increases blood volume may be stimulated, which in turn can increase the pressures on the left side of the heart and contribute to congestive heart failure.
The left atrium may enlarge due to increased pressures caused by the stiffened left ventricle's inability to fill with a normal volume of blood from the left atrium. Atrial enlargement can slow blood flow, which may in turn cause blood clots to form in the atrium. Clots that find their way into the circulatory system can become lodged in such a way as to block the flow of blood. Rear leg paralysis, a classic example, occurs when a clot lodges where the descending aorta, a major artery, branches to go to the rear legs. This situation is commonly referred to as a saddle thrombus.
Diagnosis
A cat with HCM may show no clinical signs at all, may show signs of respiratory distress, or may exhibit severe heart failure, leg paralysis, or even sudden death. Signs such as a mild increase in the respiratory rate can be so subtle that they go unnoticed. Many cats with HCM develop a fast heart rate, and/or a heart murmur, and/or a gallop rhythm, an extra heart sound, as the disease advances. These signs cannot be relied upon however to indicate the presence of disease before it becomes severe.
An echocardiogram, ultrasound of the heart, is the most conclusive means of diagnosing HCM. An echocardiogram reveals both the physical structure and dynamic functioning of the heart. It is non-invasive and poses essentially no risk to the cat. Electrocardiograms and X-rays may provide your veterinarian with additional useful information, but cannot be used by themselves to arrive at an unequivocal diagnosis. The veterinarian will also perform other tests to determine if the hypertrophy is secondary to, caused by, another illness such as hyperthyroidism or hypertension. If no other causes are found, the cat is diagnosed with HCM.
Treatment
At present there is, unfortunately, no cure for HCM. If the cat's heart is hypertrophied as the result of another disease, treatment of the primary disease may result in some improvement of the heart condition. Hypertrophy affects the heart's ability to function properly, one or more medications may be prescribed in an effort to reduce the risk of serious heart failure and to help the heart function as efficiently as possible. Treatment may also, in some cases, prevent further damage to the muscle of the heart walls.
Outlook
HCM sometimes offers no clues to its presence until it's too late for any treatment to be of much help. Sometimes there are signs that all is not well before the disease has progressed too far, but even then it is impossible to predict its course with any certainty. Some cats may develop only mild hypertrophy and suffer little compromise of heart function, while others progress to more severe disease. HCM may worsen quickly over a period of months; it may progress slowly over several years; its severity may not change for some years and then suddenly worsen, or it may not. Some cats with HCM may die very suddenly even though they seemed healthy only moments before.
The veterinarian may prescribe one or more medications to manage the cat's condition. Treatment will vary depending on the cat's clinical signs and how HCM has affected the heart. While HCM cannot be cured, medication can improve the heart's ability to function. It may help the cat live longer and more comfortably.
A cat with mild to moderate disease may enjoy an essentially normal life for a number of years. The long-term outlook is more guarded if the cat has more than a mildly enlarged left atrium, a sign of severe disease. The prognosis for a cat with heart failure, unfortunately, is guarded to poor. Survival, on average, is only a few month.