Abducens nerve

Motor function

Motor innervation to the lateral rectus muscle.

Anatomy

1. Intra-axial segment

The abducens nucleus lies medially in the pontine tegmentum, caudally in pons, anterior to the 4th ventricle. The abducens nerve exits from the ventral surface of the brainstem, medially, through the bulbopontine sulcus (between the pons and the medullary pyramids).

2. Cisternal segment

The abducens nerve courses in the prepontine cistern either anterior or posterior to the anterior inferior cerebellar artery (AICA). It penetrates the dura of the basisphenoid to enter Dorello’s canal, a channel within the basilar venous plexus that is formed between two layers of dura.

3. Intradural segment (Dorello’s canal)

Dorello’s canal extends from the point where abducens penetrates the dura to the cavernous sinus. The nerve passes over the petrous apex below the the petrosphenoidal ligament to reach the superior posterior part of the cavernous sinus.

4. Cavernous segment

Abducens is the only nerve that courses through the cavernous sinus (oculomotor, trochlear, ophthalmic and maxillary run within the dural wall of the sinus). Within the cavernous sinus abducens runs along the inferolateral aspect of the internal carotid artery (ICA).

5. Extracranial segment

Abducens exits the cranium through the superior orbital fissure. It passes through the annulus of Zinn.

It supplies motor innervation to the lateral rectus muscle.

Clinical implications

• In abducens palsy the patient is unable to abduct the ipsilateral eye.
• Abducens palsy is the most common ocular nerve palsy.
• Because it emerges near the skull base and has a relatively long course abducens is often the first nerve to be compressed when there is increase in intracranial pressure (ICP).
• The most common causes of abducens nerve palsy are microvascular infarction, trauma and idiopathic. Less common causes are increased ICP, tumors, thrombosis of the cavernous sinus, sarcoidosis, vasculitis, or as complication of lumbar puncture.