Trochlear nerve (CN IV)
Nerve · CN IV Motor
The thinnest cranial nerve with the longest intracranial course and the only one to leave the dorsal brainstem; it decussates below the inferior colliculus so each nucleus supplies the contralateral superior oblique. It runs in the lateral wall of the cavernous sinus and enters the orbit through the superior orbital fissure above the tendinous ring.
Traced from the start
- Midbrain
- Trochlear nerve (CN IV)
Detail
- Motor
- Superior oblique only: depresses the adducted eye, intorts and abducts
- Nucleus
- Midbrain at the level of the inferior colliculus; fibres cross in the superior medullary velum
- Test
- Ask the patient to look down and in (towards the nose); check for head tilt
When it goes wrong
Fourth nerve palsy from head injury (commonest acquired cause, often bilateral because the nerves cross at the velum), microvascular ischaemia or a decompensated congenital palsy
Vertical or torsional diplopia worst on looking down and in, so reading and descending stairs are difficult; the patient tilts the head away from the affected side to compensate, and the affected eye drifts up when the head is tilted towards that side
Practise this structure
1 question in the bank tagged Trochlear Nerve.