6th Cranial Nerve Palsy
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Title: 6th Cranial Nerve Palsy
Authors: Maxwell Mayeda MSIV University of Colorado School of Medicine, Griffin Jardine MD
Photographer: Griffin Jardine MD
Date: 9/8/2022
Keywords/Main Subjects: 6th cranial nerve palsy, abducens nerve, strabismus, esotropia
Diagnosis: 6th cranial nerve palsy
Video:
Summary of the Case:
The 6th cranial nerve (CN VI), also known as the abducens nerve, controls the lateral rectus muscle that is responsible for abducting the eye. The nerve originates at the pons and travels in the subarachnoid space before passing through Dorello’s Canal in the skull, where it is especially susceptible to stretching forces from increased intracranial pressure causing downward displacement of the brainstem. It then travels through the cavernous sinus and enters the orbit through the superior orbital fissure. It has the longest intracranial course of any cranial nerve.1
A palsy of CN VI will cause weakness of the lateral rectus muscle, resulting in a horizontal diplopia. The eye will turn inward and present with an incomitant esotropia that is worse with distance vision and when looking towards the affected side. It is important to try and localize the lesion in a sixth nerve palsy, which includes a careful examination of eye motility and anatomically neighboring cranial nerves. The nucleus of CN VI in the pons contains interneurons that travel to the contralateral medial rectus, responsible for conjugate gaze. A lesion at this level may will also result in a conjugate gaze palsy.2 Cranial nerves VI and VII also neighbor each other in the pons. Accompanying ipsilateral upper and lower facial weakness form an affected cranial nerve VII can also accompany a lesion in the pons. CN VI travels with the third, fourth, and fifth cranial nerves and the internal carotid artery in the cavernous sinus. Lesions at this level may affect one of these other structures.3
Abducens nerve palsy has a wide range of differential diagnoses. The most common causes in adults are idiopathic, neoplasm, trauma, and microvascular ischemia.4 Strokes and metabolic diseases like Wernicke will affect the nerve at the pons. Increased intracranial pressure from masses, sinus thromboses, hydrocephalus, pseudotumor cerebri, infection, and other etiologies can also lead to palsy. Demyelinating diseases like multiple sclerosis and inflammatory conditions including giant cell arteritis and sarcoidosis have also been implicated.2 Stretching of the nerve in the cavernous sinus from a fistula, thrombosis, or internal carotid aneurysm will also lead to palsy.1
Congenital causes in children include Duane Retraction and Mobius Syndrome. In Duane 1 and 3, co-contraction of the lateral and medial rectus muscles impairs lateral gaze and can often be distinguished from a CN VI palsy by checking for normal eye alignment in primary gaze. Mobius syndrome is a congenital facial diplegia that frequently includes a sixth-nerve palsy.2 Neoplasm is also much more likely in the pediatric population.5
Presenting patients will complain of double vision that is worse at distance and when looking towards the affected side. To minimize this diplopia patients may have their head turned to the affected side. The affected eye will be turned medially and will not be able to abduct. It is important to evaluate for signs of increased cranial pressure (headaches, papilledema, eye pain, nausea) or other cranial nerve involvement to assist in localizing the lesion. Diagnostic testing depends on suspected etiology. Children and young adults should always have neuroimaging due to increased risk of neoplasm.2 Neuroimaging is also useful in trauma. Neuroimaging in older patients (>50 years) with suspected microvascular ischemia is more controversial. One study showed a different etiology was found in 16.5% of older adults and recommended neuroimaging to rule out other causes despite cost inefficiencies.6 Lumbar puncture is recommended in suspected increased intracranial pressure. Further workup with serologies and inflammatory markers can be pursued, however it is important to remember many cases are idiopathic.5
Treatment of CN VI palsy should start with addressing the underlying cause, such as increased intracranial pressure. If this does not lead to correction of the palsy, treatment includes occlusion, prism, and surgical correction. Patching may be indicated in the short-term for patients under the age of 8 to prevent amblyopia. Surgical correction of partial nerve palsy can be considered if esodeviation has been stable for 6-12 months and prism therapy has failed. Options include medial rectus recession and/or lateral rectus resection on the affected eye. In cases of more significant nerve palsy a transposition of the superior and/or inferior rectus temporally may be indicated.2
Format: Video
References:
1. Umansky, F., Valarezo, A. and Elidan, J. (1992), The microsurgical anatomy of the abducens nerve in its intracranial course. The Laryngoscope, 102: 1285-1292.
2. Prasad, Sashank, MD, Volpe, Nicholas J., MD. Paralytic Strabismus: Third, Fourth, and Sixth Nerve Palsy. Neurologic clinics. 2010;28(3):803-833. doi:10.1016/j.ncl.2010.04.001
3. Azarmina M, Azarmina H. The six syndromes of the sixth cranial nerve. J Ophthalmic Vis Res. 2013 Apr;8(2):160-71.
4. Kung NH, Van Stavern GP. Isolated Ocular Motor Nerve Palsies. Semin Neurol. 2015;35(5):539‐548. doi:10.1055/s-0035-1563568
5. Graham C, Mohseni M. Abducens Nerve Palsy. [Updated 2022 Jul 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482177/
6. Tamhankar MA. Isolated third, fourth, and sixth cranial nerve palsies from presumed microvascular versus other causes: a prospective study. Ophthalmology. 2013;120:2264–9.
Identifier: MORAN_CORE_126149
Faculty Approval by: Griffin Jardine MD
Copyright statement: Copyright Maxwell Mayeda, ©2022. For further information regarding the rights to this collection, please visit: http://morancore.utah.edu/terms-of-use/
