Trochlear Nerve Palsy

The fourth cranial nerve, also known as the trochlear nerve, arises from the midbrain at the level of the inferior colliculus (ventral to the Sylvian aqueduct). This nerve supplies only a single muscle - the superior oblique (SO) muscle. It is the only cranial nerve that emerges from the dorsal aspect of the brainstem and decussates to supply the muscle of the contralateral side.The nerve has a very long course, making it more vulnerable to injury. Palsy of the trochlear nerve is a common presentation in ophthalmology clinics.