Trochlear nerve

Motor function

Innervation of superior oblique muscle.

Anatomy

1. Intra-axial segment

The trochlear nucleus is located in the paramedian midbrain at the level of the inferior colliculus, dorsal to the medial longitudinal fasciculus (MLF) and anterior to the cerebral aqueduct.

The fascicles of the trochlear nerve course around the cerebral aqueduct and decussate within the superior medullary velum. Each trochlear nerve thus innervates the contralateral superior oblique muscle.

The trochlear is the only cranial nerve to exit from the dorsal surface of the brainstem. It exists just inferior to the inferior colliculus.

2. Cisternal segment

The trochlear nerve courses within the ambient cistern, under the tentorium cerebelli. It passes between the posterior cerebral (PCA) and superior cerebellar (SCA) arteries, inferolateral to the oculomotor nerve. It penetrates the dura and enters the lateral wall of the cavernous sinus, inferior to the oculomotor nerve.

3. Cavernous segment

The trochlear nerve courses within the wall of the cavernous sinus, inferior to oculomotor nerve, superior to abducens nerve and the ophthalmic and maxillary branches of the trigeminal nerve and lateral to the internal carotid artery (ICA).

4. Extracranial segment

The trochlear nerve enters the orbit throug the superior orbital fissure and it runs medially to the oculomotor nerve.

It passes above the annulus of Zinn, contrary to oculomotor and abducens which pass through it.

It innervates the superior oblique muscle.

Clinical implications

• Trochlear nerve palsy causes paralysis of the superior oblique muscle which results in extorsion (outward rotation) of the affected eye and weaknes of downward gaze. The patient often tilts the head away from the affected side to ameliorate the resulting vertical diplopia.
• To find the side of the lesion:
  1. Find the side of hypertropia
  2. Determine if hypertropia is greater on left or right gaze
  3. Determine if hypertropia is greater on left or right head tilt
  4. Determine if the vertical separation is greater in upgaze or downgaze
  5. Check for cyclotropia
• The trochear nerve is the thinnest and second longest (~7.5 cm) cranial nerve after vagus. The most common cause of trochlear palsy is head trauma. If unilateral trochlear nerve palsy occurs after head trauma, bilateral trochlear nerve palsy should always be considered.
• Bilateral fourth nerve palsy presents with crossed hypertropia (the right eye is higher on left gaze, the left eye is higher on right gaze), excyclotorsion of 10° or greater (each eye rotates outwardly on Maddox rod test) and a large (≥25 D) V-pattern of strabismus.
• Less common causes of trochlear nerve palsy are brainstem or orbital tumor, PCA or SCA aneurysms, stroke and cavernous sinus thrombosis.