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What is Hypertrophic Cardiomyopathy (HCM)?
HCM is a disease of the heart muscle that results, in most cases, in an abnormal thickening of the muscle walls (Figure 1). This results in altered contraction and relaxation of the heart as it pumps blood, which can lead to clinical problems. Additionally, at least in people, the disease can also result in arrhythmias and sudden death, although this appears to be less common in cats.
Hypertrophic cardiomyopathy is rare in dogs, although some breeds appear to be possibly over-represented.
It is important to rule out other causes of thickening (hypertrophy) of the heart muscle, such as chronic hypertension, hyperthyroidism and outflow obstructions.
What Causes HCM?
HCM in humans is most commonly caused by mutations in genes that code for muscle proteins involved in contraction and relaxation of the heart muscle cells. In cats, several mutations have been identified that appear to be responsible for the disease in particular breeds. In other instances, however, no mutation has been found, but a genetic basis is suspected because the disease appears to be associated with particular breeds and can be shown to be inherited. It is important to note that the most commonly affected cat breed is the regular “domestic shorthair” cat, suggesting that many cases have no clear inheritance pattern.
To date, no viral or nutritional causes of HCM have been identified in humans or cats.
What Are the Consequences of the Thickened Left Ventricle?
Unlike thickening of the walls in response to a physical stress (e.g. weight lifting, or elite cycling) where the thickening occurs to deal with the extra workload placed on the heart, the thickening with HCM is not normal. The degree to which the muscle walls thicken ultimately determines the clinical severity of the disease – some cats (and people) have mild disease, others have severe disease. Additionally, in humans, different mutations cause different degrees of thickening.
As HCM develops and progresses, the structure and function of the heart is altered in several ways. The hallmark problem with HCM lies in the inability of the left ventricle to relax appropriately. The thickened left ventricular walls become less flexible which prevents the left ventricle from relaxing or stretching sufficiently to fill with blood from the left atrium. This abnormal relaxation and inability to stretch may ultimately result in a build-up of blood “upstream” of the left ventricle – namely the left atrium and the pulmonary circulation. As the blood backs up, fluid is forced from the pulmonary capillaries into the lungs and chest cavity causing pulmonary edema and pleural effusion, respectively (commonly called congestive heart failure, or CHF).
Very occasionally (probably <5% of the time), in cats, the thickening leads to arrhythmias and can result in sudden death (akin to what is seen in young basketball players that suddenly die on the court). It is difficult to predict which cats are likely to do, but sudden death is relatively uncommon in cats (compared to humans with HCM).
A small number of cats can suffer from feline aortic thromboembolism (FATE) – a blood clot in the aorta that causes blockage of blood flow to the back legs (most commonly), leading to sudden paralysis, severe pain, and often, death.
How Do Cats with HCM Present?
The clinical signs of HCM are variable. To some degree, the clinical signs depend on the severity – mild disease doesn’t cause obvious problems, but severe disease often does. Additionally, cats are masters at masking problems until they become severe, so cats with severe HCM may appear completely normal or have only subtle signs that go unnoticed (i.e., mildly increased respiratory rate) or they may be very nonspecific to heart disease (i.e., decreased appetite). On the other hand, an owner may notice signs such as respiratory distress secondary to congestive heart failure or leg paralysis secondary to a thromboembolic (blood clot) event.
In many cases, the first an owner knows about the disease is when a veterinarian informs them that they heard an abnormality during a regular or routine examination. A veterinarian can clue in on signs when he or she listens to your cat’s chest during their physical exam. A heart murmur, and/or gallop rhythm (extra heart sound) or an arrhythmia can be heard in many cats with HCM that are showing no outward signs of disease.
How is HCM Diagnosed?
An echocardiogram (ultrasound of the heart) with color flow and spectral Doppler imaging offers the best means to diagnose HCM. Echocardiography allows a veterinarian to observe the physical structure and dynamic function of the heart. Fortunately, the test is non-invasive and poses essentially no risk to the cat. Unfortunately, it is a relatively specialized procedure and can be expensive.
Electrocardiograms and radiographs provide additional useful information and are often used to assist a veterinarian in diagnosing HCM, but cannot be used alone to diagnose the disease. Since very subtle structural and functional changes can occur within the heart in the early stages of HCM, it is strongly recommended that a veterinary cardiologist be consulted for diagnosis as well as subsequent management of the disease.
Some veterinarians will run a blood test looking for changes in proteins produced exclusively by the heart (NT-proBNP). Elevations in this protein can be seen in cats with HCM, but are not a specific or sensitive means of detecting the disease. As previously mentioned, additional tests might be needed to rule out underlying diseases such as systemic hypertension or hyperthyroidism which can cause hypertrophy of the left ventricle similar to that seen with HCM. If no other causes are found, the diagnosis of HCM is made.
Two breeds (Maine Coon and Ragdoll) have been identified with mutations in the same gene (Myosin-binding protein C) as a cause of HCM. A genetic test has been developed to test these breeds for the mutation, and is offered through North Carolina State University. The test does not work in other breeds.
How is HCM Treated?
Currently, there is no cure for HCM. The changes occurring to the heart muscle are irreversible. However, if your pet’s left ventricular hypertrophy is secondary to some other underlying heart disease, such as hyperthyroidism, treatment of the primary disease can result in some or complete resolution of the heart condition.
