The trochlear nerve controls the superior oblique muscle of the eye, allowing for up and down and crosswise eye movement. Damage can cause trochlear nerve palsy, leading to double vision and head tilting. Diagnosis is done through the Bielschowsky head tilt test, and treatment involves surgery.
The trochlear nerve, also known as the fourth nerve or fourth cranial nerve, is located near the brain and serves the superior oblique muscle of the eye. It has several unique characteristics 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 on the opposite side of the body from the muscle it serves, and it is the only one that exits 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 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 name of the nerve, trochlear, which is Latin for “pulley”.
This nerve’s role in controlling eye movements is why damage to the trochlear nerve can lead to vision problems. In particular, a trochlear nerve injury in one eye can impede that eye’s ability to move in sync with the other eye, often causing double vision. This condition is also called trochlear nerve palsy. It is often diagnosed by the patient’s tendency to tilt their head to relieve double vision.
Alfred Bielschowsky, a German ophthalmologist, 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 remains in use today as a diagnostic tool. Most commonly, trochlear nerve palsy 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 are uncertain, because many patients simply compensate for the double vision with head movement. For those who are no longer able to obtain acceptable results with compensation, treatment usually involves surgery. Surgical innovations developed in the 1970s greatly improved treatment options and efficacy.
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