**1. Introduction**

Cardiomyopathies (CM) in children occur relatively rarely with an incidence of less than 1.3 in 100,000 children below 10 years of age and are still challenging for pediatricians, cardiologists, and cardiac surgeons [1]. Cardiomyopathies, which by definition are diseases of the heart muscle, are very heterogeneous and include a cluster of different diseases that often cannot be clearly separated from each other. Generally, CMs are classified into dilated (DCM), hypertrophic (HCM), restrictive (RCM), left ventricular non-compaction (LVNC), arrhythmogenic (ACM), and mixed phenotypes. They further may be subdivided into primary, that is, inherited, and secondary diseases (e.g., metabolic disorders or other genetic diseases as muscular dystrophy). In children, often the pathology is very severe with rapid progression. In

addition, comorbidities such as diabetes, renal and pulmonary diseases, and obesity may affect the outcome [2]. There is still a vast amount of known pediatric CM cases that are idiopathic, that is, of unknown cause, often because of lack of causal diagnosis. In consequence, this increases the inefficiency of treatments [3]. Since often suitable therapies besides heart transplantation for diagnosed CM types are not available or inefficient, prognosis is poor [4]. Here, we will describe common features of pediatric CMs concerning their etiology, pathophysiology, and molecular mechanisms, thereby underlining the challenges for classification, diagnosis, and treatment. Also, unique features will be covered, as well as current options and perspectives in diagnosis and treatment.
