Nuclei and fibre types
CN IV carries one fibre type: general somatic efferent (GSE) fibres, motor to skeletal muscle. Its nucleus lies in the midbrain at the level of the inferior colliculus, just below the oculomotor nucleus and ventral to the cerebral aqueduct.
The fibres run backward around the aqueduct and cross the midline in the superior medullary velum, the thin roof of the upper fourth ventricle. Every fibre crosses, so each trochlear nucleus supplies the superior oblique of the opposite eye. CN IV has the fewest axons of any cranial nerve.
Emergence and skull foramen
CN IV emerges from the dorsal surface of the midbrain, just below the inferior colliculus. It is the only cranial nerve to leave the posterior aspect of the brainstem. It enters the orbit through the superior orbital fissure, above and outside the common tendinous ring, in company with the frontal and lacrimal nerves.
Course and branches
CN IV has the longest intracranial course of any cranial nerve, winding from the back of the midbrain to the front of the cavernous sinus.
- Around the brainstem: it curves forward around the lateral side of the cerebral peduncle, passing between the posterior cerebral and superior cerebellar arteries with the oculomotor nerve.
- Tentorium: it runs just below the free edge of the tentorium cerebelli and pierces the dura near the posterior clinoid process.
- Cavernous sinus: it runs forward in the lateral wall of the sinus, below CN III and above the ophthalmic division of CN V. Near the front it crosses above CN III.
- Orbit: it passes through the superior orbital fissure outside the tendinous ring, runs medially above levator palpebrae superioris, and enters the orbital (upper) surface of superior oblique.
It has no named branches. Superior oblique arises from the body of the sphenoid, and its tendon turns through the trochlea, a fibrocartilage pulley on the frontal bone, before inserting on the posterolateral eyeball. Because it pulls from the front and medial side, it intorts, depresses and slightly abducts the eye. Depression is its main action when the eye is adducted.
Clinical testing
CN IV is tested by asking the patient to look down and in, the position in which superior oblique is the main depressor.
- Eye movements: ask the patient to follow a target into adduction and then downward, and report double vision. The images are vertical and furthest apart in this position.
- Head tilt test (Bielschowsky): the vertical deviation worsens when the head tilts toward the affected side.
- Intorsion with a third nerve palsy: when CN III is out of action the eye cannot adduct, so ask the patient to look down. A working superior oblique intorts the eye, seen as rotation of a conjunctival vessel.
Lesions and palsies
A fourth nerve palsy causes vertical double vision that is worst on looking down and in, so patients describe trouble reading and going downstairs. The affected eye sits higher than the other (hypertropia) and is extorted, because inferior oblique acts unopposed.
Patients compensate by tilting the head away from the affected side and tucking the chin down, which brings the eyes back into line. Old photographs showing a long-standing head tilt suggest a congenital palsy.
| Cause | Notes |
|---|---|
| Congenital | The commonest cause overall; often decompensates in adult life |
| Head injury | The commonest acquired cause; the thin nerve is stretched at the tentorial edge, and severe injury can cause bilateral palsy |
| Microvascular (diabetes, hypertension) | Usually recovers |
| Cavernous sinus or orbital apex lesion | With CN III, V1 and VI involvement |
| Midbrain lesion at the nucleus | Weakens the opposite superior oblique |