Giant cell myocarditis

Article written and reviewed by Dr Luca Moretti

Giant cell myocarditis is a rare form of myocardial inflammation characterized by rapidly progressive cardiomyocyte destruction, a heterogeneous cellular infiltrate and multinucleated giant cells. Diagnosis does not derive from the mere presence of a large cell, but from the overall tissue architecture, in which necrosis, lymphocytes, macrophages and eosinophils are distributed without forming the well-organized granulomas typical of cardiac sarcoidosis. This distinction is crucial because the rate of progression, immune treatment and transplantation strategy are not the same.

The classic presentation is an adult with new-onset heart failure who, over days or weeks, develops ventricular arrhythmias, atrioventricular block or shock not explained by coronary artery disease. Less explosive presentations also occur, initially dominated by ventricular tachycardia or a conduction disturbance with still-preserved function, as do rare predominantly atrial forms. Late diagnosis is dangerous because injury can progress while symptoms and biomarkers are interpreted as ordinary lymphocytic myocarditis.

The true incidence is unknown, and case series come from referral centers, biopsies and explanted hearts, with inevitable selection of more severe cases. The disease may be associated with autoimmune conditions, thymoma and other abnormalities of immune tolerance, but in most patients no single causal event is identified. Rarity does not justify a high diagnostic threshold when the phenotype is compatible, because biopsy information immediately changes treatment and prognosis.

Before combined immunosuppression was introduced, survival free from death or transplantation was very short. Contemporary cohorts show that timely diagnosis, multidrug regimens and early access to circulatory support can achieve stabilization, partial recovery or a bridge to transplantation. It nevertheless remains a high-risk disease in which hemodynamic improvement does not justify rapid treatment reduction or allow the arrhythmic substrate to be considered resolved.

Management requires a temporal interpretation of injury from the outset: recent necrosis, reversible edema and maturing fibrosis can coexist in the same heart and respond differently to therapy. This asynchrony explains why hemodynamic recovery does not coincide with disappearance of arrhythmias and why a still-active biopsy may coexist with a CMR dominated by scar. Discordance among tests often describes lesions of different biological ages rather than a diagnostic error.

Rarity makes large trials unlikely and requires registries and transplantation series to be interpreted with awareness of their selection bias, but it does not justify inertia in the face of a highly lethal histological subtype. Decisions arise from convergence of natural-history data, improvement observed with combined regimens, immunological plausibility and multidisciplinary discussion, with explicit acknowledgment of which components are supported by observational evidence.

Etiology, Pathogenesis and Pathophysiology

Clinical and experimental evidence supports a predominantly T-cell-mediated autoimmune pathogenesis. Animal models have shown that loss of tolerance to cardiac antigens can generate giant cell myocarditis, while in humans the association with thymoma, myositis, thyroiditis, inflammatory bowel disease and other immune conditions reinforces this paradigm. No autoantigen or circulating biomarker, however, has sufficient accuracy to replace biopsy.

Activated T lymphocytes recruit and transform macrophages, generating multinucleated cells through fusion and persistent activation signals; interferon gamma, interleukins, tumor necrosis factor and other cytotoxic pathways depress contractility and amplify necrosis. The eosinophilic component, which is frequent but variable, does not automatically convert the condition into eosinophilic myocarditis, because diagnosis depends on the dominant population and tissue arrangement.

The lesion is typically multifocal and may involve both ventricles, the septum and the conduction system; edema and cytokines generate a reversible component of dysfunction, while cardiomyocyte loss rapidly produces replacement fibrosis and re-entry circuits. This combination explains coexistence of low output, ventricular tachycardias and conduction blocks, as well as persistence of electrical risk after an apparent recovery of ejection fraction.

Giant cell myocarditis must be distinguished from granulomatous reactions to mycobacteria, fungi or foreign bodies; special stains, cultures and molecular techniques are selected according to morphology and epidemiology because intense immunosuppression could worsen an unrecognized infection. A negative microbiological result obtained from scant tissue does not have the same value as a planned investigation using multiple, properly preserved fragments.

The boundary with sarcoidosis is not always sharp; well-formed granulomas, geographic fibrosis and limited necrosis favor sarcoidosis, whereas diffuse destruction, a mixed infiltrate and dispersed giant cells favor the idiopathic form; small specimens may nonetheless show only one end of the spectrum. Review by a cardiovascular pathologist and a search for extracardiac disease reduce misclassification with important therapeutic consequences.

