Sixth nerve palsy | |
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Other names | Lateral rectus palsy, VIth cranial nerve palsy, abducens nerve palsy |
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Figure showing the mode of innervation of the Recti medialis and lateralis of the eye. | |
Specialty | Neurology |
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. [1] The inability of an eye to turn outward, 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. Diplopia is also increased on looking to the affected side and is partly caused by overaction of the medial rectus on the unaffected side as it tries to provide the extra innervation to the affected lateral rectus. These two muscles are synergists or "yoke muscles" as both attempt to move the eye over to the left or right. The condition is commonly unilateral but can also occur bilaterally. [2]
The unilateral abducens nerve palsy is the most common of the isolated ocular motor nerve palsies. [3]
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 (in this case the esotropic 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 or Lazy eye.
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:
In children, Harley [5] 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). Vallee et al. [6] report that benign and rapidly recovering isolated VIth nerve palsy can occur in childhood, sometimes precipitated by ear, nose and throat infections. [7]
The pathophysiological mechanism of sixth nerve palsy with increased intracranial pressure has traditionally been said to be stretching of the nerve in its long intracranial course, or compression against the petrous ligament or the ridge of the petrous temporal bone. Collier, however, was "unable to accept this explanation", his view being that since the sixth nerve emerges straight forward from the brain stem, whereas other cranial nerves emerge obliquely or transversely, it is more liable to the mechanical effects of backward brain stem displacement by intracranial space occupying lesions. [7]
Isolated lesions of the VI nerve nucleus will not give rise to an isolated VIth nerve palsy because paramedian pontine reticular formation fibers pass through the nucleus to the opposite IIIrd nerve nucleus. Thus, a nuclear lesion will give rise to an ipsilateral gaze palsy. In addition, fibers of the seventh cranial nerve wrap around the VIth nerve nucleus, and, if this is also affected, a VIth nerve palsy with ipsilateral facial palsy will result. In Millard–Gubler syndrome, a unilateral softening of the brain tissue arising from obstruction of the blood vessels of the pons involving sixth and seventh cranial nerves and the corticospinal tract, the VIth nerve palsy and ipsilateral facial paresis occur with a contralateral hemiparesis. [8] Foville's syndrome can also arise as a result of brainstem lesions which affect Vth, VIth and VIIth cranial nerves.[ citation needed ]
As the VIth nerve passes through the subarachnoid space it lies adjacent to anterior inferior and posterior inferior cerebellar and basilar arteries and is therefore vulnerable to compression against the clivus. Typically palsies caused in this way will be associated with signs and symptoms of headache and/or a rise in ICP.
The nerve passes adjacent to the mastoid sinus and is vulnerable to mastoiditis, leading to inflammation of the meninges, which can give rise to Gradenigo's syndrome. This condition results in a VIth nerve palsy with an associated reduction in hearing ipsilaterally, plus facial pain and paralysis, and photophobia. Similar symptoms can also occur secondary to petrous fractures or to nasopharyngeal tumours.
The nerve runs in the sinus body adjacent to the internal carotid artery and oculo-sympathetic fibres responsible for pupil control, thus, lesions here might be associated with pupillary dysfunctions such as Horner's syndrome. In addition, III, IV, V1, and V2 involvement might also indicate a sinus lesion as all run toward the orbit in the sinus wall. Lesions in this area can arise as a result of vascular problems, inflammation, metastatic carcinomas and primary meningiomas.
The VIth nerve's course is short and lesions in the orbit rarely give rise to isolated VIth nerve palsies, but more typically involve one or more of the other extraocular muscle groups.
Differential diagnosis is rarely difficult in adults. Onset is typically sudden with symptoms of horizontal diplopia. Limitations of eye movements are confined to abduction of the affected eye (or abduction of both eyes if bilateral) and the size of the resulting convergent squint or esotropia is always larger on distance fixation - where the lateral recti are more active - than on near fixation - where the medial recti are dominant. Abduction limitations that mimic VIth nerve palsy may result secondary to surgery, to trauma or as a result of other conditions such as myasthenia gravis or thyroid eye disease.
In children, differential diagnosis is more difficult because of the problems inherent in getting infants to cooperate with a full eye movement investigation. Possible alternative diagnosis for an abduction deficit would include:
1. Mobius syndrome - a rare congenital disorder in which both VIth and VIIth nerves are bilaterally affected giving rise to a typically 'expressionless' face.
