Heart disease is often called a silent killer in cats (and humans) because in many cases the first obvious symptom is sudden death. With heart disease there’s also an increased risk of blood clots which can cause pain, paralysis, or also death.
Unfortunately for many cats the first visible symptom they have of heart disease will be a saddle thrombus. Cats with heart disease often have abnormal blood flow and pooling inside the heart itself, which can cause small blood clots to form inside the heart. Not every cat with heart disease has this problem, but many do. A saddle thrombus is sudden, painful paralysis of one or both hind legs due to one of those blood clots leaving the heart and getting stuck at the base of the aorta (largest blood vessel leading away from the heart) where it splits into the femoral arteries (largest blood vessels in the legs). These clots block blood flow to the legs, which paralyzes them and causes muscle and nerve damage/death due to lack of oxygen.
There are treatments available for saddle thrombus and other types of blood clots, but if your cat experiences a saddle thrombus or other blood clot you must act immediately to begin treatment.
A saddle thrombus is a medical emergency. If your cat has sudden pain and inability to move one or both of their hind legs, get to an emergency vet right away. Do not wait. The longer you wait the less likely your cat is to recover.
Disease Information Sites
Ten Lives has an excellent site covering blood clots in cats, treatment options, and general support.
Support Groups
Cats with Blood Clots (inc. a saddle thrombus): a community support page on Facebook is an excellent group for anyone dealing with blood clots in their cat. If your cat has heart disease I would recommend becoming a member of this group even if your cat has never experienced a blood clot before.
The Heart Disease page has additional support groups that would be helpful to join
Feline Disease Management 