What is feline hypertrophic cardiomyopathy?
Feline hypertrophic cardiomyopathy (HCM) is the most common heart disease seen in cats and as the name would suggest, the main feature of HCM is an excessive thickening of the left ventricular wall, papillary muscles, and septum. Cardio=heart, myopathy=muscle disease and hypertrophic=thickened. The heart is a four-chambered pump which is divided into left and right sides, each side has two chambers, a ventricle, and atrium. The ventricles serve as pumping chambers and the atria as receiving chambers.
The left atrium and left ventricle.
The right atrium and right ventricle.
The right atrium receives deoxygenated blood from the body which is pumped into the right ventricle. From here, the blood is pumped into the lungs where it is oxygenated. The left atrium receives the oxygenated blood, pumping it into the left ventricle, from here, the blood is pumped into the rest of the body.
In some cases, systemic diseases can lead to a thickening of the heart. These include high blood pressure (hypertension), hyperthyroidism and acromegaly.
What happens with hypertrophic cardiomyopathy?
Enlargement of the heart wall causes stiffening of the muscle, preventing the heart from expanding to receive blood properly.
As the walls thicken, the size of the heart chambers decreases resulting in less blood pumping through the heart. The heart has to work harder, beating faster to maintain blood flow throughout the body.
It may also reduce the ability of the valves to work properly, and in some circumstances obstruct the flow of blood out of the heart.
Arrhythmias, irregularities of the heartbeat and conduction disturbances are also common complications of hypertrophic cardiomyopathy. The thickened wall sometimes distorts one leaflet of the mitral valve, causing it to leak.
As the left ventricle shrinks and stiffens due to a thickening of the wall, it is less able to accept as much blood, this results in a build up in the left atrium and the lungs and the lining of the chest, leading to congestive heart failure and pulmonary edema, making breathing difficult.
Blood clots can form in the left atrium and be carried into the systemic arterial system, most often lodging in the terminal artery, causing paralysis of the hind legs.
The cause of HCM hasn't been established. However, it is known that HCM in Maine Coons (MYBPC3-A31P) and Ragdolls (MYBPC3-R820W) is inherited as an autosomal dominant trait with incomplete penetrance. Cats have two sets of genes, one from the mother, one from the father. If the cat inherits one mutant allele (an allele is an alternative form of a gene), he is at greater risk of developing HCM, and an even higher chance if he inherits two mutant alleles (one from each parent). Around 30% of Maine Coons have this genetic mutation.
There is anecdotal evidence that HCM may be inherited in Norwegian Forest Cats, Scottish Folds, and American Shorthairs, but this is not the same gene mutation as the ones the Maine Coon or Ragdoll have, but possibly similar. However, the disease is seen most often in domestic (mixed breed) cats. It is still not known if this is inherited or a spontaneous mutation.
What are the symptoms of hypertrophic cardiomyopathy?
A cat with hypertrophic cardiomyopathy may display no symptoms at all but die suddenly and unexpectedly. Symptoms can often develop slowly and subtly and many are associated with fluid build up in the lungs and pleural space which make breathing more difficult:
- Rapid, laboured and noisy breathing.
- Decreased activity.
- Irregular heart rhythm.
- Heart murmur.
- Weight loss.
- Blue-tinged gums due to a lack of oxygen circulating and/or pulmonary edema and pleural effusion.
- Lameness or paralysis of the hind legs due to saddle thrombosis in which a blood clot breaks free from the heart travels down the aorta and becomes stuck in the junction which supplies blood to the legs.
- Sudden death.
Are some cats more prone to developing hypertrophic cardiomyopathy than others?
As we have already noted, some breeds can inherit a genetic mutation making them more likely to develop HCM.
The disease is seen most often in middle-aged cats and older. Male cats are more prone to developing HCM as are overweight cats.
How is hypertrophic cardiomyopathy diagnosed?
A physical examination may reveal abnormal heart or lung sounds, irregular or gallop heart rhythm or a heart murmur, this may well be the first indication that your cat has HCM.
Echocardiogram (heart ultrasound) is the best diagnostic tool for HCM. Your veterinarian will evaluate the size, shape, and functioning of the heart. The thickening can range from mild to pronounced.
X-Ray can show if there is fluid in the chest and heart enlargement.
Electrocardiogram - This test records the electrical activity of the cat's heart to check for irregular in the rhythm.
Blood tests including a complete blood count and chemistry panel. These can help provide information on the function of other organs. This information is important when determining methods of treatment.
Thyroid function tests to determine if the cause is due to hyperthyroidism.
Arterial blood pressure to check for hypertension.
A genetic test is available for the Maine Coon and Ragdoll HCM gene defect.
How is hypertrophic cardiomyopathy treated?
Asymptomatic cats may require no treatment, but your veterinarian will want to monitor him/her closely. It is not possible to cure HCM, treatment is aimed at controlling symptoms and preventing complications.
The thick ventricles of HCM contract and relax abnormally, and to assist the relaxation phase some drugs may be necessary. Beta-blocking drugs such as atenolol or propranolol to slow down the heart rate, this gives the heart more time to fill the chambers.
ACE inhibitors and calcium blocking channels may be prescribed to assist.
If the cat has congestive heart failure, diuretics may be prescribed.
Controlled doses of blood-thinning drugs such as aspirin for cats who have a high risk of clot formation.
Thoracocentesis (pleural tap) may be performed to remove fluid build up from the pleural space.
Placing your cat on a low salt diet.
Restricting activity also reduces the strain on the heart and your veterinarian may prescribe a period of cage rest.