A brain aneurysm is a bulge or ballooning in a blood vessel in the brain caused by a focal weakness in the vessel wall. Most aneurysms are unruptured and discovered incidentally on imaging performed for other reasons. When an aneurysm ruptures, it causes a life-threatening subarachnoid hemorrhage — one of the most catastrophic neurological emergencies — with mortality rates approaching 40% and significant disability among survivors.
Brain aneurysms are a leading cause of hemorrhagic stroke, specifically subarachnoid hemorrhage. They share the cerebrovascular landscape with other vascular abnormalities including arteriovenous malformations (AVMs) and cavernous malformations, all of which carry bleeding risk and are best characterized with dedicated MRI and vascular imaging. Patients with known aneurysms should also be monitored for the development of chronic small vessel disease, which shares hypertension as a common risk factor.
Causes
Arterial wall weakness
Aneurysms form at points of structural weakness in arterial walls — most commonly at bifurcation points where blood vessels branch, where hemodynamic stress is highest. Chronic hypertension, atherosclerosis, and turbulent blood flow at these branch points drive progressive wall degradation and outpouching. The vast majority occur at the base of the brain in the circle of Willis — particularly at the anterior communicating artery, posterior communicating artery, and middle cerebral artery bifurcation.
Risk factors
Modifiable risk factors include hypertension (the single most important), cigarette smoking, and heavy alcohol use — all of which accelerate arterial wall damage. Non-modifiable risk factors include female sex (women are more commonly affected, particularly after menopause), family history of intracranial aneurysms (first-degree relatives carry a 3–7x increased risk), and prior aneurysm (patients with one aneurysm have a 15–20% chance of harboring additional aneurysms). Inherited connective tissue disorders including polycystic kidney disease, Ehlers-Danlos syndrome type IV, and Marfan syndrome are associated with aneurysm formation at younger ages.
Symptoms
Unruptured aneurysms: The majority cause no symptoms and are found incidentally on MRI or CT performed for unrelated reasons such as headaches. Larger aneurysms may compress adjacent cranial nerves — a posterior communicating artery aneurysm classically compresses the oculomotor nerve (cranial nerve III), causing a painful, fixed, dilated pupil and ptosis that constitutes a neurological emergency even before rupture.
Ruptured aneurysm: Rupture produces a sudden, explosive "thunderclap headache" — universally described by patients as the worst headache of their life, reaching maximum intensity within seconds. This is accompanied by nausea, vomiting, photophobia, neck stiffness from meningeal irritation, and often brief loss of consciousness. A sentinel headache — a sudden severe headache days to weeks before major rupture — is recognized as a warning sign that is frequently missed or misattributed to migraine. Any sudden, severe, unusual headache demands emergency evaluation.
Diagnosis
CT scan (non-contrast) is the first-line emergency study when subarachnoid hemorrhage is suspected, detecting blood in the subarachnoid space in over 95% of cases within the first 12 hours. Sensitivity declines rapidly after 24 hours as blood is reabsorbed. Lumbar puncture is performed when CT is negative but clinical suspicion remains high — xanthochromia (yellow discoloration of CSF from blood breakdown products) confirms subarachnoid hemorrhage that CT missed.
CT angiography (CTA) rapidly identifies the aneurysm and characterizes its size, shape, neck morphology, and relationship to parent vessels — information essential for treatment planning. MR angiography (MRA) is the preferred tool for non-emergency screening of high-risk patients (family history, polycystic kidney disease) and for surveillance of known unruptured aneurysms, avoiding radiation. Digital subtraction angiography (DSA) — catheter-based cerebral angiography — remains the gold standard for detailed aneurysm characterization, particularly when endovascular treatment is planned.
Aneurysm Classification
Aneurysms are classified by size, shape, and rupture status, all of which inform treatment decisions:
- Small (less than 7mm): Lower annual rupture risk (approximately 0.1–1% per year for most locations). Often monitored with serial MRA surveillance. Risk varies significantly by location — posterior circulation and posterior communicating artery aneurysms carry higher rupture risk than anterior circulation aneurysms of the same size.
- Medium (7–12mm): Intermediate rupture risk. Treatment decisions individualized based on aneurysm characteristics, patient age, and health status.
- Large (13–24mm) and Giant (25mm or greater): Substantially higher rupture risk and more complex treatment due to size and often irregular morphology. Giant aneurysms may also cause symptoms from mass effect.
- Ruptured aneurysm: Urgent treatment required — the risk of rebleeding within 24 hours of initial rupture is approximately 15–20% and carries very high mortality.
Treatments
Treatment decisions for unruptured aneurysms carefully balance the annual rupture risk against the procedural risk of intervention, accounting for aneurysm size, location, morphology, patient age, and overall health. Ruptured aneurysms require urgent treatment to prevent fatal rebleeding.
Endovascular coiling (and flow diversion):
A microcatheter is guided through the blood vessels to the aneurysm, where platinum coils are packed into the sac to promote thrombosis and seal it from the circulation. This minimally invasive approach carries lower short-term procedural risk than open surgery and is used for the majority of aneurysms today. Flow diverter devices (Pipeline embolization device) are used for large or fusiform aneurysms not amenable to coiling — a stent-like implant redirects blood flow away from the aneurysm sac, causing it to thrombose over time.
Surgical clipping:
A neurosurgeon places a titanium clip across the neck of the aneurysm through an open craniotomy, permanently and immediately excluding it from circulation. Clipping is preferred for certain aneurysm shapes, locations, and in younger patients where long-term durability is particularly important, as clipping has lower recurrence rates than coiling. It provides definitive, durable treatment but carries higher short-term surgical risk.
Observation with surveillance imaging:
Small, low-risk unruptured aneurysms in older patients or those with significant medical comorbidities may be managed conservatively with serial MRA every 1–3 years, aggressive blood pressure control, and smoking cessation. Risk factor modification — particularly hypertension treatment and smoking cessation — is the most important intervention for reducing rupture risk in monitored aneurysms.
Get an MRI to Confirm Your Diagnosis
Before surgical planning or starting treatment, a clear MRI diagnosis ensures the right path forward. First Look MRI offers self-pay Brain MRI scans — no doctor's order or insurance required — at our locations in Georgia, Texas, and Colorado.