Cardiomyopathy: Epidemiology, Etiology, and Pathophysiology

"Medical Journeys" is a set of clinical resources reviewed by physicians, meant for the medical team as well as the patients they serve. Each episode of this 12-part journey through a disease state contains both a physician guide and a downloadable/printable patient resource. "Medical Journeys" chart a path each step of the way for physicians and patients and provide continual resources and support, as the caregiver team navigates the course of a disease.

The wide range of cardiomyopathies span from rare to common and touch every segment of the population. Exact numbers aren't certain, given how often the condition goes undiagnosed.

"In the absence of contemporary, population-based epidemiological studies, estimates of the incidence and prevalence of the inherited cardiomyopathies have been derived from screening studies, most often of young adult populations, to assess cardiovascular risk or to detect the presence of disease in athletes or military recruits," noted the authors of a recent review in Nature Reviews Cardiology.

An estimated 1 in 500 adults may have some form of cardiomyopathy.

The global estimates are likely conservative, however, the review added, "given that only individuals who fulfill diagnostic criteria would have been included. This caveat is highly relevant because a substantial minority or even a majority of individuals who carry disease-causing genetic variants and are at risk of disease complications have incomplete and/or late-onset disease expression."

The American Heart Association in setting out a revised classification scheme in 2006 added the caveats that the same patient's disease could appear to fall into two categories of cardiomyopathy simultaneously and that the clinical disease course for some may evolve due to remodeling, such as hypertrophic cardiomyopathy progressing from a nondilated state with ventricular stiffness to a dilated form with systolic dysfunction and failure.

Nevertheless, the organization divides the condition into primary cardiomyopathies (genetic, acquired, or mixed in etiology) in which the disease process is largely confined to the heart; and secondary cardiomyopathies that occur as part of a systemic condition.

Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is thought to be the most common of the genetic cardiomyopathies, at 1 in 500 of the general population, inherited in an autosomal dominant pattern.

This genetic cardiomyopathy has been pinned on a wide variety of errors in a number of genes that encode for the proteins that contract in the cardiac sarcomere, the most basic unit of muscle fiber. Most often (in some three-quarters or more of cases), these are β-myosin heavy chain and myosin-binding protein C mutations that lead to hypercontractility. Research suggests that HCM from β-myosin heavy chain (MYH7 gene) mutations present with more severe disease, such as a higher prevalence of mitral leaflet abnormalities and calcification of the mitral annulus.

Less common mutations linked to HCM encode for troponin T and I, regulatory and essential myosin light chains, titin, α-tropomyosin, α-actin, α-myosin heavy chain, and muscle LIM protein. Typical genetic panels screen for eight sarcomere genes: MYH7, MYBPC3, TNNI3, TNNT2, TPM1, MYL2, MYL3, and ACTC1. For atypical presentations, one should consider looking for diseases that mimic HCM, like glycogen storage disease, Danon disease, Fabry disease, and amyloidosis.

"The characteristic diversity of the HCM phenotype is attributable to the disease-causing mutations and probably to the influence of modifier genes and environmental factors," the AHA classification document noted.

For example, HCM from non-sarcomere mutations was more likely to present at older ages and with obesity, according to one study in Open Heart.

Other Genetic Cardiomyopathies

A less common genetic cardiomyopathy is arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) estimated to occur at a prevalence of 1 in 5,000, which likewise has autosomal dominant inheritance.

ARVC/D can arise from mutations in genes for the cardiac ryanodine receptor RyR2, desmoplakin, plakophilin-2, and transforming growth factor-β and largely involves progressive loss of myocytes and fatty or fibrofatty tissue replacement in the right ventricle.

Ion channel disorders including long QT syndrome, short-QT syndrome, and Brugada syndrome also fall into the AHA's genetic cardiomyopathy classification, causing arrhythmias through mutations in genes related to cell membrane transit of sodium and potassium ions.

Dilated Cardiomyopathy

Dilated cardiomyopathy is estimated at a prevalence anywhere from 1 in 250 to 1 in 2,500 in the general population, making it the third most common cause of heart failure and the most frequent cause of heart transplantation. It occurs most commonly in the third or fourth decade of life but can also present early in childhood, according to a review in the Journal of Internal Medicine.

This type of cardiomyopathy is classified as a mixed genetic and acquired condition, as some 35% of cases are caused by mutations in genes encoding for cytoskeletal, sarcomere, or nuclear envelope proteins that lead to abnormal protein-protein interaction and impaired calcium flux, as well as some of the same genetic mutations that cause HCM, such as Titin, Troponin T, and MYH7.

