A saddle thrombus, properly known as Feline Aortic Thromboembolism (FATE), is an acute, life-threatening condition in cats. This medical emergency involves a blood clot that travels from the heart and suddenly blocks the main artery leading to the hind limbs. The condition causes immediate distress and paralysis, demanding instant veterinary intervention. The name “saddle thrombus” describes the clot’s location at a major junction in the body’s largest artery.
Understanding the Location and Action of the Clot
A thrombus is a blood clot that forms within a blood vessel; an embolus is a piece of that clot that breaks off and travels through the bloodstream. Most emboli originate as a larger clot within the cat’s heart, typically in the left atrium. This traveling clot is then pumped out of the heart into the aorta, the body’s main artery.
The aorta runs down the length of the cat’s body before splitting into the two iliac arteries, which supply blood to the rear legs. This split is referred to as the aortic trifurcation, where the clot often lodges, straddling the division like a saddle. The obstruction immediately cuts off the flow of oxygenated blood to the hind limbs.
This sudden lack of blood flow results in ischemia in the affected muscles and nerves. Ischemia causes rapid tissue damage and leads to the release of inflammatory mediators, contributing to severe pain and circulatory shock. While the clot is called a saddle thrombus when it blocks flow to both hind limbs, smaller emboli can sometimes lodge in only one artery, affecting a single leg.
Immediate Signs of a Saddle Thrombus
The onset of a saddle thrombus is sudden, with the first sign often being acute pain. Cats may vocalize loudly, cry out, or exhibit frantic behavior due to the intense discomfort caused by the lack of blood flow. This is one of the most painful conditions seen in veterinary medicine, making immediate pain relief a priority.
The most recognizable physical sign is the sudden paralysis or paresis (significant weakness) of one or both hind limbs. The cat will be unable to use the affected legs and may drag them or collapse. A physical examination will reveal that the rear paws and legs are noticeably cold to the touch compared to the front paws.
This coldness, known as poikilothermy, occurs because blocked blood flow prevents warm blood from reaching the extremities. The muscles in the affected legs may feel firm and tight. Furthermore, a veterinarian will be unable to detect a pulse in the femoral arteries of the hind limbs. These signs point toward a severe lack of circulation below the obstruction.
The Primary Underlying Causes
Feline Aortic Thromboembolism is rarely a standalone event; it is almost always secondary to an underlying disease. In approximately 89% of cases, the saddle thrombus is caused by pre-existing heart disease, often a condition that had not yet shown outward signs. The most common underlying cause is Hypertrophic Cardiomyopathy (HCM), which involves the thickening of the heart muscle walls.
In cats with HCM, the left atrium often becomes enlarged, causing blood flow to slow down and become turbulent. This slow, chaotic blood movement, combined with changes to the inner lining of the heart, creates an environment ripe for clot formation. Clots (thrombi) develop in the left atrium and then break loose to become emboli that travel through the body.
The saddle thrombus episode is often the first clinical symptom an owner notices; up to 76% of affected cats have no prior diagnosis of heart disease. Less common underlying causes include certain types of cancer, particularly lung cancer, or systemic conditions that cause the blood to clot more easily. Therefore, an ATE diagnosis necessitates a thorough investigation for underlying cardiac or systemic illness.
Emergency Treatment and Prognosis
Treatment for a saddle thrombus must be initiated immediately, focusing primarily on stabilizing the cat and managing severe pain. Potent analgesia, often involving injectable opioid medications, is paramount to alleviate suffering. Supportive care, including managing concurrent heart failure and providing oxygen, is also a high priority.
Anticoagulant medications are administered to prevent new clots and stop the existing thrombus from enlarging. The goal is to allow the cat’s body to naturally dissolve the clot over time. Direct clot removal through surgery or the use of clot-busting drugs (thrombolysis) is complex and carries a high risk of complications, meaning it is only attempted in specialized centers.
The prognosis for cats with a saddle thrombus remains guarded to poor, especially in the acute phase. Survival rates for hospital discharge are typically around 30% to 50%, and the outlook is influenced by the severity of the underlying heart disease. Even cats that survive the initial crisis face a high risk of recurrence. They require lifelong management with anti-clotting medications to reduce the chance of another episode.