Posterior Circulation Aneurysm Symptoms
Most posterior circulation aneurysms remain asymptomatic until rupture, but when unruptured aneurysms do cause symptoms, they typically present with cranial nerve compression (especially isolated oculomotor nerve palsy), brainstem compression symptoms, or mass effect on surrounding structures 1, 2, 3.
Unruptured Aneurysm Presentations
Compressive Symptoms
- Cranial nerve palsies - particularly isolated oculomotor nerve (CN III) palsy, which is highly suggestive of a posterior communicating artery aneurysm 2, 3
- Brainstem compression - causing ataxia, nystagmus, diplopia, vertigo, and imbalance 4
- Visual disturbances - including visual field defects, double vision, and blurred vision 4
- Hearing loss and tinnitus 4
- Headache - typically suboccipital-posterior cervical region, though often vague and non-specific 3, 5
The development of a new third nerve palsy ipsilateral to a posterior communicating artery aneurysm implies aneurysm growth and has traditionally been regarded as an indication for urgent treatment 2.
Location-Specific Symptoms
Top of basilar syndrome includes:
- Somnolence
- Peduncular hallucinosis
- Convergence nystagmus and skew deviation
- Oscillatory eye movements
- Retraction and elevation of eyelids
- Vertical gaze paralysis 4
Mid-basilar occlusions result in various pontine syndromes 4.
Proximal basilar occlusions can cause "locked-in" syndrome 4.
Ruptured Aneurysm Presentations (Subarachnoid Hemorrhage)
Classic Presentation
The hallmark is "the worst headache of my life" - described by 80% of alert patients - characterized as:
- Extremely sudden onset
- Immediately reaching maximal intensity (thunderclap headache)
- May occur during physical exertion but most commonly during routine daily activities 1
Associated Symptoms (in order of frequency)
- Nausea/vomiting - 77% of patients 1
- Headache - 74% 1
- Loss of consciousness - 53% 1
- Nuchal rigidity (stiff neck) - 35% 1
- Photophobia 1
- Focal neurological deficits including cranial nerve palsies 1
Warning/Sentinel Hemorrhage
Critical to recognize: 10-43% of patients report a warning headache (sentinel bleed) before major rupture, typically occurring 2-8 weeks prior 1. This sentinel headache:
- Is usually milder than major rupture but may last several days
- Increases odds of early rebleeding 10-fold
- May be accompanied by nausea/vomiting
- Meningismus is uncommon 1
Clinical pitfall: Misdiagnosis occurs in approximately 12% of cases (historically up to 64%), most commonly due to failure to obtain non-contrast head CT. Misdiagnosis is associated with nearly 4-fold higher likelihood of death or disability at 1 year 1.
Seizures
Occur in up to 20% of patients after rupture, most commonly in the first 24 hours, and more frequently with intracerebral hemorrhage, hypertension, and middle cerebral/anterior communicating artery aneurysms 1.
Key Distinguishing Features
Posterior circulation aneurysms differ from anterior circulation in that:
- They have higher rupture risk 6, 7
- Clinical presentation can be non-specific and mimic other conditions, resulting in diagnostic delays 4
- There may be an extended prodrome lasting days to months 4
- Small brainstem infarctions can be clinically devastating due to eloquent neurologic territory 4
Giant posterior circulation aneurysms (≥25mm) cause substantial mass effect with 50% prevalence of cranial nerve deficits, and aneurysm volume (not degree of brainstem displacement) correlates with disability 8.
As many as 12% of patients with ruptured aneurysms die before receiving medical attention 1, emphasizing the critical importance of recognizing warning symptoms and sentinel hemorrhages.