A pseudoaneurysm, or false aneurysm, is a contained rupture of an artery that results in a hematoma in the surrounding tissue, which remains in communication with the artery through a small opening or "neck". Unlike a true aneurysm, which involves the dilation of all three layers of the arterial wall, a pseudoaneurysm lacks a true vessel wall. It is most commonly an iatrogenic complication following percutaneous procedures like cardiac catheterization or angiography, often occurring in the femoral artery in the groin. Without proper treatment, a pseudoaneurysm can continue to expand, leading to significant complications such as rupture, nerve compression, or even distal embolization.
The Mechanism: How Thrombin Induces Thrombosis
Thrombin is a central enzyme in the body's coagulation cascade. Its role is to convert the soluble plasma protein fibrinogen into insoluble fibrin. In the treatment of a pseudoaneurysm, a concentrated solution of thrombin is precisely injected into the false aneurysm sac under ultrasound guidance. Upon contact with the stagnant blood within the sac, the thrombin triggers an immediate and robust clotting reaction.
- Fibrinogen to Fibrin Conversion: The injected thrombin acts as a powerful catalyst, converting the fibrinogen in the blood inside the pseudoaneurysm sac into a mesh-like fibrin clot. This reaction occurs rapidly, often within seconds.
- Platelet Activation: Thrombin also acts on platelets, causing them to clump together and further reinforce the forming clot.
- Localized Action: The treatment is highly localized. Since the blood within the pseudoaneurysm sac is relatively static compared to the high-flow parent artery, the thrombin can induce a complete clot without being washed out into the main circulation. This is a key safety feature of the procedure.
- Effectiveness with Anticoagulation: The direct action of injected thrombin on fibrinogen means the treatment remains effective even in patients taking anticoagulant medications like heparin or warfarin. This is a major advantage over older methods like manual compression, which can fail in these patients.
The Ultrasound-Guided Thrombin Injection Procedure
This minimally invasive procedure is typically performed by an interventional radiologist or vascular surgeon with real-time ultrasound imaging. The step-by-step process is carefully executed to ensure safety and effectiveness:
- Locating the Pseudoaneurysm: The physician uses a high-frequency ultrasound transducer to visualize the pseudoaneurysm, its neck (the connection to the artery), and the parent artery. Color Doppler is used to identify the characteristic "to-and-fro" swirling blood flow within the sac.
- Needle Placement: A fine needle is inserted through the skin and guided into the center of the pseudoaneurysm sac, as far away from the neck as possible. Injecting into the center minimizes the risk of thrombin leaking into the main artery and causing distal embolization.
- Confirming Position: A small amount of saline can be injected to confirm the needle tip's position within the sac. The physician observes a "color flash" on the Doppler image.
- Thrombin Injection: A low-dose solution of thrombin is slowly injected. The doctor observes for the immediate appearance of echogenic material (the blood clot) and the characteristic "whirlpool" sign indicating thrombosis.
- Monitoring: The injection continues in small increments until the pseudoaneurysm cavity is completely thrombosed, and all flow within the sac ceases. Post-procedure ultrasound confirms successful occlusion.
- Post-Procedure: The patient is monitored for a short period, typically with bed rest for a few hours, to ensure the clot is stable and no complications arise.
Comparison: Thrombin Injection vs. Other Treatments
For decades, manual compression under ultrasound guidance or open surgical repair were the primary treatment options for pseudoaneurysms. However, as documented in studies, thrombin injection has emerged as the preferred method for many cases due to its superior efficacy, speed, and patient comfort.
| Feature | Thrombin Injection (UGTI) | Manual Compression (UGCR) | Surgical Repair |
|---|---|---|---|
| Success Rate | Very high (92–100%) | Lower (54–80%) | Very high, but more invasive |
| Procedure Time | Seconds to minutes | Often 30+ minutes, multiple sessions possible | Hours, requiring anesthesia |
| Patient Comfort | Minimal discomfort, no sedation needed | Can be very painful and difficult to tolerate | Requires anesthesia, followed by surgical site pain |
| Anticoagulation | Effective in anticoagulated patients | Lower success rates in anticoagulated patients | Requires discontinuation of anticoagulants |
| Complications | Rare (embolization, allergic reaction) | Rare (bruising, failure) | Higher risk (infection, nerve damage) |
| Hospital Stay | Usually outpatient procedure | Outpatient procedure, but can be lengthy | Longer stay for recovery |
Potential Risks and Mitigating Factors
While thrombin injection is generally safe and well-tolerated, potential risks exist, though they are infrequent.
- Native Artery Thrombosis or Distal Embolization: This is the most serious potential complication and occurs if thrombin inadvertently leaks from the sac into the parent artery. Studies confirm that injecting the thrombin slowly and deliberately into the center of the sac, away from the neck, significantly reduces this risk. Careful ultrasound guidance is crucial.
- Allergic Reaction: Using bovine (cow-derived) thrombin carries a risk of an immunogenic or allergic reaction, especially after repeated exposure. Using human or synthetic thrombin minimizes this risk, making it a safer option.
- Failed Treatment/Recanalization: A small percentage of pseudoaneurysms may not fully clot or may recanalize (reopen) later, especially larger or complex ones. These cases may require a second injection or alternative treatment, such as surgical repair.
When is Thrombin Injection Indicated?
Ultrasound-guided thrombin injection is now the first-line treatment for many pseudoaneurysms, particularly those that are iatrogenic and located in peripheral arteries like the femoral artery. However, it is not suitable for all cases. Conditions that may contraindicate thrombin injection include:
- Large, Wide Neck: Pseudoaneurysms with a neck wider than 1 cm carry a higher risk of thrombin leaking into the main artery.
- Infection: Injecting into an infected area can spread the infection.
- Rapid Expansion: If the pseudoaneurysm is rapidly expanding and causing hemodynamic instability, surgical repair is often the more appropriate treatment.
- Concomitant Arteriovenous Fistula: A connection between an artery and a vein alongside the pseudoaneurysm can increase the risk of complications.
- Short, Wide Neck: For these specific types, especially involving smaller vessels like the brachial artery, surgical or alternative endovascular options may be considered.
Conclusion
For many patients with pseudoaneurysms, particularly those arising from catheterization procedures, ultrasound-guided thrombin injection represents the optimal treatment strategy. The procedure's success is rooted in the powerful procoagulant properties of thrombin, which enables the rapid formation of a blood clot to seal the defect. Offering high success rates, minimal invasiveness, reduced discomfort, and effectiveness even in anticoagulated patients, this modern technique has largely replaced more cumbersome methods like manual compression. However, careful patient selection and precise procedural technique, as highlighted by a study published in the Journal of Thrombosis and Haemostasis, are vital to minimizing risks and ensuring the best possible outcome.
