Neuro-Ophthalmic
CN III Palsy — Surgical (Compressive)
Also known as: CN III palsy, third nerve palsy, oculomotor palsy, compressive CN III, surgical CN III, posterior communicating artery aneurysm, PComA aneurysm, dilated pupil diplopia
Filed as: Emergency
Source-cited from the record's own evidence list. Clinical content programme led by Dr Ankit Mathur, PhD, Grad Cert Ocu Thera, B.S. Optom. How the clinical content is governed
Clinical decision support only
Overview
An oculomotor nerve palsy in which the pupil is enlarged and poorly reactive, raising concern for an aneurysm compressing the nerve from outside. The eye rests down and out with ptosis and restricted adduction, elevation and depression, and the double vision is sudden. A dilated, poorly reactive pupil raises strong concern for aneurysmal compression and makes this a neurosurgical emergency. A spared pupil lowers but does not remove that concern, so every acute third nerve palsy is referred the same day for vascular imaging.
Recognition
Symptoms: what patients report
- Sudden binocular diplopia — often mixed vertical and horizontal displacement.
- Ptosis — complete or near-complete drooping of the upper lid.
- The eye may be deviated 'down and out' — exotropia and hypotropia from unopposed lateral rectus and superior oblique.
- Dilated, poorly reactive pupil — raises strong concern for compression. Its absence does not exclude an aneurysm, and mild pupil involvement can also occur in a microvascular palsy.
- Ipsilateral retro-orbital or periorbital headache — may be severe ('worst headache of my life' if SAH).
- Nausea or vomiting in the context of intracranial bleeding.
Signs: what the examination shows
- Complete or partial ptosis of the upper eyelid.
- Eye deviated down and out — exotropia with hypotropia.
- Limitation of adduction, supraduction, and infraduction.
- Fixed dilated pupil — not reactive to light or accommodation.
- Normal abduction (lateral rectus intact — CN VI not involved).
- No proptosis unless there is cavernous sinus involvement.
What OptoGuide™ covers for cn iii palsy — surgical (compressive)
- Pattern reasoning: what this combination of findings points to, and its differentiators
- Don't-miss risks and escalation triggers
- Management tiers with linked Australian therapeutics
- Referral urgency, specialty, and letter drafting
Sources
- Acquired Oculomotor Nerve Palsy — EyeWiki
- Diagnosing an Acquired, Isolated Third Nerve Palsy — AAO EyeNet
- The Neuro-Ophthalmology Survival Guide (Pane, Miller & Burdon) — standard reference text
- Intracranial aneurysm risk stratification in acute ocular motor nerve palsy based on clinical features. Ding Y, Ding X, Huang B. Scientific Reports. 2025;15(1):36871.
- Etiological distribution of isolated oculomotor nerve palsy: analysis of 633 patients and literature review. Kim HJ, Kim HJ, Choi JY, et al. European Journal of Neurology. 2024;31(6):e16261.
- The spectrum and differential diagnosis of acquired ocular motor nerve palsies: a clinical study of 502 patients. Hörner R, Kassubek J, Dreyhaupt J, Ludolph AC. Journal of Neurology. 2022;269(4):2140-2148.
- Incidence and Etiologies of Acquired Third Nerve Palsy Using a Population-Based Method. Fang C, Leavitt JA, Hodge DO, et al. JAMA Ophthalmology. 2017;135(1):23-28.
- Adult Strabismus Preferred Practice Pattern. Dagi LR, Velez FG, Holmes JM, et al. Ophthalmology. 2024;131(4):P306-P403.
Standard texts: Kanski's Clinical Ophthalmology: A Systematic Approach, Oxford Handbook of Ophthalmology, Oxford American Handbook of Ophthalmology, Signs in Ophthalmology: Causes & Differential Diagnosis.