Granulomatous myocarditis

Article written and reviewed by Dr Luca Moretti

Granulomatous myocarditis does not designate a single disease, but rather a pattern of tissue response in which epithelioid macrophages, lymphocytes and giant cells organize into recognizable aggregates within the myocardium. Cardiac sarcoidosis is the most relevant noninfectious cause, but tuberculosis, nontuberculous mycobacteria, mycoses, foreign-body reactions and rarer conditions can produce a similar appearance. The diagnostic task therefore does not end with the word granuloma: it begins there.

Etiologic distinction is urgent because treatments appropriate for one mechanism may be harmful in another. Corticosteroids and immunosuppressants can control active sarcoidosis, whereas the same strategy, when applied to an unrecognized mycobacterial or fungal infection, promotes progression and dissemination. Conversely, prolonged empirical antimicrobial treatment without evidence can delay control of immune inflammation that is destroying the conduction system and ventricular function.

A granuloma is an immunologic solution to the persistence of a stimulus that cannot easily be eliminated. Its structure and the presence of necrosis provide clues, not verdicts: caseation favors tuberculosis, well-formed nonnecrotizing granulomas favor sarcoidosis, and birefringent material suggests a foreign body, but exceptions and incomplete samples occur. A reliable diagnosis arises from integration of cardiovascular pathology, microbiology, imaging and epidemiology.

Clinically, granulomas tend to produce focal and heterogeneous disease. A small deposit in the basal septum may cause atrioventricular block before global dysfunction develops, whereas a lateral epicardial scar may sustain ventricular tachycardia despite preserved ejection fraction. This disproportion between macroscopic anatomy and electrical risk explains why a normal echocardiogram and mild symptoms are insufficient to exclude clinically significant disease.

The diagnosis should be expressed in probabilistic language that separates morphology from cause: “granulomas compatible with sarcoidosis” does not mean “proven sarcoidosis” unless infections and foreign materials have been considered. This precision does not weaken the report; it makes the remaining verification steps explicit and prevents a provisional label from becoming, over the years, a basis for automatic immunosuppression.

The pathologist describes the number, size, organization, necrosis, associated cell populations and relationship to vessels or foreign material, because each feature changes the etiologic probability without having absolute specificity. The clinician adds exposures and involved organs, constructing a matrix in which histological similarity is weighed against the actual likelihood of sarcoidosis, mycobacteria, fungi or a procedure-related reaction.

Etiology, Pathogenesis and Pathophysiology

In sarcoidosis, an undefined antigen interacts with genetic predisposition and a T-cell response, generating noncaseating granulomas composed of epithelioid macrophages and giant cells. The granuloma may regress or leave a geographic scar; the absence of an identifiable agent does not mean the disease lacks a mechanism, but it prevents a single microbiological test from being used as confirmation or exclusion.

Mycobacterium tuberculosis can reach the heart by lymphatic, hematogenous or contiguous spread from mediastinal structures and may involve the pericardium, myocardium and endocardium. Within the myocardium it forms nodules or diffuse infiltrates with variable necrosis, often in the setting of extracardiac disease; immunosuppression, origin from high-incidence areas and known contacts increase the probability without replacing microbiological demonstration when this can be obtained.

Histoplasma, Candida, Aspergillus and other fungi can cause granulomas or microabscesses, especially in immunosuppressed patients, in those with hematogenous dissemination or in the presence of prosthetic materials. Morphology depends on the organism and host response; Grocott and PAS stains are helpful, but culture and molecular methods increase identification. A negative stain on only a few micrometers of tissue does not exclude a focal infection.

Surgical material, sutures, polymers, talc or embolized particles can elicit foreign-body granulomas, sometimes identifiable under polarized light. The reaction may be localized and clinically irrelevant or may contribute to a mass, arrhythmia or dysfunction near the site of a procedure; anatomical distribution and chronology prevent every giant cell observed in a previously operated heart from being attributed to sarcoidosis.

Other causes include Whipple disease, brucellosis, Q fever, syphilis and exceptional immune disorders, whose probability varies with geography, exposure and systemic phenotype. A universal panel would be inefficient and would produce results that are difficult to interpret; the pathologist should report morphological clues, while the clinician constructs a targeted microbiological strategy and preserves unfixed tissue whenever possible.

Granulomas alter electrophysiology through local edema, compression, necrosis and fibrous replacement; the septum and conduction bundles are particularly vulnerable, although any region may develop slow conduction and re-entry. Inflammatory activity causes dynamic instability, whereas mature scar maintains the risk of tachycardia even after the underlying cause has been controlled.

