Sixth nerve palsy
Encyclopedia
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 .
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
.
, 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.
In children, Harley reports typical etiologies as traumatic, neoplastic (most commonly brainstem glioma), as well as idiopathic
. Vallee et al. report that benign and rapidly recovering isolated VIth nerve palsy can occur in childhood, sometimes precipitated by ear, nose and throat infections.
Isolated lesions of the VI nerve nucleus will not give rise to an isolated VIth neve palsy because paramedian pontine reticular formation fiber
s 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.. Foville's syndrome
can also arise as a result of brainstem lesions which affect Vth, VIth and VIIth cranial nerves.
2. Sub arachnoid space
As the VIth nerve passes through this 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.
3. Petrous Apex
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.
4. Cavernous sinus/Superior orbital fissure
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.
5. Orbit
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.
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'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.
3. Cross fixation which develops in the presence of infantile esotropia
or nystagmus blockage syndrome and results in habitual weakness of lateral rectii.
, 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.
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. 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.
1. 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.
2. 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, and less satisfactory, 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 diplopia on both left and right gaze. This procedure is rarely used, but might be 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 sequlae. In a sixth nerve palsy one would expect that, over the 9 to 12 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 fadenisation of the contraleral medial rectus.
The same approaches are adopted bilaterally where both eyes have been affected.
Lateral rectus muscle
The lateral rectus muscle is a muscle in the orbit. It is one of six extraocular muscles that control the movements of the eye and the only muscle innervated by the abducens nerve, cranial nerve VI....
to abduct
Abduction (kinesiology)
Abduction, in functional anatomy, is a movement which draws a limb away from the median plane of the body. It is thus opposed to adduction.-Upper limb:* of arm at shoulder ** Supraspinatus** Deltoid* of hand at wrist...
(i.e., turn out) the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
. The inability of an eye to turn outward results in a convergent strabismus or esotropia
Esotropia
Esotropia is a form of strabismus, or "squint", 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...
of which the primary symptom is double vision
Diplopia
Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...
or diplopia
Diplopia
Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...
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 .
Alternative names
- VIth nerve palsy
- Lateral rectus palsy
- Cranial mononeuropathy VI
- Bilateral 6 nerve palsy
Characteristics
Esotropia
Esotropia is a form of strabismus, or "squint", 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...
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
Esotropia
Esotropia is a form of strabismus, or "squint", 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...
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
Diplopia
Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...
is typically experienced by adults with VI nerve palsies, but children with the condition may not experience diplopia
Diplopia
Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...
due to suppression
Suppression
The term suppression may refer to:* Oppression, the exercise of authority or power in a burdensome, cruel, or unjust manner, also an act or instance of oppressing...
. 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
Amblyopia
Amblyopia, also known as lazy eye, is a disorder of the visual system that is characterized by a vision deficiency in an eye that is otherwise physically normal, or out of proportion to associated structural abnormalities of the eye...
.
Causes
Because the nerve emerges near the bottom of the brainBrain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
, it is often the first nerve compressed when there is any rise in intracranial pressure
Intracranial pressure
Intracranial pressure is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid . The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF...
. 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.
General Etiology
In general terms, the most common causes of VIth nerve palsy in adults are:- More common: Vasculopathic (diabetes, hypertensionHypertensionHypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...
, atherosclerosisAtherosclerosisAtherosclerosis is a condition in which an artery wall thickens as a result of the accumulation of fatty materials such as cholesterol...
), traumaPhysical traumaTrauma refers to "a body wound or shock produced by sudden physical injury, as from violence or accident." It can also be described as "a physical wound or injury, such as a fracture or blow." Major trauma can result in secondary complications such as circulatory shock, respiratory failure and death...
, idiopathicIdiopathicIdiopathic is an adjective used primarily in medicine meaning arising spontaneously or from an obscure or unknown cause. From Greek ἴδιος, idios + πάθος, pathos , it means approximately "a disease of its own kind". It is technically a term from nosology, the classification of disease...
. - Less common: Increased intracranial pressureIntracranial pressureIntracranial pressure is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid . The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF...
, giant cell arteritis, cavernous sinus mass (e.g. meningiomaMeningiomaThe word meningioma was first used by Harvey Cushing in 1922 to describe a tumor originating from the meninges, the membranous layers surrounding the CNS ....
, Brain stem Giloblastoma aneurysmAneurysmAn aneurysm or aneurism is a localized, blood-filled balloon-like bulge in the wall of a blood vessel. Aneurysms can commonly occur in arteries at the base of the brain and an aortic aneurysm occurs in the main artery carrying blood from the left ventricle of the heart...
, metastasisMetastasisMetastasis, or metastatic disease , is the spread of a disease from one organ or part to another non-adjacent organ or part. It was previously thought that only malignant tumor cells and infections have the capacity to metastasize; however, this is being reconsidered due to new research...
), multiple sclerosisMultiple sclerosisMultiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...
, sarcoidosisSarcoidosisSarcoidosis , also called sarcoid, Besnier-Boeck disease or Besnier-Boeck-Schaumann disease, is a disease in which abnormal collections of chronic inflammatory cells form as nodules in multiple organs. The cause of sarcoidosis is unknown...
