Definition and epidemiology
An aneurysm is a localized, permanent abnormal dilation of a blood vessel involving all three layers of the arterial wall (intima, media, adventitia), distinguishing it from a pseudoaneurysm, which is a contained rupture with blood held only by adventitia or surrounding tissue (not true vessel wall).
Aneurysms are classified by:
- Location: aortic (thoracic, abdominal), peripheral (popliteal, femoral, splenic, renal), cerebral (berry/saccular, usually at the circle of Willis)
- Shape: fusiform (symmetric, circumferential dilation) vs saccular (focal outpouching)
- Wall layers involved: true aneurysm (all three layers) vs false/pseudoaneurysm
Abdominal aortic aneurysm (AAA) is the most common and clinically important type. Prevalence is roughly 4-8% in men over age 65 and lower in women. Risk rises sharply with age, smoking history, and family history. Popliteal aneurysms are the most common peripheral aneurysm and are bilateral in a substantial proportion of cases. Cerebral (berry) aneurysms occur in roughly 2-3% of the general population, most are asymptomatic, and they are the leading cause of nontraumatic subarachnoid hemorrhage.
- AAA screening: one-time ultrasound for men 65-75 who have ever smoked
- Elective AAA repair threshold: diameter ≥ 5.5 cm or rapid growth > 0.5 cm/6 months
- Marfan and Loeys-Dietz syndromes classically cause thoracic aortic aneurysm/dissection
- Tertiary syphilis causes thoracic aortic aneurysm (vasa vasorum obliteration)
- Ruptured AAA: triad of abdominal pain, hypotension, pulsatile mass
- Berry aneurysms cluster at the circle of Willis and link to ADPKD
- Thunderclap headache = classic presentation of ruptured cerebral aneurysm
- Popliteal aneurysm is the most common peripheral aneurysm, often bilateral
Etiology and risk factors
Major risk factors
- Atherosclerosis (dominant cause of aortic and peripheral aneurysms)
- Smoking — strongest modifiable risk factor for AAA
- Hypertension
- Advanced age and male sex
- Family history of aneurysm (first-degree relative significantly increases risk)
- Connective tissue disorders: Marfan syndrome (fibrillin-1 defect), Ehlers-Danlos syndrome (vascular type), Loeys-Dietz syndrome — these favor thoracic aortic aneurysm and dissection, often at a younger age
Other causes
- Infection ("mycotic" aneurysm): Salmonella, Staphylococcus, syphilis (tertiary syphilis classically causes thoracic aortic aneurysm)
- Vasculitis: Takayasu arteritis, giant cell arteritis, Behçet disease
- Trauma: blunt or penetrating injury, iatrogenic (catheterization sites causing pseudoaneurysm)
- Congenital: berry aneurysms associated with autosomal dominant polycystic kidney disease, bicuspid aortic valve, coarctation of the aorta
Pathophysiology
Arterial wall integrity depends on elastin and collagen within the media and adventitia. Aneurysm formation results from progressive degradation of these structural proteins, driven by:
- Chronic inflammation and matrix metalloproteinase (MMP) activity, which breaks down elastin and collagen
- Hemodynamic wall stress — per Laplace's law, wall tension increases with vessel radius, creating a self-perpetuating cycle where dilation promotes further dilation
- Smooth muscle cell apoptosis and loss of medial elastic lamellae, seen prominently in cystic medial degeneration (classic in Marfan syndrome)
- Ischemia of the vasa vasorum, reducing the arterial wall's own blood supply and nutrient delivery
As the wall weakens and the vessel enlarges, tensile stress rises further, increasing rupture risk. This explains why rupture risk increases disproportionately once an AAA exceeds about 5.5 cm.
Clinical features
Most aneurysms are asymptomatic and discovered incidentally on imaging or physical exam.
Abdominal aortic aneurysm
- Pulsatile, nontender abdominal mass (often periumbilical)
- May cause vague abdominal or back pain as it enlarges
- Rupture: sudden severe abdominal or back pain, hypotension, pulsatile mass — a surgical emergency
Thoracic aortic aneurysm
- Often asymptomatic; found on chest imaging
- Can cause chest pain, hoarseness (recurrent laryngeal nerve compression), dysphagia, or cough from compression of adjacent structures
Peripheral (popliteal/femoral)
- Pulsatile mass behind the knee or in the groin
- Can present with limb ischemia from thrombosis or distal embolization
Cerebral (berry) aneurysm
- Usually silent until rupture
- Rupture causes sudden, severe ("thunderclap") headache, often described as "worst headache of my life," with neck stiffness from subarachnoid hemorrhage
- Unruptured large aneurysms may cause cranial nerve palsy (e.g., CN III palsy with pupil involvement from posterior communicating artery aneurysm)
Diagnosis
Imaging
- Abdominal ultrasound: initial screening and surveillance tool for AAA; inexpensive, no radiation
- CT angiography: gold standard for precise sizing, planning repair, and detecting rupture or leak
- MR angiography: alternative when contrast or radiation must be avoided
- CT angiography of the head or digital subtraction angiography: for suspected cerebral aneurysm
Screening
- One-time abdominal ultrasound screening is recommended for men aged 65-75 who have ever smoked (major US and European guidelines concur on this core recommendation; exact criteria for other groups vary)
Table: size and management thresholds (AAA)
| Diameter | Typical action |
|---|---|
| less than 4.0 cm | Ultrasound surveillance every 2-3 years |
| 4.0-5.4 cm | Ultrasound or CT surveillance every 6-12 months |
| 5.5 cm or greater, or rapid growth, or symptomatic | Elective repair |
Rapid growth is generally defined as an increase of more than 0.5 cm in 6 months, which independently prompts repair regardless of absolute size.
