Abducens nerve

The abducens nerve is the sixth cranial nerve. It courses from its nucleus located in the dorsal pons to its innervation of the lateral rectus muscle and can be divided into four parts:

1. nucleus and intraparenchymal portion
2. cisternal portion
3. cavernous sinus portion
4. orbital portion

Gross anatomy

Nucleus 

The abducens nucleus is a small nucleus situated at the upper part of the rhomboid fossa beneath the colliculus facialis within the pons.

Cisternal portion

It is the most medial of the nerves emerging immediately below the pons (facial nerve and vestibulocochlear nerve lateral to it) at the pontomedullary junction into the prepontine cistern. It courses anteriorly towards the clivus where it runs superiorly along the clivus enclosed within a fibrous sheath called Dorello canal and pierces the dura inferior to the posterior clinoid process. It then courses over the medial petrous apex towards the cavernous sinus. Its oblique course and relatively anchored position in Dorello canal make it prone to stretching when raised intracranial pressure from a space-occupying lesion causes transtentorial herniation (a sixth nerve palsy is the classic lateralising sign of an extradural hematoma).

Cavernous sinus portion

Within the cavernous sinus, the abducens nerve is located inferolateral to the internal carotid artery, medial to the lateral wall of the sinus.

Orbital portion

Having entered the orbit through the superior orbital fissure within the tendinous ring (see Figure 3) it supplies the lateral rectus. It is the most inferior nerve passing through the tendinous ring, inferior to the inferior division of the oculomotor nerve.

Related pathology

• sixth nerve palsy