The trochlear nerve, or fourth cranial nerve, is one of a few cranial nerves that has a single function - to control the motor function of the superior oblique muscle of each eye to allow the rotation of the eyeball in either a vertical or horizontal direction. Although it has fewer axons than any other cranial nerve, it is also the longest of the cranial nerves. It is also the only cranial nerve that emanates from the back of the brainstem.
Injuries to the trochlear nerve result in an inability to quickly move the eyes downwards, double vision or an inability to rotate the eyes quickly enough to compensate for head movement. Injury can result from lesions, head trauma or congenital defects.