Nuclei and fibre types
CN VI carries one fibre type: general somatic efferent (GSE) fibres, motor to skeletal muscle. Its nucleus lies in the dorsal pons, in the floor of the fourth ventricle, close to the midline.
Fibres of the facial nerve loop around the abducens nucleus, raising a bump in the floor of the fourth ventricle called the facial colliculus. A single lesion here can therefore affect both CN VI and CN VII.
The abducens nucleus is more than a motor nucleus for one muscle. It also contains internuclear neurons whose axons cross the midline and ascend in the medial longitudinal fasciculus (MLF) to the opposite medial rectus subnucleus of CN III. The nucleus is therefore the centre for conjugate horizontal gaze, moving both eyes together toward the same side.
Emergence and skull foramen
CN VI emerges from the anterior surface of the brainstem at the pontomedullary junction, close to the midline and just above the pyramid of the medulla. It enters the orbit through the superior orbital fissure, within the common tendinous ring, alongside the two divisions of CN III and the nasociliary nerve.
Course and branches
CN VI has a long course that bends sharply over bone, which explains its vulnerability.
- Pontine cistern: it runs upward and forward in the subarachnoid space on the front of the pons.
- Clivus: it pierces the dura on the clivus and continues upward between the dura and bone.
- Petrous apex: it bends sharply forward over the apex of the petrous temporal bone, passing beneath the petrosphenoidal ligament in a channel known as Dorello's canal.
- Cavernous sinus: it runs through the body of the sinus, lateral to the internal carotid artery, rather than in the lateral wall like CN III, IV, V1 and V2.
- Orbit: it passes through the superior orbital fissure within the tendinous ring and enters the medial (ocular) surface of lateral rectus.
It has no named branches. Lateral rectus arises from the common tendinous ring and abducts the eye; it is the only muscle that moves the eye purely outward.
Clinical testing
CN VI is tested by asking the patient to look to each side and watching for full abduction.
- Primary position: look for an inward deviation of one eye at rest.
- Horizontal pursuit: ask the patient to follow a target to the right and left. Note whether the white of the eye disappears fully at the lateral canthus and ask when double vision appears.
- Cover test: covering each eye in turn shows the deviation and which eye is fixing.
- Distinguishing a gaze palsy: if neither eye can look to one side, the lesion is in the abducens nucleus or pontine gaze centre, not the nerve.
Lesions and palsies
A sixth nerve palsy causes a convergent squint (esotropia) and horizontal double vision that worsens on looking toward the affected side and at distance. The eye cannot abduct past the midline in a complete palsy. Patients turn the head toward the affected side to keep single vision.
| Cause | Mechanism |
|---|---|
| Raised intracranial pressure | Downward brainstem shift stretches the nerve over the petrous apex; a false localising sign that can be bilateral |
| Low CSF pressure | After lumbar puncture or a CSF leak, by the same traction |
| Microvascular (diabetes, hypertension) | Ischaemia of the nerve; usually recovers |
| Petrous apicitis | Gradenigo syndrome: sixth nerve palsy, pain behind the eye and ear discharge after middle ear infection |
| Cavernous sinus lesion | Thrombosis, carotid aneurysm or carotid–cavernous fistula; CN VI is often first affected because it lies free in the sinus |
| Clivus tumour | Chordoma or nasopharyngeal carcinoma |
| Pontine lesion | Nuclear lesion gives an ipsilateral gaze palsy, often with facial weakness; ventral pontine lesions can combine CN VI and VII palsy with contralateral hemiplegia (Millard–Gubler syndrome) |
Damage to the MLF, rather than to CN VI, causes internuclear ophthalmoplegia: failure of adduction in one eye with nystagmus of the abducting eye. It is a classic sign of multiple sclerosis.