Pediatric Dilated Cardiomyopathy 

Idiopathic dilated cardiomyopathy (DCM) refers to congestive cardiac failure secondary to dilatation and systolic dysfunction (with or without diastolic dysfunction) of the ventricles (predominantly the left ventricle) in the absence of congenital, valvular, or coronary artery disease or any systemic disease known to cause myocardial dysfunction. DCM is the most common type of heart muscle disease in children.

All 4 cardiac chambers are dilated and are sometimes hypertrophied. Dilation is more pronounced than hypertrophy, and the left ventricle is affected more often than the right ventricle. The cardiac valves are intrinsically normal, although the mitral and tricuspid valve rings are dilated, and the valve leaflets do not appose each other in systole, giving rise to varying degrees of mitral regurgitation, tricuspid regurgitation, or both.

Persistent mitral regurgitation leads to thickening of the mitral valve leaflets, and, at times, distinguishing this thickening from other causes of mitral regurgitation is difficult. Thrombus formation (secondary to the low-flow cardiac output state) is often seen in the left ventricular apex and, at times, is seen in the atria. Occasionally, the right ventricle is preferentially involved in the cardiomyopathic process; this often indicates a familial basis.

Onset is usually insidious but may be acute in as many as 25% of patients with DCM, especially if exacerbated by a complicating lower respiratory infection. Cough, poor feeding, irritability, and shortness of breath are usually the initial presenting symptoms. In a patient with established disease, features of congestive heart failure are dominant. (See Clinical.)

Echocardiography and Doppler studies form the basis for the diagnosis of dilated cardiomyopathy (DCM) in most patients (see Workup). Cardiac transplantation is currently the optimal treatment for DCM-induced resistant chronic heart failure in children (see Treatment).

Patient education is a continuous process from the time of diagnosis. Explain the disease process, management, and prognosis to parents and older patients. In cases of familial DCM, patients and their families should be told about the genetic implications. For patient education information, see the Heart Center, as well as Heart and Lung Transplant.

Injury to the myocardial cell is the initiating factor that leads to cell death. If considerable cell loss occurs, the myocardium fails to generate enough contractile force to produce adequate cardiac output. This results in the activation of the following compensatory mechanisms:

• The renin-angiotensin-aldosterone system
• Sympathetic stimulation
• Antidiuretic hormone production
• Release of atrial natriuretic peptide

These compensatory mechanisms help to maintain cardiac output in the initial phase; however, as myocardial damage progresses, persistent and excessive activation can be detrimental to cardiac function, leading to overt congestive heart failure.

The theory that left ventricular noncompaction is an underlying factor in the development of DCM in young infants has received much attention.^{[1, 2, 3]}

Over-stretching of the ventricles causes myocardial thinning, cavity dilation, secondary valvular regurgitation, and compromised myocardial perfusion. The resulting subendocardial ischemia perpetuates myocyte damage.

Myocardial remodeling is an important contributor to worsening heart failure.^[4] Lost myocyte cells are replaced with fibrous tissue, thereby decreasing the compliance of one or more ventricles and adversely affecting performance. Aldosterone, angiotensin II, catecholamines, endothelins, and mechanical factors, such as excessive myocardial stretch and ischemia, have been identified as mediators of remodeling. The degree of left ventricular dilatation has been reported to influence short-term outcome in children listed for transplant.^[5]

Apoptosis (ie, programmed cell death) is now believed to play a role in the continuing loss of myocardial cells in chronic heart failure. Overloading of myocytes possibly triggers apoptosis without fibrosis.

Heightened peripheral vasoconstriction, abnormal and excessive remodeling of the peripheral vasculature, and abnormalities in endothelium-dependent vasodilation contribute to the progression of heart failure. Abnormal responses to muscarinic stimulation along with a defect in the endothelial nitric oxide pathway have been suggested as the potential underlying mechanisms.

Altered gene expressions resulting in calcium-handling abnormalities, downregulation of myosin or conversion to the less-active beta isoform, and abnormal beta-receptor signal transduction have all been identified at the molecular level in the chronically failing heart.

