21.3: Cardiomyopathy III: Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy, or HCM, is an autosomal dominant genetic disorder characterized by asymmetric left ventricular hypertrophy without ventricular dilation. It is more common in men and is typically diagnosed in young, athletic adults.

Etiology

HCM is primarily genetic and is caused by mutations in genes encoding sarcomeric proteins. Researchers have identified over 1400 mutations across at least 11 different genes. Among these, the most frequently occurring mutations are found in the genes for beta-myosin heavy chain, myosin-binding protein C, and cardiac troponin T.

Pathophysiology

Normally, cardiac muscle cells are aligned parallel and end-to-end with each other. However, in hypertrophic cardiomyopathy (HCM), a genetic mutation causes the cardiac muscle cells to become hypertrophied, disorganized and oriented obliquely or perpendicularly to each other. This disarray results in the thickening of the ventricular walls, especially the interventricular septum. The thickened septum can obstruct blood flow out of the left ventricle. In contrast, the thickened walls decrease ventricular compliance, making it difficult for the ventricles to fill with blood during diastole, a condition known as diastolic dysfunction.

The four main characteristics of HCM are:

• Massive ventricular hypertrophy
• Rapid, forceful ventricular contraction
• Impaired diastole
• Obstruction to aortic outflow

Clinical Manifestations

Patients with HCM may be asymptomatic or experience:

• Fatigue: Due to decreased cardiac output.
• Exertional dyspnea: Caused by elevated left ventricular diastolic pressure.
• Angina: Resulting from increased left ventricular mass or compression of small coronary arteries by the hypertrophic myocardium.
• Syncope: This can occur due to obstruction of aortic outflow during activity, leading to decreased cardiac output and reduced cerebrovascular circulation. Arrhythmias, such as supraventricular tachycardia, atrial fibrillation, ventricular tachycardia, and ventricular fibrillation, can also cause syncope.

Diagnostic Studies

Diagnosis of HCM is primarily based on imaging studies, with echocardiography being the cornerstone. It assesses heart muscle thickness and outflow tract obstruction. Other diagnostic tests include:

• Chest Palpation: This may reveal an exaggerated and left-displaced apical impulse.
• Auscultation: May detect an S4 heart sound and a systolic murmur between the apex and sternal border at the fourth intercostal space.
• ECG: Typically shows ventricular hypertrophy, ST-T wave abnormalities, prominent Q waves in inferior or precordial leads, and arrhythmias.
• Heart Catheterization and Nuclear Stress Testing: Assist in diagnosing and guiding treatment.

Interprofessional Care

Management of HCM is tailored to symptom severity and outflow tract obstruction:

• Medications: Calcium channel blockers (e.g., verapamil) or beta-blockers (e.g., metoprolol) can improve heart filling by slowing the heart rate and reducing contractility. Amiodarone or sotalol is used to treat arrhythmias.
• AV Pacing: Helpful for patients with outflow obstruction.
• Surgical Treatment: Required for patients with severe symptoms unresponsive to therapy. The procedure includes myectomy, which involves cutting into and removing part of the thickened septal muscle.
• Percutaneous Transluminal Septal Myocardial Ablation (PTSMA): It is an alternative nonsurgical procedure that reduces left ventricular outflow obstruction by injecting alcohol into the first septal artery to cause ischemia and septal wall infarction. It decreases obstruction and improves symptoms and exercise capacity.
• Implantable Cardioverter-Defibrillator (ICD): ICDs are the most effective intervention for preventing sudden cardiac death (SCD), particularly in high-risk patients with features such as prior cardiac arrest, sustained ventricular arrhythmias, or a strong family history of SCD.

Health Teaching

Instruct the patient to avoid dehydration and activities that increase systemic vascular resistance. Rest and elevation of the feet to improve venous return can help manage chest pain. Patients and their families should receive counseling to address the emotional impact of living with a genetic heart condition, discuss lifestyle modifications, and review family screening and genetic testing options. Psychological support may also help patients cope with anxiety related to activity restrictions or the risk of sudden cardiac events.