Trochlear CN IV and Abducens CN VI
Trochlear CN IV
✨ M ➡ Superior oblique, which depresses the adducted eye.
Injury classically leads to being unable to look down and inwards causing difficulty reading or descending stairs.
Abducens CN VI
✨ M ➡ Lateral rectus, which abducts the eye.
Injury classically causes an inability to look laterally, with the eye resting medially due to unopposed medial rectus action, producing horizontal diplopia that worsens when gazing toward the affected side and often prompting a compensatory head turn.
CN VI is most often injured in raised ICP or pathology around the brainstem and cavernous sinus, because its long intracranial course makes it particularly vulnerable. Classically, think intracranial hypertension (tumour, hydrocephalus, IIH) — it’s a well-known false localising sign. Other common causes include cavernous sinus lesions (thrombosis, aneurysm, infection), basal skull fracture or head trauma, pontine infarction, and diabetic microvascular ischaemia.
👩⚕️ Here's your dose of SBAs...
Cavernous sinus thrombosis from a facial infection will affect which nerve first?
Abducens CN VI
🧠 The abducens CN VI is the first to suffer injury in sinus thrombosis or compression by tumors at its base, presenting as lateral rectus palsy with an inability to abduct the eye.
👩⚕️ and here's your dose of MCQs for this topic.
A man with diplopia is unable to look down and inwards with his right eye. Which nerve is affected?
- ((Abducens nerve::Controls lateral rectus, abducts the eye))
- ((Ciliary ganglion::Parasympathetic oculomotor CN III fibres for pupil constriction and lens accommodation))
- ((Nasociliary nerve::Sensory branch of CN V1, no motor function))
- ((Trochlear nerve::☑️ Supplies superior oblique for depression of the adducted eye))
🍀 The trochlear CN IV innervates the superior oblique, responsible for depression of the adducted eye, essential for reading and descending stairs.
A 58-year-old suffering from a tumor invading the base of the cavernous sinus. Which nerve is involved?
- ((Mandibular V3::Exits the skull via the foramen ovale along with the accessory meningeal artery and lesser petrosal nerve))
- ((Abducens CN VI::☑️ Passes through the center of the cavernous sinus))
- ((Facial CN VII::Leaves the skull through the stylomastoid foramen after passing through the facial canal))
- ((Vestibulocochlear CN VIII::Travels with the facial nerve through the internal acoustic meatus))
- ((Glossopharyngeal CN IX::Exits via the jugular foramen along with the vagus and accessory nerves))
In a case of cavernous sinus thrombosis, which of the following nerves is most likely to be injured, especially considering that it lies within the sinus rather than in its lateral wall?
- ((Optic nerve::Travels through the optic canal accompanied by the ophthalmic artery))
- ((Oculomotor nerve::Runs in the lateral wall of the cavernous sinus alongside the trochlear and ophthalmic nerves))
- ((Abducens nerve::Passes centrally through the cavernous sinus next to the internal carotid artery, making it highly vulnerable to thrombosis))
- ((Ophthalmic nerve V1::Courses within the lateral wall of the cavernous sinus with oculomotor and trochlear nerves))
🧠 The Abducens CN VI is the first to suffer injury in sinus thrombosis or compression by tumors at its base, presenting as lateral rectus palsy with an inability to abduct the eye; the affected eye will turn inward, leading to horizontal double vision particularly when looking to the side of the affected eye. When venous drainage from the orbit is obstructed due to cavernous sinus thrombosis, it leads to: Oedema of the conjunctiva and eyelids, exophthalmos, transmitted pulsations from the internal carotid artery, and ophthalmoplegia with abducens palsy. On ophthalmoscopy, there is evidence of papilloedema and retinal haemorrhages.
🎯 Structures that travel through the cavernous sinus:
- Abducens CN VI
- Carotid plexus
- Internal carotid artery
🧱 Structures that travel through the lateral wall of the cavernous sinus:
- Oculomotor CN III
- Trochlear CN IV
- Trigeminal's Ophthalmic CN V1 branches
- Trigeminal's Maxillary CN V2 branches