Introduction

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

Abducens nerve anatomy

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.

Abducent nucleus

.

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

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.

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

Cisternal segement

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

Petroclival segment

Cavernous segment

Petroclival segment

Cisternal segment

MC

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

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.

Cn IV CN V1

Cn VI

Cn III

Cn V2

Orbital segment

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

Superior orbital fissure & annular ring of Zinn

Abucens nerve pathology

• Nucleus & central segment

• Cisternal segment

• Petrous segment

• Cavernous segment

• Orbital segment

Aplasia or hypoplasia of the abducens nerve

• Duane syndrome.

• Mobius syndrome.

• HGPPS (Horizontal Gaze Palsy with Progressive Scoliosis).

Duane syndrome

Mobius syndrome

It is a congenital disorder characterized by• Bilateral facial diplegia• Convergent squintSecondary to 6th & 7th cranial nerve palsiesAssociations:Other cranial nerve plasies: 5th, 9th, 10th & 12th

cranial nerves.Craniofacial abnormalites.Chest wall abnormalities.Upper & lower limb abnormalities.

http://upload.wikimedia.org/wikipedia/commons/7/7b/AAS1.jpg

Normal subject Mobius syndrome

Nucleus

• Pontine hemorrhage.

• MS

• glioma

Pontine hemorrhage

MS

Central segment

• The same as nuclear pathology.

Pontine hemorrhage

Cisternal segment

• Duplicted abucens nerve (normal variant)

Duplicated abducens nerve with normal abduction

Diabetic or viral neuropathy

Post-contrast

Post-contrast

Petroclival segment pathology

• Petroclival meningioma.

• Skull base destructive lesion.

• Neuroma.

• Usher syndrome.

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.

Petroclival meningioma

Ganglio-neurofibroma

One & half syndrome• Complete horizontal gaze palsy,

when looking toward the side of the lesion.• Half gaze palsy,

when looking toward the opposite side.

One & half syndrome

Lateral rectus muscle atrophy

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

palsy