Sixth nerve palsy, or abducens nerve palsy, is a disorder associated with dysfunction of cranial nerve VI (the abducens nerve), which is responsible for causing contraction of the lateral rectus muscle to abduct (i.e., turn out) the eye. The inability of an eye to turn outward and results in a convergent strabismus or esotropia of which the primary symptom is diplopia (commonly known as double vision) in which the two images appear side-by-side. Thus the diplopia is horizontal and worse in the distance.SIGNS AND SYMPTOMS
The nerve dysfunction induces esotropia, a convergent squint on distance fixation. On near fixation the affected individual 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 turned 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 neuroplasticity 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.CAUSE
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.
The most common causes of VIth nerve palsy in adults are:
More common: Vasculopathic (diabetes, hypertension, atherosclerosis), trauma, idiopathic.
Less common: Increased intracranial pressure, giant cell arteritis, cavernous sinus mass (e.g. meningioma, Brain stem Glioblastoma aneurysm, metastasis), multiple sclerosis, sarcoidosis/vasculitis, postmyelography, lumbar puncture, stroke (usually not isolated), Chiari Malformation, hydrocephalus, intracranial hypertension, tuberculosis meningitis.
In children, Harley reports typical causes as traumatic, neoplastic (most commonly brainstem glioma), as well as idiopathic. Sixth nerve palsy causes the eyes to deviate inward (see: Pathophysiology of strabismus). report that benign and rapidly recovering isolated VIth nerve palsy can occur in childhood, sometimes precipitated by ear, nose and throat infections.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 patient's 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 6 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.
Other management options at this initial stage include the use of botulinum toxin, which is injected into the ipsilateral medial rectus (botulinum toxin therapy of strabismus). The use of BT serves a number of purposes. Firstly, it helps to prevent the contracture of the medial rectus which might result from its acting unopposed for a long period. Secondly, by reducing the size of the deviation temporarily it might allow prismatic correction to be used where this was not previously possible, and, thirdly, by removing the pull of the medial rectus it may serve to reveal whether the palsy is partial or complete by allowing any residual movement capability of the lateral rectus to operate. Thus, the toxin works both therapeutically, by helping to reduce symptoms and enhancing the prospects for fuller ocular movements post-operatively, and diagnostically, by helping to determine the type of operation most appropriate for each patient.
A Cochrane Review on interventions for eye movement disorders due to acquired brain injury, last updated June 2017, identified one study of botulinum toxin for acute sixth nerve palsy. The Cochrane review authors judged this to be low-certainty evidence; the study was not masked and the estimate of effect was imprecise.LONGER TERM MANAGEMENT
If adequate recovery has not occurred after the 6 month period (during which observation, prism management, occlusion, or botulinum toxin may be considered), surgical treatment is often recommended.
If the residual esotropia is small, or if the patient is unfit or unwilling to have surgery, prisms can be incorporated into their glasses to provide more permanent symptom relief. When 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.SURGERY
The procedure chosen will depend upon the degree to which any function remains in the affected lateral rectus. Where there is complete paralysis, the preferred option is to perform vertical muscle transposition procedures such as Jensen's, Hummelheim's or whole muscle transposition, with the aim of using the functioning inferior and superior recti to gain some degree of abduction. An alternative approach is to operate on both the lateral and medial rectii of the affected eye, with the aim of stabilising it at the midline, thus giving single vision straight ahead but potentially diplopia on both far left and right gaze. This procedure is often most appropriate for those with total paralysis who, because of other health problems, are at increased risk of the anterior segment ischaemia associated with complex multi-muscle transposition procedures.
Where some function remains in the affected eye, the preferred procedure depends upon the degree of development of muscle sequelae. In a sixth nerve palsy one would expect that, over the 6 month observation period, most patients would show the following pattern of changes to their ocular muscle actions: firstly, an overaction of the medial rectus of the affected eye, then an overaction of the medial rectus of the contraletral eye and, finally, an underaction of the lateral rectus of the unaffected eye - something known as an inhibitional palsy. These changes serve to reduce the variation in the misalignment of the two eyes in different gaze positions (incomitance). Where this process has fully developed, the preferred option is a simple recession, or weakening, of the medial rectus of the affected eye, combined with a resection, or strengthening, of the lateral rectus of the same eye. However, where the inhibitional palsy of the contralateral lateral rectus has not developed, there will still be gross incomitance, with the disparity between the eye positions being markedly greater in the field of action of the affected muscle. In such cases recession of the medial rectus of the affected eye is accompanied by recession and/or posterior fixation (Fadenoperation) of the contraleral medial rectus.
The same approaches are adopted bilaterally where both eyes have been affected.https://en.wikipedia.org/wiki/Sixth_nerve_palsy