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The trochlear nerve is the fourth cranial nerve, and is a somatic motor (somatic efferent) nerve. The Yale School of Medicine notes that the superior oblique muscle, which the trochlear nerve innervates, is one of the six extraocular muscles involved in eye movement. The superior oblique muscle is responsible for visual tracking and eye fixation, while the trochlear nerve turns the eye downward and laterally. The trochlear nerve is only one of the cranial nerves responsible for eye movement.
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The Stritch School of Medicine states that the trochlear nerve is located in the ventral part of the brain stem, specifically in th e trochlear nucleus. The trochlear nerve then crosses over before leaving the brain stem; the trochlear nerve is also the only cranial nerve to leave the brain on the dorsal side. The superior oblique muscle on the right hemisphere is innervated by the trochlear nerve from the left side, and vice-versa.
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Damage to the trochlear nerve can affect the patient's ability to move his or her eyes. One type of trochlear nerve dysfunction, trochlear nerve palsy, is paralysis of the cranial nerve, according to the Merck Manuals Online Medical Library. The trochlear nerve palsy can be caused by a head injury, diabetes, brain tumor, aneurysm or multiple sclerosis; however, the latter three causes are more rare. One or both of the patient's eyes can be affected by trochlear nerve palsy—it depends if both of the trochlear nerves are damaged, or only one. If the left trochlear nerve is damaged, then the right eye will be effected, and vice-versa. As a result of trochlear nerve palsy, the patient cannot move his or her eye or eyes inward or down.
Another effect of damage to the trochlear nerve is visual diplopia, according to the Miller School of Medicine. During visual diplopia, the patient will see one image above another. However, if the patient turns her head contralaterally, or turns her head on the opposite side of the trochlear nerve damage, the visual diplopia will improve. If the patient turns her head ipsilaterally, or turns her head on the same side of the trochlear nerve damage, the visual diplopia will be worse.