Embolism

Introduction

• An embolus is a detached intravascular mass that is carried by the blood to a site distant from its point of origin.
  • May be solid or gaseous.
• Emboli will lodge in a vessel too small to permit further passage.
  • Results in occlusion.

Types of Embolism

Thromboembolism

• These are pieces of thrombi that have broken away from their primary site.
• The most common cause of embolism.

Air embolism

• Caused by accidental injection of air from a badly filled hypodermic syringe.
• Small air bubbles may occlude the blood vessels of the brain and cause severe ischaemic necrosis.

Fat embolism

• Pieces of adipose tissue from the bone marrow cavity may enter the blood stream following fracture.

Fibrocartilagenous embolus

• Seen in the dog.
• Thought to arise from a damaged intervertebral disc.
• Cause occlusion of spinal blood vessels.
  • Results in:
    • Spinal cord necrosis.
    • Severe sudden paralysis.

Tumour emboli

Sarcoma embolus (dog). Courtesy of T. Scase

• Otherwise known as metastasis.
• Small groups of tumour cells may break off and be carried in vessels to reach another site

in the body.

• May use the lymphatic or blood vessels.

Bacterial emboli

• Some bacteria travel in colonies.
  • Come to rest in small capillaries and incite an inflammatory response.

The results of Embolism

• The results of embolism depend on several factors.
  1. The extent of the occlusion and the availability of an adequate collateral circulation.
     • Some emboli may be irregularly shaped, allowing blood to flow around them.
  2. Duration for which the embolus blocks a vessel.
     • Some small thrombi may undergo lysis and quickly disappear.
  3. Nature of the embolus.
     • Bacterial emboli will incite an extensive inflammatory reaction.
     • Tumour emboli will multiply and grow into tumour masses.
  4. The site at which the embolus comes to rest.
     • There are two main considerations here.
       • The type of tissue supplied.
         • The brain and heart are quickly affected by anoxia.
       • The type of blood supply to the tissue.
         • Occlusion of vessels to a tissue with an end arterial supply will cause ischaemic necrosis or infarction.

Ischaemia

• This is the local reduction of blood flow to an area.
  • Usually refers to the arterial flow.
• Ischaemia may be partial.
  • Results in hypoxia and atrophy of the tissue supplied.

Infarction

• Blockage of an end artery results in acute ischaemic coagulation necrosis, or infarction.
• Some end arteries and their branches are of particluar importance.
  • The renal artery.
  • The splenic artery.
  • The coronary arteries.
  • Some cerebral and spinal cord arteries.
  • The anterior mesenteric artery.
• There are some areas that have a tendency for infarction.
  • The kidneys of pigs and dogs.
  • The spleen of the pig.
  • The intestine of the horse.
  • N.B Infarction of the heart and brain is rare in animals.

Kidney Infarcts

• Typically appear on the cortex.
• Appear as a sharply demarcated pale area surrounded by a red line of haemorrhage.
  • A white line of [[Neutrophils - WikiBlood|neutrophils]] forms inside the red line after a few days.
• The necrotic tissue undergoes liquefactive necrosis.
  • The area is invaded by fibrous tissue.
    • Eventually the lesion ends up as a pale, depressed scar.

Splenic infarcts

• Appear red in colour due to haemorrhage into the necrotic area.
• This is an important diagnostic lesion in swine fever.

Intestinal Infarcts

• Small emboli break off followinf thrombosis and occlude intestinal vessels.
  • Small intestinal infarcts develop.
    • Thought to cause bouts of colic.
• Strongylus vulgaris may also cause intestinal infacts in the horse.
  • Lodge at the root of the cranial mesenteric artery.

Lungs

• Emboli from the venous system may end up in the lung.
• The lung has an extensive collateral cirulcation, and so little damage is caused.