The document discusses the anatomy and clinical implications of the 6th cranial nerve (abducens nerve). It originates in the pons, innervates the lateral rectus muscle, and has only a motor function. Lesions along its path can cause 6th nerve palsy, including those in the pons (Foville and Millard-Gubler syndromes), at the pontomedullary junction (vestibular schwannoma), or at the petrous bone (Gradenigo syndrome). Damage to the 6th nerve results in an inability to move the eye laterally on the affected side.

1. Anatomy of 6th cranial nerve

2. Introduction
Abducens nerve is the 6th cranial nerve that has only a somatic motor
component (general somatic efferent) that supplies the lateral rectus
muscles.

3. Embriyology
The human abducens nerve is derived from the basal plate of the
embryonic pons.

4. Nucleus
Nucleus of the abducens nerve is situated near the midline in the
tegmentum of pons ventral to the facial colliculus.
Facial colliculus is an elevation in the floor of the 4th ventricle,
produced by the genu of the facial nerve.

6. Course
Abducens nerve leaves the brsinstem at the junction of the pons and
medulla superior to the pyramid and medial to the facial nerve.
It runs upward and forwards from this position and enters the
subarachnoid space when it emerges from the brainstem.

7. Course continued
It runs upward between the
pons and the clivus, and then
pierces the dura mater to run
between the dura and the
skull through Dorello’s canal.

8. Dorello’s canal is and osteofibrous
conduit located at the level of the
apex of the petrous part of the
temporal bone.

9. Course continued
At the tip of the petrous part of the temporal bone it makes a sharp
turn forward to enter the cavurnus sinus.
In the cavernous sinus it runs alongside the internal carotid artery.

11. Course continued
It then enters the orbit through
the superior orbital fissure within
the annulus of Zinn and
innervates the lateral rectus
muscle of the eye.

13. Clinical antomy
Brainstem lesion
1. Ipsilateral 6th nerve palsy
2. Ipsilateral 7th nerve palsy with 6th nerve involvement

14. Horizontal gaze control

15. Foville (inferior medial pontine)
syndrome is most frequently
caused by vascular disease or
tumours involving the dorsal pons.
It is characterized by ipsilateral
involvement of the fifth to eighth
cranial nerves, central
sympathetic fibres (Horner
syndrome) and horizontal gaze
palsy.

16. Millard–Gubler (ventral pontine)
syndrome involves the fasciculus
as it passes through the pyramidal
tract and is most frequently
caused by vascular disease,
tumours or demyelination. As well
as ipsilateral sixth nerve palsy,
there is contralateral hemiplegia
and often an ipsilateral LMN facial
nerve palsy.

17. Raymond syndrome involves the pyramidal tract and sixth nerve. In
addition to an ipsilateral sixth nerve palsy there is contralateral
hemiplegia.

18. A vestibular schwannoma may
damage the sixth nerve at the
pontomedullary junction. The
first symptom is hearing loss
and the first sign diminished
corneal sensitivity. Hearing and
corneal sensation should be
checked in all patients with
sixth nerve palsy.

19. Nasopharyngeal tumours may invade the skull and its foramina and
damage the nerve during its basilar course.
Raised intracranial pressure may cause a downward displacement of
the brainstem. This may stretch one or both sixth nerves over the
petrous tip, when paresis is a false localizing sign.
Basal skull fracture may cause unilateral or bilateral palsy.

21. Gradenigo syndrome, most frequently caused by mastoiditis or acute
petrositis, may result in damage of the sixth nerve at the petrous tip.
The latter is frequently accompanied by facial weakness and pain and
hearing difficulties.

22. Intracavernous and intraorbital
Isolated 6th nerve palsy is rare

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