The prognosis for HCM may vary. However, it should be treated as a serious condition as it has the potential to lead to congestive heart failure. There are no sex or age predispositions for HCM However, certain breeds such as Maine Coons, Ragdolls, Sphynx, and Persians have a higher chance of developing the disease due to congenital defects in cardiac proteins.

Thickening of the heart muscle is usually symptomatic of hypertrophic cardiomyopathy (HCM), the most commonly diagnosed heart condition in cats. In HCM, the walls of the left ventricle – the main heart muscle responsible for pumping blood – thicken, causing an increased heart rate and a decrease in stroke volume. Stroke volume is the amount of blood pumped out of the heart per beat. Thickening of the heart muscle may also be associated with other cardiac conditions, including high blood pressure and hyperthyroidism.

Many cats with thickened heart muscles may not present symptoms. Some cats may show signs of congestive heart failure. In any case, seek immediate veterinary attention as soon as you notice any of the following symptoms:

The primary cause of thickening of the heart muscle in cats is hypertrophic cardiomyopathy. The causes of HCM are not fully understood. However, research shows that there is a genetic link, particularly in some breeds. While the condition most commonly affects adult cats, HCM may be diagnosed in cats as young as four months old.

Other causes of thickening of the heart muscle are secondary hypertension and hyperthyroidism.

Your vet will be able to make a tentative diagnosis based on a thorough physical examination and presentation of symptoms. Be sure to inform your vet of the extent and duration of your cat’s symptoms, as well as any previous history of heart conditions or blood clots.

Your vet will first conduct tests in order to rule out other heart conditions. These tests may vary based on the suspected underlying condition but may include heart pressure, blood pressure, and blood work. Your vet will make a definitive diagnosis of HCM by taking an echocardiogram (an ultrasound of the heart). This will show the thickened heart muscle and any fluid present in the heart or lungs.

Treatment may vary depending on the severity and the underlying condition. Your vet will be able to advise you on a treatment plan based on your cat’s specific needs.

Treating high blood pressure is usually straightforward, and typically involves medication. High blood pressure may resolve or disappear completely with the help of medication.

Hyperthyroidism may be treated with medications that control hormones in the thyroid gland. Unfortunately, many of these medications are prescribed for life and can cause undesirable side effects, including vomiting, loss of appetite, and fever. Hyperthyroidism may also be treated with surgical removal of the thyroid gland, or with radioactive iodine therapy. This treatment method involves an injection of radioactive iodine, which destroys the malformed thyroid. Radioactive iodine therapy does not harm nearby organs and has no side effects. However, cats treated with radioactive iodine therapy must be hospitalized for three days following the procedure. This is so the cat’s radiation levels can return to normal.

Treating HCM usually involves treating the signs rather than attempting to cure the condition. The primary objective of treatment is to control the heart rate and prevent blood clots. This can be done with oral medications, or injections for severely ill cats. Some cats with signs of congestive heart failure may need to have excess fluid drained from their lungs.

Recovery and prognosis depend on the underlying condition. Treatment for thickened heart muscles – especially cases associated with HCM – usually require lengthy treatment. Always follow your vet’s post-treatment and/or post-operative instructions carefully. Always administer any prescribed medications for the full duration of the treatment period. 

Ensure your cat has a warm, safe place to rest during the recovery period. You may want to monitor your cat’s activity to ensure they aren’t overexerting themselves. If your cat has had surgery, do not allow them to irritate the surgery site.

Your vet will schedule follow-up appointments as needed to monitor your cat’s condition. Hyperthyroidism, especially cases treated with drug therapy, usually warrants frequent follow-ups. If you have any questions, or if the condition does not seem to be improving despite treatment, contact your vet immediately.