The abducens nerve (CN VI) supplies only the lateral rectus, so its palsy produces a convergent squint (esotropia), inability to abduct the affected eye and horizontal double vision that worsens on looking toward the affected side. Its long course up the clivus and over the petrous apex makes it the cranial nerve most often affected by raised intracranial pressure. Inside the cavernous sinus it runs free beside the internal carotid artery rather than in the sinus wall.
| Origin | Abducens nucleus in the caudal pons (near the facial nerve genu, explaining combined CN VI and VII palsies in pontine lesions) |
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| Course | Emerges at the pontomedullary junction, ascends along the clivus, pierces the dura and passes under the petrosphenoidal ligament (Dorello canal) over the petrous apex, runs through the cavernous sinus beside the internal carotid artery, and enters the orbit through the superior orbital fissure within the common tendinous ring to supply the lateral rectus |
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A unilateral or bilateral CN VI palsy is a false localising sign of raised intracranial pressure, because the nerve is stretched over the petrous apex as the brainstem is pushed downward. A pontine lesion at the abducens nucleus usually also involves the facial nerve fibres looping around it, giving facial weakness with a horizontal gaze palsy toward that side. Because it lies free in the cavernous sinus next to the carotid artery, it is often the first nerve affected by cavernous sinus thrombosis, tumour or carotid aneurysm. Petrous apex infection can cause Gradenigo syndrome (CN VI palsy, deep facial pain in the V1 territory and ear discharge). Microvascular palsy in older adults with diabetes or hypertension usually recovers within about three months.
Lateral rectus paralysis produces convergent squint and horizontal diplopia from raised intracranial pressure, cavernous sinus disease, pontine lesions or microvascular ischaemia; treatment targets the cause, with prisms or botulinum toxin for persistent diplopia.
Infection of the petrous apex from middle ear disease inflames the nerve in Dorello canal, causing CN VI palsy with retro-orbital pain and otorrhoea; it needs urgent antibiotics and sometimes surgery.
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