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Cranial nerves · Motor

VI. Abducens nerve

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The abducens nerve (CN VI) supplies the lateral rectus muscle, which abducts the eye. It has a long intracranial course and a sharp angulation over the petrous apex, so it is frequently affected by raised intracranial pressure.

Terminologia Anatomica (Latin): Nervus abducens

Function

General somatic efferent to the lateral rectus only, producing abduction of the eye. The nucleus also contains interneurons that project via the medial longitudinal fasciculus to the contralateral oculomotor nucleus for conjugate horizontal gaze.

Nuclei

Abducens nucleus in the pons, in the floor of the fourth ventricle, encircled by the fibres of the facial nerve (the facial colliculus). It lies close to the paramedian pontine reticular formation (horizontal gaze centre).

Exit from the skull

It emerges at the pontomedullary junction, ascends the clivus, bends over the petrous apex beneath the petroclinoid ligament (Dorello canal), runs through the cavernous sinus next to the internal carotid artery, and enters the orbit via the superior orbital fissure.

Testing

Ask the patient to follow a target laterally in each direction, observing for failure of abduction, the position of the eyes at rest, and diplopia that is horizontal and maximal on looking toward the affected side.

Signs of a lesion

The affected eye is medially deviated (convergent squint) at rest because of unopposed medial rectus action, with horizontal diplopia worst on looking toward the side of the lesion (lateral gaze). A pontine nuclear lesion causes an ipsilateral gaze palsy rather than an isolated abduction deficit.

Clinical correlations

Because of its long course, a sixth nerve palsy is a classic false-localising sign of raised intracranial pressure. Other causes include microvascular ischaemia (diabetes, hypertension), cavernous sinus lesions, trauma, and Gradenigo syndrome (petrous apicitis with facial pain and otorrhoea).

Common exam points

  • Supplies only the lateral rectus (LR6): abduction.
  • Palsy: medial deviation at rest and horizontal diplopia worse on lateral gaze.
  • Long intracranial course: a false-localising sign in raised intracranial pressure.
  • Runs through the cavernous sinus beside the internal carotid artery.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. Elsevier, 5th edition (2018) — Tier 3 (textbook)
  4. Bickley LS. Bates' Guide to Physical Examination and History Taking (Bickley). Wolters Kluwer, 13th edition (2020) — Tier 3 (textbook)

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