Trochlear nerve (IV) lesion
This motor nerve supplies superior oblique. It exits posteriorly from the brainstem and winds around to the front, then passes in the lateral wall of the cavernous sinus as far as the superior orbital fissue.
Supplies superior oblique (moves the eye down and in).
Rare as isolated lesion, Generally occurs as a combined III, IV and VI nerve palsies (see below) when the eye is also intorted.
Diplopia on looking down or in.