Lymphocytic myocarditis

Article written and reviewed by Dr Luca Moretti

Lymphocytic myocarditis is a histopathological pattern in which the inflammatory infiltrate of the myocardium is composed predominantly of lymphocytes, mostly T cells, accompanied to a variable extent by macrophages, rare B cells and plasma cells. In active disease, inflammation is associated with degeneration or necrosis of cardiomyocytes not explained by ischemia, biopsy-related trauma or another cause. This relationship distinguishes true myocarditic injury from the mere presence of immune cells, which may be resident, reactive or increased in many cardiac diseases without being diagnostic on their own.

It is the pattern most frequently identified in biopsies performed for suspected myocarditis, but it does not represent an etiologically uniform disease. A similar microscopic appearance may result from viral infection, a post-infectious immune response, autoimmunity, drugs, cancer immunotherapy or interaction between inflammation and genetic predisposition; in a substantial proportion of cases, the cause remains undetermined. Describing the finding as lymphocytic therefore specifies the dominant tissue mechanism, but does not automatically prove a viral origin or by itself select the treatment.

The clinical spectrum is exceptionally broad: some patients present with chest pain, elevated troponin and preserved function, followed by complete recovery; others develop progressive heart failure, arrhythmias or fulminant myocarditis with shock. This heterogeneity depends on the extent, distribution and speed of injury, but also on the genetic substrate, immune response and phase at which tissue is sampled. The term lymphocytic therefore should not be interpreted as synonymous with mild disease.

The classic Dallas morphological criteria provided the first shared terminology, while immunohistochemistry, molecular analysis and the 2025 Seaport criteria have made the distinction between active disease and an uncertain finding more precise. This evolution addresses a practical problem: a few lymphocytes in a very small fragment may be overdiagnosed, whereas a severe focal lesion may be missed. Reliable diagnosis therefore requires adequate tissue, expert cardiovascular pathology and integration with the clinical picture, CMR and temporal course.

Etiology, Pathogenesis and Pathophysiology

Cardiotropic viruses may produce initial injury through cell entry, replication, cytotoxicity and activation of innate immunity; nucleic acid pattern-recognition receptors induce interferons and chemokines, recruiting natural killer cells and monocytes before the adaptive response develops. Enteroviruses, adenoviruses, parvovirus B19 and herpesviruses have been identified in tissue in different eras and populations, but the significance of a positive PCR varies: genome, load, transcriptional activity, cellular localization and prevalence in controls must be considered together.

CD8 T lymphocytes recognize antigens displayed by infected or injured cells and induce cytotoxicity through perforin, granzymes and apoptotic signaling, while CD4 subsets coordinate macrophages, antibodies and cytokine production. A well-regulated response eliminates the stimulus and contracts; a persistent response can maintain injury through molecular mimicry, epitope spreading and loss of tolerance to cardiac antigens. Reduced regulatory lymphocyte activity or prolonged inflammatory signaling favors the transition from useful defense to pathological autoimmunity.

Macrophages are not merely accompanying cells within the infiltrate, because they produce mediators that depress contractility, clear debris and can promote repair or fibrosis according to their activation state and microenvironment. Immunohistochemistry often shows an important CD68-positive component, while B lymphocytes are fewer but can support antibody production and antigen presentation; T-cell predominance justifies the histological name without reducing the process to a single cell population.

In post-infectious forms, the agent may already have been cleared by the time the patient becomes symptomatic. In this situation, serology demonstrates at most systemic exposure and does not localize the cause to the heart, whereas a high-quality virus-negative biopsy makes an immune mechanism more plausible but does not prove it absolutely. Elapsed time, assay sensitivity and any previous therapy influence the result; distinguishing active infection from persistent immunity is essential before immunosuppressive treatment.

Systemic autoimmune diseases can produce a lymphocytic pattern through cytotoxicity, autoantibodies, vasculitis or immune complexes; lupus, myositis, systemic sclerosis and other connective tissue diseases involve different mechanisms, and cardiac findings may not parallel activity in other organs. Immune myocarditis confined to the heart is also possible and is defined by convergence of histology, exclusion of infection and the clinical course, not by the isolated absence of systemic manifestations.

