CN6 anatomy
- CN6 nuclei in the pons.
- Exits the pons ventrally in the horizontal sulcus between the pons and medulla.
- Here it is fixed at the nerve root entry zone
- In its subarachnoid course, the sixth nerve ascends along the base of the pons in the prepontine cistern and enters Dorello's canal beneath Grüber's ligament
- It travels in the subarachnoid space along the clivus before it enters the cavernous sinus .
- In the cavernous sinus, the sixth nerve lies between the carotid artery medially and the ophthalmic branch of the trigeminal nerve laterally.
- Thus, unlike the other cranial nerves, which are located in the dural wall, the sixth nerve is more free floating in the cavernous sinus.
- After passing through the superior orbital fissure to enter the orbit, the sixth nerve innervates the lateral rectus muscle.
- The arrow indicates the Dorello's channel region, and the yellow trace marks the passage of the abducens nerve.
- Its path follows the clivus in fossa posterior, passing through Dorello's channel, continuing its intracavernous passage, and then entering the superior orbital fissure to reach the lateral straight muscle in the orbit.
- Images
- Anatomical Exposure and Regions
- The optic strut is exposed in the anterior part of the clinoidal triangle.
- The clinoid segment is exposed in the midportion.
- The roof of the cavernous sinus is exposed in the posterior part.
- The posterior bend of the internal carotid artery (ICA) and the origin of the meningohypophyseal trunk are exposed in the infratrochlear triangle.
- Dural Rings and Membranes
- The upper margin of the clinoid segment is surrounded by the upper dural ring.
- The upper dural ring is formed by the dura extending medially from the upper surface of the anterior clinoid.
- The lower dural ring defines the lower margin of the clinoid segment.
- The dura on the lower margin of the anterior clinoid is referred to as the carotidoculomotor membrane.
- The carotidoculomotor membrane separates the lower surface of the anterior clinoid from the upper surface of the oculomotor nerve.
- This membrane extends medially to form the lower dural ring.
- Internal Carotid Artery (ICA) and Branches
- The meningohypophyseal trunk arises near the posterior bend of the ICA.
- The meningohypophyseal trunk gives rise to the tentorial and dorsal meningeal arteries.
- The inferolateral trunk arises from the horizontal segment of the intracavernous carotid.
- Cranial Nerve Pathways
- The abducens nerve passes through Dorello’s canal.
- The abducens nerve passes between the lateral surface of the intracavernous carotid and the medial side of the ophthalmic nerve.
- The inferolateral trunk passes above the abducens nerve.
- Aka Petroclival ligament, Petrosphenoidal ligament,
- Attachement:
- Petrous apex to just below the posterior clinoid process on the lateral border of the upper clivus
- vs petroclinoidal ligament which form the oculomotor trigone where the CN 3 enters the cavernous sinus
- Shows the various CN and the difference between the Petrolingual and petro sphenoidal ligament
- Shows the CN6 traveling below Gruber’s ligament
Grüber's ligament
CN6 lesion
General
- Most common isolated ocular motor nerve palsy.
- Presents as both because the MLF starts at CN6 nuclei
- Ipsilateral lateral rectus palsy +
- Ipsilateral gaze palsy.
- Involvement of the MLF (CN6 → CN3)
- Non-localising sign of raised or dropped in ICP
- Can pull or push the nerve between the two fixed points of nerve root entry zone and petroclinoid ligament.
- Presents as
- Diplopia at lateral version (gaze) or at far distances
- Face turn to affected side
Nuclear
- Facial nucleus does a loop round the abducens, any damage to the facial colliculus would then result in VI and Vll be affected.
- PPRF and MLF are also in close proximity to CN VI, resulting in horizontal gaze palsies (PPRF) , INO (MLF)
Fascicular
- Millard Gubler syndrome
- Aka
- Ventral pontine syndrome
- Due to
- Lesions in ventral aspect of the caudal pons
- Young
- Tumour
- Demyelination
- Viral infection
- Old
- Vascular
- Features
- Ipsilateral facial and contralateral body hemiplegia due to pyramidal tract involvement
- Isotropism and diplopia that is worsened while the patient looks toward the lesion
- Ipsilateral facial paresis and loss of corneal reflex
- Ipsilateral seventh nerve palsy
- Contralateral hemiparesis (Millard-Gubler syndrome).
Subarachnoid space
- Inc. ICP
- Most common cause of CN6 palsy
- May be associated with ipsilateral facial pain due to stretching of the root of the trigeminal nerve.
- Gradenigo's syndrome
- Inflammatory process at the petrous apex (petrous apicitis) → Inflammation spread into the neurovascular structures at the skull base:
- Clinical triad
- Lateral rectus palsy (due to involvement of the CN6)
- Retro-orbital pain (due to involvement of the CN V1 and V2 nerve)
- Otitis media → persistent otorrhea and ear pain
Cavernous sinus + supraorbital fissure
- Cavernous sinus
- Lesions here will rarely produce an isolated VI nerve palsy, due to the close proximity of Ill, IV and ophthalmic nerve
- Parkinson's syndrome
- Sixth nerve (@ cavernous sinus) palsy + Horner syndrome,
- Because the sympathetic fibers to the eye join the sixth nerve for a short distance within the cavernous sinus.
- A similar syndrome may be caused by a lesion at the superior orbital fissure, although this is frequently associated with proptosis.
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