The Abducens Nerve (CN VI)
The abducens nerve is the sixth paired cranial nerve. It has a purely somatic motor function. In this article, the anatomical course, motor functions and clinical relevance of the nerve will be examined.
The abducens nerve arises from the abducens nucleus in the pons of the brain, and exits the brainstem at the junction of the pons and the medulla.
It then enters the subarachnoid space and pierces the dura mater to run in a space known as Dorello’s canal. The nerve travels through the cavernous sinus at the tip of the petrous temporal bone, before entering the orbit of the eye through the superior orbital fissure. Within the bony orbit, the abducens nerve terminates by innervating the lateral rectus muscle.
The abducens nerve innervates a single muscle – the lateral rectus, one of the muscles of oculomotion. This muscle takes its origin from the common tendinous ring, and acts to abduct the eyeball (i.e. to rotate the gaze away from the midline).
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 (commonly the tip of a pen) with their eyes 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).
Palsy of the Abducens Nerve
Any pathology which leads to downward pressure on the brainstem (e.g. brain tumour, extradural haematoma) can lead to the nerve becoming stretched along the clivus of the skull. Wernicke-Korsakoff syndrome (caused by thiamine deficiency and generally seen in alcoholics) is a rare cause of sixth nerve palsy.
Other causes of abducens nerve damage include diabetic neuropathy and thrombophlebitis of the cavernous sinus – in these cases, it is rare for the abducens nerve to be affected in isolation.
Patients will present with diplopia and a medially rotated eye which cannot be abducted past the midline. The patient may attempt to compensate by rotating their head to allow the eye to look sideways.