/vasculitisVasculitisVasculitis refers to a heterogeneous group of disorders that are characterized by inflammatory destruction of blood vessels. Both arteries and veins are affected. Lymphangitis is sometimes considered a type of vasculitis...
, postmyelography or lumbar punctureLumbar punctureA lumbar puncture is a diagnostic and at times therapeutic procedure that is performed in order to collect a sample of cerebrospinal fluid for biochemical, microbiological, and cytological analysis, or very rarely as a treatment to relieve increased intracranial pressure.-Indications:The...
, strokeStrokeA stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
(usually not isolated).
In children, Harley reports typical etiologies as traumatic, neoplastic (most commonly brainstem glioma), as well as idiopathic
Idiopathic
Idiopathic is an adjective used primarily in medicine meaning arising spontaneously or from an obscure or unknown cause. From Greek ἴδιος, idios + πάθος, pathos , it means approximately "a disease of its own kind". It is technically a term from nosology, the classification of disease...
. Vallee et al. report that benign and rapidly recovering isolated VIth nerve palsy can occur in childhood, sometimes precipitated by ear, nose and throat infections.
Localizing Signs
1. BrainstemIsolated lesions of the VI nerve nucleus will not give rise to an isolated VIth neve palsy because paramedian pontine reticular formation fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
s 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.. Foville's syndrome
Foville's syndrome
Foville's syndrome is caused by the blockage of the perforating branches of the basilar artery in the region of the brainstem known as the pons....
can also arise as a result of brainstem lesions which affect Vth, VIth and VIIth cranial nerves.
2. Sub arachnoid space
As the VIth nerve passes through this space it lies adjacent to anterior inferior and posterior inferior cerebellar and basilar arteries and is therefore vulnerable to compression against the clivus
Clivus (anatomy)
The clivus is a part of the cranium, a shallow depression behind the dorsum sellæ that slopes obliquely backward. It forms a gradual sloping process at the anterior most portion of the basilar occipital bone at its junction with the sphenoid bone. On axial planes, it sits just posterior to the...
. Typically palsies caused in this way will be associated with signs and symptoms of headache and/or a rise in ICP.
3. Petrous Apex
The nerve passes adjacent to the mastoid sinus and is vulnerable to mastoiditis
Mastoiditis
Mastoiditis is an infection of mastoid process, the portion of the temporal bone of the skull that is behind the ear which contains open, air-containing spaces. It is usually caused by untreated acute otitis media and used to be a leading cause of child mortality. With the development of...
, leading to inflammation of the meninges, which can give rise to Gradenigo's syndrome
Gradenigo's syndrome
Gradenigo's syndrome, also called Gradenigo-Lannois syndrome and petrous apicitis, is a complication of otitis media and mastoiditis involving the apex of the petrous temporal bone.-Symptoms:Symptoms of the syndrome include:...
. 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.
4. Cavernous sinus/Superior orbital fissure
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
Horner's syndrome
Horner's syndrome is the combination of drooping of the eyelid and constriction of the pupil , sometimes accompanied by decreased sweating of the face on the same side; redness of the conjunctiva of the eye is often also present...
. 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.
5. Orbit
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 diagnoses
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 rectii are more active - than on near fixation - where the medial rectii are dominant. Abduction limitations which mimic VIth nerve palsy may result secondary to surgery, to trauma or as a result of other conditions such as myasthenia gravisMyasthenia gravis
Myasthenia gravis is an autoimmune neuromuscular disease leading to fluctuating muscle weakness and fatiguability...
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
Mobius syndrome
Möbius syndrome is an extremely rare congenital neurological disorder which is characterized by facial paralysis and the inability to move the eyes from side to side. Most people with Möbius syndrome are born with complete facial paralysis and cannot close their eyes or form facial expressions...
- a rare congenital disorder in which both VIth and VIIth nerves are bilaterally affected giving rise to a typically 'expressionless' face.
2. 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.
3. Cross fixation which develops in the presence of infantile esotropia
Infantile esotropia
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.- Name and features :Historically...
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 diplopiaDiplopia
Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...
, 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 amblyopiaAmblyopia
Amblyopia, also known as lazy eye, is a disorder of the visual system that is characterized by a vision deficiency in an eye that is otherwise physically normal, or out of proportion to associated structural abnormalities of the eye...
and because they do not experience diplopia
Diplopia
Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...
.
Other management options at this initial stage include the use of botulinum toxin
Botulinum toxin
Botulinum toxin is a protein produced by the bacterium Clostridium botulinum, and is considered the most powerful neurotoxin ever discovered. Botulinum toxin causes Botulism poisoning, a serious and life-threatening illness in humans and animals...
, which is injected into the ipsilateral medial rectus. 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.
Longer term management
where full recovery has not occurred after the 9 to 12 month 'watch and wait' period, management will take either a 'conservative' or a surgical course.1. 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.
2. 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, and less satisfactory, 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 diplopia on both left and right gaze. This procedure is rarely used, but might be 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 sequlae. In a sixth nerve palsy one would expect that, over the 9 to 12 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 fadenisation of the contraleral medial rectus.
The same approaches are adopted bilaterally where both eyes have been affected.
External links
- Animation at mrcophth.com