The abducens nerve originates in the pons and contains motor neurons that innervate the lateral rectus muscle, allowing for eye movement outward. It exits the brainstem and travels through the cavernous sinus before entering the orbit through the superior orbital fissure to innervate the lateral rectus. Pathologies of the abducens nerve can occur at the nuclear, central, cisternal, petrous, cavernous, or orbital segments and result in lateral rectus palsy and limitations in abduction of the eye. Common causes include pontine hemorrhage, MS, meningiomas, and traumatic injuries.

2. Introduction
• Abducens nerve has only a somatic motor
component (general somatic efferent) that
supplies lateral rectus muscle.

3. Abducens nerve anatomy

4. Origin
• Abducens nucleus originates from the tegmenum
pontis at the level of facial colliculus.
• The nucleus is located;
1. anterior to the 4th ventricle,
2. posterior to the medial leminiscus,
3. Lateral to the medial longitudinal fasciculus, &
4. Medial to facial nerve & trigeminal spinal nucleus.
• The facial colliculus is a focal bulge in the floor of
the fourth ventricle formed by looping fibers of the
facial nerve around the abducens nucleus.

7. Abducent nucleus
.

8. The abducens nucleus contains 3 types of
neurons:
1. Abducens motor neurons which innervate the ipsilateral lateral rectus
muscle.
2. Abducens internuclear neurons, which project to the contralateral medial
rectus subnucleus of the oculomotor nucleus via the medial longitudinal
fasciculus.
3. Neurons that project to the cerebellar flocculus

10. Central course
• The abducens nerve fascicle course
antero-inferiorly through the pontine
tegmentum adjacent to the facial nerve
and exit from the brain stem at the ponto-
medullary sulcus.

11. Intracranial course
Dorello's canal is an osteofibrous conduit located at the
level of the petrous apex through which the abducens
nerve courses to reach the cavity of the cavernous sinus
• Cisternal segment
• Petro-clival segment
• Cavernous segment
• Orbital segment

12. Cisternal segement
• courses superiorly through the prepontine cistern, to
pierce the dura over medial most aspect of the petrous
ridge.

14. Petroclival segment
Cavernous segment
Petroclival segment
Cisternal segment
MC

15. Petroclival segment
Dorello's canal is an osteofibrous conduit
located at the level of the petrous apex
through which the abducens nerve courses
to reach the cavity of the cavernous sinus

17. Cavernous segment
The cavernous segment of the abducens nerve lie
within the body of the sinus unlike the oculomotor ,
trochlear & V1 & V2 divisions of the trigeminal
nerve which lie within the lateral wall of the sinus.

19. Cn IV
CN V1
Cn VI
Cn III
Cn V2

20. Orbital segment
• The abducens nerve enters the orbit
through the superior orbital fissure and
passes through the annular ring of Zinn.

21. Superior orbital fissure & annular
ring of Zinn

24. Abucens nerve pathology
• Nucleus & central segment
• Cisternal segment
• Petrous segment
• Cavernous segment
• Orbital segment

25. Aplasia or hypoplasia of the
abducens nerve
• Duane syndrome.
• Mobius syndrome.
• HGPPS (Horizontal Gaze Palsy with
Progressive Scoliosis).

26. Duane syndrome

27. Mobius syndrome
It is a congenital disorder characterized by
• Bilateral facial diplegia
• Convergent squint
Secondary to 6th
& 7th
cranial nerve palsies
Associations:
Other cranial nerve plasies: 5th
, 9th
, 10th
& 12th
cranial
nerves.
Craniofacial abnormalites.
Chest wall abnormalities.
Upper & lower limb abnormalities.

29. Normal subject Mobius syndrome

30. Nucleus
• Pontine hemorrhage.
• MS
• glioma

31. Pontine hemorrhage

32. MS

33. Central segment
• The same as nuclear pathology.

34. Pontine hemorrhage

35. Cisternal segment
• Duplicted abucens nerve (normal variant)

36. Duplicated abducens nerve with
normal abduction

37. Diabetic or viral neuropathy
Post-contrast

38. Post-contrast

39. Petroclival segment pathology
• Petroclival meningioma.
• Skull base destructive lesion.
• Neuroma.
• Usher syndrome.

40. Usher syndrome
• Dilatation of the subarachnoid spaces
surrounding the cranial nerves with
petrous apex cephaloceles in Usher
syndrome.
• Usher syndrome is an autosomal
recessive disorder characterized by
retinitis pigmentosa & congenital SNHL.

42. Petroclival meningioma

43. Ganglio-neurofibroma

44. One & half syndrome
• Complete horizontal gaze palsy,
when looking toward the side of the lesion.
• Half gaze palsy,
when looking toward the opposite side.

45. One & half syndrome

46. Lateral rectus muscle atrophy

47. Lateral rectus muscle atrophy 1
year after post-traumatic 6th
Cn
palsy