Introduction

The abducens nerve (CN VI) is a motor nerve. It supplies the lateral rectus muscle of the eye.

VI nerve palsies are usually sudden in onset.

There is a convergent squint at rest (Fig 1).

The patient complains of horizontal diplopia when looking towards affected side (Fig 2).

With complete paralysis, the eye can’t abduct past the midline (Fig 3).

Fig 1: Convergent squint	Fig 2: Horizontal diplopia	Fig 3: Left CN VI nerve palsy

Anatomy of CN V1

The nerve originates in the pons and then travels in the subarachnoid space.

It runs between the pons and the clivus (part of the sphenoid bone). It then traverses the cavernous sinus where it runs alongside the internal carotid artery, and enters the orbit through the superior orbital fissure.

The medial longitudinal fasciculus connects the three extraocular motor nuclei (CN III, IV and VI), and is responsible for conjugate gaze. Lesions of this fasciculus cause dysfunction of this co-ordination (internuclear ophthlmoplegia).

Testing

Tested as part of eye movements with CN III and IV (Oculomotor and Trochlear).

Investigations

MRI scan is the investigation of choice as it gives greater resolution of the orbits, cavernous sinus, posterior fossa and cranial nerves.

Additionally, CT head for trauma may reveal temporal bone fractures.

Medical

The most common cause (25%) of isolated sixth nerve palsy is idiopathic.

The next most common causes are diabetes and hypertension, which cause ischaemic vasculopathy (see nerve III).

Other causes are pontine stroke and demyelination. Both of these can involve nerves V, VI and VII together, due to the close proximity of their nuclei in the brainstem.

Wernicke’s encephalopathy and giant cell arteritis are associated with sixth nerve palsies.

Tumours are another cause.

Isolated sixth nerve palsy in children is presumed due to brain tumour until proven otherwise.

Cerebellopontine angle tumours can involve the sixth nerve. More information can be found in the subsequent learning session on Cranial Nerves 7 to 12.

Basilar artery aneurysm can compress the nerve in the subarachnoid space.

Infective causes are subacute meningitis, tuberculosis and infections in the cavernous sinus.

Trauma

CN VI palsies are commonly associated with trauma (up to one-third of cases). The severity of the trauma can be relatively mild.

In severe trauma, or other causes of a space occupying lesion, the abducens nerve can be stretched against the clivus as the brainstem herniates through the foramen. Classically this produces papilloedema and bilateral sixth nerve palsies.

Fig 4: Large hyperdense extradural haemorrhage causing midline shift (click on image to enlarge)	Fig 5: Papilloedema. Image courtesy of J Comm Eye Health 2003, 16(46) p. 27