Abducens Nerve (CNVI)

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Abducens Nerve (CNVI)

Main Text

TERMINOLOGY

Abbreviations

• Abducens nerve (CNVI)

Synonyms

• Abducens nerve: 6th cranial nerve

Definitions

• CNVI: Motor nerve to lateral rectus muscle only

IMAGING ANATOMY

Overview

• CNVI is pure motor nerve, longest intracranial course
• 5 segments : Intraaxial, cisternal, interdural, cavernous, intraorbital (extracranial)

Abducens Nucleus

• Paired CNVI nuclei located in pontine tegmentum near midline, just ventral to 4th ventricle
• Facial colliculus : Axons of facial nerve (CNVII) loop around CNVI nucleus, creating bulge in floor of 4th ventricle
• Isolated lesion to facial colliculus can cause ipsilateral CNVI & CNVII palsy

Intraaxial Segment

• Ipsilateral axons from CNVI nucleus course anteroinferiorly through pontine tegmentum

Cisternal Segment

• Emerges from anterior brainstem near midline through groove between pons & pyramid of medulla oblongata (pontomedullary sulcus)
• Usually exits as single trunk but occasionally duplicated
• CNVI ascends anterosuperiorly in prepontine cistern toward site where it penetrates dura along upper clivus laterally
• Posterior to anterior inferior cerebellar artery in 85%; anterior in 15%

Interdural Segment

• Extends from point where CNVI pierces inner layer dura posteriorly to its entrance into cavernous sinus anteriorly
• Thin sleeve of arachnoid (& occasionally dura) travels with nerve through this segment
• After penetrating dura, CNVI passes superiorly through basilar venous plexus

Basilar venous plexus is dorsal to upper clivus & located between inner & outer (endosteal) layers of dura; it is interdural
• Nerve remains interdural & passes superiorly over junction of petrous apex & clivus, into adjacent venous region known as sphenopetroclival venous confluence [or simply petroclival confluence or petroclival venous confluence (PCVC)]

PCVC located at junction of posterior part of cavernous sinus, lateral part of basilar plexus, & anterior part of superior & inferior petrosal sinuses
• In this location, PCVC & interdural segment of CNVI are considered to be within classic Dorello canal
• Classic Dorello canal is zone/space bounded by petrous apex (inferolateral), clivus (inferomedial), & petrosphenoidal ligament of Gruber (superiorly)

More recent descriptions in literature have proposed modifications to classic description, often renaming canal &/or expanding limits to include portions of venous confluence above Gruber ligament, & making posterior petroclinoid fold superior boundary

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Tags: Imaging Anatomy Brain and Spine

Mar 13, 2021 | Posted by admin in NUCLEAR MEDICINE | Comments Off

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