Pediatric Viral Myocarditis Workup

Perform a complete blood count (CBC) with differential. Acute anemia of any origin may cause heart failure, and chronic anemia exacerbates heart failure; both respond to blood transfusion. The presence of lymphocytosis or neutropenia supports diagnosis of a viral infection. In addition, perform blood cultures; ruling out any bacterial infection is important.^[7]

Additional tests to perform are as follows:

• Sedimentation rate and C-reactive protein – Nonspecific inflammation markers; they are usually elevated, but a normal value does not rule out myocarditis, particularly in the presence of congestive heart failure, which may lower the sedimentation rate
• Nasopharyngeal and rectal swabs - May help to identify etiology
• Viral titers - A 4-fold increase in a specific titer from the acute to convalescent phase is strong evidence of infection
• In situ hybridization - Identifies viral ribonucleic acid (RNA) in myocardial tissue of patients believed to have myocarditis; the incidence of false-negative results is high
• Polymerase chain reaction (PCR) assay^[8] - Used to find the viral genome in myocardial cells; it is rapid and sensitive, and it may become the test of choice for the diagnosis of viral myocarditis
• Creatinine kinase–MB isoenzymes (CK-MB) - Markers of myocardial damage; they are elevated most commonly when an associated elevation of the ST segment on an electrocardiogram (ECG) is present
• Lactate dehydrogenase isoenzyme 1 - May be elevated in idiopathic myocarditis
• Troponin I - Another indicator of myocardial damage; it is usually elevated up to a month after infection, but it is not specific for this disease^[9]

Other techniques are under investigation to determine a specific viral etiology of myocarditis, such as immunohistochemical stains, inflammatory mediators, and autoantibody measurements.

Chest radiography and MRI

On chest radiography, cardiomegaly and pulmonary edema may be depicted. Incidentally noted cardiomegaly on chest radiography may be the initial presentation.

Magnetic resonance imaging (MRI) with gadolinium can be used to evaluate the cardiac muscle inflammation via a special protocol for myopericarditis.

Echocardiography is the most cost-effective test used for evaluation of myocardial function. It is sensitive but not specific. Findings include the following:

• Global hypokinesis (the most common finding)
• Increased left ventricular end diastolic and systolic dimensions
• Left ventricular dysfunction, primarily systolic with decreased ejection fraction and shortening fraction
• Segmental wall motion abnormalities
• Pericardial effusion

Radionuclide imaging may be helpful as a screening tool. Gallium 67 (⁶⁷ Ga) citrate myocardial scintigraphy is useful for revealing chronic inflammatory processes. It is a sensitive test but is limited by its low specificity and predictive value.

Indium 111 (¹¹¹ In) antimyosin antibody imaging is highly sensitive for myocardial necrosis, but it has a high incidence of false-positive results. However, absence of antimyosin uptake is highly predictive of negative biopsy findings (92-98%).

Myocardial perfusion imaging with technetium 99m–labeled methoxyisobutyl isonitrile single-photon emission computed tomography (^99m Tc-MIBI SPECT) scanning is usually a tool used to evaluate the severity of myocardial ischemia.^[10] Because the uptake and clearance of^99m Tc-MIBI is affected by cell viability and membrane integrity, clinicians have also used it as a marker for the severity of myocardial necrosis and inflammation in patients with myocarditis, with results comparable to those obtained with enzymatic cell damage markers.

In some patients with mild cardiac involvement, electrocardiographic changes may be the only abnormal findings suggestive of myocarditis. Low-voltage QRS (< 5 mm throughout the limb leads) is the classic pattern. Pseudoinfarction patterns with pathologic Q waves and poor progression of R waves in the precordial leads may also be present. T-wave flattening or inversion is a common finding associated with small or absent Q waves in V₅ and V₆.

Left ventricular hypertrophy with strain may be present. Other, nonspecific findings include prolonged PR segment and prolonged QT interval.^[11]

Sinus tachycardia is the most common finding. Premature ventricular contractions and atrial tachycardias have been reported. Junctional tachycardia is common and may worsen congestive heart failure. Occasional second-degree and third-degree atrioventricular block may be present, requiring temporary pacing. Ventricular tachycardia is commonly associated and may be the initial presentation.

Biopsy is the criterion standard for the diagnosis of myocarditis.^[12] Myocardial biopsy findings establish diagnosis and classify disease stage. Biopsy is a relatively safe and effective way to sample heart muscle in older children; however, a risk of perforation in sick or younger infants is observed.

The use of endomyocardial biopsy is controversial because of the possibility of a high false-negative result rate and because no proven therapy is available, even when a positive biopsy finding is obtained. Some advocate using radionuclide imaging techniques as screening tools before considering endomyocardial biopsy.

Biopsy specimens may be useful for PCR assay diagnosis of viral etiology. Viral serology in adults has been shown to have poor sensitivity and specificity compared with endomyocardial biopsy with PCR assay detection of the viral genome under diagnostic testing.^[13]

Gross evaluation of the heart reveals flabby and pale muscle with petechiae. Ventricular muscle is usually thin and may be hypertrophied. Heart valves and the endocardium are not usually involved, but in cases of chronic myocarditis, they might appear as they appear in endocardial fibroelastosis. Some experts believe that endocardial fibroelastosis is a result of viral myocarditis.

The microscopic hallmark of acute myocarditis is focal or diffuse interstitial infiltrate of mononuclear cells, lymphocytes, plasma cells, and eosinophils. Viral particles are rarely seen unless searched for with special techniques (ie, PCR assay). Necrosis and disarrangement of the myocytes are typical and often are seen with coxsackievirus infection. In the chronic and healing stages, myocytes are replaced by fibroblasts (scar tissue).^[14]

In adenoviral myocarditis, the infiltrate seen histologically is less severe than is seen in cases of enteroviral infection.