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Hypertrophic cardiomyopathy (HCM in cats) is a disease of the heart muscle in which it becomes thicker than normal (it hypertrophies) and is unable to relax appropriately.
What causes HCM in cats ?
The exact causes of this disease are not yet completely understood. The heart muscle may undergo hypertrophy in response to certain stimuli (i.e. high blood pressure and overactive thyroid gland) or this may be due to a primary cardiac disease. In humans, hundreds of genetic mutations that may cause HCM have been identified and this disease tends to be familial. In cats only a few genetic mutations are presently known to be associated with the disease.
What happens ?
As the heart muscle progressively thickens, the heart cavities become smaller. Additionally a hypertrophied muscle is not able to relax normally and becomes “stiff”. As a consequence there will be less space left in the heart (ventricles) to accommodate blood that needs to be pumped to the body. Blood “backs up” in the heart cavities that receive blood (atria) and in the veins in the lungs and the rest of the body. The atria will enlarge in order to accommodate this extra amount of blood but this can only happen to a certain extent. Beyond that point too much blood will back up in the veins and fluid will start accumulating in the lungs, chest or abdomen - congestive heart failure.
In some cases hypertrophy of the muscle close to the exit of the heart will lead to obstruction of blood flow exiting the heart in the direction of the whole body. This is no more or less serious than conventional HCM but it may occasionally lead to fainting during periods of stress or excitement.
All cats may develop HCM although a few breeds are more affected.
Onset and natural progression
HCM is an adult onset disease - often around 5 years of age when first diagnosed - but may be detected in cats as young as 4 months of age. Many cats live a long time with this condition without any sign of disease. It may even go undetected throughout their whole lives. Some however develop heart failure at some point in their lives and need medical care. Ideally we would identify these patients as early as possible so that we can hopefully avoid and control heart failure.
Often a heart murmur is the only sign of disease. Cats are experts in “hiding” any sign of disease and when we do detect them (i.e. lethargy, difficulty breathing) the disease is usually already in a quite advanced stage.
HCM can be easily identified on an ultrasound scan of the heart - echocardiographic examination. Other diseases are also excluded with this exam. It is very important that a heart scan be performed as soon as a heart murmur is detected.
If irregularities in heart rhythm are noticed an electrocardiogram is performed in order to assess it’s significance and need for treatment.
Chest X-rays allow identification of signs of congestive heart failure - fluid in the lungs or chest cavity.
Blood pressure measurement is necessary to assess for systemic hypertension.
Blood tests are also performed to assess thyroid hormone levels and assess kidney function and electrolytes.
Unfortunately a cure does not exist for this condition. A combination of drugs is used to control signs of congestive heart failure and prolong life with the best quality of life possible.
Furosemide - to excrete the excess fluids in the body and treat congestive heart failure
ACE inhibitor - to counteract fluid retention by the kidneys and dilate the blood vessels and lessen the load on the heart
Other drugs- other drugs may also be necessary (i.e. antiarrhythmic drugs)
Fluid drainage- if a significant amount of fluid accumulates in the chest or abdomen, drainage may be necessary.
Presently there is not enough evidence to suggest that starting treatment before heart failure occurs is effective in delaying onset of failure or prolonging life. If signs of imminent risk of heart failure are detected on the heart scan or chest radiographs then treatment is started.
Regular examinations (i.e. every 6 months) will be necessary to appropriately manage HCM.
What will happen during a re-examination ?
At each visit you will be asked a number of questions to assess how your cat is coping with treatment and with the condition. After being examined, blood pressure will be measured and an echocardiographic examination will normally be repeated. Blood tests may be performed to assess how the kidneys are coping with treatment and disease and also provide further important information needed for treatment adjustments.
After any treatment adjustments a brief re-check at your vet may be necessary. The same blood tests will be repeated to assess the kidneys response to therapy, blood pressure and eventually chest x-rays if there has been previous congestive heart failure. Usually this is performed 5-10 days after the initial examination.
Depending on response to therapy, some cats may cope well with the disease for a fair amount of time whereas others unfortunately do not. This varies with each case and is very difficult to predict. Sudden death is possible, particularly if rhythm disturbances occur.