Aortic Thromboembolism in Cats

By Courtney Barnes, BSc, DVM; Malcolm Weir, DVM, MSc, MPH; Robin Downing, DVM, DAAPM, DACVSMR, CVPP, CRPP

What is aortic thromboembolism?

Aortic thromboembolism (ATE) is a devastating condition. The aorta is the main artery of the body and carries oxygenated blood from the heart out to the rest of the body.

The word thromboembolism combines the words thrombus and embolism. A thrombus is a blood clot that occurs inside a blood vessel, and the word embolism indicates that the clot has traveled through a blood vessel to a location distant from where it formed.

An aortic thromboembolism results when a blood clot is dislodged and travels through the aorta, becoming lodged in a distant location. This clot causes severely reduced blood flow to the tissues receiving blood from that section of the aorta, leading to decreased oxygen in the tissues. This condition is also sometimes called a saddle thrombus.

What cats are at risk of aortic thromboembolism?

While aortic thromboembolism is a rare occurrence in dogs, it is much more common in cats. Certain breeds are more commonly affected (mixed breed, Abyssinian, Ragdoll, and Birman), and males are more likely to be diagnosed than females. The age range of affected cats is reported as 1–21 years of age, but most commonly occurs between 8 and 12 years of age.

Affected cats often have underlying heart disease called hypertrophic cardiomyopathy, in which the heart muscle thickens and loses function. The lower heart chambers (ventricles) decrease in size and contracting ability (see handout "Cardiomyopathy in Cats" for further information on this type of heart disease). Blood flow through the heart is compromised, and clots can form that later become dislodged into the aorta.

Aortic thromboembolism can also be associated with cancer or with a body-wide generalized infection called sepsis.

What are the signs of aortic thromboembolism?

  • The most common clinical signs of aortic thromboembolism are sudden paralysis and pain, usually in the rear legs, although weakness and lameness may be seen. 
  • If the rear limbs are affected, there may be decreased or absent pulses in the femoral arteries of the rear legs. Sometimes, a front leg is involved. 
  • Rapid breathing or difficulty breathing may be seen. 
  • The cat may vocalize from pain and may act anxious. 
  • Occasionally, the cat will vomit. 
  • The nailbeds and foot pads may be pale or bluish. 
  • The cat may experience a lower-than-normal body temperature. 
  • Sometimes, the heart will sound abnormal through a stethoscope, with a murmur or irregular heartbeat being heard.

How is aortic thromboembolism treated?

Unfortunately, the prognosis for a full recovery is guarded to poor. As the condition can be extremely painful, humane euthanasia is a viable option for many cats. If treatment is attempted, most cats need to be hospitalized to manage pain and any concurrent heart disease. They may be anxious and in pain. Supplemental oxygen therapy may be beneficial.

"If treatment is attempted, most cats need to be hospitalized to manage pain and any concurrent heart disease."

Initially, affected legs should be handled minimally. As blood flow returns, physical therapy (passive extension and flexion of the legs) may speed full recovery. Affected cats may need assistance with urinating and defecating. It is best to restrict activity as treatment starts, and the main goal is to keep the cat stress-free. There may be sudden death, usually associated with irregular heartbeats, secondary to increased levels of potassium in the blood. Potassium and other products are released when blood flow returns to oxygen-deprived tissues.

Stressed cats tend not to eat, so it is important to encourage food intake in any way possible. Cats who stop eating suddenly can accumulate fats in the liver, a condition called hepatic lipidosis or fatty liver syndrome. This condition can be fatal.

Surgical removal of the aortic thromboembolism is typically not recommended, as these are high-risk patients due to their severe heart disease.

Aspirin, and a similar drug called clopidogrel (brand name Plavix®), is theoretically beneficial during and after an episode of aortic thromboembolism but should be used only under the direct supervision of your veterinarian. These drugs prevent platelets (blood cells that assist with blood clotting) from activating to clump together and form a clot while flowing through the blood vessels.

Recently, an anti-blood-clotting medication called low molecular weight heparin (LMWH) has been proposed for long-term prevention of aortic thromboembolism in cats. Medications to break up the clot have been tried but must be started right away and should only be attempted at specialized referral centers as rates of complications are high.

Finally, the cat’s heart disease should be treated as appropriate for the type and severity of the disease.

Is any monitoring required? What is the expected outlook?

Blood work, chest X-rays, and cardiac ultrasound should be done routinely to assess the management of heart disease. Blood clotting tests will be performed if anti-clotting medication is used. The cat’s legs will be evaluated to assess the clinical response to therapy. Unfortunately, there is a high rate of recurrence of blood clot formation. There may be permanent nervous system damage, or the hind leg muscles may be adversely affected.

The expected course of this disorder is days to weeks for full recovery of function in the legs, but the prognosis in general is very poor. Long-term survival varies between two months to several years, with the average being a few months with treatment. Most cats that survive the initial episode will be on some type of medication to prevent blood clotting (anticoagulant therapy) and may require frequent re-evaluations and an indoor lifestyle.

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