Cranial nerve VI, also known as the abducens nerve, innervates the ipsilateral lateral rectus (LR), which functions to abduct the ipsilateral eye. The sixth cranial nerve has a long subarachnoid course. The sixth nerve nucleus is located in the pons, just ventral to the floor of the fourth ventricle and just lateral to the medial longitudinal fasciculus (MLF). About 40% of its neurons project into the ipsilateral MLF only to cross over to the contralateral side and ascend to innervate that contralateral medial rectus subnucleus to participate in contralateral eye adduction.[1, 2, 3]
The abducens nerve emerges from the brainstem at the pontomedullary junction to enter the subarachnoid space, coursing upward between the pons and clivus to enter the Dorello canal. At the petrous apex, it angulates to enter the cavernous sinus and travels in close proximity to the internal carotid artery. The abduces nerve then proceeds through the superior orbital fissure and innervates the lateral rectus muscle.
Patients usually present with binocular horizontal diplopia (double vision producing a side-by-side image with both eyes open), worse in the distance, and esotropia in primary gaze. Patients also may present with a head-turn to maintain binocularity and binocular fusion and to minimize diplopia. Congenital sixth nerve palsy (Duane syndrome) is a well-recognized entity.
Examination for a sixth nerve palsy involves documenting the presence or absence of papilledema, examining the ocular motility, evaluating the eyelids and pupils, and excluding involvement of other cranial nerves (eg, V, VII, VIII).
Occasionally checking deep tendon reflexes (DTRs) and motor function to exclude corticospinal tract involvement may be important. MRI is indicated for any brainstem findings to exclude pontine glioma in children (most have papilledema and nystagmus without other cranial nerve involvement) and in adults who show no improvement.
In young adults, a lumbar puncture (LP) for cerebrospinal fluid (CSF) analysis is completed to exclude meningitis in patients who have no history of diabetes or hypertension and who have a head scan negative for other pathology. Elderly patients should undergo blood testing for erythrocyte sedimentation rate (ESR), C-reactive protein, and platelets to screen for giant cell arteritis (temporal arteritis). Poor or no resolution of sixth nerve palsy should prompt a full neurologic evaluation.
Only the ipsilateral lateral rectus that is solely innervated by the involved peripheral sixth cranial nerve is affected; therefore, only deviations in the horizontal plane are produced. In isolated cases of peripheral nerve lesions, no vertical or torsional deviations are present. Central nervous system lesions of the abducens nerve tract are localized easily secondary to the typical findings associated with each kind of lesion. Damage to the sixth nerve nucleus results in an ipsilateral gaze palsy. The lack of a contralateral adduction defect makes it easy to differentiate a nuclear lesion from a fascicular or nonnuclear lesion.[2]
Abducens palsy can be a false localizing sign with lesions that cause increased intracranial pressure and stretching of the sixth nerve as it ascends the clival area.
Abducens nerve palsy is frequently seen as a postviral syndrome in younger patients and as an ischemic mononeuropathy in the adult population.
Sixth nerve palsies fall into the following categories: 3%-30% trauma, 0%-6% aneurysm, 0%-36% ischemic, 8%-30% idiopathic, and 10%-30% demyelination/miscellaneous.
The sixth cranial nerve is the most commonly affected of the ocular motor nerves. In children, it is the second most common after the fourth nerve, with an incidence of 2.5 cases per 100,000 in the population.
Mortality/Morbidity
A young patient should have an aggressive workup because of the greater likelihood of a neoplasm causing the palsy. Patients older than 55 years with isolated sixth nerve palsies may require a less aggressive initial workup if they have predisposing microvascular ischemic risk factors, but no history of cancer.
Age
Cranial nerve VI palsy can occur in all age groups; however, the etiology varies depending on the age group.
Physical findings of abducens nerve palsy include the following:
An esodeviation that increases on ipsilateral gaze and is often greater at a distance; prism measurements in different positions of gaze can reveal the magnitude of misalignment and its incomitance (asymmetry)
An isolated abduction deficit
Slowed ipsilateral saccades
Papilledema (if increased intracranial pressure)
Altered sensation in the V1 or V2 distribution with cavernous sinus lesions
Nystagmus (usually in children, ie, secondary to pontine glioma)
Otitis media
Petrous bone fracture
Tender, enlarged, nonpulsatile temporal arteries in giant cell arteritis
Not all abduction deficits are cranial nerve VI palsies. Mimickers are orbital lesions, medial wall fractures, Duane syndrome, thyroid-associated orbitopathy, myasthenia gravis, and spasm of the near reflex.[4]
Elevated intracranial pressure can result in downward displacement of the brainstem, causing stretching of the sixth nerve secondary to its anatomic location within the Dorello canal. This is believed to be the reason that about 30% of patients with pseudotumor cerebri have an isolated abducens nerve palsy and explains how lesions remote from the sixth cranial nerve can cause abducens paresis (false localizing sign).
