What is Hypertrophic Cardiomyopathy?
Hypertrophic Cardiomyopathy (HCM) is a primary defect of the myocardium leading to mild to severe concentric hypertrophy. HCM is most commonly seen in cats with breeds such as Maine Coon cats being predisposed. The concentric hypertrophy of HCM can appear similar to other hypertrophy caused by hyperthyroidism, anemia, hypertension or acromegaly but the name HCM is reserved for the primary defect seen with no underlying systemic cause for this myocardial disorder. Hypertrophy can be seen symmetrically or asymmetrically and can affect the ventricular free wall, septum or combination of both. This thickening can make the ventricular chamber itself smaller and stiffer and will decrease afterload with progression.
Diastolic filling is impaired by the thickened chamber, leading to increasing filling pressures during diastole and ultimately leading to a decrease in end diastolic volume. Mitral valve regurgitation can be seen secondary to systolic anterior motion of the mitral valve when the abnormally shaped ventricle and/or papillary muscles allows the mitral valve to move into the left ventricular outflow tract during systole. Maintenance of ejected blood volume is attempted as the heart rate increases to maintain stroke volume. Decreased perfusion may still occur as cardiac output is eventually decreased. This scenario can lead to congestive heart failure manifested most commonly as pulmonary edema or pleural effusion.
Thromboembolic disease can be a complication of HCM with stagnation of blood in an enlarged left atrium. Supraventricular or ventricular tachyarrhythmias are also possible findings in cats with HCM. Auscultation of the heart often reveals a low to moderate grade systolic heart murmur most commonly due to mitral valve regurgitaiton and/or left ventricular outflow tract obstruction. A gallop sound and/or an arrhythmia may also be heard. The gallop sound is usually a third heart sound of ventricular filling or fourth heart sound of atrial contraction.
Clinical signs of HCM are usually not observed by the owner until the animal is in congestive heart failure. Left sided heart failure is the most commonly seen as the left side of the heart is most commonly affected. Clinical signs of left heart failure may be subtle such as increased respiratory rate at rest to mild decreases in activity or exercise intolerance secondary to pulmonary edema or pleural effusion. Other clinical signs may include hiding, decreased appetite and lethargy.
Signs may progress to fulminate respiratory distress. Coughing cats can be mistaken for vomiting cats. Clinical signs of right heart failure may include weight gain, abdominal distension, inappetance, difficulty breathing, exercise intolerance and increased lethargy. A way for owners to monitor to their pet at home for early signs of congestive heart failure is to count the resting respiratory rate. This can be done when the pet is sound asleep and the number of breathes per 15 seconds is counted.
This number is then multiplied by 4. If this number is consistently above 40 when the pet is sound asleep, this may be an early sign of congestive heart failure and the veterinarian should be contacted. Other signs in cats of HCM include signs associated with at thromboembolic event. These signs include weakness of the hind limbs or fore limb, dragging the hind limbs, cold and discolored limbs and paws, tense hind limb muscles and generalized limb pain. Other clinical signs of HCM include sudden death most likely related to a fatal arrhythmia or thromboembolism to the brain.
Treatment for this disease varies amongst clinicians. A general treatment protocol is the institution of a loop diuretic such as furosemide and an angiotensin converting enzyme (ACE) inhibitor when these patients present in heart failure. Most clinicians will also institute a beta-blocker to decrease the heart rate and decrease systolic anterior motion of the mitral valve. Calcium channel blockers may be added to help with early diastolic filling. In severe or recurrent cases of heart failure, diuretics with effects in the distal nephron and other afterload reducers may be added for treatment. Finally, thromboembolic events are treated with superior nursing care with attention to pain relief and electrolyte disturbances that can occur with reperfusion.
Cats with mild HCM and no evidence of congestive heart failure should have thoracic radiographs and echocardiograms performed every 6-12 months. Moderate to severely effected cats should be examined every 6 months or more frequently once congestive heart failure has been diagnosed. The prognosis and disease progression can be quite variable between cats and follow up can help determine your pet’s prognosis and visit frequency.