Written by Dr Kathryn Knipe

One day, out of the blue, your cat struggles to get up; her hind quarters are paralysed, her paws are ice-cold. She’s panting heavily, struggling to breathe, and when you go to pick her up, she cries in pain. It’s a nightmare scenario for any cat lover.

Your first thought may be that she’s been knocked by a car or has injured herself in some way. But these could actually be signs of a type of blood clot – a serious complication of heart disease called saddle thrombus. It occurs in 25% of cats with underlying hypertrophic cardiomyopathy (the most common form of heart disease in cats), which you may not even know your cat has.

It’s important to understand what it is and how it happens so that you know how to help your cat.   

Cat cardiology in a nutshell

To understand saddle thrombus and its effects and prognosis, you need to know how your cat’s cardiovascular system works. The heart pumps blood around the body carrying oxygen, nutrients, hormones, etc. to cells, and taking carbon dioxide and other waste products away.

Veins go into the heart for blood to be oxygenated; arteries exit the heart to take the blood to the cells. The main artery is the aorta and, after it leaves the heart, it forms two branches: one supplying blood to the upper part of the body and one to the lower half.

In the lower half of the body, the aorta splits into several large blood vessels that supply blood to the hindquarters, including the legs and tail. This is called the aortic trifurcation (or terminal abdominal aorta).

Saddle up

Thrombus is the technical term for a blood clot. When a thrombus forms in the aortic trifurcation, it’s referred to as a saddle thrombus or, as it’s known to veterinarians, feline aortic thromboembolism (FATE).

Picture a river which splits into streams: the area where the river splits is the most likely place for rocks, sticks, etc. to get stuck. Likewise, that spot where the aorta splits is the most likely place for a thrombus to form, blocking blood flow to the lower body. This can lead to paralysis and even death.

The external iliac arteries form a Y-shape as they branch off from the aorta. When a thrombus forms here, extending downwards into those arteries, it resembles a saddle – hence “saddle thrombus”.

Kitty heart disease

In cats, the single most common cause for thrombus formation is heart disease – most commonly hypertrophic cardiomyopathy (HC) – basically, thickened heart muscle (“hyper” = excessive; “trophic” = growth; “cardio” = heart; “myopathy” = disease of the muscle). It is irreversible.

There are various causes for HC, including:

• Congenital abnormality (born with it)
• Systemic hypertension (high blood pressure)
• A response to overactive thyroid (hyperthyroidism), a condition in which benign nodules in the thyroid gland secrete excess thyroid hormones in the body.

Note that saddle thromboembolism can occur in dogs, but it’s very rare.

How HC happens

1. Changes in the body lead to increased pressure against which the heart must pump the blood out.
2. Because the heart has to work harder to pump the blood against this increased resistance, the heart tissues change, becoming stronger and thicker.
3. Initially, these changes help improve the heart’s ability to pump blood.
4. If the problem causing the pressure is not corrected, the heart has to work harder and harder, and the muscle gradually becomes thicker.
5. Eventually, the heart muscle becomes so thickened and stiff that the heart chambers can’t collect enough blood and pump it out.
6. When the heart can’t keep pumping blood out, congestive heart failure develops. The complications of this include blood clot formation.

How does a blood clot form?

In a nutshell: blood should move forward throughout the body in a smooth, continuous flow. If it can’t, there’ll be abnormal pressure and muscle changes; this creates the ideal circumstances for a blood clot (thrombus) to form.

Here’s how this happens:

1. If blood doesn’t move forward as it should, it starts “back-damming”, which leads to increased pressure and fluid seeping out of the blood vessels into, for example, the chest cavity and lungs. This is why people or animals with congestive heart failure often struggle to breath and have a chronic cough.
2. All these abnormal pressure and muscle changes lead to very turbulent blood flow in the heart; this creates the perfect situation for blood clot formation. Blood emboli can also form because of other diseases such as infections or certain cancers, but this is less common.
3. If this clot gets dislodged or a piece breaks off and moves into the circulation, it’s known as an embolus (plural: emboli); this can lodge elsewhere in the blood vessels of the body, impeding or even completely blocking blood flow to organs and limbs (embolism).
4. Normally, the body can break down microscopic emboli and thrombi. In saddle thrombus these mechanisms fail to break down the clot forming in the aortic trifurcation. This leads to the blood flow being obstructed to the hindquarters. Although this is the most common place for clots to form in cats, emboli can also lodge in one of the large blood vessels supplying one of the front legs, or important internal organs such as the intestines or kidneys.

Why blood clots are bad and what are the symptoms?

All tissues in the body need nutrients and oxygen to be supplied by the blood as well as toxins removed in the same fashion. Blood clots prevent this from happening.

Although the body has mechanisms by which it can shift blood through other blood vessels along the spine to supply blood flow to where it’s been cut off, sometimes these mechanisms are insufficient.

When a thrombus blocks the blood flow to an area, the consequences are very severe. In the most common presentation, where the thrombus is in the aortic trifurcation and the blood supply to the hindquarters is cut off, the back legs will appear paralysed. The muscles of the back legs become very swollen and painful, the toes will be cold to the touch and often cats will not be able to feel you touching them.

Emboli that lodge in large blood vessels supplying the front legs will lead to the same symptoms of swollen, painful muscles, coldness of the extremity and loss of function of the leg. If the embolus lodges in a blood vessel supplying an internal organ, symptoms seen will be most likely vomiting, abdominal pain, lethargy and an unwillingness to eat.

