Trochlear Nerve

visual perception with vertical diplopia secondary to trochlear nerve palsy

The trochlear nerve (CN IV) is a motor nerve supplying the superior oblique muscle of the eye.

The superior oblique primarily rotates the top of the eye toward the nose (intorsion). Secondarily, it moves the eye downward (depression) and outward (abduction). It prevents the unopposed action of the superior rectus which would otherwise rotate the globe.

Trochlear palsy causes weakness of downward gaze. The patient complains of difficulty reading or walking downstairs. The patient may also complain of vertical diplopia (see image).


CN IV leaves the dorsal aspect of the midbrain. It runs forwards towards the eye in the subarachnoid space and then passes between the posterior cerebral and superior cerebellar arteries.

It enters the cavernous sinus where it joins CN III and CN VI, and the ophthalmic branch of CN V. Finally, it enters the orbit through the superior orbital fissure.


CN IV is tested as part of eye movements with CN III and CN VI (Oculomotor and Abducens).


Investigation for CN IV is the same as for CN III.


The most common cause of fourth nerve palsy is idiopathic followed by trauma (often minor). Other causes include:

  • Microvasculopathy (associated with diabetes and hypertension)
  • Tumours or aneurysms in the subarachnoid space

Multiple sclerosis rarely involves the fourth nerve.

Lesions in the midbrain may damage the trochlear nucleus. Due to the close proximity of the sympathetic fibres to the trochlear nucleus such lesions may also cause a Horner’s syndrome(Fig.1) (interruption of the sympathetic supply to the eye causing ptosis, miosis and enophthalmos)

The fourth nerve can be affected by cavernous sinus disease.


CN IV is damaged, uncommonly, in trauma affecting the orbit.

Note that direct damage to the superior oblique muscle can mimic a CN IV nerve palsy.