Cardiac Path Online

Pathology of Myocarditis

Dr Sampurna Roy MD





Myocarditis is characterized by myocardial inflammation (inflammation of the muscular layer) that result in injury to the cardiac myocytes.

An inflammatory infiltrate is present in the myocardium together with necrosis and degeneration of the myocytes.  

Clinical presentation:  Patients may remain asymptomatic or there may be abrupt onset of arrhythmia, congestive cardiac failure or sudden death.

Most patients recover quickly and without sequelae.


Causes of Myocarditis:

Related post: Infectious Disease Online

1)  Microbial infection: 

Bacteria- Stapylococci, beta-hemolytic streptococci, diphtheria, tuberculosis, brucellosis, clostridia, meningococci.

Chlamydia- Psittacosis

Fungi- Candidiasis cryptococcosis histoplasmosis, actinomycosis , blastomycosis, coccidioidomycosis aspergillosis.

Viruses (viral myocarditis is most common amongst the infectious causes)-   Coxsackievirus types A and B (specially Type B) , Adenovirus, Echovirus, Epstein Barr virus, Hepatitis C virus, Herpes Simplex Virus Infection, Measles, Influenza , Rubella Poliomyelitis virus, Human immunodeficiency virus , Mumps, Parvovirus B19 Infection ,Cytomegalovirus. 

Protozoa-  Chagas' Disease (Trypanosoma crusie) , toxoplasmosis , malaria, leishmaniasis.

Helminth- Trichinosis, schistosomiasis, echinococcosis, cysticercosis.

Rickettsia-  Scrub Typhus, Q Fever, Rocky Mountain Spotted Fever.

Spirochetal- Syphilis, Lyme disease,  Weil disease, Borrelia.

2) Immune mediated reaction:

Postviral ; Poststreptococcal ; Systemic lupus erythematosus ; Drug hypersensitivity ; Transplant rejection.

3) Unknown:

Sarcoidosis ; Giant cell myocarditis 

4) In connective tissue disease:

Rheumatic fever ; Rheumatoid arthritis ; Polyarteritis nodosa ; Dermatomyositis

5) Caused by physical agents , chemical poisons, drugs, metabolic disorders:

Trauma ; Irradiation ; Chemical poisons (arsenic, lead, hydrocarbons, mercury) ; Radiation ; Heatstroke ; Bites  and stings ; Drugs ( Adriamycin , streptomycin , catecholamines, acetaminophen, methyldopa, penicillin, sulphonamide, phenytoin) ; Uremia ; Hypokalemia.


Gross features: 

Flabby ventricular myocardium, often with four chamber dilatation and patchy or diffuse hemorrhagic mottling. There may be mural thrombi in dilated chambers.

The endocardium and valves are unaffected.

Microscopic features: 

Myocardial inflammatory infiltrate associated with myocyte necrosis or degeneration. In late stage, delicate interstitial fibrosis and focal scarring may occur.

Characteristic microscopic features in each condition:

Microbial infection:

Viral Myocarditis:   Initially, interstitial tissue is infiltrated by lymphocytes, and neutrophils and there is necrosis of individual muscle fibres ; Later lymphocytes and histiocytes predominate and some degree of connective tissue proliferation occurs.

Rickettsia:  Interstitial edema ;  Focal or patchy infiltration by inflammatory cells ;  Anitschkow cells, mast cells and eosinophils are noted in close association with small vessels ;  Neutrophils are few unless necrosis is present ;  Vascular changes - capillary and arteriolar endothelial swelling and phlebitis with thrombosis.  ;  Necrotizing arteritis of the myocardium (epidemic typhus).

Bacteria and Fungus:  Abscess may develop in the myocardium as a result of infection by pyogenic (staphylococci and beta-hemolytic streptococci) and fungal organisms.

Pyogenic organisms may produce a more diffuse suppurative myocarditis. Fibrosis is seen in the later phase of certain myocardial lesions.

Granulomatous inflammation of the myocardium caused by certain bacterial infections (tuberculosis , tularemia and brucellosis). 

Visit - Giant cell myocarditis.

Protozoa: Trypanosoma cruzi (Chaga's disease): Dense inflammatory infiltrate in the myocardium ;  Cellular infiltration consisting of histiocytes, plasma cells, lymphocytes, and some polymorphonuclear leukocytes ; Degeneration and necrosis of muscle fibres may be prominent.

Toxoplasma gondii:  Focal necrosis, edema and inflammatory infiltrate in the myocardium ; Infiltrate composed of plasma cells, macrophages, and lymphocytes ; Pseudocysts containing organisms are seen in the myocardium within or outside muscle fibres often  separated from the foci of inflammation.

Helminth :  Trichinella spiralis:  Inflammatory reaction is a response to the larvae ; Larvae are rarely identified in the lesions ; Focal necrosis of the muscle fibres ; inflammatory  infiltrate composed of lymphocytes and eosinophils.

