Pseudoaneurysm & the Yin-Yang Sign
1. What is a Pseudoaneurysm (False Aneurysm)?
Unlike a true aneurysm, which involves all three layers of the arterial wall (intima, media, and adventitia), a pseudoaneurysm is a collection of blood leaking out of an artery into the surrounding tissue, contained only by a fibrous capsule or adventitia.
- Common Causes: Iatrogenic (post-catheterization, e.g., femoral artery), trauma, or infection (mycotic).
2. The Yin-Yang Sign
The Yin-Yang sign is a classic appearance seen on Color Doppler Ultrasound:
- Mechanism: It represents the swirling, bidirectional blood flow within the sac of the pseudoaneurysm.
- Appearance: A circular area filled with swirling red and blue color flow, representing blood flowing toward and away from the transducer as it rotates within the cavity.
3. Imaging & Diagnostic Clues
| Modality | High-Yield Finding |
|---|---|
| Doppler US | Yin-Yang sign; “to-and-fro” spectral waveform at the neck of the pseudoaneurysm. |
| CT Angiography | Focal contrast-filled sac adjacent to an artery with a detectable “neck” connecting them. |
4. Clinical Pearls
- “To-and-Fro” Flow: The most specific Doppler sign. It refers to blood flowing into the sac during systole and out of the sac during diastole.
- Management: Depends on size and location. Options include ultrasound-guided compression, thrombin injection into the sac, or surgical/endovascular repair.