The trochlear nerve is also known as cranial nerve IV (CN-IV). It is the only cranial nerve that emerges dorsally from the brain (near the back), giving it the longest pathway. It is the smallest nerve to service the eye.

CN-IV passes through the superior orbital fissure, and it provides motor function, or movement. It serves the superior oblique eye muscle and connects to the annular tendon. As a result, it processes brain signals to move the eyes up and down, and also outwards.

Whether due to a head injury or a complication of surgery, damage to this nerve will compromise some ability to use the superior oblique eye muscle. Without the use of the nerve, the superior oblique eye muscle will no longer function properly. The muscle, not the trochlear nerve, is what physically moves the eyeball. Double vision, otherwise known as diplopia, results from problems with either the muscle or the nerve. Complications from these issues will result in a diminished ability to walk, especially down stairs.