The Abducens Nerve (CN VI)
The abducens nerve is the sixth paired cranial nerve. It has a purely somatic motor function – providing innervation to the lateral rectus muscle.
In this article, we shall look at the anatomy of the abducens nerve – its anatomical course, motor functions and clinical relevance.
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 tip of petrous temporal bone, the abducens nerve leaves Dorello’s canal and enters the cavernous sinus (a dural venous sinus). It travels through the cavernous sinus and enters the bony orbit via the superior orbital fissure.
Within the bony orbit, the abducens nerve terminates by innervating the lateral rectus muscle.
The abducens nerve provides innervation to the lateral rectus muscle – one of the extraocular muscles.
The lateral rectus originates from the lateral part of the common tendinous ring, and attaches to the anterolateral aspect of the sclera. It acts to abduct the eyeball (i.e. to rotate the gaze away from the midline).
Clinical Relevance – Examination of the Abducens Nerve
The abducens nerve 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).
Clinical Relevance – Abducens Nerve Palsy
Abducens nerve palsy can be caused by any structural pathology which leads to downwards pressure on the brainstem (e.g. space-occupying lesion). This can stretch the nerve from its origin at the junction of the pons and medulla.
Other causes include diabetic neuropathy and thrombophlebitis of the cavernous sinus (in these cases, it is rare for the abducens nerve to be affected in isolation).
Clinical features of abducens nerve palsy include diplopia, the affected eye resting in adduction (due to unopposed activity of the medial rectus), and inability to abduct the eye. The patient may attempt to compensate by rotating their head to allow the eye to look sideways.