Cranial Nerve IV- Trochlear Nerve

The trochlear nerve (IV cranial) innervates the superior oblique muscle and contains only somatic motor fibers. CN IV originates from the trochlear nucleus, which is a small, oval mass (yellow in the pseudocolored section below) situated in the ventral part of the central gray matter of the cerebral aqueduct at the level of the upper part of the inferior colliculus. In the figure, the cerebral aqueduct is near the top which is the posterior part of the section.
The cells of the trochlear nucleus are large, irregular and yellowish in color. The nuclei of the two sides are separated by the raphé through which dendrites extend from one nucleus to the other. They receive many collaterals and terminals from the posterior longitudinal bundle which lies on the ventral side of the nucleus.
The nucleus then is located at the level of the pons but posteriorly. The axons from the nucleus pass downward in the tegmentum toward the pons, but turn abruptly dorsalward before reaching it, and pass around the cerebral aqueduct into the superior medullary velum, in which they cross horizontally, to decussate with the nerve of the opposite side, and emerges from the surface of the velum, immediately beneath the inferior colliculus. The nerve descends to the base of the brainstem, crosses the superior cerebellar peduncle and turns forward against the cerebral peduncle. The nerve passes between the posterior cerebral and superior cerebellar arteries but more lateral than the third cranial nerve (see Figure). It pierces the dura in the free border of the tentorium cerebelli just behind the posterior clinoid process and passes forward to the cavernous sinus. The course of this nerve in the cranium is long~about 75 mm .
CN IV travels in the lateral wall of the cavernous sinus. The nerve is initially below the 3rd cranial nerve but then crosses the oculomotor nerve and enters the orbit through the superior orbital fissure as the most superior of all nerves entering the orbit. In the orbit the trochlear nerve is medial to the frontal nerve. CN IV crosses over the levator muscle and enters the superior lateral border of the superior oblique muscle in its posterior third where it provides innervation (arrow 1).
The function of the superior oblique muscle is to rotate and depress the eye. Paralysis of the trochlear nerve causes vertical diplopia which is increased when the eye is directed downward and inward such as when going down stairs. The patient compensates by tilting the head to the side opposite the paresis. This relieves them of the need to move the eye downward in the field of action of the paretic muscle. On examination there is a hyperdeviation that increases on contralateral gaze and with ipsilateral head tilt. Congenital fourth nerve palsies may be evident with a head tilt seen in old photographs. The most common cause of an acquired 4th nerve palsy is head trauma. Other causes of 4th nerve palsy include tumors of the pineal gland, A-V malformation, demyelination, meningitis, carotid-cavernous fistula, and iatrogenic causes (neurosurgical cut at the edge of the tentorium).