbar puncture (raised opening pressure); treat with weight loss and acetazolamide.

Q2.

A 24-year-old woman looks to the right: her left eye does not adduct, and her right eye has nystagmus on abduction. Convergence is normal. Where is the lesion?

Show answer

C

Failure of adduction on lateral gaze with preserved convergence is a left internuclear ophthalmoplegia from a lesion of the left MLF. In a young adult, especially if bilateral, MS is the leading cause.

Q3.

Why is the abducens nerve often the first cranial nerve affected in cavernous sinus thrombosis?

Show answer

B

CN VI runs inside the cavernous sinus next to the internal carotid artery, while CN III, IV, V1 and V2 lie in the lateral wall, so CN VI is often affected first.

Q4.

A pontine lesion of the abducens nucleus (rather than the nerve) causes which finding?

Show answer

D

The abducens nucleus contains interneurons that project through the MLF to the opposite medial rectus, so a nucleus lesion causes an ipsilateral horizontal gaze palsy, often with an ipsilateral facial palsy (facial colliculus). A nerve lesion affects only the lateral rectus.

Q5.

What forms the facial colliculus in the floor of the fourth ventricle?

Show answer

A

The facial colliculus is formed by facial motor fibers looping around the abducens nucleus in the caudal pons, so a lesion there can cause both a gaze palsy and a facial palsy.

Flashcards

See also