The sixth cranial nerve, the abducens nerve, is the longest cranial nerve in the body. It is responsible for ipsilateral eye abduction/horizontal gaze. Dysfunction of the abducens nerve can occur at any point of its transit from the pons to the lateral rectus muscle, resulting in sixth nerve palsy. To understand the causes of abducens nerve palsy, one must have a good grasp of the anatomy of the nerve as it transverses the brain. The abducens nerve begins in the pons near the seventh cranial nerve before exiting the brainstem. At this point, it travels into the subarachnoid space and moves along the skull at the clivus. It then travels to the basal skull at the petrous apex of the temporal bone where it enters the cavernous sinus. In the cavernous sinus, the internal carotid will be located medially as compared to the abducens nerve. The trigeminal nerve will be found just laterally in the cavernous sinus. The abducens nerve then enters the orbit via the superior orbital fissure and innervates the lateral rectus muscle, resulting in eye abduction.
As discussed above, the determining etiology of abducens nerve palsy requires knowledge of the nerve path. Neoplasm and trauma may affect the abducens nerve at any point along its path and cause a resulting palsy. Other causes are best broken down by the location of the abducens nerve.
Nuclear and fascicular causes include etiologies that affect the pons directly, including ischemic stroke and metabolic disease such as Wernicke disease. Demyelinating lesions may also affect this portion of the nerve. These nuclear causes may be associated with facial nerve palsies, secondary to the close proximity of the two cranial nerves in the pons.
As the abducens nerve enters the subarachnoid space, other etiologies may arise resulting in palsy. In these cases, the palsy is primarily because of the increased intracranial pressure. As such, other symptoms such as a headache, nausea, vomiting, and papilledema may be noted. Causes include an aneurysm, carcinomatous meningitis, procedure-related injury (e.g., spinal anesthesia, post-lumbar puncture), inflammatory lesions (e.g., sarcoid, lupus), infection (e.g., Lyme disease, syphilis, tuberculosis, Cryptococcus).
As the abducens nerve courses over the petrous apex, causes of related palsy include complicated otitis media or mastoiditis, sinus thrombosis, and basal skull fracture.
As the abducens nerve traverses the cavernous sinus, the resultant palsy is typically secondary to stretching of the abducens nerve. The etiologies to be considered include cavernous sinus thrombosis, cavernous sinus fistula, and internal carotid aneurysm or dissection.
Finally, orbital lesions also may cause an abducens nerve palsy. These include neoplasm, inflammatory disease, infection, or trauma.
The sixth cranial nerve is the most commonly affected ocular motor nerve in adults. In children, it is the second most common, following the fourth cranial nerve, with an incidence of 2.5 cases per 100,000. Poorly controlled diabetes mellitus is a significant risk factor for an abducens nerve palsy.
The abducens nerve is the longest cranial nerve in the body, traveling from the pons to the lateral rectus of the ipsilateral eye. It is primarily responsible for ipsilateral eye abduction. Those diseases result in an inability of the abducens nerve to transmit to the lateral rectus and cause a motor palsy, resulting in the inability to move the affected eye laterally as well as horizontal diplopia.
Patients who develop abducens nerve palsy often present with binocular horizontal diplopia, which is a double vision when looking at objects side by side. There will be a notable weakness of the ipsilateral lateral rectus muscle with loss of eye abduction on the affected side. Some patients may present with a constant head turning movement to maintain binocular fusion and to lessen the degree of diplopia. Other findings in the clinical history may include vision loss, headache, vomiting, trauma, hearing loss, recent lumbar puncture, and recent viral illness.
The workup for sixth cranial nerve palsy depends on what cause is suspected. In children, an aggressive workup should be performed as there is a significantly higher risk of neoplasm as a primary cause. In the setting of abducens nerve palsy associated with trauma, neuroimaging should be performed at the time of injury. If there is a concern of elevated intracranial pressure as a cause, lumbar puncture should be performed. If the palsy is suspected to be an ischemic cause, MRI is recommended as the preferred modality because of its superior capability of imaging the posterior fossa.
Laboratory studies include a complete blood cell count, diabetes testing, and erythrocyte sedimentation and C-reactive protein to assess for possible giant cell arteritis. Other laboratory studies to be considered include acetylcholine receptor antibodies if there is a concern for myasthenia gravis, rapid plasma reagin test, and fluorescent treponemal antibody-absorption test if there is a concern for syphilis, Lyme titer, thyroid function tests, and an antinuclear antibody test.
Treatment therapies in children include alternate patching, prism therapy, strabismus surgery, and botulism toxin. Alternate patching consists of patching each alternating eye for a few hours each day. This is used to prevent amblyopia in the affected eye. Prism therapy requires placement of a temporary press-on prism on the lens of the affected eye. If a child fails prism therapy, he or she would be eligible for strabismus surgery. Botulism can be injected into the medial rectus of the affected eye to prevent contracture and nasal deviation.
Treatment of abducens nerve palsy in adults is less well defined. The majority of cases will be self-limited and not require any intervention, only observation. Other treatments will be dictated by the determined cause of the abducens nerve palsy. For example, steroids will be given to those suffering from temporal arteritis. In cases related to intracranial pressure, such as pseudotumor cerebri and cancer, the pressure would need to be reduced through surgery or lumbar puncture. Further treatment of persistent sixth nerve palsy would be similar to that in children, save for alternative patching, which has not proven to be effective in adults.
Abducens nerve palsy is a disease that requires careful consideration of the underlying pathology that resulted in the physical finding of lateral rectus paresis. This is best done by using a differential diagnosis that follows the anatomy of the nerve to elucidate the cause. Treatment can then be directed to the underlying cause.