Persistence of the process depends on a T-cell response that does not spontaneously resolve as often as in common lymphocytic myocarditis; corticosteroids reduce inflammatory transcription and permeability, but monotherapy alone often does not control all active pathways. Calcineurin inhibition interferes with T-cell activation and is one of the mainstays of combined regimens, to which other immunosuppressive agents are added according to organ function and protocol.

The disease can recur in the transplanted heart, demonstrating that the mechanism does not reside exclusively in the original myocardium. Recurrence may be subclinical and detected during surveillance biopsies or may present with dysfunction and arrhythmias, requiring differentiation from cellular rejection. Immunophenotype, morphology, chronology and response to treatment guide this complex diagnosis.

Autoimmune predisposition does not imply that relatives should undergo systematic cardiac screening because Mendelian transmission of the disease has not been demonstrated. A family history of cardiomyopathy or sudden death nevertheless retains independent significance and may reveal a genetic substrate unmasked by inflammation. In discordant phenotypes, genetic evaluation complements the diagnosis without diminishing the value of histological evidence.

Clinical Manifestations

Rapidly progressive heart failure is the most recognizable presentation; dyspnea, orthopnea, congestion, fatigue and hypotension can evolve over a few days despite diuretics and conventional therapy, while ventricular dilation may be absent in the early phases. A nondilated chamber with severe dysfunction does not guarantee spontaneous recovery and, when associated with arrhythmias or conduction blocks, should accelerate biopsy.

Ventricular tachycardias may be monomorphic because of re-entry in scar or polymorphic during diffuse activity and metabolic instability. Palpitations, syncope, electrical storm and cardiac arrest may precede pump shock, making an assessment based only on ejection fraction inadequate. An increasing arrhythmic burden during treatment may indicate persistent activity, scar maturation or toxicity and requires integrated reassessment.

Atrioventricular block results from involvement of the septum and conduction system and can progress rapidly from PR prolongation to advanced block. A new conduction disturbance in an adult without an evident cause, especially when associated with troponin elevation or ventricular tachycardia, should not be dismissed as degenerative disease. Temporary pacing protects against bradycardia but does not treat the underlying process or concomitant ventricular risk.

Chest pain and an infarct-like phenotype are possible but less characteristic than in lymphocytic myocarditis. Nonobstructive coronary arteries and nonischemic LGE point toward inflammation without defining the histological subtype; a generic clinical diagnosis becomes insufficient if instability, block or lack of response develops. Troponin may be markedly elevated or only moderately increased and does not quantify the speed of progression.

Known autoimmune diseases, thymoma, myositis, colitis or other immune manifestations increase pretest probability, but their absence is common; the history also searches for drugs and immunotherapies capable of producing giant cell myocarditis or overlapping presentations. Temporal relationships help formulate the mechanism, while tissue remains necessary to distinguish idiopathic disease, hypersensitivity and other lesions.

On physical examination, cold extremities, reduced pulse pressure, oliguria, altered mental status and rising lactate indicate hypoperfusion; jugular venous distension, hepatomegaly and edema may reflect right-sided impairment, often associated with left-sided dysfunction. Repeated assessment is more important than a single measurement because an apparently compensated patient can rapidly lose reserve.

A slower presentation does not exclude the diagnosis; some patients develop dysfunction, arrhythmias and conduction blocks for weeks or months before sudden acceleration, and a previous negative biopsy may have missed a focal lesion. Discordance between electrical severity, imaging and the initial diagnosis justifies a second review or new targeted sampling.

Clinical probability rises especially when heart failure, ventricular tachycardia and conduction block occur together, but a single axis may dominate for weeks; reconstructing previous ECGs, imaging and functional capacity can reveal a destructive trajectory hidden by a single visit. A patient treated for idiopathic tachycardia or degenerative block deserves urgent reassessment if function changes or troponin remains elevated.

Investigations and Diagnosis

ECG and telemetry assess ventricular arrhythmias, block, ST-T abnormalities and dynamic changes. Troponin, natriuretic peptides, complete blood count, inflammatory markers, renal and hepatic function and electrolytes define injury and treatment tolerance without providing a specific marker; relatively normal C-reactive protein does not exclude a destructive myocardial process.