Ventricular function deteriorates when multiple lesions reduce contractile mass, when the right ventricle is involved, or when arrhythmias and blocks cause dyssynchrony. Edema and cytokines can produce a reversible component, whereas focal aneurysms and wall thinning reflect structural damage; recovery of ejection fraction therefore does not erase the electrical vulnerability embedded in the scar.

The most challenging differential diagnosis is giant cell myocarditis. In the latter, giant cells are dispersed within a mixed infiltrate with more aggressive cardiomyocyte necrosis and without the typical sarcoid organization; however, small samples may create overlap. Clinical tempo, the search for extracardiac disease and review of multiple fragments complement morphology.

Necrosis and organization must be interpreted in the context of the host's immune status; a neutropenic patient or one already treated with corticosteroids may fail to form mature granulomas, whereas partially treated tuberculosis may show few bacilli and minimal necrosis. The same image therefore carries different probabilities before and after immunosuppression, previous antibiotic use and geographic origin have been considered.

The granulomatous response attempts to confine a persistent antigen, but in the heart the anatomical price may be high even when the total volume involved is modest, because the septum and conduction bundles poorly tolerate edema and scar. A lesion measuring only a few millimeters in a critical location can produce complete block, whereas more extensive foci in a free wall may be discovered only during an episode of tachycardia.

Clinical Manifestations

New-onset atrioventricular block, especially in a nonelderly adult, is a characteristic presentation of cardiac granulomatous disorders; it can progress from fascicular conduction abnormalities to complete block and cause presyncope or syncope. Pacemaker implantation corrects bradycardia but neither identifies the cause nor protects against ventricular tachyarrhythmias that may arise from the same substrate.

Monomorphic ventricular tachycardia, complex ventricular ectopy and cardiac arrest often result from multifocal and epicardial scars; risk is not proportional to ejection fraction alone, because relatively small lesions may disrupt electrical architecture at critical sites. Palpitations in a patient with systemic sarcoidosis or tuberculosis should therefore prompt at least an ECG, rhythm monitoring and appropriate imaging.

Heart failure may present as dilated cardiomyopathy, noncoronary regional dysfunction or predominant right ventricular impairment. Dyspnea, congestion and edema may evolve slowly during chronic scarring or rapidly during intense activity; diffuse myocarditis, sepsis and concomitant pericarditis can lead to shock, particularly in disseminated infections.

Fever, weight loss, night sweats and fatigue increase suspicion of infection, but they may also occur in active sarcoidosis or malignancy. Cough, lymphadenopathy, skin lesions, uveitis and neurological involvement suggest multisystem sarcoidosis; respiratory symptoms and epidemiological contacts support tuberculosis or mycoses; the absence of extracardiac manifestations does not exclude an isolated form.

Chest pain is less specific and may result from pericardial involvement, microvascular ischemia or myocardial inflammation; fluctuating troponin indicates injury but does not distinguish an active granuloma from other forms of myocarditis. Concomitant coronary artery disease should be excluded according to age and risk, without treating two processes that can coexist as mutually exclusive alternatives.

In immunosuppressed patients, manifestations may be blunted and granuloma formation incomplete despite a high microbial burden; fever may be absent and the first evidence may consist of block, shock or multiple lesions on imaging. Immunosuppressive drugs, transplantation, HIV and malignancies therefore alter both the probability of the causes and the histological appearance.

The clinical trajectory provides complementary information; fulminant progression with arrhythmias and refractory failure heightens concern for giant cell disease or an aggressive infection, whereas a multiyear alternation of activity and scar favors sarcoidosis. These associations should not become shortcuts, because atypical presentations remain possible.

Fever and weight loss do not reliably separate infection from immune disease, because corticosteroids, age and immunosuppression can attenuate fever and sarcoidosis itself can cause systemic symptoms. It is more informative to describe duration, the extracardiac pattern, microbiology and response to therapy already received; malignancies and granulomatous disorders share many manifestations, making any shortcut based on a single symptom inadequate.

The physical examination also searches for findings in the skin, eyes, lymph nodes, joints and respiratory system that can provide a biopsy site or alter the differential diagnosis. Surgical scars and devices are correlated with granuloma distribution, while occupational exposures may suggest foreign materials, turning apparently peripheral details into decisive pathological information.

