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The abducens nerve, also called cranial nerve VI, innervates the lateral rectus — the muscle that turns the eye outward. It is the longest of all of the cranial nerves, running from the midbrain to the eye, and is, consequently, more susceptible to injury than all of the other cranial nerves. One abducens nerve courses along each side of the brain. About 40 percent of the nerve fibers cross over to the opposite eye to partially innervate the medial rectus — the muscle that turns the opposite eye inward. By sending fibers to muscles of both eyes, the abducens nerve helps both eyes to move together in lateral gaze toward the side on which the nerve runs.
Injury to the abducens, or sixth, nerve causes double vision, due to the unopposed action of the opposite medial rectus muscle, which is also innervated by the oculomotor nerve. The eye on the affected side deviates inward. In order to avoid double vision, an individual will turn his or her head toward the side of the weakness, so that both eyes are looking toward the opposite side. The weak eye muscle cannot turn the eye out past the midline. As a result, the double vision gets worse as the patient attempts to look laterally.
Trauma accounts for up to 30 percent of cases of abducens nerve dysfunction. An additional six percent can be traced to aneurysms and approximately 36 percent to strokes. Anything that stretches, squeezes, or inflames the abducens nerve — including fractures, meningitis, tuberculosis, or multiple sclerosis — can cause damage to the nerve and result in palsy.
The most common cause of sixth nerve dysfunction is diabetes-related nerve impairment, which occurs due to faulty blood flow to the nerve and muscle. A rare but preventable cause of sixth nerve palsy, called Wernicke-Korsakoff syndrome, is due to thiamine deficiency brought about by alcoholism. The classic signs of this condition are jiggly eyes and lateral rectus weakness.
Symptoms reported by patients with abducens nerve palsies include crossed eyes, increased double vision at a distance, the need to turn the head to see straight, and a feeling of strain when attempting a lateral gaze. Depending on the cause of the abducens injury, the patient may also experience associated hearing or vision loss, weakness, numbness, fever, or pain. Abducens nerve palsy, however, is not the sole reason an eye will not turn outward. The great imitators of sixth nerve palsy include thyroid disease, myasthenia gravis, tumors in the orbit or eye socket, and fractures of the wall of the orbit with entrapment of tissue.