Third (Oculomotor) Nerve Palsy
A third nerve (oculomotor) palsy is a focal cranial neuropathy affecting the motor and parasympathetic fibres of cranial nerve III. It commonly produces ptosis, diplopia and the characteristic “down and out” eye position.
Causes range from benign microvascular ischaemia to life‑threatening compressive lesions (posterior communicating artery aneurysm, pituitary apoplexy, tumour, trauma). Emergency clinicians must recognise red flags, perform a focused bedside assessment, and arrange urgent imaging and specialist referral when indicated.
Key points
- Classic triad: ptosis, diplopia and an eye resting “down and out.”
- Pupillary involvement (dilated, poorly reactive pupil) strongly suggests an external compressive lesion, but this is not absolute.
- Any new paralytic squint or new cranial nerve palsy requires urgent neurological assessment and early specialist referral (NICE NG127).
- Acute severe headache with a new ocular motor palsy should prompt immediate consideration of subarachnoid haemorrhage or pituitary apoplexy and rapid imaging (NICE NG228; Society for Endocrinology/RCEM guidance).
Anatomy and pathophysiology (why the pattern appears)
- Cranial nerve III supplies levator palpebrae (lid elevation), the medial, superior and inferior rectus muscles, and inferior oblique (eye movements), plus parasympathetic fibres to the pupil (Edinger-Westphal nucleus).
- Pupillary fibres run peripherally in the nerve and are more vulnerable to external compression from aneurysm or mass.
- Motor fibres are more central and are relatively more susceptible to ischaemic microvascular injury (for...