Echocardiography assesses biventricular function, wall thickness, effusion, valves and the hemodynamic profile; noncoronary regional abnormalities and reduced strain may precede global depression, while right ventricular dysfunction identifies greater complexity. Serial studies document the trajectory and guide support but cannot distinguish giant cells, lymphocytes and eosinophils.

CMR localizes edema, injury and fibrosis with a pattern that is often multifocal, subepicardial or intramyocardial. It can guide the biopsy site and quantify the arrhythmic substrate, but a negative study does not exclude disease, especially in early phases, focal lesions or patients already treated. Instability should not be prolonged in pursuit of ideal imaging when biopsy and support are urgent.

Urgent endomyocardial biopsy is the decisive examination; multiple fragments are obtained from different sites and, when distribution and safety permit, a biventricular strategy or guidance by imaging and mapping can increase yield. Sampling remains fallible, and an initial negative biopsy does not close the case if shock, conduction blocks and arrhythmias continue to support a high probability.

Histology shows cardiomyocyte necrosis, multinucleated giant cells, lymphocytes, macrophages and eosinophils in variable proportions; well-formed sarcoid granulomas are absent or not dominant, and necrosis does not follow an ischemic distribution. Immunohistochemistry and stains for microorganisms complete the assessment, while PCR is interpreted cautiously to avoid attributing causality to incidental genomes.

The differential diagnosis includes sarcoidosis, eosinophilic myocarditis, hypersensitivity, granulomatous infections, lymphomas and foreign-body reactions. Chest CT and PET can identify lymphadenopathy, thymoma or extracardiac sites, but biopsy of a sarcoid lymph node does not prove that rapid cardiac deterioration has the same histological subtype. When the phenotype is aggressive, cardiac tissue retains specific value.

Right-heart catheterization defines pressures, cardiac output and resistances in shock and helps select support and a transplantation strategy. Coronary angiography is performed when ischemia remains plausible or before devices and transplantation according to context; diagnostic value derives from integration rather than from searching for a single number capable of confirming the disease.

Once the pathology report is available, clinicopathological review verifies adequacy, extent, possible agents and overlaps. The diagnosis should be communicated immediately to the intensive-care, electrophysiology and transplant teams because every delay separates potentially recoverable tissue from irreversible necrosis. The center should also plan the samples and controls required to monitor immunosuppression and infection.

Before immunosuppression is intensified, complete blood count, immunoglobulins, renal and hepatic function, latent infections and vaccination status are documented without turning screening into a delay for an unstable patient. Residual fragments are preserved for possible additional stains and microbiological analyses; a diagnosis stated only as severe myocarditis instead deprives the team of information required to balance the combined regimen, prophylaxis and transplantation strategy.

Treatment and Prognosis

Management takes place in a center capable of cardiac intensive care, biopsy, electrophysiology, mechanical support and transplantation. In shock, vasopressors and inotropes are used at the lowest effective dose while hypoxia, acidosis and arrhythmias are corrected; early escalation to venoarterial ECMO or ventricular support prevents increasing catecholamine doses from worsening oxygen consumption and instability. Device type depends on uni- or biventricular involvement and the prospect of recovery or transplantation.

Immunosuppression is started rapidly when giant cell myocarditis is strongly suspected, coordinating urgent endomyocardial biopsy and without necessarily awaiting tissue confirmation in high-risk patients. Effective regimens are combined and generally include a corticosteroid, a calcineurin inhibitor and another antiproliferative or cytotoxic agent; there is no universal randomized protocol, and choice, intensity and duration are tailored to severity, renal and hepatic function, cytopenias and infection.

Corticosteroid therapy alone is associated with less reliable control than multidrug regimens in available cohorts; the calcineurin inhibitor is maintained over time when tolerated because early withdrawal may be followed by relapse. Dose reductions and substitutions require a stable clinical trajectory, therapeutic drug monitoring and assessment of cumulative risks of nephrotoxicity, infection and malignancy.

Heart failure therapy is introduced after stabilization and titrated according to blood pressure and organ function; diuretics control congestion, while neurohormonal drugs should not be forced during hypoperfusion. Recovery of function does not demonstrate disappearance of disease and does not justify automatic withdrawal of either immunosuppression or heart-failure protection.

