What is the pathophysiology of third cranial nerve palsy (oculomotor cranial nerve palsy)?

Updated: Oct 08, 2018
  • Author: James Goodwin, MD; Chief Editor: Andrew G Lee, MD  more...
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Answer

The anatomical relationship of the various portions of the oculomotor (third) cranial nerve accounts for many of the clinical features of third cranial nerve palsy as outlined below.

Nuclear portion: The cell bodies for axons that travel in the oculomotor nerve reside in the column-shaped nuclear groups on either side of the midbrain tegmentum. The axons destined for each extraocular and intraocular muscle derive from a specific subnucleus. [1] Interestingly, the superior rectus muscle is innervated by the contralateral subnucleus. Nuclear third nerve lesions may be isolated or may be accompanied by other neurologic symptoms given the proximity of the nucleus to other important brainstem structures.

Fascicular intraparenchymal midbrain portion: The fascicular portion of the oculomotor nerve courses ventrally from the nucleus in the dorsal midbrain tegmentum, passes through the red nucleus near the corticospinal tract, and emerges from the medial aspect of the cerebral peduncle. Because of its proximity to other midbrain structures, lesions in the fascicular portion of the nerve typically produce associated neurological findings in addition to oculomotor palsy. [2]

Subarachnoid portion: The cisternal portion of the nerve courses in the subarachnoid space anterior to the midbrain after exiting between the superior cerebellar artery and posterior cerebral artery and running in close proximity to the posterior communicating artery. Berry aneurysm at the junction between the posterior communicating artery and the internal carotid artery is an important cause of an otherwise isolated oculomotor nerve palsy.

Cavernous sinus portion: The next segment of the oculomotor nerve runs through the lateral wall of the cavernous sinus superiorly. It enters the cavernous sinus just above the petroclinoid ligament and inferior to the interclinoid ligament. Although the sixth cranial nerve is the most affected by intrinsic lesions of the cavernous sinus, masses invading the cavernous sinus from within the sella often cause third cranial nerve dysfunction prior to involvement of the other cranial nerves. This is probably because of the oculomotor nerve's close proximity to the unyielding interclinoid ligament above and the petroclinoid ligament below. Cavernous sinus lesions that produce a third nerve palsy are typically accompanied by one or more other neurologic findings (eg, IV, V, VI, or Horner syndrome).

Orbital portion: The oculomotor nerve enters the orbit through the superior orbital fissure adjacent to the fourth cranial nerve. The nerve branches into superior and inferior divisions, usually within the posterior orbit, but, occasionally, the branching occurs as far back as the cavernous sinus segment. The superior division innervates the levator palpebrae and the superior rectus muscles; the inferior division innervates all the other muscles innervated by the third nerve and supplies the preganglionic parasympathetic fibers to the ciliary ganglion, which innervates the iris sphincter to constrict the pupil. Orbital lesions may show proptosis if large enough.

The axons for most of the muscles are uncrossed from the nucleus to the eye, but there are 2 exceptions: (1) Axons for the levator palpebrae come from both sides of the central caudal subnucleus via bilateral, crossed, and uncrossed pathways. (2) Those for the superior rectus muscle come from the superior rectus subnucleus on the contralateral side.

The pupillomotor and ciliary muscle neurons derive from the Edinger-Westphal subnucleus, which is in the midline in the most rostral and anterior part of the oculomotor nerve nucleus. These autonomic pathways are all ipsilateral or uncrossed.

The oculomotor nerve innervates the following extraocular muscles of each eye: superior rectus, inferior rectus, medial rectus, inferior oblique, levator palpebrae superioris, ciliary muscle, and iris sphincter.


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