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. In addition, out of all of the cranial nerves, it is the only one that originates on the opposite site of the body from the muscle it serves, and it is the only one that exits from the back of the brain stem. There are two trochlear nerves, one for each eye, and they are found not only in humans but in all vertebrates that have jaws.
Trochlear nerve function focuses on a single muscle that works to move the eye. Movement made possible by the superior oblique muscle of the eye 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.
This nerve's role in controlling movements of the eye is why trochlear nerve damage can lead to problems with vision. In particular, a trochlear nerve injury in one eye can impede that eye's ability to move in synchronization with the other eye, often causing double vision. This condition is also referred to as trochlear nerve palsy. It often is diagnosed by the patient's tendency to hold his or her head in a tilted position in order to alleviate double vision.
Alfred Bielschowsky, an ophthalmologist from Germany, developed the head tilt test used to diagnose trochlear nerve palsy. The majority of Bielschowsky's work was performed in the first half of the 20th century. Although tilting of the head can be caused by other conditions, the Bielschowsky head tilt test remains in use today as a diagnostic tool. Most commonly, trochlear nerve palsy occurs as a result of head trauma, though it also has been diagnosed in conjunction with conditions such as multiple sclerosis, diabetes and atherosclerosis.
Exact data regarding frequency of trochlear nerve palsy is uncertain, because many patients simply compensate for the double vision through head movement. For those who are no longer able to achieve acceptable results with compensation, treatment generally involves surgery. Surgical innovations developed in the 1970s have greatly improved treatment options and effectiveness.