The Diagnostic and Clinical Approach to Pediatric Myocarditis: A Review of the Current Literature
(2019)
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Arrhythmias
Symptoms such as palpitations and syncope occur in pediatric myocarditis patients even in the absence of heart failure or demonstrable reduction of left ventricular function. Pediatric ventricular arrhythmias in structurally normal hearts and ventricular tachyarrhythmias in athletes have been associated with myocarditis [28], [29]. Myocarditis should always be considered in a child with acquired complete heart block. Lyme carditis and Chagas disease have been associated with complete heart block. Although the majority of children may recover atrioventricular conduction, most patients need implantation of a permanent pacemaker.
Myocarditis in children is associated with a high rate of congestive heart failure, hospitalisation, intensive care unit stay, and use of inotropic support at the time of diagnosis compared with children with idiopathic dilated cardiomyopathy. A recent study of hospitalised patients in the United States found that nearly half of the patients required inotropic support, 37.5% required mechanical ventilation, and 7.4% required extracorporeal membrane oxygenator (ECMO) support.
Diagnosis
History and physical examination
The diagnosis of pediatric myocarditis is especially challenging because of the variable clinical presentation, ranging from asymptomatic patients with only subtle findings on an electrocardiogram to fulminant cardiac failure and sudden death. Diagnosis remains largely based on clinical judgement, supported by ancillary tests; therefore, a high index of suspicion is necessary. Clinical features are often those of congestive cardiac failure and myocarditis is identified as the most common cause of new-onset cardiac failure in previously well children [22], [46]. Even patients with mild symptoms are at risk of deterioration, and therefore early diagnosis is important in establishing appropriate monitoring and supportive care.
The majority of patients present with a resting tachycardia, but other cardiac-specific signs such as pallor, hypotension, oedema, and hepatomegaly occur in only a minority of cases. Chest pain, syncope, and palpitations may also be presenting complaints. Fever may or may not be present. There are several paediatric studies that have examined the most common signs and symptoms of patients with myocarditis. Shortness of breath, respiratory distress, or an abnormal respiratory exam were commonly observed [22], [47]. In two studies, 84% of patients required more than one visit to a physician within 14 days before the diagnosis of myocarditis or dilated cardiomyopathy was made [23].
Children were often given an initial diagnosis of asthma or pneumonia [5]. Resting tachycardia is also an important subtle feature, and although this is not always a consistent finding, it may be the only finding of mild disease [5], [48]. Isolated vomiting is observed, and this may be due to gut ischaemia secondary-to-low cardiac output. Isolated gastrointestinal symptoms of anorexia, abdominal pain, and vomiting may also occur. Hepatomegaly is a common feature on physical exam and may present as non-specific abdominal pain [5], [23]. In contrast to adults with myocarditis, chest pain is seldom reported in younger children though it may be a more common feature in adolescents. A case series of several teenage patients diagnosed with myocarditis described symptoms of severe chest pain, in combination with elevated cardiac troponin-I, ST-segment, and T-wave changes
In the setting of myocarditis, the pattern of myocardial fibrosis varies considerably but typically involves patchy focal fibrosis of the sub-epicardium or mid-myocardial wall, rarely extending to the sub-endocardium [41], [43]. The presence of myocardial fibrosis detected by cardiac magnetic resonance late enhancement is more common in males and younger patients, and the pattern of myocardial injury sustained in young patients tends to be more regional and more severe, with a higher incidence of irreversible myocardial scarring [28]. This pattern of myocardial injury may explain why younger patients are at risk of adverse cardiac outcomes â the presence of gadolinium late enhancement 4 weeks after the acute presentation is also correlated with reduced left ventricular ejection fraction and clinical symptomatology at 30-month follow-up
Animal studies have suggested that myocarditis has a 3-phased course [31]. Phase 1 involves initial direct myocardial injury from the actively replicating virus or the innate immunological response from infection of cardiac myocytes, fibroblast, or endothelial cells. Phase 2 is marked by activation of antigen-specific immunity involving T cells, B cells, and antibody production. Various chemokines are present that may contain the inflammatory response but extend tissue injury. Development of autoantibodies and persistent T-cell activation can be induced by antigens intrinsic to the myocardium that cross-react with viral peptides (molecular mimicry). Ultimate outcomes may vary. Negative immune modulation may occur rapidly after the elimination of the infectious pathogens, leading to a cessation of the inflammatory response with a complete recovery or little long-term myocardial damage. However, phase 3 may occur in which acute myocarditis leads to a chronic dilated cardiomyopathy. This may result from severe myocardial injury caused by the acute event; an ongoing inflammatory, autoimmune process that may occur without the persistent presence of virus in the myocardium (inflammatory dilated cardiomyopathy); or ongoing direct injury from virus with or without a persistent myocardial inflammatory response (viral heart disease).
Outcomes and prognosis
As described, it is difficult to estimate the true incidence and prevalence of myocarditis because of the broad spectrum of symptoms. It is said that the prognosis for children with viral myocarditis tends to be more positive than the outcomes with dilated cardiomyopathy [2], [88]. Survival rates for paediatric patients with myocarditis can be as high as 93% [81]. However, a large, multi-centre study including all age groups showed that there was significant mortality in neonates and infants (33-45% survival, 23-32% improvement) with better outcomes in children between 1 and 18 years of age (78-80% survival, 46-67% improvement) [18].
Overall, 1-8% of patients with acute myocarditis eventually go on to transplant [25]. Owing to the potential for recovery, even with severe disease at presentation, patients are not typically listed for cardiac transplant unless a recovery is considered extremely unlikely despite reasonable management and a period of observation.
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