Myocarditis is inflammation of the heart muscle, and it can range from a mild condition that resolves on its own to a life-threatening emergency. Globally, it affects roughly 16 people per 100,000 each year. Most cases are triggered by viral infections, though the immune system’s response to the virus, not the virus itself, often does the real damage to heart cells.
What Causes Myocarditis
Viruses are the most common trigger. Coxsackievirus B3, a type of enterovirus, is the single most frequent culprit. Adenoviruses, which typically cause respiratory infections, are another well-established cause. Enteroviruses as a group account for nearly a quarter of all viral myocarditis cases. Other viruses linked to the condition include influenza, HIV, hepatitis C, and SARS-CoV-2.
Bacteria can also cause myocarditis, though this is far less common. Staphylococcus aureus is the leading bacterial cause, and the bacterium behind Lyme disease, Borrelia burgdorferi, is another recognized trigger. Fungal infections from organisms like Candida and Aspergillus occasionally cause myocarditis, almost exclusively in people with weakened immune systems.
Not all myocarditis comes from infections. Autoimmune diseases like lupus and sarcoidosis can drive the same kind of heart muscle inflammation. Certain medications, drugs, and toxins can trigger it too. In many cases, no specific cause is ever identified.
How It Damages the Heart
When a virus infects the heart muscle, the immune system sends inflammatory cells to fight it off. This inflammatory response can injure or kill heart muscle cells (called myocytes) in the process. As these cells are damaged, the heart’s ability to pump blood effectively decreases. The inflammation can also disrupt the heart’s electrical system, leading to abnormal rhythms. In most people, the inflammation subsides and the heart heals. But in a significant minority, the damage becomes permanent.
Symptoms in Adults
Myocarditis often starts with symptoms that feel like a bad flu: fatigue, body aches, and sometimes fever. Within days to weeks, heart-specific symptoms emerge. Chest pain is the hallmark, and it can mimic a heart attack. Shortness of breath, especially during physical activity or when lying down, is common. Some people notice heart palpitations, a fluttering or racing sensation caused by irregular rhythms.
In severe cases, called fulminant myocarditis, symptoms escalate rapidly. The heart’s pumping ability drops dramatically, leading to signs of heart failure: swelling in the legs, difficulty breathing at rest, and lightheadedness. Fulminant myocarditis is a medical emergency, but paradoxically, patients who survive the acute phase often recover more completely than those with a slower, smoldering course.
Symptoms in Children and Infants
Myocarditis looks different in young children, which makes it harder to catch. About two-thirds of pediatric cases start with a recognizable viral illness, like a cold or stomach bug, in the days before heart symptoms appear. Arrhythmias show up in roughly 45% of pediatric cases, and about 10% of children experience fainting.
Infants present the biggest diagnostic challenge. They can’t describe chest pain, so the signs are indirect: rapid breathing that doesn’t improve, poor feeding or tiring quickly during feeds, excessive sweating, and sometimes a bluish tint to the skin. Older children and adolescents tend to look more like young adults, with fatigue, shortness of breath, poor appetite, and abdominal pain, particularly when heart function has dropped significantly.
How Myocarditis Is Diagnosed
Diagnosis typically starts with a blood test measuring troponin, a protein released when heart muscle cells are injured. Elevated troponin confirms heart damage is occurring, though it doesn’t pinpoint the cause. Levels of troponin T above 50 nanograms per liter are associated with a worse outlook, though they only loosely correlate with how well the heart is actually pumping.
Cardiac MRI has become the gold standard for confirming myocarditis without a biopsy. Doctors use a set of imaging criteria called the Lake Louise criteria, updated in 2018, which look for two things: swelling in the heart muscle (a sign of active inflammation) and evidence of inflammatory injury. The updated criteria detect myocarditis with about 88% sensitivity and 96% specificity, a substantial improvement over the original version. An echocardiogram, or ultrasound of the heart, is also used to assess how well the heart is pumping and whether it has enlarged.
Treatment and Recovery
There is no specific cure for most viral myocarditis. Treatment is largely supportive, meaning it focuses on managing symptoms and protecting the heart while it heals. When the heart’s pumping ability has dropped, the same medications used for heart failure come into play: drugs that reduce fluid buildup, medications that lower the workload on the heart by relaxing blood vessels, and drugs that slow the heart rate to let it pump more efficiently.
Physical activity restriction is a critical part of recovery. Patients are typically told to avoid exercise for three to six months after diagnosis. This isn’t just about feeling tired. Exercising with an inflamed heart can trigger dangerous arrhythmias. Restrictions are lifted only after follow-up testing, including imaging and sometimes exercise stress tests, shows the inflammation has resolved and heart function has returned to normal.
Long-Term Outlook
Most people with acute myocarditis recover fully, especially when the condition is caught early and managed appropriately. However, somewhere between 6% and 30% of cases progress to dilated cardiomyopathy, a condition where the heart muscle stretches and weakens permanently. One pediatric study found that about 15% of children with acute myocarditis developed dilated cardiomyopathy during short-term follow-up. A small number of these patients eventually require a heart transplant.
The risk of progression depends on several factors, including how enlarged the heart was at the time of diagnosis, how much the pumping function was reduced, and whether the inflammation fully resolves. People who have had myocarditis generally need periodic cardiac follow-up for at least the first year, and sometimes longer, to watch for signs of lasting damage.
COVID-19 and Vaccine-Related Myocarditis
Both COVID-19 infection and mRNA vaccination can trigger myocarditis, but the risks are not equal. The largest study comparing the two in children and young people found that COVID-19 infection caused 2.24 extra cases of myocarditis or pericarditis per 100,000, while vaccination caused 0.85 extra cases per 100,000. In the first week after a COVID-19 diagnosis, the risk of myocarditis was roughly threefold higher than baseline.
The vaccine-associated risk was concentrated in the first four weeks after the shot, with no increased risk observed beyond that window. The infection-associated risk, by contrast, was both higher and longer lasting. Myocarditis in children is rare to begin with, affecting about 5 in 100,000 per year regardless of cause. Vaccine-related myocarditis cases have generally been mild, with most patients recovering fully with standard supportive care.