Checkpoint inhibitors remove physiological restraints on lymphocyte activation and may trigger T-cell myocarditis, sometimes associated with myositis and myasthenia. The infiltrate may be multifocal and initial CMR is not always positive; conduction blocks, arrhythmias and persistent troponin elevation can occur despite preserved function. Tissue findings overlap with conventional lymphocytic myocarditis, so exposure, temporal interval and extracardiac toxicities are integral parts of the diagnosis.

Pathogenic variants in desmosomal and cardiomyopathy genes, particularly DSP but also other substrates, can present with episodes of pain and troponin elevation, edema and subepicardial scar. Death of genetically vulnerable cardiomyocytes recruits lymphocytes, while inflammation accelerates structural progression, making it difficult to establish a single direction of causality. Recurrence, family history, disproportionate arrhythmias and ring-like LGE patterns justify genetic evaluation and help avoid attributing every hot phase to an incidental virus.

Depression of function results from a potentially reversible component, consisting of edema, altered calcium handling, mitochondrial dysfunction and cytokines, and an irreversible component caused by cell loss and fibrosis. Severe initial dysfunction may therefore recover if functional injury predominates, whereas an apparently limited episode may leave an arrhythmogenic scar. If inflammation and remodeling persist, the condition evolves toward inflammatory cardiomyopathy, in which heart failure treatment remains necessary even after immune remission.

Clinical Manifestations

The infarct-like phenotype is common in young and middle-aged adults: acute chest pain, elevated troponin and ST-T abnormalities occur with nonobstructive coronary arteries and often preserved function. Pain may result from pericardial involvement or myocardial injury and lacks features sufficiently specific to exclude ischemia; the initial priority remains to distinguish acute coronary syndrome, spontaneous coronary dissection, vasospasm, Takotsubo syndrome and pulmonary embolism before attributing the presentation to myocarditis.

A respiratory or gastrointestinal prodrome may precede onset, but its frequency in the general population limits its causal value. Fever, myalgia and fatigue support an inflammatory context without identifying the agent; the absence of a prodrome does not reduce probability to zero. The history should include new drugs, immunotherapies, autoimmune diseases, substances of abuse, vaccinations, travel and documented infections, recording dates and doses to construct a verifiable chronology.

The heart-failure phenotype ranges from slowly progressive exercise intolerance to pulmonary edema and shock; dyspnea, orthopnea, oliguria, cold extremities and altered mental status indicate hemodynamic compromise and require intensive monitoring. A nondilated ventricular chamber with walls apparently thickened by edema and severe dysfunction suggests an acute form, whereas dilation and wall thinning point toward a more prolonged process; neither appearance alone defines the histology.

The fulminant presentation is a clinical syndrome, not an exclusive histological subtype; lymphocytic myocarditis can cause severe shock and, if the patient survives the critical phase with adequate support, may show substantial recovery. Tissue is nevertheless indispensable when giant cells, eosinophils, sarcoidosis or another specific cause would immediately change treatment. Mechanical support should not be delayed while awaiting the pathology report when perfusion and organ function are deteriorating.

Palpitations, ectopy, ventricular tachycardias, conduction blocks and syncope may precede or accompany dysfunction; active inflammation changes conduction and refractoriness, while residual fibrosis creates re-entry circuits that persist after troponin normalizes. Syncope without prodromes, sustained tachycardia and advanced block define a high-risk presentation and should prompt reconsideration of histological and genetic differential diagnoses even when ejection fraction is preserved.

Pericardial involvement creates a continuum between myopericarditis with preserved function and perimyocarditis with predominant myocardial injury; a small effusion does not measure the extent of myocarditis and typically pericarditic pain does not guarantee a benign course. Troponin, function, arrhythmias and CMR define the weight of the myocardial component, while biopsy is reserved for phenotypes in which the result can modify treatment.

In children, tachypnea, feeding difficulties, vomiting, lethargy and reduced perfusion may be mistaken for sepsis or respiratory disease. Adolescents more often present with pain and troponin elevation, whereas neonates may deteriorate rapidly; in older adults, symptoms and inflammatory markers may be attenuated and ischemic comorbidities complicate interpretation. The threshold for investigations therefore depends on clinical risk rather than on the presence of a complete textbook picture.

Recurrence presents with new pain, troponin elevation, arrhythmias or dysfunction after a phase of remission. Before being labeled idiopathic, autoimmunity, repeated exposures, immunotherapy and genetic cardiomyopathy should be reconsidered. Comparing ECGs, biomarkers and images from different episodes helps distinguish a new inflammatory phase from persistent scar or a noninflammatory cause; automatically repeating the previous diagnosis may conceal a different mechanism.