Subarachnoid space lesions can be causes of abducens nerve palsy (eg, hemorrhage, infection, inflammation, space-occupying tumor, cavernous sinus mass). Inflammatory (eg, postviral, demyelinating, sarcoid, giant cell arteritis)
Patients younger than 55 years with no vasculopathic history
Associated pain or other neurologic abnormality[11]
History of cancer
Bilateral sixth nerve palsy
Papilledema
In the event no marked improvement is seen or other nerves become involved
An LP should be considered if MRI results are negative.
If a presumed microvascular ischemic sixth nerve palsy does not improve within 3-4 months or if other cranial nerves become involved, a full medical, neurologic, and imaging workup should be performed.
Truly isolated cases of abducens nerve palsy are often benign. They can be followed with a serial examination, at least every 6 weeks, over a 6-month period to note decreasing symptoms (diplopia) and resolution of the paretic lateral rectus (increasing motility).[12, 3] Prism measurements are a simple objective method of documenting any changes in the esotropia.
Children with sixth nerve palsy who are in the amblyopic age group can be treated with an alternating patching to decrease their chances of developing any amblyopia in the paretic eye. Additionally, prescribing the full amount of hyperopic correction helps to decrease the esodeviation by relaxing the child's accommodative effort.
Adult patients and those children beyond the amblyopic age can be patched or have their lenses "fogged" with clear tape or nail polish to reduce their diplopia. Fresnel prisms also can be prescribed as an alternative.
Older patients in whom giant cell arteritis is suspected should start the standard treatment with prednisone or intravenous methylprednisolone.
If, after 9-12 months of follow-up care, the remaining deviation is still unacceptable and is too large to be corrected with prisms, surgical corrective options should be discussed with the patient. The procedure that is chosen depends on the remaining function of the lateral rectus and the experience of the surgeon.
If some residual function exists in the lateral rectus, a graded recession of the medial rectus or botulinum toxin to the medial rectus, and resection of the lateral rectus or lateral rectus bupivacaine (Marcaine) injection can be performed.
When little or no residual function is present, a transposition of the vertical recti toward the lateral rectus (eg, Hummelsheim, Jensen, or Nishida procedure), can be considered in conjunction with weakening of the ipsilateral medial rectus.
Patients with abducens palsy can benefit from consultation with a neurologist, ophthalmologist, or neuro-ophthalmologist, especially if the lesion does not resolve.
Patients who occlude an eye to alleviate diplopia should be warned that the resulting effects on depth perception may interfere with their ability to drive or perform certain occupations safely.
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Michael P Ehrenhaus, MD, Director, Department of Cornea, External Disease & Refractive Surgery, Assistant Professor, Department of Ophthalmology, State University of New York Downstate Medical Center
Disclosure: Nothing to disclose.
Coauthor(s)
Mohammedyusuf E Hajee, MD, Clinical Instructor, Staff Physician, Department of Ophthalmology, Director, Blood Flow Laboratory, State University of New York-Downstate Medical Center
Disclosure: Serve(d) as a director, officer, partner, employee, advisor, consultant or trustee for: Allergan<br/>Received research grant from: Alcon, novartis.
Specialty Editors
Simon K Law, MD, PharmD, Clinical Professor of Health Sciences, Department of Ophthalmology, Jules Stein Eye Institute, University of California, Los Angeles, David Geffen School of Medicine
Disclosure: Nothing to disclose.
Chief Editor
Edsel Ing, MD, MPH, FRCSC, Associate Professor, Department of Ophthalmology and Vision Sciences, University of Toronto Faculty of Medicine; Active Staff, Michael Garron Hospital (Toronto East Health Network); Consulting Staff, Hospital for Sick Children and Sunnybrook Hospital, Canada
Disclosure: Nothing to disclose.
Additional Contributors
Andrew W Lawton, MD, Neuro-Ophthalmology, Ochsner Health Services
Disclosure: Nothing to disclose.
Hampton Roy, Sr, MD, Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences
Disclosure: Nothing to disclose.
Acknowledgements
Brian R Younge, MD Professor of Ophthalmology, Mayo Clinic School of Medicine
Brian R Younge, MD is a member of the following medical societies: American Medical Association, American Ophthalmological Society, and North American Neuro-Ophthalmology Society
Disclosure: Nothing to disclose.
Acknowledgments
The authors and editors of Medscape Reference gratefully acknowledge the assistance of Ryan I Huffman, MD, with the literature review and referencing for this article.
References
Evans NM. Ophthalmology. 2nd ed. Oxford University Press Inc; 1995.