A 68-year-old man with a history of hypertension and smoking presents to the emergency department with sudden-onset severe back pain radiating to the left flank. On examination, he is diaphoretic, blood pressure is 160/95 mmHg in the right arm and 140/85 mmHg in the left arm, and femoral pulses are diminished bilaterally. Abdominal examination reveals a pulsatile mass. A stat CT angiography of the chest and abdomen with contrast shows a large infrarenal aortic aneurysm with a crescent-shaped area of high attenuation within the aneurysm wall and periaortic blood. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Contained Rupture of Infrarenal AAA ### Key Presentation Features **Key Point:** The classic triad of sudden severe back/flank pain, hypotension (or relative hypotension with hypertension history), and a pulsatile abdominal mass is pathognomonic for ruptured AAA. **High-Yield:** The crescent sign (crescent-shaped high-attenuation area within the aneurysm wall) on contrast-enhanced CT is a hallmark of contained rupture — blood is actively extravasating but contained by the retroperitoneal fascia and surrounding tissues, preventing exsanguination. ### Imaging Interpretation | Finding | Significance | |---------|-------------| | Crescent sign (high attenuation within wall) | Active extravasation; contained rupture | | Periaortic blood / stranding | Retroperitoneal hemorrhage | | Infrarenal location | Allows for emergency repair without cross-clamping suprarenal aorta | | Blood pressure differential between arms | Possible aortic involvement, but dissection ruled out by imaging | ### Pathophysiology of Contained Rupture When an AAA ruptures, the initial hemorrhage is often contained by: 1. The retroperitoneal fascia (posterior and lateral containment) 2. Anterior peritoneum (limited anterior containment) 3. Surrounding viscera and tissues This temporary containment allows the patient to reach the hospital alive, but the risk of free rupture is imminent without urgent surgical intervention. **Clinical Pearl:** A contained rupture is a surgical emergency requiring immediate operative repair. The mortality rate for contained rupture is ~30–50%, compared to >80% for free rupture. **Warning:** Do not delay imaging or operative planning. Even a "stable" patient with contained rupture can deteriorate rapidly if the contained hematoma ruptures freely into the peritoneal cavity. ### Why This Is Not Dissection Although blood pressure differential between arms can occur in aortic dissection, the imaging findings (crescent sign, periaortic blood, aneurysm morphology) are diagnostic of rupture, not dissection. Dissection typically shows an intimal flap and false lumen, not extravasation.