Sixth nerve palsy

A sixth cranial nerve palsy refers to dysfunction of the abducens nerve that causes a lateral rectus palsy.

The abducens nerve is the sixth (VI) cranial nerve, which provides innervation to the lateral rectus muscle. This is one of the extra-ocular muscles that causes eye abduction, which is needed for horizontal gaze.

A VI nerve palsy is the most common cranial nerve palsy in isolation. A palsy can develop due to a lesion anywhere along the path of the nerve from pons to lateral rectus muscle.

The abducens nerve innervates the lateral rectus muscle.

Nerve course

The sixth nerve nucleus is located in the dorsal (posterior) pons of the brainstem. The nerve then begins on a long intracranial route.

It exits the brainstem at the pontomedullary junction and enters the subarachnoid space. It subsequently travels over the petrous apex (pyramid-shaped anteromedial part) of the temporal bone and passes through Dorello’s canal. It then passes through the cavernous sinus lateral to the internal carotid artery before entering the orbit via the superior orbital fissure.

Function

The VI nerve has predominantly motor function and provides innervation to the ipsilateral (same-side) lateral rectus muscle that is needed for eye abduction involved in horizontal gaze. In addition, several interneurons travel to the contralateral third nerve nucleus via the medial longitudinal fasciculus (MLF).

Communication between the third and sixth cranial nerves is needed for coordinated conjugate lateral gaze. This means when we look laterally to the left or the right, both eyes move together in a symmetrical manner and prevent diplopia (double vision). Damage to the MLF can result in a conjugate lateral gaze palsy known as intranuclear ophthalmoplegia (INO). For more information see Multiple sclerosis notes.

A lesion at any point during the course of the sixth nerve can cause palsy.

A variety of pathologies can lead to a VI nerve palsy. These are generally divided according to location. Each location has several possible causes and associated clinical manifestations.

Locations

• Pons: commonly ischaemia or neoplasia. Deficiency in thiamine (Vitamin B1) can cause bilateral abducens nerve palsy as part of the syndrome Wernicke’s encephalopathy (although nystagmus is more commonly seen).
• Subarachnoid space: meningeal inflammation (e.g. meningitis), infiltration (e.g. malignancy), increases in intracranial pressure or aneurysm formation can all compress the sixth nerve. An ischaemic mononeuropathy, often due to diabetes mellitus or hypertension, may also affect the nerve at this location (typically isolated)
• Petrous apex: trauma, neoplasia or inflammatory lesions.
• Cavernous sinus: abducens palsy at the cavernous sinus is usually in combination with other nerves that run through the sinus. Pathologies include thrombosis, infections, aneurysm/dissection (carotid artery) and pituitary adenoma.
• Orbital lesions: typically due to neoplasia, infections or inflammatory conditions. May cause a global syndrome (e.g. orbital apex syndrome or superior orbital fissure syndrome).

Raised intracranial pressure

Increases in intracranial pressure may result in unilateral or bilateral abducens nerve palsies. This is because of the long intracranial course of the VI nerve, which is particularly prone to compression along the skull base.

Adults versus children

Particularly among older adults, an isolated abducens nerve palsy is commonly due to ischaemic mononeuropathy, which occurs in relation to diabetes mellitus and/or hypertension. In children, an abducens nerve palsy is more commonly related to tumours, trauma, an increase in ICP or a congenital lesion.

Unilateral versus bilateral

A sixth nerve palsy may affect one or both of the paired cranial nerves. Bilateral sixth nerve palsies are more likely to occur in global pathologies such as raised ICP or trauma.

Isolated versus nonisolated

An isolated sixth nerve palsy refers to the absence of additional neurological deficits. On the contrary, a nonisolated sixth nerve palsy refers to the presence of additional neurological deficits. Characteristic neurological features in a nonisolated sixth nerve palsy are often specific to a region such as the cavernous sinus (e.g. cavernous sinus thrombosis) or orbit (e.g. orbital apex syndrome).

A sixth nerve palsy is characterised by failure in eye abduction leading to horizontal diplopia.

Symptoms

• Diplopia: worse on horizontal gaze in direction of lesion
• Blurred vision
• Dizziness
• Eye pain

Signs

• Failed eye abduction: ipsilateral to the lesion (unilateral or bilateral)
• Strabismus: abnormal eye alignment. May only be present on lateral gaze initially

Accompanying features

These may be seen in patients with a nonisolated sixth nerve palsy.

• Brainstem signs (e.g. hemiparesis, hemisensory loss, dysarthria/dysphagia)
• Raised ICP (e.g. headache, nausea, papilloedema)
• Orbital signs: proptosis (eye protrusion), chemosis (conjunctival swelling), visual changes
• Additional cranial nerve involvement: lesions of III, IV, VI, V1 may be seen in orbital apex syndrome or cavernous sinus pathology

Identification of a sixth nerve palsy is a clinical diagnosis.

A sixth nerve palsy is usually easily recognised at the bedside based on the presence of diplopia and failed eye abduction. Further investigations are warranted to determine the underlying cause.

Neuroimaging

This is the investigation of choice for a sixth nerve palsy. Magnetic resonance imaging (MRI) is the modality of choice, but a computed tomography (CT) of the head may be completed initially to exclude major differentials (e.g. tumour, intracerebral haemorrhage) whilst awaiting an MRI.

Neuroimaging is needed to exclude the majority of causes of a sixth nerve palsy. The urgency of imaging depends on the cause. In adults, a sixth nerve palsy is commonly isolated due to microvascular damage to the nerve (i.e. ischaemic mononeuropathy). In these cases, the need for urgent imaging of the head is less pressing.

Other investigations

Additional investigations may be required depending on the suspected cause. Examples include ESR/CRP in suspected giant cell arteritis or lumbar puncture in suspected idiopathic intracranial hypertension.

The treatment of a sixth nerve palsy is directed towards the underlying cause.

There are numerous causes of a sixth nerve palsy and treatment should be directed towards the underlying cause. This is particularly relevant for infectious, inflammatory of neoplastic lesions, whereby treatment may alleviate diplopia.

In patients with an isolated, unilateral, nontraumatic sixth nerve palsy (usually due to microvascular disease), spontaneous recovery is common. This is also true for traumatic sixth nerve palsies.

In patients with persistent symptoms, or those unlikely to recover (e.g. bilateral at presentation or complete palsy), treatments can include patching (covering one eye to prevent binocular diplopia), use of prisms, strabismus surgery or even botox injections. Patients should be referred to an ophthalmologist.

Last updated: July 2021