What causes myocarditis

Overview

Myocarditis is a condition characterized by inflammation of the heart muscle, known as the myocardium. This inflammation can weaken the heart muscle, making it harder for the heart to pump blood to the rest of the body and causing the heart to beat faster or irregularly. The severity of myocarditis can range from mild, with no symptoms, to severe, leading to heart failure, stroke, or sudden cardiac death.

What Causes Myocarditis?

The causes of myocarditis are diverse and can be broadly categorized into several groups:

Infections

Infections are the most frequent culprits behind myocarditis. These can be:

• Viral Infections: These are by far the most common cause, particularly in developed countries. Numerous viruses can infect the heart muscle. In children, enteroviruses like coxsackievirus B are a common cause. In adults, adenoviruses, influenza viruses, parvovirus B19 (the cause of Fifth Disease), herpes simplex virus, and coronaviruses (including SARS-CoV-2, the virus that causes COVID-19) have all been implicated. The virus can directly invade the heart muscle cells or trigger an immune response that leads to inflammation.

• Bacterial Infections: While less common than viral causes, certain bacteria can also cause myocarditis. These include Staphylococcus, Streptococcus (including the one that causes rheumatic fever), and Corynebacterium diphtheriae (which causes diphtheria). Bacterial infections can reach the heart through the bloodstream or by spreading from a nearby infection.
• Fungal Infections: In individuals with weakened immune systems, fungal infections like Candida, Aspergillus, and Pneumocystis can infect the heart muscle.
• Parasitic Infections: In certain regions of the world, parasitic infections such as Chagas disease (caused by the parasite Trypanosoma cruzi) and toxoplasmosis are significant causes of myocarditis.

Autoimmune Diseases

In autoimmune diseases, the body's immune system mistakenly attacks its own tissues, including the heart muscle. Conditions that can lead to myocarditis include:

• Systemic lupus erythematosus (lupus)
• Rheumatoid arthritis
• Scleroderma
• Polymyositis and dermatomyositis
• Vasculitis (inflammation of blood vessels)

The inflammation caused by these conditions can affect the myocardium, leading to myocarditis.

Allergic Reactions and Toxins

Certain substances can trigger an inflammatory response in the heart muscle:

• Medications: A wide range of drugs have been associated with myocarditis. These include certain antibiotics (like sulfonamides), anti-seizure medications, diuretics, antipsychotics, and chemotherapy drugs (such as doxorubicin). The reaction can be a direct toxic effect or an allergic response to the medication.
• Illicit Drugs: Stimulants like cocaine and amphetamines can cause direct damage to the heart muscle and trigger inflammation.
• Environmental Toxins: Exposure to certain heavy metals or chemicals can, in rare cases, lead to myocarditis.

Idiopathic Myocarditis

In a significant number of cases, the precise cause of myocarditis cannot be identified even after thorough investigation. This is known as idiopathic myocarditis. It is believed that many of these cases may be due to viral infections that are no longer detectable or subtle autoimmune responses.

Other Causes

Less common causes include:

• Certain types of radiation therapy to the chest.
• Postpartum myocarditis, a rare but serious form that occurs in the final month of pregnancy or within five months after delivery.

Understanding the cause of myocarditis is crucial for appropriate diagnosis and treatment. While viral infections are the most common cause, it's important to consider other possibilities, especially in individuals with underlying health conditions or those exposed to specific toxins or medications.