2. Duane 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.
3. Cross fixation which develops in the presence of infantile esotropia or nystagmus blockage syndrome and results in habitual weakness of lateral recti.
4. Iatrogenic injury. Abducens nerve palsy is also known to occur with halo orthosis placement. The resultant palsy is identified through loss of lateral gaze after application of the orthosis and is the most common cranial nerve injury associated with this device. [9]
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.[ citation needed ]
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.[ citation needed ]
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.[ citation needed ]
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.[ citation needed ]
A Cochrane Review on interventions for eye movement disorders due to acquired brain injury, [10] last updated June 2017, identified one study of botulinum toxin for acute sixth nerve palsy. [11] The Cochrane review authors judged this to be low-certainty evidence; the study was not masked and the estimate of effect was imprecise.
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.[ citation needed ]
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.[ citation needed ]
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. [12] [13] [14] An alternative approach is to operate on both the lateral and medial recti 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.[ citation needed ]
The same approaches are adopted bilaterally where both eyes have been affected.[ citation needed ]
Esotropia is a form of strabismus in which one or both eyes turns inward. The condition can be constantly present, or occur intermittently, and can give the affected individual a "cross-eyed" appearance. It is the opposite of exotropia and usually involves more severe axis deviation than esophoria. Esotropia is sometimes erroneously called "lazy eye", which describes the condition of amblyopia; a reduction in vision of one or both eyes that is not the result of any pathology of the eye and cannot be resolved by the use of corrective lenses. Amblyopia can, however, arise as a result of esotropia occurring in childhood: In order to relieve symptoms of diplopia or double vision, the child's brain will ignore or "suppress" the image from the esotropic eye, which when allowed to continue untreated will lead to the development of amblyopia. Treatment options for esotropia include glasses to correct refractive errors, the use of prisms, orthoptic exercises, or eye muscle surgery. The term is from Greek eso meaning "inward" and trope meaning "a turning".
The abducens nerve or abducent nerve, also known as the sixth cranial nerve, cranial nerve VI, or simply CN VI, is a cranial nerve in humans and various other animals that controls the movement of the lateral rectus muscle, one of the extraocular muscles responsible for outward gaze. It is a somatic efferent nerve.
The trochlear nerve, also known as the fourth cranial nerve, cranial nerve IV, or CN IV, is a cranial nerve that innervates a single muscle - the superior oblique muscle of the eye. Unlike most other cranial nerves, the trochlear nerve is exclusively a motor nerve.
The medial longitudinal fasciculus (MLF) is an area of crossed over tracts, on each side of the brainstem. These bundles of axons are situated near the midline of the brainstem. They are made up of both ascending and descending fibers that arise from a number of sources and terminate in different areas, including the superior colliculus, the vestibular nuclei, and the cerebellum. It contains the interstitial nucleus of Cajal, responsible for oculomotor control, head posture, and vertical eye movement.
Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally or vertically in relation to each other. Also called double vision, it is a loss of visual focus under regular conditions, and is often voluntary. However, when occurring involuntarily, it results in impaired function of the extraocular muscles, where both eyes are still functional, but they cannot turn to target the desired object. Problems with these muscles may be due to mechanical problems, disorders of the neuromuscular junction, disorders of the cranial nerves that innervate the muscles, and occasionally disorders involving the supranuclear oculomotor pathways or ingestion of toxins.
Duane syndrome is a congenital rare type of strabismus most commonly characterized by the inability of the eye to move outward. The syndrome was first described by ophthalmologists Jakob Stilling (1887) and Siegmund Türk (1896), and subsequently named after Alexander Duane, who discussed the disorder in more detail in 1905.
Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms to fixate, inspect and track visual objects of interests. A special type of eye movement, rapid eye movement, occurs during REM sleep.
The lateral rectus muscle is a muscle on the lateral side of the eye in the orbit. It is one of six extraocular muscles that control the movements of the eye. The lateral rectus muscle is responsible for lateral movement of the eyeball, specifically abduction. Abduction describes the movement of the eye away from the midline, allowing the eyeball to move horizontally in the lateral direction, bringing the pupil away from the midline of the body.
The medial rectus muscle is a muscle in the orbit near the eye. It is one of the extraocular muscles. It originates from the common tendinous ring, and inserts into the anteromedial surface of the eye. It is supplied by the inferior division of the oculomotor nerve (III). It rotates the eye medially (adduction).
