08 TROCHLEAR CN IV AND ABDUCENS CN VI
# 09 TROCHLEAR CN IV AND ABDUCENS CN VI
Both nerves are purely motor and supply a single extraocular muscle each. They are small, easy to overlook, and disproportionately loved by examiners because their lesions produce highly characteristic gaze defects that the MRCS Part A demands you recognise instantly.
Remember the rhyme: LR6 SO4 — the Lateral Rectus is supplied by CN 6, the Superior Oblique by CN 4. Everything else is CN 3.
Trochlear nerve (CN IV)
👩⚕️ M ➡ Supplies the superior oblique, which depresses the adducted eye and intorts it.
The trochlear nerve is unique among the cranial nerves for two reasons:
- It is the only cranial nerve to exit the brainstem dorsally (from the posterior surface of the midbrain, just below the inferior colliculus).
- It is the only cranial nerve that decussates before innervating its target — the left nucleus supplies the right superior oblique and vice versa.
It is also the thinnest cranial nerve and has a long intracranial course, wrapping around the midbrain to reach the cavernous sinus. This makes it particularly vulnerable to closed head trauma, where shearing forces can stretch or tear it.
Course in brief
Nucleus in the midbrain at the level of the inferior colliculus → exits dorsally → wraps around the cerebral peduncle → pierces the dura → runs in the lateral wall of the cavernous sinus → enters the orbit through the superior orbital fissure (outside the common tendinous ring) → supplies superior oblique.
Action of superior oblique
The superior oblique originates near the sphenoid, runs forward to the trochlea (a pulley on the frontal bone), then reflects backwards and laterally to insert on the sclera. Because of this pulley, contraction pulls the eye down and out when the eye is adducted. The cleanest clinical test of CN IV is therefore: ask the patient to look medially and then down — a palsy makes this impossible.
Functions:
- Depression of the adducted eye (main action)
- Intorsion (inward rotation) of the abducted eye
- Abduction (minor contribution)
Trochlear palsy
A patient with a CN IV palsy reports vertical diplopia worst on looking down and inwards — classically when reading or walking down stairs. The two images are tilted with respect to one another (torsional diplopia).
To compensate, the patient tilts the head AWAY from the affected side (the Bielschowsky head tilt). Tilting the head removes the need for intorsion, so the diplopia improves. Park's three-step test confirms the diagnosis:
1. Which eye is higher? (The affected eye sits higher — hypertropia)
2. Is the hypertropia worse on left or right gaze? (Worse on gaze AWAY from the affected side)
3. Is it worse on head tilt left or right? (Worse on tilt TOWARDS the affected side)
Causes: head trauma (most common — long thin nerve), congenital, microvascular ischaemia (diabetes, hypertension), and cavernous sinus pathology.
Abducens nerve (CN VI)
👩⚕️ M ➡ Supplies the lateral rectus, which abducts the eye.
The abducens nucleus sits in the pons, in the floor of the fourth ventricle. The fibres of the facial nerve (CN VII) loop around it on their way out, raising the facial colliculus — a frequently tested landmark.
CN VI has the longest intracranial course of any cranial nerve. After leaving the pons it climbs over the petrous temporal bone, runs through Dorello's canal, and enters the cavernous sinus — where, uniquely, it travels inside the sinus itself, adjacent to the internal carotid artery, rather than within the lateral wall. It then enters the orbit through the superior orbital fissure (inside the common tendinous ring) to reach lateral rectus.
Abducens palsy
Lateral rectus fails to abduct the affected eye. At rest the eye is pulled medially by unopposed medial rectus action, producing a convergent squint (esotropia) and horizontal diplopia that worsens on gaze towards the affected side. Patients often adopt a compensatory head turn towards the affected side to keep both eyes in the field of binocular vision.
Why CN VI matters: the false localising sign
Because of its long, tethered intracranial course, CN VI is the first cranial nerve affected by raised intracranial pressure. Downward displacement of the brainstem stretches the nerve over the petrous ridge, producing an isolated VI palsy that does NOT indicate the actual site of pathology — hence the classic "false localising sign." Any patient with a new CN VI palsy needs imaging to exclude raised ICP.
Causes of CN VI palsy:
- Raised ICP — tumour, hydrocephalus, idiopathic intracranial hypertension (false localising sign)
- Cavernous sinus pathology — thrombosis, aneurysm, tumour, infection
- Pontine stroke — often with ipsilateral CN VII palsy (because of the looping fibres)
- Basal skull fracture / head trauma
- Diabetic microvascular ischaemia
Cavernous sinus thrombosis
When the cavernous sinus thromboses (classically from spreading facial infection via the ophthalmic veins, the "danger triangle"), CN VI is the first nerve to fail because it sits free within the venous channel rather than protected in the lateral wall. Clinical features:
- Painful ophthalmoplegia (CN III, IV, VI all affected eventually)
- Proptosis and chemosis (impaired venous drainage from the orbit)
- V1/V2 sensory loss (forehead, cheek)
- Pulsatile exophthalmos (transmitted from the internal carotid)
- Papilloedema and retinal haemorrhages on fundoscopy
Contents of the cavernous sinus (high-yield)
| Inside the sinus (free in venous blood) | In the lateral wall |
|---|---|
| Internal carotid artery | Oculomotor (CN III) |
| Abducens (CN VI) | Trochlear (CN IV) |
| Sympathetic carotid plexus | Ophthalmic (CN V1) |
| Maxillary (CN V2) |
Memory aid: "OTOM CAt" lateral wall, top to bottom — Oculomotor, Trochlear, Ophthalmic, Maxillary; Carotid and Abducens inside.
