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Sixth cranial nerve palsy has many causes, including damage to small blood vessels by diabetes, but the cause is often unidentified.
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The affected eye cannot turn fully outward and may turn inward when people look straight ahead.
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Doctors can identify 6th cranial nerve palsy based on the symptoms, but tests, including brain imaging, are done to try to identify the cause.
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Sixth cranial nerve palsy usually resolves whether the cause is identified or not.
(See also Overview of the Cranial Nerves.)
Palsy refers to paralysis, which can range from partial to complete.
Many disorders can cause this palsy:
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Diabetes when it damages small blood vessels in the eyes
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Tumors
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A bulge (aneurysm) in an artery in the skull
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Brain infections, such as meningitis, a brain abscess, or a parasitic infection
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Complications of an ear or eye infection
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Blockage of an artery supplying the nerve, as can result from diabetes, a stroke, a transient ischemic attack, or vasculitis (inflammation of blood vessels)
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Wernicke encephalopathy (commonly due to chronic alcoholism)
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Idiopathic intracranial hypertension (pseudotumor cerebri)
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Sometimes high blood pressure
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Rarely, upper respiratory infections (in children)
Some of these disorders, such as tumors and brain abscesses, increase pressure within the skull and put pressure on (compress) the nerve. Other disorders, such as a blocked artery, interfere with blood flow to the nerve.
If 6th cranial nerve palsy occurs alone (without other cranial nerve palsies), its cause is often never identified.
Symptoms
The affected eye cannot turn fully outward and may turn inward when people look straight ahead. Double vision occurs when people look toward the side of the affected eye.
Other symptoms of 6th cranial nerve palsy depend on the cause. For example, disorders that increase pressure inside the skull may cause a severe headache and temporary blurring of vision when people move their head suddenly. Also, the face and mouth may be numb, and people may be unable to move the affected eye in other directions.
Diagnosis
Usually, doctors can easily identify a 6th cranial nerve palsy, based on results of a neurologic examination and an eye examination, including ophthalmoscopy. However, the cause is usually less obvious.
An ophthalmoscope is used to look into the eye and check for evidence of increased pressure within the skull and abnormalities in blood vessels.
Computed tomography (CT) or, preferably, magnetic resonance imaging (MRI) of the brain is done to exclude tumors and other abnormalities that may be increasing pressure within the skull. If results of imaging are normal, a spinal tap (lumbar puncture) may be done to determine whether an infection or bleeding is present.
If symptoms suggest vasculitis, blood is withdrawn to check for evidence of inflammation, such as certain abnormal antibodies (antinuclear antibodies and rheumatoid factor) and an abnormal erythrocyte sedimentation rate (ESR—how quickly red blood cells settle to the bottom of a test tube containing blood).
Doctors may do blood tests to check for diabetes, especially if people have risk factors for it, such as older age, high blood pressure, a family history of diabetes, or obesity. Doctors may measure fasting blood glucose levels and hemoglobin A1C level or do an oral glucose tolerance test.
After all tests are done, the cause may remain unknown.
