What it is
A healthy heart muscle is strong yet supple; in this disease it becomes thicker and stiffer. The chambers of the heart shrink as a result, blood lingers in them, and the heart finds it harder to supply the whole body.
The disease can run in families, but it also appears in cats with no family history, so it is sometimes first noticed by a vet at a routine check-up.
At home, watch how your cat breathes while resting: calm, even breaths are a good sign, while quicker or laboured ones are a reason not to wait. Cats are skilled at hiding discomfort, so any change you notice deserves attention.
Signs you may notice
A cat may tire sooner, breathe harder or faster even at rest, play less and hide more. Sometimes the back legs suddenly turn weak.
When to see a vet
If your cat breathes with difficulty, with an open mouth or very fast, or its back legs suddenly weaken, a vet is needed immediately. For smaller changes, such as tiring quickly or losing interest in play, book a check-up.
This is general information, not veterinary advice. For your own cat's health, tests and treatment, see a vet.