The trochlear nerve, also known as the fourth nerve or the fourth cranial nerve, is located near the brain and serves the superior oblique muscle of the eye. It has several unique features in that it contains the fewest axons of any of the 12 cranial nerves and is the longest. Also, of all the cranial nerves, it is the only one that originates at the opposite site in the body from the muscle it serves, and it is the only one that exits from the back of the brainstem. There are two trochlear nerves, one for each eye, and they are found not only in humans but in all vertebrates that have jaws.
The function of the trochlear nerve is focused on a single muscle that works to move the eye. Movement made possible by the superior oblique eye muscle includes rolling the eye up and down and moving it toward the nose, or "crossing" the eyes. The muscle itself attaches to the back of the eyeball, but a tendon extending from it attaches to the top of the eyeball and exerts pressure through a pulley-like structure. This structure explains the nerve's name, trochlear, which means "pulley" in Latin.
The role of this nerve in controlling eye movements is why trochlear nerve damage can lead to problems with vision. In particular, a trochlear nerve injury in one eye can impede the eye's ability to move in sync with the other eye, often causing double vision. This condition is also known as trochlear nerve palsy. It is often diagnosed by the patient's tendency to keep their head tilted to relieve double vision.
Alfred Bielschowsky, an ophthalmologist from Germany, developed the head tilt test used to diagnose trochlear nerve palsy. Most of Bielschowsky's work was done in the first half of the 20th century. Although head tilt can be caused by other conditions, the Bielschowsky head tilt test is still in use today as a diagnostic tool. Trochlear nerve palsy most often occurs as a result of head trauma, although it has also been diagnosed in conjunction with conditions such as multiple sclerosis, diabetes, and atherosclerosis.
Exact data on the frequency of trochlear nerve palsy is uncertain, because many patients simply compensate for double vision through head movement. For those who can no longer achieve acceptable results with compensation, treatment usually involves surgery. Surgical innovations developed in the 1970s have greatly improved treatment options and effectiveness.