VI - Abducens Nerve

Question 1

What is the function of the abducens nerve?

Answer 1

One of the nerves responsible for extraocular motor functions of the eye.
It controls the lateral rectus muscle and is responsible for lateral movement of the eye.

Question 2

Where is the abducens located?

Answer 2

The Abducens nucleus is located in the pons
beneath the fourth ventricle in the caudal portion of the pons near the midline, medial to the sulcus limitans.

Question 3

Outline the pathway of the abducens nerve

Answer 3

The abducens nerve arises from the abducens nucleus in the pons of the brainstem. It exits the brainstem at the junction of the pons and the medulla.
It then enters the subarachnoid space and pierces the dura mater to travel in an area known as Dorello’s canal.
At the petrous apex, it angulates to enter the cavernous sinus and travels in close proximity to the internal carotid artery.
The abducens nerve then proceeds through the superior orbital fissure and innervates the lateral rectus muscle.

Question 4

How do you examine the abducens nerve?

Answer 4

It is examined in conjunction with the oculomotor and trochlear nerves by testing the movements of the eye.
The patient is asked to follow a point with their eyes (commonly the tip of a pen) without moving their head.
The target is moved in an ‘H-shape’ and the patient is asked to report any blurring of vision or diplopia (double vision).

Question 5

Describe acquired abducens nerve palsy

Answer 5

Most commonly acquired abducens nerve palsy
• Damage to CN VI results in unopposed activity of the medial rectus which leads to medial deviation of the eye
• Causes: any structural pathology which leads to downwards pressure on the brain stem eg. tumors, aneurysms, fractures, or increased intracranial pressure (ICP)
• Other causes include: diabetic neuropathy and thrombophlebitis of the cavernous sinus
Clinical features: binocular horizontal diplopia (double vision producing a side-by-side image with both eyes open), worse in the distance, and esotropia in primary gaze
• The patient may attempt to compensate by rotating their head to allow the eye to look sideways