Types of Myocarditis

FAQ – Types of Myocarditis

Written By Monte Willis MD, PhD, FASCP, FCAP

What are the clinical classifications of myocarditis?

A:  Myocarditis can be classified in several different ways. Recently, experts have proposed a three-tiered classification system for acute myocarditis, based on increasing the certainty of the diagnosis and presentation.

  1. Possible subclinical acute myocarditis. Subclinical myocarditis, without specific symptoms, may be inferred by transient increases in troponin (a biomarker of cardiac damage) or ECG, which measures the electrical activity of the heart, after an acute viral illness, adverse drug reaction, or rarely vaccination. The risk of these patients developing heart failure or arrhythmias is not known. More research is needed to determine the long-term significance of subclinical myocarditis.
  2. Probable acute myocarditis.  The diagnosis of probable acute myocarditis is made if one of four clinical syndromes can be identified that are consistent with myocarditis.
    1. Chest pain
    2. Acute heart failure (inability for heart to perfuse the body)
    3. Presyncope or syncope
    4. Myopericarditis
  3. Definite myocarditis.  The diagnosis of definite myocarditis is made with confirmed histological analysis of the heart tissue. Here, a heart specimen is investigated by microscope for the presence of specific inflammatory cells in the heart, which are normally absent. A clinical classification is also frequently used in practice. Clinical scenarios include:
  4. Myocarditis resembling a heart attack.  Myocarditis can present with chest pain due to inflammation of the blood vessels supplying the heart. While the heart function is largely preserved, reversible coronary closing due to spasm can result from the influx of inflammatory cells. Chest pain that is worse with leaning backward and better leaning forward can signify inflammation of the pericardium or pericarditis. Pericarditis frequently accompanies myocarditis of the outside layers of the heart muscle.
  5. Myocarditis resembling acute or chronic heart failure.  Patients with myocarditis can present with shortness of breath, fatigue, and the inability to tolerate exercise. These patients generally have enlarged hearts, with symptoms that occur about 2 weeks to a few months after gastrointestinal or upper respiratory infections (acute). A more chronic form may be seen where the body’s immune system continues to attack the heart in the absence of infection., Such cases of chronic myocarditis can occasionally result from a previous acute infection or heart muscle injury.

Can myocarditis be confused with other heart diseases?

A: Yes. For example, the symptoms of heart failure, including shortness of breath, fatigue, inability to tolerate exercise, and associated with difficulty breathing while laying down/sleeping are common to many heart diseases besides myocarditis. One feature that distinguishes myocarditis from other causes of heart failure is that it often follows an upper respiratory or gastrointestinal infection and is due to a specific immune response against the heart itself.

Myocarditis can also mimic a heart attack. When cardiac inflammation occurs in the regions of the heart nearest to the outside surface, it can present as chest pain. However, in myocarditis, the coronary vasculature, which supplies the heart and is generally blocked in heart attacks, usually appears normal1.

Myocarditis can cause arrhythmias caused by defects in the conduction system of the heart. When the conduction (electrical) system in the heart experiences “blocks”, it is unable to coordinate pumping blood adequately. Loss of blood perfusion to the brain can result in “passing out”. Cardiac sarcoidosis and infections with Lyme disease, and Diphtheria are associated with these types of defects (see FAQ of types of viruses and bacteria that cause myocarditis for disease details).

What is a biopsy and when is it used in the diagnosis of myocarditis?

A: A biopsy is performed when myocarditis is suspected and when making the diagnosis of myocarditis may impact treatment options or prognosis (expected outcomes as in life-threatening outcomes). It is recommended that if a patient has an indication (reason) for an endomyocardial biopsy and they are at a medical center where this expertise is unavailable, the patient should be transferred to a medical center with this expertise.

What does it mean to diagnose something by histology?

A:  Many disease processes occur on the microscopic level and cannot be seen by other means. In order to see what is going on in tissue, physicians will look at finely cut and stained tissue sections to identify the disease process present. This involves looking at the types of cells present and their shapes and spatial orientation. The disease process in myocarditis can only be diagnosed by histological investigation of very small pieces of heart tissue (biopsies).

How is heart tissue collected to evaluate it by histology?

A:  Heart tissue is collected by an endomyocardial biopsy to get very small pieces of tissues that can be analyzed histologically. It is collected by accessing the blood vessels connected to the heart. Briefly, the internal jugular vein or femoral vein (the large veins in the front of the neck or leg) are accessed and a bioptome is guided to the right side of the heart.

With the advent of flexible bioptomes, access through the femoral veins (the large veins in the groin) can equally allow access to the right heart. Endomyocardial biopsies are generally performed under the guidance of imaging techniques that allow the visualization of the bioptome in the vessels and heart. In addition to being used in the diagnosis of myocarditis, endomyocardial biopsies are used for the diagnosis of transplant rejections 3.3.

Based on established recommendations in the 2007 American Heart Association/American College of Cardiology/European Society of Cardiology (AHA/ACC/ESC) scientific statement on endomyocardial biopsies 2, samples are taken from more than one region in the right ventricle (5-10 samples taken). At least 4-5 samples should be submitted for light microscopic examination (lower power magnifications). The samples can sometimes be examined under very high power magnification using transmission electron microscopes, which can help identify certain adverse drug reactions and metabolic diseases.

What are the histological classifications of myocarditis?

A:  Interpretation of endomyocardial biopsy samples is guided by the Dallas criteria by physicians that developed a working standard for diagnosing myocarditis 4.

  1. Active myocarditis is defined as “an inflammatory infiltrate of the myocardium with necrosis and/or degeneration of adjacent myocytes not typical of the ischemic damage associated with coronary heart disease”.-Interpretation: There are small inflammatory cells that are found in the heart that are causing damage to the heart itself. These cells include monocytes and neutrophils and occasionally eosinophils, each with roles in killing infections and infected cells.
  2. Borderline myocarditis is diagnosed when the inflammatory infiltrate is sparse and injury to the heart cells themselves (myocytes) is not demonstrated.
  1. Yilmaz A, Mahrholdt H, Athanasiadis A, et al. Coronary vasospasm as the underlying cause for chest pain in patients with PVB19 myocarditis. Heart 2008;94:1456-63.
  2. Cooper LT, Baughman KL, Feldman AM, et al. The role of endomyocardial biopsy in the management of cardiovascular disease: a scientific statement from the American Heart Association, the American College of Cardiology, and the European Society of Cardiology. Circulation 2007;116:2216-33.
  3. Cooper LT, Jr., ed. Clinical manifestations and diagnosis of myocarditis in adults. Waltham, MA: Wolters Kluwer Health; 2012.
  4. Aretz HT, Billingham ME, Edwards WD, et al. Myocarditis. A histopathologic definition and classification. The American journal of cardiovascular pathology 1987;1:3-14.

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