A 38-year-old woman is referred from ophthalmology with a 3-week history of progressive right-sided ptosis and double vision. On examination she has complete right CN III palsy — ptosis, eye deviated down and out, and a fully dilated unreactive right pupil. She has no headache.
What is the most likely diagnosis? This is not Bell's palsy — what's the key difference and what do you do next?
What is a Cerebral Aneurysm?
A cerebral aneurysm is a localised abnormal dilatation of an intracranial artery wall, arising at points of structural weakness — typically at arterial branching points on the Circle of Willis. The wall lacks the normal tunica media (muscular layer), making it vulnerable to expansion and rupture under haemodynamic stress.
Prevalence in the general population is approximately 3–5% — meaning millions of people are walking around with unruptured aneurysms. Most never rupture. The annual rupture risk for a small unruptured aneurysm is approximately 0.5–1% per year — but rupture, when it occurs, carries 40–50% mortality.
Types and Locations
| Type | Features | Clinical Relevance |
|---|---|---|
| Saccular (berry) | Round outpouching at branching point — 90% of all aneurysms | Ruptures → SAH. Most common at ACoA, PCoA, MCA bifurcation |
| Fusiform | Diffuse dilatation of entire vessel circumference | Basilar artery most common. Compressive symptoms. Rarely rupture. |
| Mycotic | Infected emboli — vessel wall destruction | Infective endocarditis. Peripheral location. Treat infection first. |
| Giant (>25mm) | Large enough to act as space-occupying lesion | Compressive symptoms + high rupture risk |
How Aneurysms Present
1. Rupture — Subarachnoid Haemorrhage
The most dramatic presentation — sudden thunderclap headache, meningism, altered consciousness. Read our full SAH article for complete management. Approximately 80% of non-traumatic SAH is caused by ruptured aneurysms.
2. Mass Effect — Compressive Symptoms
Large unruptured aneurysms compress adjacent structures. The most important example is the posterior communicating artery (PCoA) aneurysm causing CN III palsy — as in our case.
Why the pupil? The pupillomotor parasympathetic fibres run on the outer surface of CN III. An expanding PCoA aneurysm compresses these fibres first — causing a dilated, unreactive pupil. This is the key distinguishing feature from a medical CN III palsy (e.g. diabetic mononeuropathy) where ischaemia affects the core motor fibres and typically spares the pupil.
Surgical CN III palsy (aneurysm): Pupil INVOLVED — dilated, unreactive. Painful. Urgent CT angiography.
Medical CN III palsy (diabetic/ischaemic): Pupil SPARED — normal size and reaction. Painless. MRI + vascular risk factor management.
Any doubt → treat as surgical until proven otherwise. A missed aneurysm will rupture.
3. Incidental Discovery
Increasingly, aneurysms are found incidentally on MRI/MRA done for other reasons (e.g. headache workup). Management depends on size, location, patient age, and risk factors.
Investigation
Imaging Pathway
- CT angiography (CTA): First-line for suspected unruptured aneurysm — fast, widely available, 95%+ sensitive for aneurysms >3mm
- MR angiography (MRA): No radiation, good for screening and follow-up of known aneurysms
- Digital Subtraction Angiography (DSA): Gold standard — gives dynamic flow information, essential before treatment planning. Used when CTA/MRA inconclusive or pre-operatively
- For ruptured aneurysm: CT head first (SAH), then CTA to find source, then DSA before intervention
Treatment
Two proven methods exist — the choice depends on aneurysm anatomy, patient factors, and centre expertise:
| Method | Technique | Best For | Advantages |
|---|---|---|---|
| Endovascular coiling | Catheter via femoral artery → coils packed into aneurysm sac → thrombosis | Most posterior circulation aneurysms, elderly patients, poor surgical candidates | Less invasive, faster recovery, now preferred globally for most aneurysms |
| Surgical clipping | Craniotomy → titanium clip across aneurysm neck | MCA aneurysms, complex neck anatomy, young patients, large haematoma needing evacuation | Definitive — permanently excludes aneurysm. Lower recanalization rate. |
| Flow diversion | Pipeline embolisation device — stent redirects flow away from aneurysm | Large/giant fusiform aneurysms, wide-neck aneurysms not suitable for coiling | Treats aneurysm without entering sac |
Unruptured Aneurysm Management
Not all aneurysms need treatment. The ISUIA (International Study of Unruptured Intracranial Aneurysms) provides risk data. General principles:
Treatment Decision Factors
- Size: <7mm in anterior circulation — low rupture risk, may observe. >7mm or posterior circulation — higher risk, consider treatment.
- Location: Posterior circulation and PCoA aneurysms rupture at smaller sizes.
- Shape: Irregular/multilobulated aneurysms have higher rupture risk than smooth ones.
- Symptoms: Any symptomatic aneurysm (compressive CN palsy, sentinel headache) should be treated regardless of size.
- Patient factors: Age, comorbidities, family history of SAH, hypertension, smoking.
- Observation: Small asymptomatic aneurysms in elderly patients may be safely monitored with serial MRA.
Painful CN III palsy with pupil involvement = PCoA aneurysm until proven otherwise. Urgent CT angiography confirmed a 7mm right posterior communicating artery aneurysm. Neurosurgery and interventional neuroradiology reviewed together. Given her age (38), good surgical candidate, and aneurysm morphology, surgical clipping was chosen. Right pterional craniotomy — aneurysm clipped successfully. CN III palsy resolved completely over 6 weeks as the nerve decompressed. She made a full recovery — because the ophthalmologist recognised the pupil involvement and referred urgently.
Saccular (berry) aneurysms most common — at Circle of Willis branching points. Rupture → SAH (thunderclap headache). Compression → CN III palsy (PCoA). Surgical CN III: pupil involved, painful. Medical CN III: pupil spared, painless. CTA first-line imaging. Coiling vs clipping — depends on anatomy and patient. Symptomatic aneurysm = treat regardless of size. >7mm anterior or any posterior = higher rupture risk.
References
- Wiebers DO et al. Unruptured intracranial aneurysms: natural history, clinical outcome, and risks of surgical and endovascular treatment (ISUIA). Lancet. 2003;362(9378):103–110.
- Molyneux AJ et al. International Subarachnoid Aneurysm Trial (ISAT) of neurosurgical clipping versus endovascular coiling. Lancet. 2002;360(9342):1267–1274.
- Thompson BG et al. Guidelines for the Management of Patients With Unruptured Intracranial Aneurysms. Stroke. 2015;46(8):2368–2400.
