Hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most common type of acquired heart disease in cats. This condition causes the muscular walls of a cat’s heart to thicken, decreasing the heart’s efficiency and sometimes creating symptoms in other parts of the body. 

While cats may show no outward symptoms at first, the disease can be progressive and may cause clinical signs in time.  The domestic shorthair cat is most commonly diagnosed with HCM, although there is a form of HCM that is familial and manifests at a young age (usually less than 3 years old) in Maine Coon, Sphynx, and Ragdoll cats. Evidence also shows that American shorthair, Persian, and oriental breeds may be predisposed to HCM. HCM may develop at any age, but most cases are diagnosed in middle age to older cats. Males are diagnosed more frequently than females.

Effects of HCM

When the heart muscle becomes hypertrophied, it cannot relax normally after contraction. Relaxation is necessary so that blood can refill the ventricle in order to be pumped to the body. The blood that should be shuttled to the body then builds up and the pressure forces fluid from the blood vessels into the lungs or chest cavity (this is commonly known as congestive heart failure (CHF)). In some cats, HCM will cause abnormal heart rhythms or arrhythmias and can result in sudden death. The build-up of blood within the heart may also cause the formation of thrombi, or blood clots. These clots are then pumped through the blood stream and may cause blockage of blood flow.  This occurs most commonly when blood reaches the end of the aorta and splits to deliver blood to the back legs. Clots may also occur in the front limbs, and happen more commonly on the right side. HCM cats that develop an aortic thromboembolism (ATE) will develop sudden paralysis.

Symptoms

The clinical signs of HCM are variable. In most cases, the degree of heart muscle thickening determines the severity of the disease. Cats with early or mild forms of the disease may show no evidence of disease while more advanced cases may have non-specific symptoms like decreased appetite or lethargy or evidence of CHF, like difficulty breathing. Cats with ATE will often have acute, sudden paralysis.

Other common symptoms consist of:

• Loss of appetite (anorexia)
• Lethargy
• Weak pulse
• Difficulty breathing
• Short, rough, snapping breathing sounds (crackles)
• Abnormal heart sounds (i.e., muffled, galloping rhythm, murmurs)
• Inability to tolerate exercise or exertion
• Sudden hind-limb paralysis with cold limbs due to clot in the terminal aorta
• Bluish discoloration of foot pads and nailbeds (indicates a lack of oxygen flow to the legs)
• Collapse
• Sudden heart failure

Diagnosis

The earliest indication that HCM may be present is detection of a heart murmur or arrhythmia upon chest auscultation, although some cats with clinically significant HCM will not have detectable abnormal heart sounds. Many times this is noted during routine physical examinations. Your veterinarian will make determinations as to the next appropriate diagnostic step, which may include blood work, chest x-rays or a cardiac ultrasound (echocardiogram). An echocardiogram can identify how thick the heart muscle is and whether or not blood clots are present  in order to determine the severity of the disease and if treatment is needed.

Treatment

Although HCM has no known cure, a specialized care plan can help manage clinical signs of the condition in your cat. Treatment goals include controlling the heart rate, alleviating lung congestion (congestive heart failure), and preventing the formation of blood clots that can lead to thromboembolism.

Medication can help manage HCM, and can be administered orally to stable patients or by injection in more serious situations. Other drugs, such as nitroglycerine, may be applied to the cat’s skin for absorption.

Unfortunately, no therapy has been shown to prevent the progression of HCM when started before clinical signs are observed.

Prognosis (predicted outcome)

The prognosis for cats with HCM is variable. Cats that do not display any clinical signs are often able to survive for years with only mildly compromised heart function.

 Cats that develop heart failure have a guarded prognosis and survival times are usually estimated to be 12 to 18 months after diagnosis. Only 30% of cats that develop ATEs survive to discharge from the hospital and they often require intensive at home care.

HCM is most commonly a progressive disease, and findings that suggest a worse prognosis include congestive heart failure, thromboembolism, and hypothermia (low body temperature). However, in many cases, medical therapy can significantly improve your cat’s quality of life.

Talk to your veterinarian today to learn more about your cat’s health and treatment options. 

Living and Management

The cat should be put on a sodium-restricted diet, especially if there is congestive heart failure, to keep the pressure in the blood stable. Providing a quiet and safe space for your cat, away from other pets and active children, is important to its recovery. Environmental stress may activate the nervous system, placing excess stress on the already overstressed left ventricle, and possibly leading to heart failure.

You will need to monitor your cat closely during the recovery period, watching for difficulty breathing, lethargy, weakness, lack of appetite, and painful hind-limb weakness or paralysis. If your cat is being treated with warfarin, its blood will need to be tested to determine whether the drug is effectively decreasing the likelihood of a blood clot. Use of warfarin can also lead to uncontrolled bleeding and bruising. You will need to take care that your cat is kept from activities that may lead to injury while it is on this drug. If your cat is taking an ACE inhibitor, or spironolactone, kidney function and electrolytes will need to be monitored. After six months, a repeat ultrasound examination of the heart will be required to determine progress, and whether further treatment will be required.