Echinococcus disease:  Hydatid cysts occasionally occur in the myocardium ; Hydatid cyst of the heart tends to rupture into the lumen of the cardiac chamber or into the pericardial sac.

Polyarteritis nodosa :   Necrotizing vasculitis with extension of the inflammatory reaction to the adjacent perivascular tissue ; Formation of thrombi in the vessels ;  Secondary ischemic changes including infarction may occur in the myocardium.

Dermatomyositis :   Degeneration ;  Loss of striation ; Vacuolization and necrosis of muscle fibres may be seen ;  Interstitial edema may be present ;  Inflammatory infiltrate  composed of mainly lymphocytes and histiocytes is present interstitially or perivascularly  ;  Hyalinized thickening of the intramyocardial arterioles with narrowing of the lumens.

Certain chemical poisons or drugs : Myofiber swelling, fatty change and individual cell lysis ;  Edema and small hemorrhage may be present ;  A nonspecific inflammatory reaction consisting of lymphocytes and histiocytes occur in response to necrotic tissue ;  Myocarditis characterized by an interstitial exudate with many eosinophils has been noted as a complication of therapy with a variety of drugs.

Cardiac trauma : Nonspecific myocarditis may be seen in association with with contusions of the myocardium ; Lesion consisting of an infiltrate of neutrophils and a few eosinophils and mononuclear cells together with focal necrosis of muscle fibers and hemorrhage.

Uremia :  Fatty degeneration and edema of the myocardium due to uremic intoxication.

Cardiac transplant rejection :  Characterized by the presence of interstitial lymphocytes and myocyte degeneration.

Related post: Giant cell Myocarditis 


Dallas Classification (1987)

Myocarditis:   Myocardial necrosis, degeneration, or both,  in the absence of significant coronary artery disease with adjacent inflammatory infiltrate with or without fibrosis.

Borderline myocarditis:   Inflammatory infiltrate too sparse or myocyte damage not apparent.

No myocarditis:

WHO Marburg Criteria (1996)

Acute (active) myocarditis:  A clear-cut infiltrate (diffuse, focal or confluent) of >14 leukocytes/mm˛ (preferably activated T-cells).

The amount of the infiltrate should be quantitated by immunohistochemistry. Necrosis or degeneration are compulsory, fibrosis may be absent or present and should be graded.

Chronic myocarditis:  An infiltrate of >14 leukocytes/mm˛ (diffuse, focal or confluent, preferably activated T-cells).

Quantification should be made by immunohistochemistry.

Necrosis or degeneration are usually not evident, fibrosis may be absent or present and should be graded.

No myocarditis:  No infiltrating cells or <14 leukocytes/mm˛.

Further reading:

Incidental myocarditis with intravenous drug abuse: the pathology

HIV-associated myocarditis. Pathology and immunopathology.

Influenza myocarditis--pathophysiology and developmental  

Myocarditis in mixed connective tissue disease

Survival in biopsy-proven myocarditis: a long-term retrospective analysis of the histopathologic, clinical, and hemodynamic predictors.

Indication for myocardial biopsy in myocarditis and dilated cardiomyopathy.

Transplantation for myocarditis: a controversy revisited.

Endomyocardial biopsy for non-transplant-related disorders. 

Mortality in primary and secondary myocarditis.

Acute myocarditis.

Diagnosis and course of myocarditis: a survey in the medical clinics of Zurich University Hospital 1980 to 1998.  

Sudden death caused by myocardial tuberculosis: case report and review of the literature.

Ten-year experience with endomyocardial biopsy in myocarditis presenting with congestive heart failure: frequency, pathologic characteristics, treatment and follow-up.

Myocarditis. Epidemiology, etiology and clinical aspects.

Pathological findings in 2300 consecutive endomyocardial biopsies.

Phenotypic analysis of infiltrating cells in human myocarditis. An immunohistochemical study in paraffin-embedded tissue.

Myocarditis: the Dallas Criteria. Hum Pathol. 1987. 18:619-24.

Giant-cell myocarditis in a newborn with congenital herpes simplex virus (HSV) infection: an immunohistochemical study on the origin of the giant cells.



Dr Sampurna Roy  MD

Consultant  Histopathologist (Kolkata - India)






Infectious Disease Online


Pathology Quiz Online 


Paediatric Pathology Online


Pancreatic Pathology Online




Endocrine Pathology Online


Eye Pathology Online


Ear Pathology Online


Cardiac Path Online


Pulmonary Pathology Online


Lung Tumour Online




Nutritional Pathology Online


Environmental Pathology Online


Soft Tissue Tumour Online


GI Path Online-India


Gallbladder Pathology Online


E-book - History of Medicine  


Microscope - Seeing the Unseen





Privacy Policy  

Advertising Policy

Copyright © 2020