As previously mentioned, hypertrophy of the heart muscle affects the ability of the left ventricle to relax properly, and therefore, function appropriately. While veterinarians might prescribe one or more medications to try to improve the heart’s relaxing ability, it is important to recognize that no drugs have been shown to be effective in achieving this. Thus, attempts to reduce the risk of heart failure and to help the heart function efficiently are largely theoretical and may ultimately be of no value.
Some treatment options that may be prescribed include:
- Drugs that are thought to alter relaxation of heart muscle, or slow down the heart rate to allow a longer time for the heart to fill, or both. No proof exists that either of these approaches actually benefits cats with HCM. It is quite acceptable to not treat cats with HCM prior to the onset of CHF – when information becomes available showing a benefit of any treatment, guidelines might change.
- Drugs to treat congestive heart failure (diuretics and ACE inhibitors). These medications are not specific for HCM, but are used in cats with heart failure secondary to any heart condition. With severe fluid build-up in the chest cavity, the veterinarian may physically remove the fluid with a catheter to help the cat breathe.
- Drugs that are thought to reduce the risk of clot formation, or clot recurrence. A medication that reduces the ability of the blood to clot might be prescribed if the veterinarian believes the cat is at risk for blood clot formation or currently has a blood clot in one of its arteries or heart chambers. The use of certain drugs for this purpose must be closely monitored to insure the patient is not placed at risk for hemorrhage. Treatment does not guarantee that a blood clot will not form, nor is it designed to break down previously formed clots. One study has demonstrated that cats given clopidogrel after surviving an initial clotting event have a lower risk of another event, compared with cats receiving aspirin.
As an owner of a cat with HCM, you should be very sensitive to changes in your pet's condition and should not hesitate to seek veterinary advice. Your veterinarian might show you how to monitor your cat's respiratory rate as an increased rate may be a sign that congestive heart failure is developing or worsening. Cats with congestive heart failure do not generally cough, but often exhibit open mouth breathing and panting. A cat that is having difficulty breathing from heart failure or has loss of function of hindlimbs or front limbs, requires veterinary care as quickly as possible. In the acute setting, these problems often need specific treatments (oxygen therapy, injectable medications, anticoagulation medications, or pain medications) that can only be offered by a veterinarian.
What is the Prognosis of a Cat with HCM?
The prognosis of a cat with HCM is highly variable. Some cats might develop only mild hypertrophy and suffer little compromise of heart function, while others progress to more severe disease. HCM might worsen quickly over a period of months, or it may progress slowly over several years. Its severity might not change for many years and then suddenly worsen. Some cats with HCM die very suddenly even though they had no clinical signs of heart disease.
A cat with mild to moderate disease may enjoy an essentially normal life for a number of years. However, the prognosis is much more guarded once the cat has more severe disease. The risk of developing congestive heart failure is proportional to disease severity, which is often classified by measuring wall thicknesses and left atrial size. Although congestive heart failure can be treated medically, severe heart failure may become difficult to manage over time as the disease progresses. The prognosis for a cat with heart failure, unfortunately, is guarded to poor. On average, survival for cats with HCM and heart failure is 12 to 18 months after diagnosis.
Thromboembolism is a severe uncommon complicating condition in HCM. It can cause acute pain and various clinical signs such as loss of function of the hindlimbs (most commonly, although other organs or limbs can be affected). Although treatment to break down or remove the clot is available, the high treatment costs of the procedure, death during administration of the drugs and high recurrence rate of thromboembolism dissuades most from attempting this type of therapy. With supportive care, about 40 to 50% of patients with thromboembolic disease will break down clots on their own and regain limb function over time. However, despite the best medical efforts to prevent their reoccurrence, a cat that has survived a thromboembolic event has a significant risk of developing another over the following weeks to months.
To Breed or Not to Breed?
In some forms of HCM, an autosomal dominant pattern of inheritance has been documented in some breeds. Not only does that mean it can be passed on genetically, but also that males and females are equally affected; every affected cat will have at least one affected parent; and all carriers of even a single copy of the gene can show the disease and transmit a mutant gene to half of their offspring.
In two breeds (Maine Coon and Ragdoll), a specific genetic mutation has been identified and a test has been developed to identify cats with the mutation. So what does this mean if you want to breed Maine Coon or Ragdoll cats?
If the DNA test is positive for that mutant gene, the cat may develop HCM. As in people, not all individuals with the mutation will develop the disease. Breeding recommendations are currently all over the place. Some people currently recommend that if the cat has two copies of the gene, the cat should not be used for breeding. Rather, the cat should be screened periodically to see if he has the disease. But if the cat has one copy of the gene, he should also be screened periodically for the disease, although his status for breeding is much better.
On the flip side, some veterinarians feel that any cat with the mutation should be spayed or neutered. However, the gene pool for purebred cats is pretty small, which colors all breeding considerations. If all the cats with the mutation in one generation are not bred, you might end up with far too much inbreeding, which would ruin the breed anyway. Therefore, some people recommend that cats who have one copy of the mutation and who have no clinical evidence of the disease may be used to breed to a mutation-negative cat. Offspring of that cat should be carefully evaluated and, if possible, a mutation-negative kitten should be used for a breeding replacement.
Before breeding your cat, be sure you understand all of the possibilities of what could happen. Genetics is an area that not all veterinarians are comfortable discussing. Discuss with your veterinarian whether referral to a geneticist or cardiologist or would be best for you and your pet.
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