Oculomotor nerve

Motor function

Motor innervation of extraocular muscles except lateral rectus (abducens) and superior oblique (trochlear).

Visceral motor function

Parasympathetic innervation of pupillary sphincter and ciliary muscle.

Anatomy

1. Intra-axial segment

• The oculomotor nucleus is located in the midbrain at the level of the superior colliculus. It is located ventral to the cerebral aqueduct and medially and superior to the medial longitudinal fasciculus (MLF).
• The Edinger-Westphal nucleus is located dorsal to the oculomotor nucleus, in the periaqueductal grey matter. It is the origin of preganglionic parasympathetic fibers for the innervation of the pupillary sphincter and ciliary muscle.
• The Perlia nuclei are parasympathetic nuclei located between the Edinger-Westphal nuclei. They are involved in ocular convergence.
• The oculomotor fascicles course through the MLF, red nucleus, substantia nigra and medial part of the cerebral peduncle. They exit the midbrain ventrally and run in the interpeduncular cistern.

2. Cisternal segment

• This segment courses anterolaterally in the interpeduncular and prepontine cisternae.
• It runs between the posterior cerebral (PCA) and superior cerebellar (SCA) arteries.
• It runs medially to the free edge of the tentorium cerebelli.
• It runs inferior to the posterior communicating artery (PCom).
• It crosses the petroclinoid ligament and penetrates the dura to enter the roof of the cavernous sinus.

3. Caverouns segment

• This segment courses into the dural wall of the cavernous sinus.
• It lies superolateraly to the internal carotid artery (ICA).
• The occulomotor nerve is the most superiorly situated of all the cranial nerves within the cavernous sinus. (In cranio-caudal order: oculomotor, trochlear, abducens, ophthalmic branch of trigeminal, maxillary branch of trigeminal).

4. Extracranial segment

• This segment enters the orbit through the superior orbital fissure, through the annulus of Zinn.
• It divides into superior and inferior branches.
• The superior branch innervates the levator palpebrae superioris and the superior rectus muscles.
• The inferior branch innervates the inferor and medial rectus and the inferior oblique muscles.
• The preganglionic parasympathetic fibers follow the inferior branch and reach the ciliary ganglion. From the ciliary ganglion, short ciliary nerves enter the globe with the optic nerve and innervate the iris (pupilary sphincter) and the ciliary muscle (accomodation).

Clinical implications

• Fully developed oculomotor ophthalmoplegia includes diplopia, pathologic accomodation, mydriasis, ptosis and downward, abducted globe.
• The parasympathetic fibers course in the medial outer part of the nerve and are most susceptible to compression. An abnormal pupil will be the earliest sign of oculomotor compromise.
• The oculomotor nerve is susceptible to compression by PCom, PCA and rarely, large basilar artery aneurysms. Uncal herniation or downward shift of brainstem on head impact stretches the oculomotor nerve over the petroclinoid ligament.
• Pupil-sparing oculomotor deficit is most often caused by microvascular infarction. Rare cases of compression from a basilar artery aneurysm have been reported.
• Other causes of oculomotor palsy include: microvascular infarction, infection, trauma, demyelinating disease, myasthenia gravis, iatrogenic injury and cavernous sinus thrombosis.