Affects: Cats
Hypertrophic cardiomyopathy (HCM, or HOCM when obstructive) is a condition in which muscle tissues of the heart become thickened without an obvious cause. The parts of the heart most commonly affected are the interventricular septum and the ventricles. This results in the heart being less able to pump blood effectively and also may cause electrical conduction problems. Specifically affected are the bundle branches that conduct impulses through the interventricular septum and into the Purkinje fibers, as these are responsible for the depolarization of contractile cells of both ventricles.
People who have hypertrophic cardiomyopathy may have a range of symptoms. People may be asymptomatic, or may have fatigue, leg swelling, and shortness of breath. It may also result in chest pain or fainting. Symptoms may be worse when the person is dehydrated. Complications may include heart failure, an irregular heartbeat, and sudden cardiac death.
Hypertrophic cardiomyopathy is most commonly inherited in an autosomal dominant pattern. It is often due to mutations in certain genes involved with making heart muscle proteins. Other inherited causes of left ventricular hypertrophy include Fabry disease and Friedreich's ataxia. Other considerations for causes of an enlarged heart are athlete's heart and hypertension (high blood pressure). Making the diagnosis of hypertrophic cardiomyopathy often involves a family history or pedigree, an electrocardiogram, echocardiogram, and stress testing. Genetic testing is recommended for affected people and their family members. Hypertrophic cardiomyopathy can be distinguished from other inherited causes of cardiomyopathy by its autosomal dominant pattern, whereas Fabry disease is X-linked, and Friedreich's ataxia is inherited in an autosomal recessive pattern.
Signs And Symptoms: Many people with hypertrophic cardiomyopathy are asymptomatic or mildly symptomatic, and many of those carrying disease genes for hypertrophic cardiomyopathy do not have clinically detectable disease. The symptoms of hypertrophic cardiomyopathy include shortness of breath due to stiffening and decreased blood filling of the ventricles, exertional chest pain (also known as angina) due to reduced blood flow to the coronary arteries, uncomfortable awareness of the heart beat (palpitations), lightheadedness, weakness, fainting, and sudden cardiac death.
Shortness of breath is largely due to increased thickness of the left ventricle (LV) wall and interventricular septal wall, which impairs the filling of the ventricles, but also leads to elevated pressure in the left ventricle and left atrium as a result of increased thickness. This impaired heart filling can cause blood to back up in the lungs' circulation, causing further symptoms Often, symptoms of hypertrophic cardiomyopathy are due to congestive heart failure (especially activity intolerance and dyspnea), but also lower extremity edema.
Major risk factors for sudden death in individuals with hypertrophic cardiomyopathy include prior history of cardiac arrest or ventricular fibrillation, spontaneous sustained ventricular tachycardia, abnormal exercise blood pressure and non-sustained ventricular tachycardia, unexplained syncope, family history of premature sudden death, and left ventricular wall thickness greater than 15 mm to 30 mm, on echocardiogram.
Hypertrophic cardiomyopathy also presents with a systolic ejection murmur that increases in intensity with decreased preload (as in the Valsalva maneuver or standing), or with decreased afterload (as in vasodilator administration). On the other hand, the murmur decreases in intensity with increased preload (as in squatting) or increased afterload (as in the handgrip maneuver). "Spike and dome" pulse and "triple ripple apical impulse" are two other signs that can be discovered in physical examination. Pulsus bisferiens may also be found during examination.
Pathophysiology: Ventricular hypertrophy causes a dynamic pressure gradient across the left ventricular outflow tract (LVOT), which is associated with further narrowing of the outflow during systole. Pulling of the mitral valve leaflets towards the septum contributes to the outflow obstruction. This pulling is thought to occur by several proposed mechanisms, including the flow of blood through the narrowed outflow tract resulting in a higher velocity, and less pressure via the Venturi effect. This low pressure then causes the anterior leaflet of the mitral valve to be pulled into the outflow tract, resulting in further obstruction.
The hypertrophied (thickened) left ventricular wall in hypertrophic cardiomyopathy requires a greater oxygen demand. This increased oxygen demand of the enlarged heart muscle, combined with fibrosis (scarring) of the heart muscle and a reduced flow through thick-walled coronary arteries results in an oxygen deficit to the heart muscle. These pathophysiological changes result in the anginal (exertional chest pain) symptoms sometimes seen in hypertrophic cardiomyopathy.
Diagnosis: A diagnosis of hypertrophic cardiomyopathy is based upon several features of the disease process. While there is use of echocardiography, cardiac catheterization, or cardiac MRI in the diagnosis of the disease, other important considerations include ECG and genetic testing. Genetic testing is recommended for those affected by hypertrophic cardiomyopathy and their family members.
In about 60 to 70% of the cases, cardiac MRI shows thickening of more than 15 mm of the lower part of the ventricular septum. T1-weighted imaging may identify scarring of cardiac tissues while T2-weighted imaging may identify edema and inflammation of cardiac tissue which is associated with acute clinical signs of chest pain and fainting episodes.
ECG is the most sensitive diagnostic test.
Prognosis: The annual mortality rate in those with hypertrophic cardiomyopathy is 1%. 70% of those with hypertrophic cardiomyopathy have a left ventricular outflow tract obstruction (HOCM).
A family history of sudden cardiac death, left ventricular wall thickness greater than 30 milimeters, an aneurysm at the left ventricular apex, unexplained syncope (passing out) multiple episodes of sustained ventricular tachycardia, late gadolinium enhancement on cardiac MRI (a marker of heart muscle fibrosis or scar formation) and a left ventricular ejection fraction of less than 50% are all risk factors for sudden cardiac death in hypertrophic cardiomyopathy. The incidence of sudden cardiac in those older than 60 with hypertrophic cardiomyopathy is rare.