Blood Clots and Aneurysms in Cats: Causes, Symptoms, and Treatment

Tyler Olson / Shutterstock.com

Blood clot formation in cats is a very serious condition which requires rapid veterinary assessment and treatment. This is made difficult as commonly the presence of a blood clot may be unknown until it leads to an emergency situation. In this article, you’ll learn about different types of blood clots, conditions that cause them in cats, how these may be treated and steps you can take to reduce your cat’s risk.

Quick Overview: Blood Clots and Aneurysms in Cats

Other Names: Thromboembolism

Common Symptoms: Sudden onset of breathing difficulty, difficulty walking, balance issues, distressed vocalizing, sudden death

Requires Ongoing Medication: Yes

Vaccine Available: No

Treatment Options: Relief of secondary pain, respiratory distress, high blood pressure, or heart dysfunction. Clopidogrel (Plavix) for the prevention of blood clots in cats at risk

Home Treatment: Regular health screening, immediate veterinary visit for any sudden occurrence of respiratory distress, pain, or difficulty walking

What are Blood Clots and Aneurysms in Cats?

Understanding more about what blood clots and aneurysms are and do is important to help you recognise the warning signs.

Blood Clot

A blood clot is a collection of red blood cells, platelets, and a protein called fibrin. On the outside of the body, they form as a normal response by the body to a cut or scrape. But when they form inside the body like inside the heart or a blood vessel, this is abnormal and a health issue is to blame.

There are two types of blood clots:

• Thrombus: a blood clot that forms in a vessel or in the heart as a result of reduced or abnormal blood flow. They can also form if a blood vessel is damaged or if another condition causes the abnormal formation of blood clots throughout the body. A thrombus generally describes a blood clot that forms in place and remains there. They can cause partial or full blockage of a vessel.
• Embolus: a blood clot that dislodges (either in part or completely), is carried down the bloodstream, and then gets lodged somewhere else when the blood vessel gets too narrow. An embolus can also be composed of something other than clotted blood like bacteria, fat, or air. But for now, we’re only focusing on blood clots.

Aneurysm

An aneurysm is a different situation but is related to blood clots. It’s an enlargement or bulging of a blood vessel, such as the aorta. They can also form on one of the chambers of the heart. This occurs because of a weakening of the wall of the vessel or heart chamber. The bulging and abnormal blood vessel wall can lead to secondary blood clot formation. Aneurysms are considered very rare in cats.

Causes of Blood Clots and Aneurysms in Cats

Heart disease is a common cause of blood clots in cats. Tyler Olsen / Shutterstock.com

Blood Clots

Blood clots can form in a few different places of the body. Here, we’ll look at those locations and potential causes when blood clots form there.

Heart

Heart disease that leads to dilation of the left atrium heart chamber (and sometimes the left ventricle as well) is the most common cause of blood clot formation in cats. In the dilated chambers, blood flow becomes stagnant, especially along the sides of the chamber. Slower, swirling blood flow in these areas allows clot components to start to merge together. A larger and larger clot forms over time.

To understand better how this happens, think of a stream that is narrow with rushing water. That’s a normal blood vessel or heart chamber. Blood should be constantly flowing and moving. Now imagine that the stream suddenly becomes wider. The water generally slows to fill the space. Water in the center more or less continues to move straight. But water along the widened sides of the stream tends to flow even slower. Leaves, sticks and other debris in the center of the stream will continue on. But debris along the sides will collect with other debris, and swirl around, sometimes getting stuck on the banks. This is similar to how blood clot formation works in these cases.

Hypertrophic cardiomyopathy is the most common type of heart disease in cats and the most common contributor to blood clots in cats. Hyperthyroidism (an overactive thyroid gland) in cats can also have significant cardiovascular effects. We can see increased blood pressure, as well as a form of heart disease called thyrotoxic cardiomyopathy.

Aorta

A blood clot in the aorta almost always occurs due to an embolus that dislodges from the heart and travels downstream. At the top of a cat’s back legs, the aorta branches off into three smaller vessels and this is most often where the blood clot lodges. This is what causes feline aortic thromboembolism (FATE), also called a saddle thrombus.

