Hypertrophic cardiomyopathy (HCM)
The most common heart disease in cats: the muscle of the left ventricle thickens, so the chamber holds less blood and fills less easily.
A thicker heart muscle sounds like a stronger heart and is the opposite. The wall grows inward, the chamber it encloses shrinks, and the stiffened muscle cannot relax enough to fill properly between beats. Blood that moves sluggishly is blood that can clot, which is why one of the first signs is sometimes a sudden, painful loss of use of the back legs.
Many affected cats show nothing at all until they are in trouble, and a heart murmur is an unreliable guide either way. Ultrasound is how it is actually diagnosed. In some breeds a known mutation can be DNA-tested, though a clear test does not rule the disease out.