Various factors have been identified as causes of myocardial damage. These are presented in Table 1, below. However, in the vast majority of patients, no specific etiology is demonstrable (ie, idiopathic DCM). Systemic carnitine deficiency and anthracycline-induced cardiomyopathy are notable exceptions. Three major factors have been implicated in the pathogenesis of myocardial damage in DCM: preceding viral myocarditis, autoimmunity, and underlying genetic predisposition.

Table 1. Factors Identified as Causes of Myocardial Damage (Open Table in a new window)

Viral myocarditis

Epidemiologic, serologic, and molecular studies have detected evidence of enteroviral infection, in particular coxsackievirus B, in 20-25% of patients. Evidence also implicates various other viruses. In fact, the most common associated viruses appear to vary over time.^{[6, 7]} Currently, coxsackievirus B is likely a less common cause of DCM than in the past.

Currently, no methods can be used to distinguish cardiovirulent strains of enteroviruses from those that are not virulent. Furthermore, the presence of a virus in a patient with DCM does not necessarily establish a causal relationship. Demonstration of viral DNA or RNA by polymerase chain reaction (PCR) is a more reliable method for revealing viral myocarditis. Unfortunately, obtaining myocardial tissue is invasive.

The exact mechanism of myocardial damage (rapid destruction or a long-term slowing of cardiomyocyte function) also remains unclear.^[8]

Autoimmunity

Animal studies have shown that DCM is an autoimmune disease in genetically predisposed strains of mice. In humans, approximately 30-40% of adult patients with DCM have organ-specific and disease-specific autoantibodies. The absence of these antibodies in the remaining patients may be related to the stage of disease progression.

It has been postulated that an insult such as viral myocarditis initiates an autoimmune process with superantigen-triggered immune responses, resulting in massive T-lymphocyte activation and myocardial damage.

Genetic predisposition

Genetic causes account for 25-50% of DCM cases.^[9] The role of genetic factors is exemplified by the studies on familial DCM.^{[10, 11]} Patients with familial DCM have an increased frequency of human leukocyte antigen (HLA)-DR4. The frequency of HLA-DQA1 0501 alleles has been reported to be significantly higher in patients with idiopathic DCM.^[12]

Autosomal dominant and recessive inheritance, X-linked transmission, and polygenic and mitochondrial inheritance have all been documented. Presently known DCM genetic loci are summarized in Table 2, below.

Table 2. Summary of Genetic Loci and Disease Genes for Familial Dilated Cardiomyopathy (Open Table in a new window)

Mutation screening of the exons that code for actin, β myosin heavy chain (MYH7 gene), cardiac troponin T (TNNT2 gene), phospholamban (PLN gene), titin, αβ-crystallin, and the cardio-specific exon of metavinculin (VCL gene) could be helpful in detecting some forms of familial DCM.

Anthracycline cardiotoxicity

Anthracyclines, which are widely used in the management of childhood malignancies, account for as many as 30% of cases of DCM in the United States and a lesser percentage in other countries.

Besides DCM, the other manifestations of anthracycline cardiotoxicity include restrictive cardiomyopathy (symptomatic and asymptomatic), cardiac arrhythmias^[13] , asymptomatic left ventricular enlargement, and more subtle changes of cardiac function.

Cardiotoxicity has 2 types: early onset and late onset. The early-onset type may be acute nonprogressive or chronically progressive.

Acute-onset type is defined as left ventricular dysfunction during or immediately following infusion of anthracycline and is attenuated by discontinuation of therapy. With use of low-dose regimens, this type is becoming rare.

Electrocardiographic (ECG) changes include nonspecific ST segment and T wave changes, decreased QRS voltage, prolonged QT interval, and sinus tachycardia. Less commonly, ventricular, junctional, or supraventricular tachycardia or atrioventricular and bundle branch blocks. Blood levels of cardiac troponin T (cTnT) are a specific marker of this type of injury.