Differential diagnosis
- Aortic dissection: tearing chest/back pain, intimal tear with false lumen, can coexist with or complicate an aneurysm
- Pseudoaneurysm: contained rupture, often iatrogenic (post-catheterization), lacks all three wall layers
- Arteriovenous malformation or fistula: abnormal vascular connection rather than wall dilation
- Retroperitoneal or abdominal tumor: can mimic a pulsatile mass but is not truly pulsatile (transmitted pulsation only)
- Diverticular abscess or other intra-abdominal mass: for presentations with abdominal pain and mass
- Migraine or tension headache: must be distinguished from sentinel or ruptured aneurysm headache by onset and severity
Management
Unruptured, asymptomatic, below threshold
- Surveillance imaging at intervals based on size (see table above)
- Risk factor modification: smoking cessation (single most important intervention), blood pressure control, statin therapy for atherosclerosis
- Beta-blockers or other antihypertensives are used particularly in thoracic aortic disease and connective tissue disorder-associated aneurysms to reduce wall stress
Elective repair indications
- AAA ≥ 5.5 cm (lower threshold, around 5.0 cm, often used in women due to smaller baseline aortic size and relatively higher rupture risk at a given diameter)
- Rapid growth (> 0.5 cm/6 months)
- Symptomatic aneurysm (pain, tenderness)
- Saccular morphology or connective tissue disease (lower thresholds apply)
Repair options
- Endovascular aneurysm repair (EVAR): catheter-based stent graft; lower perioperative morbidity and mortality, preferred in suitable anatomy, but requires long-term surveillance for endoleak
- Open surgical repair: durable, preferred when anatomy is unsuitable for EVAR or in young, fit patients; higher immediate perioperative risk but less long-term reintervention
Ruptured aneurysm
- Surgical emergency: immediate hemodynamic resuscitation (permissive hypotension to avoid dislodging clot) and emergent open or endovascular repair
- Mortality is very high, and many patients do not survive to reach the hospital
Cerebral aneurysm
- Unruptured: observation vs. endovascular coiling or surgical clipping based on size, location, and patient factors
- Ruptured (subarachnoid hemorrhage): urgent securing of the aneurysm (coiling or clipping), nimodipine to reduce vasospasm-related ischemia, and management in a neurocritical care setting
Complications and prognosis
- Rupture: the feared complication; mortality is extremely high, especially outside the hospital
- Thrombosis and distal embolization: particularly in peripheral aneurysms (e.g., popliteal), causing acute limb ischemia
- Compression of adjacent structures: thoracic aneurysms can compress trachea, esophagus, or recurrent laryngeal nerve
- Aortic dissection: can arise from or complicate an aneurysm
- Post-repair complications: endoleak after EVAR, graft infection, and the need for lifelong imaging surveillance after EVAR
Prognosis is excellent with elective, timely repair of asymptomatic aneurysms above threshold size, but survival drops sharply once rupture occurs, underscoring the importance of screening and surveillance in at-risk populations.
CRASH and BURN for ruptured AAA: Cardiac risk, Rupture pain, Abdominal mass, Shock, Hypotension — and remember BURN = Back pain, Unstable, Ruptured, Needs emergency surgery.
Practice questions
1. A 70-year-old man who smoked for 40 years presents for a routine physical exam. He is asymptomatic. Abdominal ultrasound reveals an infrarenal aortic diameter of 4.2 cm. What is the most appropriate next step in management?
2. A 58-year-old woman with Marfan syndrome has a dilated aortic root on echocardiogram. Which medication class is most appropriate to reduce the rate of aortic dilation while she undergoes serial monitoring?
3. A 62-year-old man presents to the emergency department with sudden severe abdominal and back pain, diaphoresis, and a blood pressure of 80/50 mmHg. On exam there is a pulsatile, tender abdominal mass. What is the most appropriate immediate step?
Frequently asked questions
What causes an aneurysm?
Aneurysms are caused by progressive weakening of the arterial wall, most often from atherosclerosis combined with smoking and hypertension. Other causes include connective tissue disorders like Marfan syndrome, infections, vasculitis, and trauma. The common pathway is degradation of elastin and collagen in the vessel wall, which allows it to stretch and balloon over time.
What are the first symptoms of an aneurysm?
Most aneurysms cause no symptoms at all and are found incidentally on imaging. When symptoms occur, they depend on location: a pulsatile abdominal mass or vague back pain with abdominal aortic aneurysm, chest pain or hoarseness with thoracic aneurysm, or a painless pulsatile lump behind the knee with popliteal aneurysm. Sudden severe pain usually signals rupture or expansion.
How is an aneurysm diagnosed?
Aneurysms are diagnosed with imaging, most often abdominal ultrasound for initial detection and surveillance, and CT angiography for precise measurement and surgical planning. Cerebral aneurysms are evaluated with CT or MR angiography. Diagnosis is often incidental, found while imaging for an unrelated problem.
Is an aneurysm curable?
An aneurysm itself does not resolve on its own, but it can be effectively treated. Small aneurysms are monitored with regular imaging and risk factor control, while larger or rapidly growing ones are repaired surgically or with an endovascular stent graft, which essentially eliminates the risk of rupture from that segment of vessel.
Aneurysm vs aortic dissection — what is the difference?
An aneurysm is a permanent dilation of the entire vessel wall, while a dissection is a tear in the inner layer of the aortic wall that allows blood to split the layers apart, creating a false lumen. Dissection typically presents with sudden tearing pain, while aneurysms are usually asymptomatic until they rupture or grow large enough to compress nearby structures.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: I72
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.