Lungs

A blood clot in the lungs is called a pulmonary thromboembolism (PTE). Blood clots that occur in the arteries of the lungs are often caused by heartworm disease in cats, though other conditions lending to blood clot formation may cause them too. Heartworms commonly live in the pulmonary artery, the vessel that carries blood from the heart to the lungs.

Other locations

Other possible sites include the kidneys, brain, front legs and other internal organs. When clots form in these areas, they are most often caused by emboli that originated in the heart because of underlying heart disease.

Aneurysms

As mentioned, Aneurysms are considered very rare in cats. When they do occur, they appear to be associated with other congenital defects (birth defects) of the heart.

One study from 2005 found aneurysms in 3 cats with a defect called a ventricular septal defect (where the wall between the two ventricle heart chambers doesn’t form completely before birth). The aneurysm in all cases was also in the ventricle wall. An aneurysm in the caudal vena cava was documented in a young kitten with multiple heart defects and an aneurysm of the right atrium heart chamber was documented in a kitten that also had a congenital defect in the cranial vena cava blood vessel.

Symptoms of Blood Clots and Aneurysms in Cats

Sudden hindlimb paralysis with pain and limbs that are cool to the touch is the most common sign of a blood clot event in cats. Thoreau / Shutterstock.com

Blood Clots

Symptoms of a blood clot will depend on where the clot has formed. The two most recognized locations are:

• Feline aortic thromboembolism (FATE, saddle thrombus) where the clot most often lodges where the aorta branches into smaller vessels near the hips
• Pulmonary thromboembolism (PTE) where a clot lodges in the lungs

Symptoms of FATE/saddle thrombus include:

• Sudden paralysis and pain in the rear legs. Affected cats may fall over or lay down and not be able to get up. They may attempt to walk or pull themselves with their front legs
• Less commonly, paralysis and pain in a front leg (usually the right front)
• Increased vocalizing (related to distress and pain)
• Decreased or absent pulses in the affected limbs
• Paw pads and nail beds of affected limbs may appear pale or blue in color
• Affected limbs are cool to the touch
• Rapid or difficulty breathing (most often pain related). Open-mouth panting is not uncommon.
• Vomiting (as a result of noxious stimulus from pain)
• Lower than normal body temperature

Symptoms of PTE include:

• Acute onset of difficulty breathing. This may look like panting or increased breathing effort. Cats often breathe with an open mouth.
• Poor or absent appetite
• Fainting or collapse (from low oxygen)
• Lethargy
• Coughing
• Spitting up or coughing blood
• Inability to lay down or get comfortable, especially on one side
• Tiring out quickly during exercise

Aneurysm

Many people think of an aneurysm as an event that happens but an aneurysm by definition only describes a structural change. The problem is, most aneurysms go undiagnosed in people until they rupture, causing a critical life event which is why we think of them that way.

There is very little information on aneurysms in cats and most documented cases are found incidentally while investigating other aspects of congenital heart disease. Most cats with aneurysms actually have no signs attributed to the aneurysm itself.

The exception is if one starts to bleed or if a blood clot develops. In the cases previously mentioned where an aneurysm was found, none of the cats had signs of illness directly related to the aneurysm.

Complications of Having Blood Clots and Aneurysms in Cats

The sudden presence of a blood clot can be very serious. It is not uncommon for a critical health change to suddenly occur as the first and only sign of a blood clot problem, requiring an emergency veterinary visit.

FATE (Saddle Thrombus)

A blood clot that lodges in the aorta, causing a saddle thrombus, is extremely painful. The degree of pain itself may be difficult to effectively manage.

Cats with FATE-induced limb paralysis have a low chance of getting full limb function back. Studies have shown a success rate ranging from about 25% to 70% in cats that are hospitalized, receive aggressive anticoagulant therapy, and dedicated home care. In cats that do regain function, this may take anywhere from 2-6 weeks or longer before a cat can walk acceptably again.

