An aneurysm is a permanent localized pathologic dilation of a blood vessel segment exceeding the normal diameter.
An aortic aneurysm is a permanent, localized dilation of the aortic wall.
Age-related Changes in Arteries
- Arteries become stiffer , wider (aneurysm formation), and longer (tortuosity) with age.
Pathogenesis & Causes of Aneurysms
Main Mechanism:
- Degradation or abnormal production of aortic wall structural proteins: collagen and elastin.
- Most aneurysms arise due to degenerative disease — predominantly atherosclerosis .
Specific Causes:
- Structural weakness and altered haemodynamic forces on the vessel wall.
- Intimal damage and loss.
- Reduced elastin and collagen content in the media and adventitia:
- Collagen: Provides tensile strength (mainly in adventitia).
- Elastin: Provides recoil capacity (mainly in media).
Risk Factors for Aortic Aneurysms
- Smoking (strongest modifiable risk factor)
- Hypertension
- Hypercholesterolemia
Laplace’s Law
- Wall tension (T) = Pressure (P) × Radius (r)
- As the radius increases, wall tension rises , causing progressive aneurysm enlargement and risk of rupture.
Rare Causes of Aneurysms
| Cause | Examples/Notes |
|---|---|
| Congenital | Marfan’s syndrome (mutation in fibrillin-1 gene ), Berry aneurysms (cerebral) |
| Post-stenotic | Coarctation of the aorta, cervical rib, popliteal artery entrapment syndrome |
| Traumatic | Gunshot, stab wounds, arterial punctures (often descending thoracic aorta just beyond ligamentum arteriosum) |
| Inflammatory | Vasculitides such as Takayasu’s arteritis, Behcet’s disease |
| Mycotic (infectious) | Infection with Staph, Strep, Salmonella, fungi; often saccular; associated with bacterial endocarditis, syphilis, TB |
| Pregnancy-associated | Splenic, cerebral, aortic, renal, iliac, coronary aneurysms |
Classification of Aortic Aneurysms
1. By wall involvement
- True aneurysm: Involves dilation of all three vessel wall layers (intima, media, adventitia).
- False aneurysm (pseudoaneurysm): Disruption of intima and media; dilated segment lined by adventitia or perivascular tissue and clot; sac formed by compressed surrounding tissue.
2. By gross morphology
- Fusiform aneurysm: Spindle-shaped, involves entire circumference → diffuse dilation.
- Saccular aneurysm: Localized outpouching involving a segment of the vessel wall.
3. By location
- Thoracic aortic aneurysm (TAA): Ascending, arch, descending thoracic aorta.
- Abdominal aortic aneurysm (AAA): Below diaphragm, most commonly infra-renal.
- Thoracoabdominal aneurysm: Continuous from thoracic to abdominal aorta.
Epidemiology
| Feature | Detail |
|---|---|
| Most aneurysms (>90%) | Abdominal aorta |
| Most common segment | Infra-renal abdominal aorta (95%) |
| Gender ratio | Male:Female = 4:1 |
| Geographic prevalence | More common in Western countries |
| Prevalence | 5% of people >50 years, 15% >80 years |
| Associated aneurysms | Iliac (30%), Popliteal (10%) |
Thoracic Aortic Aneurysm (TAA)
Clinical Features
- Mostly asymptomatic , found incidentally.
- Symptoms if compressing adjacent structures:
- Acute chest pain
- Dyspnea, cough, hoarseness (due to recurrent laryngeal nerve involvement)
- Aortic regurgitation → congestive heart failure (if ascending aorta involved)
- Superior vena cava compression → head, neck, upper extremity congestion
Diagnosis
- Chest X-ray: Widened mediastinum, tracheal or bronchial displacement.
- Echocardiography: Especially transesophageal echo for proximal ascending and descending thoracic aorta.
- Contrast-enhanced CT, MRI, aortography: Gold standard for size and branch involvement.
- Additional: ECG, ESR, Urea & Electrolytes.
Treatment
- Medical:
- β-blockers (especially in Marfan’s) to reduce shear stress and expansion rate.
- Control hypertension.
- Angiotensin receptor blockers (ARBs) may help in Marfan’s (reduce TGF-β signaling).
- Surgical:
- Symptomatic aneurysms or ascending aortic diameter ≥5.5 cm.
- Growth >0.5 cm/year.
- Marfan’s syndrome: surgery at 4–5 cm.
- Degenerative descending thoracic aneurysms: surgery at >6 cm; consider endovascular repair at >5.5 cm.
Abdominal Aortic Aneurysm (AAA)
Anatomy Reminder
- Begins at T12 , ends at L4 .
- Relations:
- Anterior: splenic vein, pancreas, duodenum
- Right: IVC, azygos vein
- Left: sympathetic trunk
- Branches:
- Paired visceral: suprarenal, renal, gonadal arteries
- Unpaired visceral: celiac trunk, SMA, IMA
- Paired abdominal wall: subcostal, inferior phrenic, lumbar arteries
Epidemiology & Pathophysiology
- More common in males, incidence increases with age.
-
90% of AAA >4 cm are atherosclerotic.
- Most are infra-renal.
- Rupture risk correlates with size.
Clinical Features
- Usually asymptomatic, found on routine exam or imaging.
- Palpable, pulsatile, expansile, nontender abdominal mass.
- Expansion may cause:
- Abdominal or back pain
- Pulsations felt by patient
- Chest, lower back, or scrotal pain
- Rupture: Acute severe pain, hypotension, requires emergency surgery.
