29th December 2023

3ʳᵈ nerve palsies

The oculomotor nerve has three main functions: innervation to the pupil and lens, innervation to the upper eyelid and innervation of the extraocular muscles (SR, MR, IR, IO). Complete third nerve palsies are likely to present with ptosis, a dilated pupil with sluggish to no reactions and a down-and-out eye with an inability to adduct, infraduct or supraduct. Partial third nerve palsies are more common, and these can present with a mix of the previously mentioned signs to varying degrees of severity.

The third nerve originates from two nuclei in the midbrain and exits near the midline at the base of the midbrain. From here, the nerve passes through the cavernous sinus towards and through the superior orbital fissure and to the orbit of the eye. Therefore, the same pathologies which can affect the 6ᵗʰ cranial nerve in the cavernous sinus or orbital fissure can also affect the 3ʳᵈ nerve. Patients presenting with new signs and symptoms of 3ʳᵈ nerve palsy raise suspicion for aneurysms, which may be compressing the nerve between it exiting the midbrain and entering the cavernous sinus, mainly an aneurysm of the posterior communicating artery. Also, CN III has parasympathetic fibres controlling pupil constriction running along the outside of the nerve. Therefore, external compression of the 3ʳᵈ nerve may lead to pupil dilation. If any ocular motility examination suggests involvement of CN III + deficits of duction with or without a dilated pupil, urgent non-contrast CT head with possibility of additional CT angiogram should be discussed. MRA should be discussed for patients who are pregnant, with poor renal function or allergy to contrast agents.

Lesions affecting the 3ʳᵈ nerve nucleus in the midbrain can present with bilateral complete ptosis due to there being only one sub-nucleus innervating both levator palpebrae superioris muscles. If the corticospinal tract has been affected, patients can present with contralateral hemiplegia or ataxia if the superior cerebellar peduncle is involved. The 3rd nerve partly exists within the subarachnoid space and is therefore at risk of damage secondary to infections, inflammation or masses within the subarachnoid space. In patients presenting and not requiring urgent CT imaging of the brain, it is not unreasonable to consider an MRI  with contrast to evaluate the entire path of the 3rd nerve to rule out lesions/processes along its course.