25th June 2023

CNIII – Oculomotor nerve

The oculomotor nerve extends from the ventral midbrain through the superior oblique fissure to travel to the extrinsic eye muscles. It has two nuclei in the midbrain, the motor nucleus which controls the extraocular muscles, and the parasympathetic nucleus, also known as the Edinger-Westphal nucleus, which controls the light reflex and accommodation reflex.

An oculomotor palsy consists of the following:

• Ptosis – paralysis of levator palpebrae superioris
• Lateral strabismus – unopposed CN 6 action
• Loss of pupillary light reflex
• Loss of accommodation

Care should be taken to distinguish the ptosis of an isolated CN 3 palsy from other causes of ptosis, including Horner’s Syndrome. The main differentiators are whether the ptosis is complete or partial, bilateral or unilateral, pupil size, along with other suggestive features.

In the presence of a painful ophthalmoplegia, always be aware of intracavernous carotid artery dissections, particularly if the history involves trauma to the neck or head, or any Valsalva manoeuvre action, e.g. weightlifting, straining against a load. In a painful isolated CN 3 palsy, a posterior communicating artery dissection or aneurysm should always be excluded.

Other causes of a painful ophthalmoplegia include vascular causes, e.g. cerebral venous sinus thrombosis, neoplastic causes, infective causes, e.g. aspergillosis, inflammatory causes, e.g. sarcoidosis, and Tolosa Hunt syndrome, which is an idiopathic inflammatory diagnosis of exclusion that manifests as periorbital headache, ipsilateral painful ophthalmoplegia, and often pain in the distribution of the trigeminal nerve.