The abducens nerve controls the lateral rectus muscle, which moves the eye outward. Damage to the nerve causes double vision and inward deviation of the affected eye. Trauma, aneurysms, strokes, and diabetes-related nerve damage are common causes of abducens nerve dysfunction. Other conditions can mimic sixth nerve palsy.
The abducens nerve, also called cranial nerve VI, innervates the lateral rectus, the muscle that rotates the eye outward. It is the longest of all cranial nerves, running from the midbrain to the eye, and is, as a result, more susceptible to injury than all other cranial nerves. An abducens nerve runs along each side of the brain. About 40% of the nerve fibers pass through the opposite eye to partially innervate the medial rectus, the muscle that rotates the opposite eye inward. By sending fibers to the muscles of both eyes, the abducens nerve helps both eyes move together in lateral gaze to the side on which the nerve runs.
Lesion of 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. To avoid double vision, an individual will turn their head to the side of weakness, so that both eyes are looking to the opposite side. The weak eye muscle cannot turn the eye beyond the midline. As a result, double vision gets worse when the patient tries to look sideways.
Trauma accounts for up to 30% of cases of abducens nerve dysfunction. A further 36% can be traced to aneurysms and around 36% to strokes. Anything that stretches, pinches, or inflames the abducens nerve, including fractures, meningitis, tuberculosis, or multiple sclerosis, can cause damage to the nerve and lead to paralysis.
The most common cause of sixth nerve dysfunction is diabetes-related nerve damage, which occurs due to defective 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 caused by alcoholism. The classic signs of this condition are flickering eyes and lateral rectus weakness.
Symptoms reported by patients with abducens nerve palsy include crossed eyes, increased distance double vision, the need to turn the head to see straight ahead, and a feeling of tension when attempting a side gaze. Depending on the cause of the abducens injury, the patient may also experience hearing or vision loss, weakness, numbness, fever, or pain. Abducens nerve palsy, however, isn’t the only reason one eye doesn’t turn outward. Great mimics of sixth nerve palsy include thyroid disease, myasthenia gravis, tumors of the orbit or eye socket, and fractures of the wall of the orbit with tissue entrapment.
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