Aortic aneurysm and dissection for USMLE Step 1
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Aortic aneurysm and aortic dissection for the USMLE Step 1 Exam
- Definition:
- An aortic aneurysm is an abnormal dilation of the aorta, usually classified as either thoracic aortic aneurysm (TAA) or abdominal aortic aneurysm (AAA).
- AAA is most common and usually occurs below the renal arteries.
- Pathophysiology:
- The aortic wall weakens due to factors affecting collagen and elastin, resulting in dilation.
- Inflammation and degradation of extracellular matrix proteins, particularly in AAAs, play a central role.
- Risk factors contribute to vascular wall stress, including:
- Smoking: Strongest modifiable risk factor for AAA.
- Hypertension: Increases shear stress on the aortic wall.
- Genetic conditions: Marfan syndrome and Ehlers-Danlos syndrome (particularly for TAA).
- Clinical Presentation:
- Asymptomatic: Most AAAs remain silent until rupture.
- Symptoms if present: May include a pulsatile abdominal mass, or vague back or abdominal pain, especially if expansion is rapid.
- Diagnosis:
- Imaging:
- Ultrasound: First-line for screening and diagnosis, especially for AAA.
- CT Angiography (CTA): Provides detailed visualization and is used preoperatively or in suspected rupture.
- Screening Recommendations: One-time ultrasound screening for men aged 65-75 who have smoked.
- Management:
- Small AAAs: Regular imaging surveillance.
- Large or Rapidly Expanding AAAs: Surgical repair (e.g., open surgery or endovascular aneurysm repair).
- Medical Management: Blood pressure control and smoking cessation are critical.
Aortic Dissection
- Definition:
- A tear in the aortic intima that allows blood to separate layers of the aortic wall, forming a true and false lumen.
- Classification:
- Stanford Classification:
- Type A: Involves the ascending aorta; typically managed surgically.
- Type B: Involves the descending aorta; often managed medically unless complications develop.
- DeBakey Classification (for reference):
- Type I: Both ascending and descending aorta.
- Type II: Ascending aorta only.
- Type III: Descending aorta only.
- Pathophysiology:
- Initiated by a tear in the intima, allowing blood to penetrate the media and create a false lumen.
- Risk Factors:
- Hypertension: Most common predisposing factor, increasing shear force.
- Connective Tissue Disorders: Marfan syndrome, Ehlers-Danlos syndrome.
- Bicuspid Aortic Valve: Associated with increased risk of aortic dissection.
- Clinical Presentation:
- Classic Symptom: Sudden, severe, “tearing” chest or back pain.
- Type A Complications: Aortic regurgitation, myocardial ischemia, cardiac tamponade.
- Type B Complications: May affect blood flow to abdominal organs or lower limbs.
- Diagnosis:
- Imaging:
- CT Angiography: Preferred method for acute diagnosis, visualizing the true and false lumens.
- Transesophageal Echocardiography (TEE): Useful in unstable patients, particularly for Type A dissections.
- Management:
- Type A Dissection: Surgical emergency.
- Type B Dissection: Typically managed with blood pressure control unless complications arise.
Key Points
- Aortic Aneurysm:
- Defined by abnormal aortic dilation, commonly in the abdomen (AAA).
- Main risk factors: Smoking, hypertension, and genetic disorders (Marfan, Ehlers-Danlos).
- Screening with ultrasound is recommended in older male smokers.
- Managed based on size; larger aneurysms may require surgical repair.
- Aortic Dissection:
- Life-threatening tear in the aorta leading to a false lumen.
- Classified by location (Stanford Type A and B).
- Acute chest or back pain is characteristic, and CTA is the imaging of choice.
- Type A requires surgery; Type B is often managed with blood pressure control.