Third Cranial Nerve Lesions

• oculomotor nerve
• innervates: superior rectus, inferior rectus, medial rectus, inferior oblique, levator palpebrae, cillary muscle and iris sphincter

SYMPTOMS/SIGNS

• “down and out” – because of antagonism of the trochlear nerve (superior oblique) and abducens nerve (lateral rectus)
• ptosis – weakness of levator palpebrae
• diplopia + strabismus – unable to maintain normal alignment when looking straight ahead
• dilated, fixed pupil (anisocoria) + blurred vision – parasympathetic fibres originate from the Edinger-Westphal subnucleus of IIIrd nerve complex

ANATOMICAL BASIS (causes of IIIrd nerve dysfunction)

Nuclear Portion

• column shaped
• either side of the midbrain tegmentum
• pathological causes: infarction, haemorrhage, neoplasm, abscess

Fascicular Midbrain Portion

• courses ventrally
• passes through the red nucleus
• emerges from the medial aspect of the cerebral peduncle
• pathological causes: infarction, haemorrhage, neoplasm, abscess

Fascicular Subarachnoid Portion

• nerve runs in the subarachnoid space anterior to the midbrain and in close proximity to the posterior communicating artery
• pathological causes: aneurysm, meningitis, meningeal infiltration, ophthalmoplegic migraine, compression from ipsilateral or mass effect (uncal herniation)

Fascicular Cavernous Sinus Portion

• the nerve runs through the lateral wall of the cavernous sinus
• it enters the sinus just above the petroclinoid ligament and inferior to the interclinoid ligament
• pathological causes: tumour, pituitary apoplexy/infarction, vascular (giant intracavernous aneurysm, carotid artery-cavernous sinus fistula, cavernous sinus thrombosis), ischaemia, inflammatory (Tolosa-Hunt Syndrome)

Fascicular Orbital Portion

• it enters the orbit through the superior orbital fissure
• it then branches into superior and inferior divisions
• superior -> levator palpebrae and superior rectus
• inferior -> innervates the rest
• axons are mostly uncrossed with 2 exceptions (axons to levator palpebrae are from both sides, those for superior rectus come from the contralateral side)
• pathological causes: inflammatory (orbital inflammatory pseudotumour), endocrine (thyroid orbitopathy), tumour (haemangioma, lymphoma)