Pediatric myocarditis is inflammation and weakness of the heart muscle in an infant or young child.

Myocarditis is an uncommon disorder. In children it is usually caused by viral infections that reach the heart, such as the influenza (flu) virus, Coxsackie virus, and adenovirus. However, it may also occur during or after other viral or bacterial infections such as polio, rubella, Lyme disease, and others.

When you have an infection, your body's immune system produces special cells that release certain chemicals to fight off disease. If the infection affects your heart, the disease-fighting cells enter the heart. However, the chemicals they produce can damage the heart muscle, causing it to become thick and swollen. This leads to symptoms of heart failure. In addition, the virus or bacteria may damage the heart muscle.

Other causes of pediatric myocarditis include:

• Allergic reactions to certain medications
• Exposure to certain chemicals in the environment
• Infections due to fungus or parasites
• Radiation
• Some diseases (autoimmune disorders) that cause inflammation throughout the body
• Some drugs

Pediatric myocarditis tends to be more severe in newborns and young infants than in children over age 2.