About 80% of cats with FATE also have concurrent heart disease. 50-70% are actively in congestive heart failure (CHF) at the time of a saddle thrombus episode. The presence of heart failure is often a surprise to a cat parent, as the FATE episode may be the first sign of heart disease in over 75% of cats. Heart failure in cats is tricky to manage on its own and survivability is significantly reduced in cats that have both a saddle thrombus and heart failure at the same time. In cats that have one FATE episode, about 25% will have another episode within 6-12 months.

Unfortunately, due to a combination of complications of extreme pain, paralysis, the chance of recurrence, and concurrent heart disease, about 50-70% of cat parents will elect to euthanize their cat at the time of initial presentation for a saddle thrombus.

Pulmonary thromboembolism (PTE)

With PTE, complications can also be very serious. Sudden death, especially when heartworm disease is present, is unfortunately not uncommon. The sudden onset of respiratory distress may be very severe and even in cats that make it to a veterinary hospital shortly after, a cat’s deterioration may outpace treatment efforts.

In cats who are treated successfully initially, longer term outcomes depend on the underlying cause of the PTE. If a chronic disease (like heart disease) is related, the long term outcome can be very poor because of the chance of recurrence.

Aneurysms

As mentioned before, cats with aneurysms usually don’t have complications from the aneurysm itself. In all cats where an aneurysm was found in clinical studies, other heart or vascular defects were also present and contributed more to a poor outcome.

Aneurysms can contribute to blood clot formation, leading to potential complications. Aneurysms are considered so rare in cats that there is little data on other complications. Sudden death from a ruptured aneurysm, depending on the location, could theoretically occur.

Diagnosis of Blood Clots and Aneurysms in Cats

An echocardiogram, or heart ultrasound, is one way to diagnose cats with heart disease and risk for clotting disorders. Sungthong / Shutterstock.com

Diagnosis of blood clots in cats often depends on the cat’s symptoms. The signs of a cat with a saddle thrombus can be recognized very quickly by an experienced veterinarian.

Because PTE presents with respiratory distress for which there can be many other causes, diagnosing a PTE depends on ruling out other causes.

Here are some tests a veterinarian may use to help diagnose a blood clot condition.

Ultrasound

With FATE/saddle thrombus, the clot is always located where the aorta narrows and branches near the hips (at least if the hind limbs are involved). Veterinarians experienced in ultrasound may be able to visualize the clot and disruption in blood flow.

Because most cats with FATE also have heart disease, the heart may be looked at with an ultrasound. Sometimes a thrombus can be found in one of the heart chambers, or the blood flow in the right atrium will appear cloudy and turbid.

Ultimately, the best assessment for underlying heart disease is an echocardiogram (full heart ultrasound) with a board-certified veterinary cardiologist. This may be a more immediate option in cats presenting to emergency/specialty centers. If presenting to a general practice, a visit to a cardiologist will only be possible if a cat is able to be initially stabilized.

X-rays

X-rays are valuable for assessing cats for heart failure since it is so commonly associated with clotting disorders in cats. In cats with heart disease but no active signs of congestive heart failure, no evidence of PTE may be visible on x-ray.

Treatments for Blood Clots and Aneurysms in Cats

Treatment for blood clots in cats depends on the location where it occurs. In general, treatment outcomes will be better in cats that are assessed and treated at well-equipped emergency and specialty veterinary centers. This is due to the high rate of complications that can occur, training and experience of the veterinary teams involved for such critical disorders, and the supplies/resources that are available.

We’ll split scenarios into FATE/saddle thrombus and PTE as these are the most common presentations.

FATE (Saddle Thrombus)

When you arrive at a veterinary hospital, there are some immediate actions that are taken. Aggressive treatment within the first 6 hours has been associated with better outcomes. Hospitalized care for 2-5 days to stabilize a cat with FATE is not uncommon.