Ventricular tachycardia and electrical storm require correction of precipitating factors, antiarrhythmics, sedation and sometimes ablation, but control of immune activity remains part of treatment. Temporary pacing treats unstable block; the choice between a permanent pacemaker and defibrillator takes recovery, documented arrhythmias and persistent risk into account. A device should neither delay biopsy nor create a false sense that the disease is controlled.

Heart transplantation is evaluated early in patients with refractory shock, progression despite treatment or irreversible injury. Mechanical support can serve as a bridge to decision and allow immunosuppression to produce recovery, but not all patients can be maintained indefinitely while waiting. Systemic diseases, infections and the function of other organs influence candidacy and timing.

Prognosis has improved compared with historical series, but remains more severe than in most lymphocytic myocarditis. Age, shock, extent of injury, right ventricular function, arrhythmias, initial response and tolerance of immunosuppression define the trajectory; transplant-free survival does not always equal normalization because dysfunction and scar may persist.

Follow-up includes ECG, Holter monitoring, echocardiography, biomarkers, renal and hepatic function, drug levels and infectious surveillance; CMR and, in selected cases, repeat biopsies help distinguish activity, scar and toxicity when the therapeutic decision is uncertain. After transplantation, biopsies must be interpreted with recurrence as well as rejection in mind.

There is no isolated marker by which treatment tapering can be scheduled; normalized troponin may coexist with focal infiltrates, while a modest persistent elevation may result from renal failure, tachycardia or procedures. The decision integrates hemodynamic stability, arrhythmic burden, function, imaging and, when the stakes are high, new histology, with a pathway arranged in advance to recognize and treat relapse.

Complications

Cardiogenic shock can progress to multiorgan failure before immunosuppression has time to work; renal, hepatic and neurological injury reduces drug tolerance and can compromise transplant candidacy, making early support decisive. Bleeding, peripheral ischemia and device infection must be balanced against the risk of unsupported inadequate perfusion.

Ventricular arrhythmias can recur despite hemodynamic improvement because inflammation and scar respond on different timelines. Electrical storm, ICD shocks and prolonged sedation worsen function and quality of life, while ablation in still-active tissue may have incomplete efficacy; the strategy combines immune control, drugs, device therapy and ablation without relying on a single intervention.

Advanced block may require pacing and persist after edema has subsided if the conduction system has been destroyed. Chronic ventricular pacing may worsen already impaired function, and the choice of system takes into account the need for defibrillation and resynchronization; implantation must be coordinated with vascular access, infection and possible mechanical support.

Opportunistic infections are a major complication of combined immunosuppression; screening for latent infections, appropriate prophylaxis, non-live vaccines when indicated and monitoring of leukocytes and immunoglobulins reduce but do not eliminate risk. Fever, cytopenia or respiratory deterioration requires a differential diagnosis among infection, immune activity and toxicity because empirically increasing immunosuppression may be dangerous.

Nephrotoxicity, hypertension, neurotoxicity, diabetes, osteoporosis and malignancies complicate prolonged regimens; interactions with antiarrhythmics, antifungals and antibiotics can rapidly alter calcineurin-inhibitor levels. Therapeutic drug monitoring and collaboration among cardiology, transplant, infectious-disease and nephrology teams are therefore integral to efficacy.

Relapse during treatment tapering can present with a new troponin elevation, arrhythmias, block or dysfunction and requires exclusion of infection, rejection in transplant recipients and other causes. A repeat biopsy can alter treatment when procedural risk is acceptable; a single normal biomarker does not provide sufficient protection in a disease historically capable of reactivation.

After transplantation, recurrence in the graft can overlap with cellular rejection and lead to complex treatment intensification; expert pathology, comparison with previous biopsies and assessment of giant cells and necrosis distribution help separate the processes. Despite this possibility, transplantation remains a life-saving therapy with acceptable outcomes in appropriately selected patients.

In survivors, long-term planning includes contraception or reproductive planning, compatible vaccinations, cardiovascular prevention and review of interactions with every new prescription. Vulnerability is not limited to recurrence: nephrotoxicity, infections and scar can alter reserve for years; a coordinating clinician integrates the different specialties and prevents isolated decisions from disrupting a carefully achieved therapeutic balance.