Sixth Nerve Palsy

Sixth nerve palsy, or abducens nerve palsy, is a disorder associated with dysfunction of cranial nerve VI (the abducens nerve), which is responsible for contracting the lateral rectus muscle to abduct (i.e., turn out) the eye. The inability of an eye to turn outward results in a convergent strabismus or esotropia of which the primary symptom is double vision or diplopia in which the two images appear side-by-side. The condition is commonly unilateral but can also occur bilaterally.



The unilateral abducens nerve palsy is the most common of the isolated ocular motor nerve palsies [1].

Alternative names

VIth nerve palsy

Lateral rectus palsy

Cranial mononeuropathy VI

Characteristics

The affected individual will have an esotropia or convergent squint on distance fixation. On near fixation the affected indiviual may have only a latent deviation and be able to maintain binocularity or have an esotropia of a smaller size. Patients sometimes adopt a face turn towards the side of the affected eye, moving the eye away from the field of action of the affected lateral rectus muscle, with the aim of controlling diplopia and maintaining binocular vision.



Diplopia is typically experienced by adults with VI nerve palsies, but children with the condition may not experience diplopia due to suppression. The neural plasticity present in childhood allows the child to 'switch off' the information coming from one eye, thus relieving any diplopic symptoms. Whilst this is a positive adaptation in the short term, in the long term it can lead to a lack of appropriate development of the visual cortex giving rise to permanent visual loss in the suppressed eye; a condition known as amblyopia.

Causes

Because the nerve emerges near the bottom of the brain, it is often the first nerve compressed when there is any rise in intracranial pressure. Different presentations of the condition, or associations with other conditions, can help to localize the site of the lesion along the VIth cranial nerve pathway.

Differential diagnoses

Mobius syndrome - a rare congenital disorder in which both VIth and VIIth nerves are bilaterally affected giving rise to a typically 'expressionless' face.

Duane's syndrome - A condition in which both abduction and adduction are affected arising as a result of partial innervation of the lateral rectus by branches from the IIIrd oculomotor cranial nerve.

Cross fixation which develops in the presence of infantile esotropia or nystagmus blockage syndrome and results in habitual weakness of lateral rectii.


Management

The first aims of management should be to identify and treat the cause of the condition, where this is possible, and to relieve the patients symptoms, where present. In children, who rarely appreciate diplopia, the aim will be to maintain binocular vision and, thus, promote proper visual development.

Thereafter, a period of observation of around 9 to 12 months is appropriate before any further intervention, as some palsies will recover without the need for surgery.

Symptom relief and/or binocular vision maintenance

This is most commonly achieved through the use of fresnel prisms. These slim flexible plastic prisms can be attached to the patient's glasses, or to plano glasses if the patient has no refractive error, and serve to compensate for the inward misalignment of the affected eye. Unfortunately, the prism only correct for a fixed degree of misalignment and, because the affected individual's degree of misalignment will vary depending upon their direction of gaze, they may still experience diplopia when looking to the affected side.



The prisms are available in different strengths and the most appropriate one can be selected for each patient. However, in patients with large deviations, the thickness of the prism required may reduce vision so much that binocularity is not achievable. In such cases it may be more appropriate simply to occlude one eye temporarily. Occlusion would never be used in infants though both because of the risk of inducing stimulus deprivation amblyopia and because they do not experience diplopia.

Longer term management

Conservative management

Where the residual esotropia is small and there is a risk of surgical overcorrection, or where the patient is unfit or unwilling to have surgery, prisms can be incorporated into their glasses to provide more permanent symptom relief. Where the deviation is too large for prismatic correction to be effective, permanent occlusion may be the only option for those unfit or unwilling to have surgery