A 72-year-old hypertensive man presents with acute onset of severe chest pain radiating to the back. CT angiography shows a Type B aortic dissection with an intimal flap. Which finding best distinguishes an acute aortic dissection from a ruptured thoracic aortic aneurysm in the acute phase?

Explanation

## Distinguishing Acute Aortic Dissection from Ruptured Thoracic AAA ### Key Clinical Discriminator **High-Yield:** Differential blood pressure or pulse deficits between the upper extremities (or between upper and lower limbs) is the most specific discriminating feature of acute aortic dissection. This occurs because the dissecting flap can selectively compromise branch vessel ostia, reducing perfusion to one arm or leg. Ruptured thoracic AAA does not typically produce this regional perfusion deficit. ### Comparison Table | Feature | Acute Aortic Dissection | Ruptured Thoracic AAA | |---------|------------------------|---------------------| | **Intimal flap** | Present (pathognomonic) | Absent | | **Differential BP/pulses** | Common (20–30% of cases) | Rare | | **Pulse deficits** | May affect single limb/carotid | Generalized shock | | **Hemodynamics** | Hypertension common early | Hypotension/shock | | **Pain character** | Tearing, sudden onset | Sudden, severe | | **Aortic regurgitation** | May occur (Type A) | Unlikely | ### Clinical Pearl **Key Point:** While both dissection and ruptured AAA present with sudden severe chest/back pain and hemodynamic derangement, dissection may preserve regional perfusion initially because the false lumen can remain patent. The intimal flap may selectively obstruct branch vessels (left subclavian, carotid, iliac), producing asymmetric pulses or blood pressures. This finding is highly specific for dissection and absent in simple rupture. ### Mnemonic **DISSECTION → Different Perfusion**: Dissection creates an intimal flap that can obstruct branch vessels asymmetrically, causing differential pulses and blood pressures between limbs. Rupture causes global hemorrhage without regional obstruction. ### Pathophysiology In acute aortic dissection, the intimal tear allows blood to track into the media, creating a false lumen. Depending on the dissection's extent and location, branch vessels (subclavian, carotid, renal, iliac) may be compressed or occluded by the flap, reducing perfusion to that territory. A ruptured AAA causes uncontrolled bleeding into the pleural or peritoneal space, leading to generalized shock without selective branch obstruction. [cite:Harrison 21e Ch 256]