Abducens Nerve (Sixth Cranial Nerve)
The nucleus of the abducens nerve (CN VI) is situated in the floor of the fourth ventricle, beneath the facial colliculus in the caudal pons. The medial longitudinal fasciculus lies medial to the CN VI nucleus. The fascicular portion of CN VI runs ventrally through the paramedian pontine reticular formation and the pyramidal tract and leaves the brainstem in the pontomedullary junction (see Figs 3-1, 3-2).
CN VI then takes a vertical course along the ventral face of the pons and is crossed by the anterior inferior cerebellar artery. It continues through the subarachnoid space along the surface of the clivus to perforate the dura mater below the petrous apex, approximately 2 cm below the posterior clinoid process (see Fig 3-24). It then passes intradurally through or around the inferior petrosal sinus and beneath the petroclinoid (Gruber) ligament through the Dorello canal, after which it becomes extradural and enters the cavernous sinus. This long route (especially along the surface of the clivus and beneath the petroclinoid ligament) is responsible for this nerve’s susceptibility to stretch injury leading to paresis in the context of increased intracranial pressure. In the cavernous sinus, CN VI runs below and lateral to the ICA and may transiently carry sympathetic fibers from the carotid plexus (see Fig 3-25). It passes through the superior orbital fissure within the annulus of Zinn to enter the medial surface of the lateral rectus muscle, which it innervates.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.