Investigations and Diagnosis

ECG, Holter monitoring and telemetry quantify conduction blocks, ectopy and tachycardias, while echocardiography assesses function, wall thinning, aneurysms, effusion and valves; troponin and natriuretic peptides help monitor injury and hemodynamic burden. Complete blood count, inflammatory markers, organ function, HIV testing and other studies are selected according to context, because no blood biomarker specifically identifies a cardiac granuloma.

CMR shows multifocal LGE, often intramyocardial or subepicardial and frequently septal, with or without edema; its distribution can guide biopsy and quantify scar, but it does not reliably distinguish sarcoidosis from infection or other forms of myocarditis. A negative study reduces probability only in relation to technical quality, disease phase and pretest suspicion.

Fluorodeoxyglucose PET evaluates metabolic activity after preparation designed to suppress physiological myocardial uptake. Focal uptake with perfusion defects supports inflammation, whereas a whole-body study identifies lymph nodes, lung, spleen or bone that may be more accessible for biopsy; diffuse uptake due to inadequate preparation is indeterminate and should not automatically be interpreted as active disease.

High-resolution chest CT characterizes lymphadenopathy, nodules, cavitations and interstitial abnormalities, helping differentiate certain causes and plan tissue sampling. Bronchoscopy, endoscopic ultrasound-guided lymph-node biopsy, skin biopsy or sampling of other organs may obtain tissue at lower risk than cardiac biopsy; an extracardiac biopsy demonstrating granulomas supports systemic disease, but the relationship to the cardiac phenotype must still be established.

Targeted endomyocardial biopsy remains decisive when giant cell disease, sarcoidosis and infection must be distinguished or when imaging and extracardiac sites do not resolve the case. Focal distribution reduces the sensitivity of random sampling; multiple specimens and guidance by CMR, PET or electroanatomical mapping can improve yield. A negative result is not equivalent to absence of disease when the clinical probability remains high.

Tissue is examined with hematoxylin and eosin, stains for acid-fast organisms and fungi, polarized light, immunohistochemistry, cultures and PCR when appropriate. Before immunosuppression, geographic origin, travel, contacts and immune status should be reviewed, together with collection of targeted respiratory or blood samples; a positive PCR requires morphological and clinical correlation, whereas a negative PCR does not overcome sampling limitations.

Diagnostic criteria for cardiac sarcoidosis are not identical across international documents and produce different levels of certainty. It is more transparent to express a clinical probability and the data supporting it than to force every patient into an absolute category; the required level of certainty increases when the proposed treatment involves intense immunosuppression or when a credible infectious alternative exists.

The preanalytical phase determines a substantial part of the diagnostic yield: fixing all fragments in formalin precludes cultures and limits some molecular analyses, whereas sending nonsterile tissue creates contamination. Before the procedure, cardiologist, pathologist and microbiologist agree on the number, container and priority of specimens; preparation is essential when a second biopsy would be risky or when antibiotics and immunosuppressants are about to alter the findings.

A negative test is interpreted according to analytical sensitivity and the probability of sampling the lesion, distinguishing absence of a signal in the organism from absence in a few fragments. When an infectious alternative remains credible, an extracardiac specimen or prolonged culture may be more useful than indiscriminately repeating the same test on exhausted material.

Treatment and Prognosis

Treatment diverges according to cause; a mycobacterial granulomatous disease requires an antimicrobial combination based on species, susceptibility, involved organs and interactions; a mycosis requires antifungal treatment and sometimes surgical source control; a foreign-body granuloma may require removal. Infectious-disease and microbiology specialists should be involved before an ambiguous morphological finding is converted into prolonged immune therapy.

In clinically manifest cardiac sarcoidosis with evidence of activity, corticosteroids are generally first-line treatment, often accompanied or followed by a steroid-sparing agent. Dose, taper and duration depend on presentation, response, comorbidities and center protocol, because large randomized trials capable of defining a single regimen are lacking. PET can contribute to monitoring, but it should not be the sole therapeutic target.

Immunosuppression does not remove mature scar, and disappearance of edema does not eliminate re-entry risk; heart failure treatment, congestion control and device therapy therefore follow independent cardiological criteria. Improved function during corticosteroid therapy confirms a reversible component, but it does not retrospectively prove the etiology, because many inflammatory processes respond transiently.

Symptomatic block requires pacing, whereas the choice of a defibrillator takes into account documented arrhythmias, function, LGE, syncope and probability of progression. In sarcoid phenotypes, the need for pacing is often associated with a ventricular risk that makes a simple pacemaker insufficient; in active infections, device type and timing must minimize the risk of colonization.

