- Saddle Thrombus in cats (also known as fatal aortic thromboembolism) comes on very suddenly and is often fatal
- It happens when a large blood clot forms in the left atrium, then a smaller clot breaks off, exits the heart, and gets stuck at the base of the aorta
- Heart disease is the main cause of saddle thrombus in cats
- Signs can include blue/cold rear paw pads, hyperventilation, and behavior changes, among others
- Saddle thrombus treatment goals are: pain control, heart disease management, blood flow improvement, and rear limb reperfusion
- The prognosis for saddle thrombus in cats is poor, but it can be prevented by detecting and managing heart disease early
Saddle thrombus is a scary medical condition that no pet parent ever wants their cat to experience. It occurs without warning and is often fatal. That’s why it’s critical to be able to recognize the signs of saddle thrombus in cats quickly. Doing so will help ensure that you can get your cat immediate veterinary care.
What is Saddle Thrombus?
Saddle thrombus is also known as fatal aortic thromboembolism. That’s quite a mouthful, so let’s define some terms first:
- Thrombus. A thrombus is a large blood clot that forms within a blood vessel and gets stuck.
- Embolism. An embolism is a small clot that breaks off from the thrombus, travels through the bloodstream, then gets lodged in another blood vessel elsewhere in the body.
With saddle thrombus, the thrombus forms in the left atrium (upper left chamber of the heart). The embolism that breaks off exits the heart through the aorta and eventually gets stuck at the base of the aorta. In some cases, the entire thrombus moves through the aorta.
The base of the aorta is located near the rear limbs, where it splits into the major arteries that supply the hind limbs and tail. This area looks like an upside-down ‘V,’ hence the name “saddle.”
The saddle thrombus causes inflammation where it gets stuck, decreasing circulation to the legs and setting off a sudden cascade of symptoms.
More common in cats than dogs, saddle thrombus is most seen in male adult cats aged 8 to 12 years.  The cat breeds at the highest risk of saddle thrombus include Abyssinians, Birmans, and Ragdolls. 
Causes of Saddle Thrombus in Cats
The most common cause of saddle thrombus in cats is heart disease, namely hypertrophic cardiomyopathy (HCM). HCM is an inward enlargement of the heart muscles, which causes the heart to pump poorly and increases the risk of thrombus formation.
Nearly 90 percent of cats with saddle thrombus have heart disease.  However, somewhat confusingly, many cats with heart disease never develop saddle thrombus. Currently, there is no way to predict whether a cat with heart disease will develop the condition.
Less common causes of saddle thrombus cats are cancer and sepsis (bacterial infection in the blood). For some cats, the cause of saddle thrombus remains unknown.
Signs of Saddle Thrombus in Cats
Saddle thrombus occurs suddenly; a cat may be fine one second and in excruciating pain the very next second.
Because it comes on without warning, detecting early signs of saddle thrombus in cats is nearly impossible. Here are the symptoms to look for:
- Loud vocalization
- Rear paw pads are blue
- Rear legs and paw pads feel cold
- Hyperventilation (rapid breathing)
- Paralysis and lameness in one or both rear limbs
- Behavioral changes (aggression, anxiety, frenzy)
Saddle thrombus may be the first sign of heart disease in cats.
Diagnosing Saddle Thrombus in Cats
Saddle thrombus is a medical emergency, so a diagnosis must be made quickly and accurately to give a cat the best chance of survival.
A veterinarian will typically use the presence of telltale symptoms of saddle thrombus, particularly sudden rear limb paralysis, to diagnose the condition. They will also perform a physical exam and potentially conduct other testing to support the diagnosis.
During the physical exam, the veterinarian will search for a pulse in the rear limbs and determine if they feel cooler than the front limbs. No pulse in the rear limbs and cool-feeling rear limbs are key clues for saddle thrombus.
The veterinarian will listen closely to the heart for abnormal heart rhythms, known as arrhythmias, that may indicate heart disease. They may also perform imaging tests like chest X-rays and an echocardiogram (heart ultrasound), which could demonstrate heart disease.
Cats with saddle thrombus often need to be stabilized before all the diagnostic testing can be performed.
Treatment for Saddle Thrombus in Cats
Cats with saddle thrombus must be hospitalized for treatment for at least several days.
The main treatment goals are:
- Controlling pain
- Managing underlying heart disease
- Improving blood flow through the heart
- Reperfusion (restoring circulation) of the rear limbs
Pain medications are used to control pain, while other medications are given to improve heart function and blood flow.
Blood thinners are administered to prevent both the formation of future clots and the saddle thrombus from getting bigger. Sometimes, the blood clot will resorb on its own.
Intravenous fluid therapy helps improve and restore circulation but can lead to fluid overload in cats with heart disease because their hearts are not pumping well and they have poor circulation.
While it is one of the goals when treating saddle thrombus, reperfusion of the rear limbs is risky. When the saddle thrombus initially cuts off circulation to the rear limbs, cells die and release potassium. Reperfusion can cause a massive rush of potassium to the heart, which can be fatal because too much potassium in the blood can stop the heart.
In addition to these treatments, cats may be given supplemental oxygen and encouraged to eat. Cats that do not eat for several days are at high risk for fatal hepatic lipidosis (fatty liver disease).
For cats that survive treatment (many don’t), physical therapy helps them regain movement in their rear limbs after blood flow has been restored. Physical therapy could take anywhere from several days to several weeks.
After returning home from saddle thrombus treatment, cats require frequent monitoring, lifelong anti-clotting medication, and heart disease management.
Unfortunately, saddle thrombus treatment for cats is often difficult and unrewarding.
The prognosis for feline saddle thrombus is usually very poor, with about half of affected cats not surviving treatment. 
Frequently, cats who survive saddle thrombus have permanent nervous system damage. Only about 20 percent of cats survive more than one year after their saddle thrombus. 
Euthanasia is often selected over treatment for several reasons:
- Extreme pain and discomfort
- Likelihood of saddle thrombus recurrence
- Long-term management of underlying heart disease
How to Prevent Saddle Thrombus in Cats
The best way to prevent saddle thrombus is to detect and start managing heart disease early.
A consultation with a veterinary cardiologist is the ideal way to diagnose heart disease. However, these consults are very expensive and may not be readily available, depending on where you live. Regular wellness exams can help your veterinarian detect heart disease early and begin your cat on a heart disease management plan.
- Aherne, M. Dec. 23, 2020. Feline Arterial Thromboembolism. Today’s Veterinary Practice. Retrieved from https://todaysveterinarypractice.com/cardiology/feline-arterial-thromboembolism/
- Brooks, W. Feb. 27, 2012. Feline Aortic Thromboembolism (FATE or Saddle Thrombus). Veterinary Partner. Retrieved from https://veterinarypartner.vin.com/default.aspx?pid=19239&catId=102903&id=5307199
- Borgeat, K et al. “Arterial thromboembolism in 250 cats in general practice: 2004-2012.” Journal of Veterinary Internal Medicine vol. 28,1 (2014): 102-8.