The most likely cause of a saddle thrombus in cats (a blood clot that blocks arteries in the legs) is cardiomyopathy or congestive heart failure. Cardiomyopathy is a disease of the heart muscle that prevents the muscle from being able to pump effectively.
I spoke with Dr. Prueter, an internal medicine veterinary specialist here in Cleveland, to learn more and here is my summary of our conversation. Note that I've taken the liberty of summarizing our conversation in lay person's language (because that's what I am) and so any inaccuracies are all mine!
In the case of hypertrophic cardiomyopathy (HCM), the left side of the heart thickens so much that abnormalities can occur where the blood exits the heart (the areas in red in the diagram). HCM is generally found to be hereditary, particularly in Persians and Maine Coon cats.
Restrictive cardiomyopathy occurs when the inner lining of the heart becomes stiff, causing the heart muscle to thicken to try to pump harder, setting the stage for blood clots as well. Not much is known about the cause of restrictive cardiomyopathy.
Dilated cardiomyopathy (DCM) occurs when the heart muscle weakens. DCM used to occur frequently before it was discovered that the lack of the amino acid Taurine in cat food caused the condition. Taurine is very heat sensitive and the taurine in meat was destroyed in the pet food processing. Now that we know better and pet food is supplemented by taurine after the heat processing, this condition is much less common than before.
How exactly is a blood clot caused by heart disease in the case of a saddle thrombus?
The abnormalities from cardiomyopathy cause turbulence or "swirling" of blood inside the heart instead of the usual flow of blood out into the artery (aorta), which creates prime conditions for blood clots to form.
If a clot does form and moves out of the heart, it very often goes down the aorta and gets stuck where the artery splits (bifurcates) into the iliac artery, which lead down into the legs.
But a clot can go anywhere: into one of the legs where the whole or part of the leg is blocked off; the front legs; the brain. None of it good although it's much easier to tackle a lower leg blockage with surgery than a total leg blockage. Once the thrombus blocks the artery, it is very hard for a cat to recover from the damage caused.
Treatment Options for Saddle Thrombus Once It Has Happened
Veterinarians used to recommend surgery after a dye study is performed to determine where exactly the clot is located but even if the clot was removed successfully, many times another clot would replace it, sometimes within minutes. Nowadays, vets are not very likely to take this course of action.
Current treatments use drugs to break down the clots and to keep the clots from forming such as Acepromozine, which dilates the blood vessels. But there are adverse effects of these drugs and they don't always work, certainly not in the time frame needed to recover the use of the cat's legs.
How can you tell if a cat has heart disease? What are the symptoms?
Cats are notorious for not showing their symptoms externally. It is quite possible for your cat to have advanced heart disease and appear completely normal. Heart disease usually occurs as a cat ages but it is possible for younger cats and even kittens can get heart disease.
The symptoms can be found internally through a veterinarian's examination. A heart murmur or gallop rhythm (arrhythmia) can indicate an issue with the heart that has the potential for a clot to form.
The good news is that a cat can have a heart murmur for several years without any issue so it's important you take your cat to the vet for her regular check up to catch issues early. Heart disease can be managed well for many years before any heart failure occurs.
Treatment of Heart Disease and Blood Clots in Cats
If your cat has signs of heart disease, your vet may take an X-Ray of your cat's heart for indications of an enlarged or weakened heart muscle. For more refined measurements, he/she may also do an ultrasound. It is possible to see a thrombus in the left atrium if it's there but it may not always be visible.
If there is evidence of a clot, the vet will usually prescribe aspirin immediately and maybe even heparin to prevent further growth of the clot and to encourage the body's own internal mechanisms to break down the clot.
In short, take your cat to your veterinarian regularly so you might catch the early signs of heart disease and be able to prevent a saddle thrombus before it happens.
As with any of your pet’s health conditions, if you are worried about your pet, talk to your vet.