Intracranial lesion involving the orbit

The trochlear nerve (), also called the fourth cranial nerve or CN IV, is a motor nerve (a somatic efferent nerve) that innervates only a single muscle: the superior oblique muscle of the eye, which operates through the pulley-like trochlea. The trochlear nerve is unique among the cranial nerves in several respects: It is the smallest nerve in terms of the number of axons it contains. [Source: Wikipedia ]