The fourth cranial nerve (CNIV) is also named the trochlear nerve. It only controls eye movement from the superior oblique muscle of the eye. The primary action is to help the eye rotate in and out; the secondary action helps make the eye move down. CNIV is unique in that it has a long path to its origin and is the only cranial nerve that exits the brainstem dorsally (towards the back). The long pathway, as well as the dorsal exit of the nerve, makes this nerve one of the most susceptible to damage from a head injury.
All cranial nerves (all nerves for that matter) have a nucleus (cell body) and axon (that carries nerve impulses away from the nucleus to other structures). The nucleus of CNIV is located in the midbrain. The axons from CNIV exit dorsally (as stated earlier, CNIV is the only cranial nerve to do so), decussate (crossover), and take a long pathway eventually penetrating through the cavernous sinus and ending in the superior oblique muscle.
A quick review of the location and destination of CNIV: -Starts in the MIDBRAIN -Exits DORSALLY -Crosses over (decussates) so the left nucleus eventually controls the right superior obliques muscle -Ends in the superior oblique extraocular muscle
Damage to the trochlear nerve can occur anywhere along its long path. The posterior (dorsal, towards the back of the head) exit from the midbrain makes the trochlear nerve uniquely susceptible to damage from a head injury.
The most common cause of a trochlear nerve palsy is a congenital palsy. Patients often have large vertical fusion abilities (if fusion is possible) and may adopt a tilted head position to help compensate for the underaction of the affected superior oblique muscle.
Head trauma is a common cause of acquired trochlear nerve palsy, as is microvascular damage due to high blood pressure, diabetes, or other vascular disorders. Patients with an acquired palsy often complaint of vertical diplopia (double vision).
Evaluation of a trochlear nerve palsy involves a thorough evaluation of eye movement. The action of the superior oblique muscle is best evaluated when the eye is moved in towards the nose. When the superior oblique received reduced or absent input from CNIV, the eye tends to deviate upward when the eye is moved in towards the nose. This occurs due to the absent depression (downward movement) of the superior oblique muscle.
Root Atlas has a great set of videos that help explain how damage to CNIV affects a patient. Check it out below: