25th June 2023

CN IV – Trochlear nerve

The trochlear nerve is a motor nerve that innervates the superior oblique muscle of the eye, allowing the eye to look down and outwards.

Palsy of the trochlear nerve primarily causes diplopia when looking downwards, e.g. when going downstairs. The trochlear nerve decussates in the brainstem after originating from its nuclei in the midbrain. A unilateral lesion to the trochlear nucleus affects the contralateral nerve and superior oblique muscle, whereas a lesion to the trochlear nerve itself results in ipsilateral signs. If there is evidence of involvement of other ocular motor nerves, a lesion in the cavernous sinus or midbrain is likely implicated as these nerves are in close proximity in this space and share blood supply.