Abducens nerve (CN VI)

Nervus abducens

Also known as: CN VI, sixth cranial nerve, abducent nerve

A purely motor nerve (GSE) to the lateral rectus, which abducts the eye. Its long course over the petrous apex and through the middle of the cavernous sinus makes it the cranial nerve most often affected by raised intracranial pressure.

Overview

NucleusDorsal caudal pons (facial colliculus)

Exit from brainstemPontomedullary junction

Exit from skullSuperior orbital fissure (within tendinous ring)

Unique relationRuns inside the cavernous sinus (not its wall)

The abducens nucleus lies in the dorsal caudal pons, in the floor of the fourth ventricle. Fibres of the facial nerve loop around it (the internal genu), producing the facial colliculus. The nucleus holds two cell types:
• motor neurons to the ipsilateral lateral rectus;
• interneurons whose axons cross and ascend in the medial longitudinal fasciculus (MLF) to the contralateral medial rectus subnucleus of CN III.

This pairing coordinates conjugate horizontal gaze.

The nerve emerges at the pontomedullary junction near the midline. It ascends steeply along the clivus and pierces the dura. It then bends sharply over the petrous apex beneath the petrosphenoidal (Gruber) ligament in Dorello's canal. Unlike nerves III, IV, V1 and V2, which run in the lateral wall, it passes through the cavernous sinus itself beside the internal carotid artery. It enters the orbit through the superior orbital fissure inside the common tendinous ring.

Location

Pontomedullary junction → clivus → Dorello's canal over the petrous apex → within the cavernous sinus → superior orbital fissure → lateral rectus.

Parts & subdivisions

Functional components

Motor only (GSE).

Clinical testing

Ask the patient to follow a target laterally. Failure of abduction, and horizontal diplopia that worsens on looking toward the affected side, indicate a CN VI palsy. Cover testing shows an esotropia that is greater at distance.

Lesion signs

The affected eye is deviated medially (convergent squint / esotropia) at rest and cannot abduct. Horizontal diplopia is worst on looking toward the lesion. A lesion of the nucleus causes an ipsilateral horizontal gaze palsy (both eyes), often with an LMN facial palsy because of the facial colliculus.

Anatomy

Innervation

Lateral rectus muscle only.

Course

Nucleus (dorsal pons, beneath the facial colliculus) → emerges at the pontomedullary junction → ascends in the prepontine cistern along the clivus → pierces the dura and angles over the petrous apex (Dorello's canal) → runs inside the cavernous sinus lateral to the ICA → superior orbital fissure (within the common tendinous ring) → lateral rectus.

Clinical significance

False-localising sign of raised ICP

Raised intracranial pressure from any cause (tumour, hydrocephalus, idiopathic intracranial hypertension) displaces the brainstem downward. This stretches CN VI over the petrous apex, causing a unilateral or bilateral abducens palsy that has no localising value for the actual lesion.

Cavernous sinus lesions

Because CN VI lies free within the sinus next to the ICA, it is often the first nerve affected in cavernous sinus thrombosis (spread from facial "danger triangle" infection), carotid–cavernous fistula and intracavernous ICA aneurysm. III, IV, V1 and V2 follow.

Internuclear ophthalmoplegia (INO)

A lesion of the MLF, from MS in young people or stroke in older people, disconnects the abducens interneurons from the contralateral CN III. On lateral gaze the ipsilateral eye fails to adduct and the abducting eye shows nystagmus, while convergence is preserved. CN VI itself is intact.