Corpus: Trochlear nerve

The trochlear nerve is the 4th cranial nerve and carries somatomotor fibers. Together with the oculomotor nerve and the abducens nerve, it is responsible for the movement of the eyeball and innervates the superior oblique muscle.

The trochlear nerve is the only cranial nerve that originates on the dorsal side of the brain: its origin is caudal to the inferior colliculi in the tectum of the mesencephalon. The nerve is also remarkable in other respects:

• It is the weakest cranial nerve — measured by the number of axons.
• It has the longest intracranial course of all cranial nerves.

In its course, the trochlear nerve runs laterally past the crura cerebri and breaks through the dura mater dorsolateral to the oculomotor nerve, at the anterior edge of the tentorium cerebelli. It then runs in the lateral wall of the cavernous sinus and passes through the superior orbital fissure into the orbit. In the orbit, it runs laterally and cranially past the annulus tendineus communis, the origin of the eye muscles.

The trochlear nerve supplies the superior oblique muscle.

The core area of the trochlear nerve is called the nucleus nervi trochlearis and is located at the level of the inferior colliculi in the mesencephalon. The fibers from the nuclei on both sides cross over to the opposite side.

Trochlear nerve palsy results in the loss of the superior oblique muscle. As a result, the function of the antagonistic inferior oblique muscle predominates. The following disorders of ocular motility can be observed:

• Lack of lowering of the eyeball during adduction and elevation of the bulb
• Slight nasal strabismus deviation in abduction
• Lack of internal rotation

The symptoms consist of oblique, vertical double images, with a maximum when looking nasally and downwards. The greatest strabismus deviation occurs when the head is tilted to the diseased side. A conspicuous elevation of the affected eye becomes visible, which is known as the Bielschowsky phenomenon. The patient compensates by tilting the head to the opposite side — a torticollis opticus develops.