This article includes discussion of isolated sixth nerve palsy, abducens nerve palsy, and lateral rectus palsy. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
An isolated sixth nerve palsy may be a harbinger of underlying intracranial disease. Due to its long subarachnoid course, it may be damaged by downward shift of the brainstem as often occurs in increased or decreased intracranial pressure (“false-localizing sixth nerve palsy”). Alternatively, the sixth nerve may be involved in isolation by a compressive lesion in the cavernous sinus or along the clivus (“true-localizing sixth nerve palsy”), and specific attention on neuroimaging should be paid to these areas, especially in chronic or progressive cases. The most common causes of a sixth nerve palsy in an adult include ischemia, head trauma, and compression by a mass lesion, but inflammation, primary demyelination, and intracranial hypotension may also produce a sixth nerve palsy.
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• A deficit in abduction is not always due to a sixth nerve palsy; therefore, patients should be considered to have an “abduction deficit” rather than a “sixth nerve palsy” until a diagnosis is confirmed. Alternative causes of an abduction deficit are medial rectus muscle restriction (as in dysthyroid orbitopathy, myositis, orbital wall fracture), myasthenia gravis, Duane type I retraction syndrome, and convergence spasm.
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• A minimal sixth nerve palsy can be present without causing a visible abduction deficit. Therefore, the evaluation of patients reporting diplopia may require testing of ocular alignment using prisms.
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• Sixth nerve palsy may be caused by increased or decreased intracranial pressure (“false-localizing palsy”) or by lesions in the pons (nucleus and fascicle), subarachnoid space, clivus, cerebellopontine angle, cavernous sinus, superior orbital fissure, and orbit along the course of the nerve (“true-localizing palsy”).
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• Chronic or slowly progressive sixth nerve palsies may reflect life-threatening intracranial disease.
Historical note and terminology
The terms sixth nerve palsy, abducens nerve palsy, and lateral rectus palsy are essentially interchangeable.