An understanding of cardiomyopathy (CMP) is very important, as it is a common cause of heart failure in children and the most common indication for heart transplantation in children older than 1 year1).

Primary CMP is rare2-4). In Asia, reports on the incidence of primary CMP in pediatric age groups are extremely rare; hence, further research is needed in this area. Among Asian countries, Korea is uniquely comprised of a relatively homogeneous ethnic group; therefore, a study of the incidence of CMP in Korean children would be especially interesting4).

Dilated CMP (DCMP) is the most common form of CMP worldwide and has many causes. In 30% to 48% of patients, DCMP is genetically inherited. Moreover, inflammatory disorders such as myocarditis, or toxic agents such as medications and alcohol can result in DCMP5). Of all the DCMP cases, 20% to 48% have a family history of the disease6).

The etiology of hypertrophic CMP (HCMP) in the pediatric population is heterogeneous, including inborn metabolism errors, neuromuscular disorders, and malformation syndromes. However, most cases of apparently idiopathic HCMP in childhood are caused by mutations of cardiac sarcomere protein genes.

In the pediatric population, risk stratification is necessary because most cases of HCMP have a family history of the disease. Evaluation of first-degree relatives and any at-risk family members should be a routine component of clinical management7).

Recent technological developments in genetic studies on CMP have elucidated the potential advantage of genetic testing in the diagnosis and in understanding the pathogenetic basis of inherited CMP7).

The World Health Organization (WHO) defines CMP as "a disease of the myocardium associated with cardiac dysfunction" and classifies it as dilated, hypertrophic, restrictive, arrhythmogenic right ventricular, or unclassified (Table 1)8).

Some consider this definition by the WHO as inadequate to emphasize the complexity of the disease9). Therefore, the American Heart Association (AHA) issued a consensus statement that revised the WHO's classification of CMP based on genetics, heart structural changes, cellular events, and multiorgan involvement10).

Most classify CMP as either primary or secondary. CMP can be considered as primary when the disorder is solely or predominantly confined to the heart muscle and has a genetic, nongenetic, or acquired etiology. Meanwhile, secondary CMP is diagnosed when the disorder involves myocardial damage because of a systemic or multiorgan disease11).

The European Society of Cardiology has adopted an alternative classification which is similar to the WHO definition in some ways but is more clinically orientated12).

Each of the CMP forms is determined by the nature of muscle damage. Moreover, CMP may be classified as more than 1 type or may change from 1 type to another over time in some patients. Although the WHO has not formally categorized left ventricular noncompaction cardiomyopathy, it is increasingly being identified8).

Reports on the incidence of CMP in children are rare. The incidence of CMP in different countries has been inconsistent owing to the varying research methods used in studies with different age groups and study durations (Table 2).

According to a study based on the AHA classification, the incidence of CMP between 1998 and 2006 in Korean children was 0.28 per 100,000 children younger than 15 years4). The point prevalence of primary CMP in Korean children was 2.11 per 100,000 children on December 31, 2006. DCMP, HCMP, restrictive cardiomyopathy (RCMP), and others accounted for 66.4%, 23.5%, 6.5%, and 3.6%, respectively4).

According to the United States (US) Pediatric Cardiomyopathy Registry, the annual incidence of CMP was 1.13 per 100,000 children younger than 18 years13), with DCMP as the most common (58%), followed by HCMP (30%). There were relatively few cases of RCMP (5%) and arrhythmogenic right ventricular cardiomyopathy (ARVC) (5%).

The incidence of CMP in children was 1.24 per 100,000 children younger than 10 years in Australia14) and 0.65 per 100,000 children aged 20 years or younger in Finland3).

The incidence of DCMP in Korean children was 0.18 per 100,000 children4), whereas that in American children was 0.57 per 100,000 children younger than 18 years15). These findings suggest that the incidence of primary CMP is significantly lower in Korean children than in other ethnic groups.

The incidence of HCMP in Korean children was 0.07 per 100,000 children. Epidemiological studies in Finland, Australia, and the US revealed that the incidence of new cases of HCMP was between 0.24 and 0.47 per 100,000 children3,14,15).

The differences in the incidence of HCMP between these studies may reflect the diversity in racial composition and dynamic fluxes in the study population, the inclusion of secondary cases of HCMP, the influence of family cascade screening, the age group under study, and the inclusion of cases identified at the time of autopsy. For example, a Korean study on HCMP that included only patients younger than 15 years as subjects might have excluded a larger proportion of HCMP patients because HCMP usually presents during the late adolescent years.

Further studies with a more consistent approach in data collection and an international agreement on a unified classification system of CMP are needed.

The symptoms of CMP may differ in every child. Symptoms can be absent, mild, or severe. Heart murmurs are not always present in children with CMP. Only approximately one-third to one-half of HCMP patients experience heart murmurs, which are usually because of ventricular obstruction or leaking in 1 of the heart valves. Because CMP is not easily detected on physical examination, it is often diagnosed in its later stages19).

The CMP type and disease stage should be considered to determine the overall long-term prognosis of pediatric patients. The 9-year survival rate was estimated to be 69.8% in DCMP, 90.3% in HCMP, 47.2% in RCMP, and 42.0% in other CMP patients with ion channelopathy and mitochondrial CMP according to the Korean registry.4) Echocardiography has important implications for prognosis. A high degree of LVH could indicate a worse prognosis44,45).

It is recommended that all first-degree relatives of a patient be screened because CMP can be inherited and can present without any signs or symptoms. It is also advisable to screen second-degree relatives. This is strongly recommended if there is a family history of sudden infant death or sudden cardiac arrest.

Children with an affected family member but without symptoms should undergo echocardiography and ECG every 1 to 3 years before the age of 12 years and then more frequently from age 12 to 21 years7,20).

Those who have a family history of CMP may be advised to continue screening at least every 5 years throughout their adult life20).

The generally accepted practice is that immediate family members of patients with HCMP should undergo periodic screenings for HCMP, including echocardiography, ECG, history taking, and physical examination19). Whether all children need to be screened for HCMP before participating in competitive athletics is controversial. Nevertheless, the Bethesda guidelines recommend universal screening, including historical evaluation and physical examination, before participation in competitive athletics because of the indolent nature of the disease, heterogeneity of clinical presentation, and potential for sudden unexpected death during peak exercise46).

The identification of the causes of gene mutations in CMP is critical because presymptomatic interventions have been shown to prevent morbidity and mortality. Identification of the mutated genes has the potential to lead to an understanding of the disease, including its etiology, pathophysiology, and potential therapy46). Genetic counseling should be regarded as an essential part of the genetic testing process and is recommended for all patients with HCMP47). Furthermore, as already discussed, early diagnosis, treatment, and potential risk stratification may result in improved patient survival. However, further international studies with a more consistent data collection approach for determining disease prevalence and an international agreement on a unified classification system for CMP are needed.