Common causes of acquired dilated cardiomyopathy are inflammation (as in the case of viral myocarditis or autoimmune disease), toxicity from substance use or cancer medications, and metabolic disorders.

The condition leads to a left ventricle with patches of cardiomyocytes, necrosis, fibrosis, and calcifications, which increases diastolic pressure and ventricular dilatation and reduces ejection fraction. As diastolic dysfunction worsens, clinical symptoms arise.

"Neuro-humoral activation upon the decreased cardiac ejection fraction (catecholamine secretion, reduction of vagal nerve activity, activation of the renin–angiotensin–aldosterone system) further contributes to ventricular wall stress," the review noted.

Restrictive Cardiomyopathy

This mixed cardiomyopathy can arise from mutations in the sarcomere subunits, such as troponin T, troponin I, α-actin, and MYH7, typically in an autosomal dominant fashion, or be acquired through scleroderma, diabetes, sarcoidosis, use of drugs like ergotamine or anthracyclines, or a number of other causes.

Acquired cases are most common. Of the inherited causes, one of the most common, and most severe, is amyloid light-chain amyloidosis (or primary amyloidosis), which is caused by extracellular deposition of misfolded immunoglobulin light chains that are synthesized by a clonal population of plasma cells in the bone marrow and that can damage almost all organs in the body.

The rate of identification of amyloidosis has been rising over time with awareness and use of noninvasive imaging, noted a study that found a prevalence of 17 per 100,000 person-years in the Medicare population as of 2012. While typically diagnosed at ages 50-65, it has been identified in youth as well. In a U.S. claims analysis, prevalence in adults overall was 40.5 cases per million population in 2015.

Sarcoidosis, a multisystem inflammatory disorder characterized by T lymphocytes, mononuclear phagocytes, and noncaseating granulomas in involved tissues, doesn't have a known cause but largely affects young and middle-age adults, more often women, notes a review in Circulation Research. Prevalence in the U.S. is estimated at 10.9 per 100,000 in white patients and 35.5 per 100,000 in Black patients, but only some 2.5-5% have cardiac involvement.

Acquired Cardiomyopathies

Myocarditis, or inflammatory cardiomyopathy, most often arises from infection but can also result from exposure to toxins or drugs or from conditions like Whipple disease. The rare giant cell myocarditis develops for unknown reasons.

Myocarditis progresses from inflammatory cell infiltration to interstitial edema and focal myocyte necrosis to ultimately replacement fibrosis. This electrically unstable substrate predisposes to ventricular tachyarrhythmias. Autoimmune reaction against the myocardium can also be triggered.

While the incidence of myocarditis is estimated at 10 to 20 cases per 100,000 persons, it's likely underdiagnosed because many cases are subclinical. Most cases occur in young, healthy patients.

Stress cardiomyopathy -- also known as Takotsubo cardiomyopathy -- most often occurs in postmenopausal women following emotional or physical stress but also occurs in men and has even been reported in children. The prevalence of about 2% (up to 10% if only women are considered) among patients presenting with acute coronary syndrome rose substantially during the COVID-19 pandemic to 7.8% in one series.

It's characterized by regional left ventricular wall motion abnormality with a peculiar circumferential pattern resulting in a conspicuous ballooning of the left ventricle during systole that gives it the "takotsubo" (Japanese for octopus pot) moniker. The right ventricle is also involved in about one-third of cases.

Peripartum cardiomyopathy -- systolic heart failure occurring toward the end of pregnancy or in the months following delivery -- had its definition expanded because the classic "presentation in the last month of pregnancy and up to 5 months postpartum" wasn't clinically indistinguishable from cases occurring outside that window.

The incidence was 4.84 per 10,000 live births in one series, but estimates range from 1 in 1,000 to 1 in 4,000 deliveries. According to a review in the Journal of the American College of Cardiology, incidence may be rising, "due to the rise in maternal age, increased rates of multifetal pregnancies due to contemporary fertility techniques, and possibly to increased recognition of the disease." Peripartum cardiomyopathy is more common with older maternal age, obesity, multiple gestation pregnancies, African-American ethnicity, and preeclampsia.

What causes peripartum cardiomyopathy remains unknown, although potential mechanisms include nutritional deficiencies, viral myocarditis, autoimmune processes, and hemodynamic stress. Genetic predisposition may also play a role.

Read previous installments in this series:

Part 1: Cardiomyopathy: What are the Signs, What are the Symptoms?

Part 2: Diagnosing Cardiomyopathy: History, Examination, and Testing

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