Early-onset chronically progressive toxicity presents within 1 year of completion of therapy and persists or progresses even after discontinuation of therapy. Clinical features are similar to any other type of cardiomyopathy and include ECG changes, left ventricular dysfunction, arrhythmias, reduced exercise-stress capacity, and even overt signs of heart failure. Blood levels of cTnT are elevated. Presence of early-onset cardiotoxicity is believed to be a harbinger of poor patient outcome.

Late-onset toxicity clinically manifests after a latent period of 1 or more years following completion of therapy. This type of manifestation is presumably due to diminished left ventricular contractility and an inappropriately thin left ventricular wall, resulting in elevated wall stress and progressive left ventricular dysfunction.

Myocyte loss underlies all these sequelae, and alteration of myocellular protein transcription by anthracyclines may contribute. Thus, the latent period is not latent at all, and more sensitive markers may be able to detect the changes earlier. Late-onset asymptomatic toxicity has also been reported, but more detailed questioning often reveals easy fatigability or dyspnea in many of these patients.

Risk factors contributing to the development of anthracycline cardiotoxicity include the following:

• Total cumulative dose (20% mortality with cumulative dose >550 mg/m², 65% frequency of subtle echocardiographic changes with dose >400 mg/m², and histologic evidence of toxicity with >240 mg/m²)
• Female sex (higher cellular concentrations because of higher body fat percentage)
• Young age
• Rate of drug administration (maximum risk with doses >50 mg/m²/dose)
• Concomitant cardiac radiation exposure or use of amsacrine
• Black race
• Trisomy 21

In the United States, the reported incidence rate of DCM is 0.57 cases per 100,000 children.^[14] The incidence rate in Finland is 2.6 cases per 100,000 children.^[15] In the United Kingdom, the incidence rate is 0.87 cases per 100,000 individuals older than 16 years.^[16] No reliable figures are available for the rest of the world. Genetic causes account for more than 30% of DCM cases.

DCM is reportedly more common in boys than in girls, and some forms are clearly X-linked.^[14] All age groups are affected. However, studies suggest that DCM is more common in infants (age < 1 y) than in children.^[14] Fetal presentation is uncommon.

Approximately one third of patients with DCM die of the disease, one third continue to have chronic heart failure requiring therapy, and one third of patients experience improvement in their condition. Causes of death include heart failure, ventricular arrhythmias, and transplantation-related complications (less common).

Children with underlying muscle disorders with progressive dilatation of the heart and worsening heart failure have a worse prognosis compared with patients with idiopathic DCM.^[17] If a treatable cause is discovered, prognosis is better. A history of viral illness in the 3 months before onset may suggest a better prognosis. Prognosis is worst for cardiomyopathy secondary to storage diseases that do not have effective therapy.

In DCM with no obvious detectable etiology, outcome depends on severity of myocardial dysfunction, improvement during the first year after onset, compliance with therapy, and availability of timely transplant.

The degree of depression of fractional shortening or ejection fraction on initial echocardiography, elevation of left ventricular end diastolic pressure, and cardiothoracic ratio have all been applied as predictors of outcome, although they are not often predictive. Other possible prognostic factors include age at onset (better for infants), presence of symptomatic arrhythmias, and thromboembolic episodes. A recent review of outcomes from the Pediatric Cardiomyopathy Registry places the incidence of sudden cardiac deaths at 3% and suggests age at diagnosis younger than 14.3 years, left ventricular dilation, and left ventricular posterior wall thinning as predictors of risk.^[18]

Arrhythmic death can occur even after the left ventricular ejection fraction has returned to normal.

Following cardiac transplant, survival rates of as much as 77% at 1 year and as much as 65% at 5 years have been reported in children.

Mortality and morbidity have greatly decreased because of advances in medical management. Studies from 1975-1990 reported 70% survival at 2 years and 52% survival at 11.5 years of follow-up.^{[19, 20, 21, 22, 15, 23]} Studies from 1992-1997 document more than 85% survival at 5 years. However, a study from Texas that included patients diagnosed from 1990-2004 found reported a survival of only 40% at a mean followup of 6.2 years.^[24]