If blood flow is not restored, the tissues will eventually die off. In the case of hind limbs, this means that the muscles, bones, tendons, skin and nerves of the back legs can die. If the embolus affects organs, it will lead to death of those internal organs.

In order to diagnose a thromboembolism, the vet will do a physical examination; chest x-rays, blood tests and heart ultrasound may also be suggested.

Heart failure

Symptoms of heart failure may also be present in a cat presenting with thromboembolism, most notably increased respiratory rate (“panting”) or difficulty breathing. Although the occlusion of a blood vessel by an embolus or thrombus is a sudden event, the development of congestive heart failure is protracted.

You may have noticed a change in your cat’s respiratory pattern or behaviour. However, as cats are sedentary in nature, these changes are often subtle and ascribed to old age; cats are also good at hiding illness.

Cats with heart disease are often presented only once they start to suffer severe difficulty breathing or if they show symptoms of a thromboembolism. This is why being vigilant and visiting your vet regularly is crucial, particularly if your cat is elderly.

Can you fix or prevent it?

If your pet has a heart murmur, hypertrophic cardiomyopathy, hyperthyroidism or high systemic blood pressure, they are at risk for developing an aortic thromboembolism.

Although congenital abnormalities and advanced heart disease in cats can’t be fixed, you can try to lower the chance of complications such as saddle thrombus by keeping their stress levels down, ensuring they eat a healthy, species-appropriate diet, and ensuring their weight is at a healthy level. Preventative treatment for blood clots may be indicated, including medication.

Once a saddle thrombus has occurred, recovery depends on how advanced it is – the prognosis is around 50% survival rate. If the cat survives, there is a high chance of it developing again, plus the underlying heart problems may already be advanced. All this should be taken into account when considering starting treatment.

If your vet considers treatment to be possible, your cat will need to be admitted to hospital, for some time. Because a thromboembolism is very painful, strong painkillers will be given, along with drugs to help break up blood clots. At the same time, treatment may be instituted for the underlying cause, e.g. hypertrophic cardiomyopathy or hyperthyroidism.

Loss of blood flow to a limb can lead to necrosis (tissue death) of that limb; if only one limb is affected, amputation may be considered, depending on several factors (see below).

Worst-case scenarios

Unfortunately, around half of the cats admitted to hospitals for this condition will suffer such severe consequences that they’re humanely euthanised so as not to prolong their suffering. If they survive, lifelong treatment will be required.

Things to consider:

• If blood supply to an internal organ is cut off and cannot be restored in good time, it’s likely that the organ involved will die; this is excruciating and invariably fatal.
• If only one limb is affected, one can consider amputation, but if blood supply to both hind legs is lost, this is not a viable option. Additionally, anaesthesia is risky due to underlying conditions such as heart disease. There’s also increased risk of post-operative bleeding because they will likely have been treated with drugs to help dissolve blood clots.
• Essentially, these patients will be admitted to hospital for pain control, nursing care and intensive monitoring for additional complications. If after a few days of treatment in hospital there appears to be no improvement, one may need to consider euthanasia as the kind alternative.
• If your cat does recover use of the affected body parts, they will be sent home with lifelong treatment to try to prevent another thromboembolic incident, as well as treatment for the underlying disease that caused it in the first place. You need to be prepared to monitor your cat carefully, give it medicine daily (sometimes several times a day), feed it specific food, and make regular visits to the vet to monitor its health.

All of these things should be taken into consideration when making a decision.

Anything out of the ordinary

Cats tend to hide ill health quite successfully, so be vigilant – take careful note of anything out of the ordinary and discuss this with your vet at the annual examinations (or, if sudden onset, contact them immediately).

Early recognition of diseases such as hyperthyroidism and chronic renal disease may prompt early intervention from your vet, which may improve the symptoms or delay the clinical onset of the disease.

Look out for changes in:

• Appetite (not just the usual feline habit of picky eating!)
• Litter box habits, such as urinating large amounts, missing the litter box, or going in unusual places, e.g. on the carpet
• Water intake – “is my cat drinking more than is normal for him/her?” (an adult cat’s average water intake is around a cup, depending on what they eat, outside temperature, etc.)
• Body weight – if your cat has a very thick or long coat, you may not notice weight changes straight away.
• Breathing, such as panting, shallow breathing, etc.
• Activity levels
• Behaviour
• Intermittent lameness in different limbs – this may be an indicator of smaller blood clots occurring in the body and being dissolved by the body’s natural mechanisms. In a patient with an underlying disease this should prompt one to discuss preventative therapy for blood clots with your vet.

See your vet

Symptoms of illnesses leading to thromboembolism are often subtle. Early detection and treatment of many conditions – including heart disease – can mean a better outcome, so regular visits are important. Always follow their recommendations regarding your pet’s nutrition, vaccinations and parasite control.

By listening to your cat’s heart, the vet could pick up a heart murmur or hear sounds of congested lungs, which could indicate heart failure. In senior cats, it’s also a good idea to do blood tests, including annual testing for hyperthyroidism (overactive thyroid) and kidney disease. Hyperthyroidism is present in approximately 10% of senior cats and can predispose cats to form blood clots.

And if you have any suspicions about your pet’s health, do not delay (even if you visited recently). Rather a trip to the vet to have your pet’s good health confirmed than a trip to the emergency hospital once a disease has progressed to a life-threatening condition.

Remember that your vet wants the same thing you do: to help your pet have the longest, happiest life it can. Many blessings and good health to all our furry babies.