• Increasing body temperature with warming devices
• Injectable pain medication
• If deemed safe and appropriate, sedation may be needed for some cats as the pain and distress may make it too difficult and unsafe to examine and treat them
• Anticoagulant and antiplatelet therapies
• Treating for congestive heart failure, if present

Long term care approaches depend on the degree of illness, the concurrent presence of heart disease, and response to therapy. Long-term anticoagulant/antiplatelet therapy is usually needed for life. Clopidogrel (Plavix) is most often recommended for long term treatment and prevention of FATE. Cats surviving an initial treatment for FATE will also need pain medication for several weeks. Buprenorphine and gabapentin are commonly used.

PTE

Immediate care for PTE focuses more on the respiratory distress that most cats present with. As with FATE, hospitalized care for 2-5 days is also common for cats with PTE.

• Oxygen therapy
• Sedation or relaxants to ease breathing stress (if appropriate)
• Injectable pain medication
• Anticoagulant and antiplatelet therapies
• Treating for other concurrent diseases, if present

In cats that recover from a PTE event, long term care depends largely on the underlying cause of PTE and how well that condition can be managed. Long-term anticoagulant therapy (usually clopidogrel) is often necessary.

Aneurysm

There is no documented case that describes a cat being treated for a bleeding or ruptured aneurysm.

Cat Care Tips

If they survive the initial blood clot event and hospital care, cats with FATE may take weeks of nursing care to regain limb function. ERIK Miheyeu / Shutterstock.com

If your cat ever has signs of limb paralysis or breathing difficulty, see a veterinarian as soon as possible. These are considered critically emergent situations.

How a cat’s condition progresses over the first 2-3 days can often dictate a prognosis for recovery at home. Even so, cats with FATE will require several weeks of home nursing care to give them that chance.

• Cats recovering from FATE often cannot walk normally for at least a couple weeks. Keep your cat in a smaller, enclosed area.
• Litter box use may be difficult. Get a wider box with a low entry lip. Have lots of cleaning agents on hand.
• Waterless shampoo can be used to clean up a cat’s hind end from urine, feces, and litter.
• A pain scale like this one from CO State University can be helpful to assess your cat’s level of pain while providing care at home.
• When your cat is less painful, range of motion exercises (like bicycle motions with the hind legs) with the affected limb(s) and gently massaging the muscles can help prevent muscle loss from disuse.

Prevention of Blood Clots and Aneurysms in Cats

As mentioned earlier, in just over 75% of cats, a blood clot emergency event like FATE is the first sign that heart disease is present. And with 50-70% of cats with FATE also having concurrent heart disease, you can imagine that early detection of heart disease can be crucial as a preventative strategy.

Keeping at least an annual exam with your veterinarian can go a long way towards prevention. Although heart disease may not be easily detected with an exam alone, subtle clues may prompt your vet to recommend one of the above screening tests. In cat breeds like Maine Coons and Ragdolls that have genetic predisposition to heart disease, these screening tests should be considered every year, even from a young age.

However, in many cats with underlying heart disease, no abnormalities will be found during an annual physical exam. Because heart murmurs and arrhythmias may not occur until more advanced stages of heart disease are present, these are not reliable indicators of early risk.

There are 3 best ways to detect heart disease in cats without signs or symptoms.

• X-rays: heart enlargement and changes in blood vessels can be used to screen for heart disease.
• N terminal pro B-type natriuretic peptide (NT-proBNP) test: this test detects the stretch or thickening of heart muscle, as seen in heart enlargement with most forms of heart disease in cats. Many vet practices have an in-hospital test requiring a small blood sample that only takes about 20-30 minutes to run.
• Ultrasound: A full echocardiogram with a cardiologist provides the best and most detailed information about heart function. I do recommend this step in any cat with heart enlargement on x-rays or an elevation in NT-proBNP. Some general practice veterinarians skilled in ultrasound may be able to get some basic measurements of the left atrium size and its ratio to the size of the aorta, which can be used as an indicator of underlying heart disease.

In cats found to have underlying heart disease and at risk for blood clot formation, starting clopidogrel is the most common initial recommendation.