Trochlear nerve (IV)

At a glance

English

Trochlear nerve (IV)

Latin (TA2)

Nervus trochlearis (IV)

System

Nerve

Region

Head

Side

Paired, left and right

About the trochlear nerve (iv)

Cranial nerve IV is a pure somatic motor nerve with one target, the superior oblique, whose tendon runs through a fibrous pulley, the trochlea, at the front of the orbit's medial wall; the nerve takes its name from that pulley. It is unusual in several ways. It has fewer axons than any other cranial nerve, the longest course inside the skull, and it is the only one to leave the dorsal surface of the brainstem. Its fibres also cross before they emerge, so each nucleus serves the muscle of the opposite eye. Embryologically it grows out of the midbrain's basal plate.

The nucleus sits in the lower midbrain, ventral to the cerebral aqueduct, just caudal to the oculomotor nucleus. Its axons curve backwards around the aqueduct, decussate in the superior medullary velum and appear just below the inferior colliculus. The nerve then winds forwards around the side of the brainstem in the subarachnoid space. It pierces the dura beneath the free edge of the tentorium near the posterior clinoid process, runs forwards in the lateral wall of the cavernous sinus, and passes via the superior orbital fissure into the orbit, outside the common tendinous ring, to reach the upper surface of the muscle.

Because the superior oblique tendon is redirected by the trochlea and meets the globe from in front at an angle, its pull both depresses the eye and rotates its upper pole towards the nose (intorsion). Depression is strongest with the eye turned in, intorsion with the eye turned out. At the bedside the examiner checks it by having the patient gaze down and inwards.

A palsy leaves the affected eye riding higher than its fellow, with vertical and tilted double vision that is worst on reading or going downstairs. Patients tuck the chin and tilt the head away from the affected side to fuse the images; Bielschowsky's head tilt test helps confirm the diagnosis. Head injury is the usual acute cause, since the long, thin nerve is easily stretched against the tentorium. Congenital palsies may only surface in adult life and can be treated surgically. Damage to the nucleus affects the opposite eye, whereas damage to the nerve after it emerges affects the eye on the same side.

Course and supply

Arises from

The trochlear nucleus in the caudal midbrain; the fibres cross and leave the back of the brainstem immediately caudal to the inferior colliculus.

Ends in

Reaches the orbit by the superior orbital fissure, outside the common tendinous ring, and ends in the superior oblique muscle.

Supplies

Motor to the superior oblique, which depresses the eye (mainly when turned in) and intorts it (mainly when turned out).