Thrombosis

• Define thrombus
• List risk factors for thrombus formation
• Describe the morphological features of a thrombus (macro & micro)
• Discuss the consequences of thrombosis

• Thrombus – a solidification of blood constituents formed within the vascular system during life
• Haematoma (blood clot) – solidification of blood constituents outside the vascular system or after death
• Thrombosis – the process of thrombus formation

Abnormal flow prevents the dilution of clotting factors, stops clotting factor inhibitors from reaching the endothelium and promotes endothelial cell activation. There are 2 main types of blood flow found in the human body.

Stasis is another type of blood flow which increases the risk of thrombosis. Stasis is particularly relevant in the development of deep vein thromboses.

Risk factors can be divided into inherited and acquired. Please match the risk factor under the suitable headers.

Thrombosis can occur in both arteries and veins

Please be aware that although thrombosis can occur in arteries and veins, atheroma can only occur in the arterial system.

The risk factors for arterial thrombosis and venous thrombosis are generally different.

This is a cross section through the ventricles of the heart

The trabeculae are visible at point (b).

Notice much of the space in the left ventricle is occupied by a pale lesion marked (a). It is a large (mural) thrombus.

It is much paler than the thrombi in previous pictures. This is because in the heart or large arteries blood flow is so fast that it is mostly platelets and leucocytes which form the thrombi.

In smaller arteries and veins the blood flows much slower and red cells get entangled with the fibrin and make the thrombus look more red.

This is a microscopic picture of a deep vein thrombosis located in fat tissue.

Note the thin wall of the vein and the large thrombus located within the lumen.

It is composed of fibrin, platelets and red blood cells. The alternating pattern of red/white lines are known as the ‘Lines of Zahn’

There are 5 main outcomes of thrombosis: lysis, organisation, occlusion, embolism & propagation. Embolism & occlusion will be covered in more detail in later Pathologia modules.

1. Lysis – the body naturally dissolves via fibrinolytic mechanisms

2. Propagation – occurs if a vein is completely occluded. The column of blood above the thrombus will clot and enlarge the thrombus. If this encounters the tributary of another vein then the blood in that can also thrombose. The process of thrombosis extends into bigger and bigger veins.

3. Embolism – part of the thrombus can detach and travel to a distant site

4. The thrombus enlarges and occludes the blood flow through the artery or vein. Depending on the organ that is involved it can have varying consequences

5. Organisation – an occluding thrombus is reorganised (see next slide for more detailed steps)

1. An acute inflammatory reaction occurs at the edge of the thrombus

2. A chronic inflammatory reaction with mostly lymphocytes infiltrates the thrombus

3. Capillary loops grow into the thrombus from the endothelial surface, revascularizing it

4. Fibroblasts infiltrate the thrombus and lay down collagen to form granulation tissue and then mature scar tissue

5. Macrophages phagocytose the thrombus to remove it

6. New blood vessels form running lengthwise through the thrombus to restore blood flow through the blocked vessel (AKA recanalisation)

• A 75-year-old female presents to the ED with a right neck of femur fracture. She was treated with a hip replacement and had an uneventful recovery period.
• 6 days post-op she developed a red, hot swollen right calf
• She received a Doppler Ultrasound of his leg to confirm the diagnosis.
• What is the most likely differential diagnosis?

• Doppler USS confirmed a DVT in her right popliteal vein
• She was treated with treatment dose dalteparin and long-term warfarin and recovered
• Often a DVT can result in an embolus breaking off and travelling to the pulmonary circulation
• This will be covered in more detail in the Embolism module.

By the end of this module you should:

• Understand the risk factors for thrombosis (inherited vs. acquired & arterial vs. venous)
• Identify the macroscopic & microscopic morphology of a thrombus
• Discuss the outcomes of a thrombus