Hypertrophic Cardiomyopathy: When Bigger Does Not Mean Better
Hypertrophic cardiomyopathy (HCM) is a hereditary disease that, in some patients, proceeds asymptomatically, while others struggle with arrhythmias or heart failure, and in some individuals, the first manifestation of the disease can be sudden cardiac death. More about the etiology and diagnosis of HCM is discussed in the following overview.
What is the prevalence of HCM?
HCM is a relatively common genetically determined disease that occurs equally in both sexes. The prevalence of asymptomatic hypertrophy in young adults in the USA ranges from 2 to 5 cases per 1000 people.
How does the disease manifest?
The pathophysiology of HCM involves dynamic obstruction of the left ventricular outflow tract (LVOTO), mitral regurgitation, diastolic dysfunction, myocardial ischemia, arrhythmias, and autonomic dysfunction. For each individual patient with HCM, the clinical manifestation of the disease may be dominated by only one of these components or a combination thereof. Some patients have no limiting symptoms, but up to 40% of patients experience undesirable manifestations over their lifetime – sudden death, progression of limiting symptoms of LVOTO or diastolic dysfunction, heart failure symptoms related to systolic dysfunction, or atrial fibrillation with the risk of stroke.
How is the diagnosis made?
Suspected HCM may arise from the detection of a heart murmur or abnormalities in a 12-lead ECG during a routine health check, clinical examination prompted by the occurrence of certain symptoms, or an unexplained cardiac event. HCM can also be detected through heart imaging in the screening of blood relatives of patients.
HCM is characterized by left ventricular hypertrophy that cannot be explained by another cardiac, systemic, or metabolic disease, with the involvement limited exclusively to the heart. Clinical diagnosis of HCM in an adult patient can be established using imaging methods (2D echocardiography or cardiovascular magnetic resonance), which show a maximum thickness of the left ventricular wall at the end of diastole (in any location) ≥ 15 mm. For patients with a familial history of HCM or positive genetic testing, a thickness of 13–14 mm is considered diagnostic.
What causes HCM?
Around 1990, gene variants leading to left ventricular hypertrophy were identified in families with HCM. These include mutations in genes for cardiac muscle proteins, most commonly in the gene for myosin heavy chain 7 (MYH7) or myosin-binding protein C 3 (MYBP-C3). Changes in one of these genes are found in approximately 70% of patients with an identified pathogenic variant, while changes in other genes for cardiac sarcomere proteins occur in only a small population of patients (1–5% per gene).
Currently, the causative mutation can be identified in about 30–60% of patients with HCM. More than 1500 pathogenic variants of sarcomere proteins have been found, most of which are unique to a particular family. The inheritance of HCM is autosomal dominant, meaning each offspring has a 50% chance of having the disease. The likelihood of clinical development of HCM in individuals with a pathogenic variant is high, but the onset time of the disease is highly variable.
What is the prognosis for patients?
Given the large number and diversity of pathogenic gene variants associated with HCM, it is not possible to use a specific genotype to determine individual risk. In recent years, algorithms for risk stratification of sudden cardiac death (SCD) have been developed, taking into account numerous non-invasive risk markers. Adult patients at the highest risk of SCD are candidates for implantable cardioverter-defibrillator therapy. The use of modern cardiovascular therapies and interventions has reduced the mortality associated with HCM to levels <1% per year.
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Source: Ommen S. R., Mital S., Burke M. A. et al. 2020 AHA/ACC guideline for the diagnosis and treatment of patients with hypertrophic cardiomyopathy: a report of the American College of Cardiology/American Heart Association joint committee on clinical practice guidelines. J Am Coll Cardiol 2020; 76 (25): e159–e240, doi: 10.1016/j.jacc.2020.08.045.
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