The inferior oblique muscle or obliquus oculi inferior is a thin, narrow muscle placed near the anterior margin of the floor of the orbit. The inferior oblique is one of the extraocular muscles, and is attached to the maxillary bone (origin) and the posterior, inferior, lateral surface of the eye (insertion). The inferior oblique is innervated by the inferior branch of the oculomotor nerve.
The extraocular muscles, or extrinsic ocular muscles, are the seven extrinsic muscles of the human eye. Six of the extraocular muscles, the four recti muscles, and the superior and inferior oblique muscles, control movement of the eye and the other muscle, the levator palpebrae superioris, controls eyelid elevation. The actions of the six muscles responsible for eye movement depend on the position of the eye at the time of muscle contraction.
The abducens nucleus is the originating nucleus from which the abducens nerve (VI) emerges—a cranial nerve nucleus. This nucleus is located beneath the fourth ventricle in the caudal portion of the pons near the midline, medial to the sulcus limitans.
The one and a half syndrome is a rare weakness in eye movement affecting both eyes, in which one cannot move laterally at all, and the other can move only in outward direction. More formally, it is characterized by "a conjugate horizontal gaze palsy in one direction and an internuclear ophthalmoplegia in the other". Nystagmus is also present when the eye on the opposite side of the lesion is abducted. Convergence is classically spared as cranial nerve III and its nucleus is spared bilaterally.
Hypertropia is a condition of misalignment of the eyes (strabismus), whereby the visual axis of one eye is higher than the fellow fixating eye. Hypotropia is the similar condition, focus being on the eye with the visual axis lower than the fellow fixating eye. Dissociated vertical deviation is a special type of hypertropia leading to slow upward drift of one or rarely both eyes, usually when the patient is inattentive.
Millard–Gubler syndrome is a lesion of the pons. It is also called ventral pontine syndrome.
Conjugate gaze palsies are neurological disorders affecting the ability to move both eyes in the same direction. These palsies can affect gaze in a horizontal, upward, or downward direction. These entities overlap with ophthalmoparesis and ophthalmoplegia.
Oculomotor nerve palsy or oculomotor neuropathy is an eye condition resulting from damage to the third cranial nerve or a branch thereof. As the name suggests, the oculomotor nerve supplies the majority of the muscles controlling eye movements. Damage to this nerve will result in an inability to move the eye normally. The nerve also supplies the upper eyelid muscle and is accompanied by parasympathetic fibers innervating the muscles responsible for pupil constriction. The limitations of eye movement resulting from the condition are generally so severe that patients are often unable to maintain normal eye alignment when gazing straight ahead, leading to strabismus and, as a consequence, double vision (diplopia).
Infantile esotropia is an ocular condition of early onset in which one or either eye turns inward. It is a specific sub-type of esotropia and has been a subject of much debate amongst ophthalmologists with regard to its naming, diagnostic features, and treatment.
Alternating hemiplegia is a form of hemiplegia that has an ipsilateral cranial nerve palsies and contralateral hemiplegia or hemiparesis of extremities of the body. The disorder is characterized by recurrent episodes of paralysis on one side of the body. There are multiple forms of alternating hemiplegia, Weber's syndrome, middle alternating hemiplegia, and inferior alternating hemiplegia. This type of syndrome can result from a unilateral lesion in the brainstem affecting both upper motor neurons and lower motor neurons. The muscles that would receive signals from these damaged upper motor neurons result in spastic paralysis. With a lesion in the brainstem, this affects the majority of limb and trunk muscles on the contralateral side due to the upper motor neurons decussation after the brainstem. The cranial nerves and cranial nerve nuclei are also located in the brainstem making them susceptible to damage from a brainstem lesion. Cranial nerves III (Oculomotor), VI (Abducens), and XII (Hypoglossal) are most often associated with this syndrome given their close proximity with the pyramidal tract, the location which upper motor neurons are in on their way to the spinal cord. Damages to these structures produce the ipsilateral presentation of paralysis or palsy due to the lack of cranial nerve decussation before innervating their target muscles. The paralysis may be brief or it may last for several days, many times the episodes will resolve after sleep. Some common symptoms of alternating hemiplegia are mental impairment, gait and balance difficulties, excessive sweating and changes in body temperature.
The management of strabismus may include the use of drugs or surgery to correct the strabismus. Agents used include paralytic agents such as botox used on extraocular muscles, topical autonomic nervous system agents to alter the refractive index in the eyes, and agents that act in the central nervous system to correct amblyopia.