Ablation can control recurrent tachycardias arising from complex, frequently epicardial scars, but untreated inflammatory activity increases recurrences. Antiarrhythmic drugs, immunotherapy when appropriate and ablation are sequenced according to urgency and mechanism; electrical storm requires sedation, correction of precipitating factors and access to an advanced center.

Prognosis depends more on etiology and phenotype than on the term granulomatous; ventricular arrhythmias, advanced block, right or left ventricular dysfunction, extensive LGE, diagnostic delay and disseminated infection identify higher risk. A small lesion treated early may stabilize, whereas a multifocal scar persists even after eradication or remission.

Follow-up integrates activity and damage; symptoms, ECG, rhythm monitoring, echocardiography and biomarkers are repeated, while CMR and PET are selected according to the clinical question. Microbiological investigations follow the causal infection and treatment toxicities, without using normalization of inflammatory markers alone as proof of microbiological clearance.

The duration of antimicrobial therapy is not shortened because PET becomes negative, just as persistence of a scar does not require indefinite prolongation of immunosuppression. Each examination retains its own question: microbiology for eradication, metabolic imaging for activity, CMR for damage and rhythm monitoring for electrical risk; confusing these objectives is a frequent cause of over- or undertreatment.

Discontinuation follows criteria different from initiation, because benefit decreases when only scar remains while cumulative toxicities and infections continue to increase. A plan defines response indicators, minimum duration, conditions for tapering and signs of failure, allowing the regimen to be modified without interpreting every fluctuation in inflammatory markers as cardiac reactivation.

Complications

Complete atrioventricular block and ventricular arrhythmias can cause syncope, cardiac arrest and sudden death; scar persists after inflammation is controlled, making it necessary to distinguish biological response from electrical protection. Devices and prolonged monitoring are therefore decided on the basis of clinical history and substrate, not solely on the most recent troponin value.

Heart failure may progress to biventricular dysfunction, pulmonary hypertension and the need for mechanical support or transplantation. In infections, eligibility and timing depend on eradication and dissemination; in sarcoidosis, extracardiac involvement and the possibility of recurrence enter the assessment. Advanced therapy remains possible when the case is managed with multidisciplinary expertise.

Immunosuppression administered before tuberculosis or a mycosis has been excluded can cause rapidly fatal dissemination; a careful initial work-up does not eliminate all risk, because immunologic tests may be falsely negative and tissue sampling is limited. Fever or new lesions during therapy require the diagnosis to be reopened rather than automatically attributed to immune reactivation.

Prolonged antimicrobial therapy can cause hepatotoxicity, nephrotoxicity, cytopenias, neuropathy and interactions with antiarrhythmics or immunosuppressants. Electrocardiographic monitoring is particularly important for drugs that prolong the QT interval in an already vulnerable heart; adherence must be actively supported when treatment lasts for months.

Focal aneurysms and wall thinning can harbor thrombi, cause embolism or provide arrhythmogenic circuits. Contrast echocardiography and CMR define their anatomy and guide anticoagulation when an indication exists; an area that appears inactive on PET may remain mechanically and electrically dangerous.

Overlap with malignancies or genetic cardiomyopathies constitutes a diagnostic complication; a pathogenic variant may explain scar and arrhythmia while extracardiac granulomas are incidental, or inflammation may unmask a pre-existing susceptibility. Family history and a discordant phenotype justify genetic evaluation without dismissing inflammation a priori.

Relapses and reactivations may occur after immunosuppression is reduced or with new immunosuppression, whereas reinfections depend on the organism and exposure; a surveillance plan should specify which signs require urgent reassessment. Continuity among cardiologist, infectious-disease specialist, pulmonologist and pathologist prevents new images from being interpreted without comparison with the initial findings.

In empirically treated cases, scheduled reassessment prevents the initial hypothesis from becoming permanent merely because the patient improved. Antimicrobials, corticosteroids and correction of heart failure can produce simultaneous improvement, and response alone does not identify the mechanism; the team re-examines cultures, imaging and tissue after a defined interval. If the data remain discordant, a second pathology opinion or an extracardiac biopsy may reduce uncertainty more effectively than another unguided treatment change.

Communication with the patient should clarify that “granulomatous” describes the microscopic appearance and does not imply contagiousness or definite sarcoidosis. This distinction reduces stigma and improves adherence to investigations that may appear contradictory, such as simultaneously searching for infection and autoimmunity; a concise document recording probabilities, tests performed and preserved samples facilitates future reassessment when new symptoms or knowledge emerge.