Comparison: CN IV vs CN VI palsy
| Feature | Trochlear (CN IV) | Abducens (CN VI) |
|---|---|---|
| Muscle | Superior oblique | Lateral rectus |
| Action lost | Depression of adducted eye | Abduction |
| Diplopia | Vertical / torsional, worse looking down and in | Horizontal, worse looking towards lesion |
| Resting eye position | Slightly elevated (hypertropia) | Convergent (esotropia) |
| Head posture | Tilt away from lesion | Turn towards lesion |
| Classic clue | Trouble reading / descending stairs | False localising sign of raised ICP |
| Unique features | Only dorsal exit; only decussates; thinnest CN | Longest intracranial course; runs inside cavernous sinus |
| Foramen | Superior orbital fissure (outside ring) | Superior orbital fissure (inside ring) |
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[Image: MCQs banner]
Test yourself
A man with diplopia is unable to look down and inwards with his right eye. Which nerve is affected?
- ((Abducens nerve::Supplies lateral rectus; palsy causes failure of abduction, not depression.))
- ((Ciliary ganglion::Parasympathetic relay for CN III — pupil constriction and accommodation.))
- ((Nasociliary nerve::Sensory branch of CN V1; no motor function to extraocular muscles.))
- ((Trochlear nerve::☑️ CN IV supplies superior oblique, which depresses the adducted eye.))
👩⚕️ Trouble reading or going downstairs is the classic CN IV palsy story.
A 58-year-old has a tumour invading the centre of the cavernous sinus. Which nerve is most likely involved first?
- ((Mandibular V3::Exits via foramen ovale — never enters the cavernous sinus.))
- ((Abducens CN VI::☑️ Runs free inside the sinus next to the ICA, so hit first.))
- ((Facial CN VII::Leaves the skull via stylomastoid foramen after the facial canal.))
- ((Vestibulocochlear CN VIII::Travels with CN VII through the internal acoustic meatus.))
- ((Glossopharyngeal CN IX::Exits via the jugular foramen with CN X and XI.))
In cavernous sinus thrombosis, which nerve is affected first because it lies within the sinus rather than its lateral wall?
- ((Optic nerve::Travels through the optic canal with the ophthalmic artery, not the sinus.))
- ((Oculomotor nerve::In the lateral wall — protected from venous thrombus.))
- ((Abducens nerve::☑️ Sits free in the sinus beside the ICA, vulnerable to thrombus.))
- ((Ophthalmic nerve V1::In the lateral wall with CN III and CN IV.))
👩⚕️ Inside the sinus: ICA and CN VI. Everything else is in the wall.
A patient with a CN IV palsy is most likely to adopt which compensatory head posture?
- ((Head turn towards the affected side::Compensates for a CN VI (lateral rectus) palsy.))
- ((Head tilt away from the affected side::☑️ Removes the need for intorsion by superior oblique.))
- ((Chin elevation::Compensates for bilateral inferior rectus / CN III palsy.))
- ((Chin depression::Compensates for bilateral superior rectus weakness.))
👩⚕️ Bielschowsky tilt — Park's three-step test confirms the diagnosis.
Which cranial nerve is the classic "false localising sign" of raised intracranial pressure?
- ((Oculomotor CN III::Compressed by uncal herniation — a true localising sign.))
- ((Trochlear CN IV::Vulnerable to head trauma, not specifically to raised ICP.))
- ((Abducens CN VI::☑️ Longest intracranial course; stretched over the petrous ridge.))
- ((Facial CN VII::Affected in pontine lesions, not classically by ICP alone.))
Which feature is unique to the trochlear nerve?
- ((Longest intracranial course::That describes CN VI.))
- ((Supplies a muscle outside the common tendinous ring::True, but not unique — CN IV shares the SOF with others.))
- ((Only cranial nerve to exit dorsally and to decussate::☑️ Both features are unique to CN IV.))
- ((Carries parasympathetic fibres::CN III, VII, IX and X carry parasympathetics; CN IV is purely motor.))
Revision summary
- LR6 SO4 — lateral rectus = CN VI; superior oblique = CN IV; everything else = CN III.
- CN IV (trochlear): midbrain nucleus at inferior colliculus; only dorsal exit; only decussates; thinnest CN; runs in lateral wall of cavernous sinus.
- CN IV palsy: vertical/torsional diplopia, worst reading or going downstairs; head tilt AWAY from lesion; Park's three-step test.
- CN VI (abducens): pontine nucleus at facial colliculus; longest intracranial course; runs inside the cavernous sinus next to the ICA.
- CN VI palsy: failure of abduction, convergent squint, horizontal diplopia worse on gaze towards the lesion; head turn TOWARDS the lesion.
- CN VI = false localising sign of raised ICP; first nerve affected in cavernous sinus thrombosis.
- Cavernous sinus inside: ICA + CN VI. Lateral wall: CN III, IV, V1, V2.
- Both nerves are purely motor and enter the orbit via the superior orbital fissure.