Investigations and Diagnosis

The electrocardiogram may show ST elevation or depression, T-wave inversion, low voltages, ectopy, tachycardias and conduction disturbances, but it may also be only mildly abnormal. A nonterritorial distribution favors an inflammatory cause without excluding ischemia. Serial ECGs and telemetry document dynamics and instability; a normal initial tracing does not eliminate the possibility of evolution over the following hours.

Troponin confirms myocardial injury but not its cause or the amount of tissue involved; kinetics, time from onset and renal function influence the value, and a normal troponin does not exclude focal or late disease. Natriuretic peptides reflect hemodynamic stress; complete blood count, C-reactive protein, organ function and electrolytes define severity and etiologic clues. Indiscriminate viral serology panels have poor ability to identify the myocardial agent and should not replace targeted investigation.

Echocardiography assesses global and regional function, wall thickness, effusion, valves, pressures and the right ventricle; longitudinal strain may reveal subtle dysfunction despite normal ejection fraction, but it is not specific for myocarditis. Repeated imaging is particularly useful for recognizing a rapidly changing trajectory, guiding shock management and documenting reverse remodeling; normal initial findings do not exclude injury detectable on CMR.

CMR combines edema, hyperemia, cellular injury and scar through T1 and T2 mapping, extracellular volume and LGE; the typical pattern is subepicardial or intramyocardial, often inferolateral, but septal, diffuse and atypical distributions are possible. The updated Lake Louise criteria increase diagnostic accuracy by requiring T1-based and T2-based evidence in the appropriate context; sensitivity and specificity decrease if the study is delayed, disease is focal or the protocol is inadequate.

CMR supports a clinical diagnosis of myocarditis, but it cannot label an infiltrate as lymphocytic because it does not visualize histology; it does not reliably distinguish persistent virus, autoimmunity and immunotherapy-related disease and may be negative in high-risk forms. In shock, advanced block, malignant arrhythmias, rapidly progressive heart failure or lack of response to treatment, the need for tissue should not be negated by a nondiagnostic imaging result.

Endomyocardial biopsy is performed when identifying the histological subtype, activity or agent can change management; multiple samples from different sites increase sensitivity, and CMR, PET or electroanatomical mapping can guide sampling in focal lesions. Part of the tissue is fixed for histology and immunohistochemistry, while another part is preserved under sterile protocols for molecular analyses; late planning can make reliable testing for infectious genomes impossible.

Hematoxylin and eosin evaluation assesses lymphocyte density and distribution, relationship to cardiomyocytes, necrosis, edema, vasculitis and other cell populations; forceps artifact, ischemia and nonspecific perivascular infiltrates must be recognized. Immunohistochemistry identifies pan-leukocyte, T-cell, B-cell and macrophage markers and may document endothelial activation or HLA expression; historically used quantitative thresholds increase sensitivity but should not be separated from the morphological quality of the injury.

According to the Dallas criteria, active myocarditis combines inflammatory infiltrate with nonischemic myocyte injury, whereas the former borderline category described an infiltrate without unequivocal necrosis. Limited reproducibility and sensitivity led to variable use of the term. The Seaport criteria instead propose, for endomyocardial biopsy, three grades of severity for lymphocytic myocarditis and a distinct category of scattered increase in T lymphocytes of indeterminate significance (SITUS).

The Seaport distinction accounts for the different material available: biopsy provides few fragments and is highly affected by sampling, whereas surgical and autopsy specimens allow extent and distribution to be assessed over larger areas. The report should indicate which system was applied and describe the findings rather than being limited to a label; an infiltrate of uncertain significance may gain weight in a coherent clinical phenotype, but it does not automatically justify immunosuppression.

Tissue PCR requires controls and an experienced laboratory. Detection of a genome does not prove replication or causality, especially for viruses capable of persisting in nonmyocyte compartments; conversely, a negative result is not absolute if the sample is small or obtained late. Histology, viral load, transcriptional activity when available, cellular localization and the systemic picture together define significance; discordance between tissue and clinical findings should prompt review of slides and the preanalytical pathway.

The differential diagnosis includes myocardial infarction with nonobstructive coronary arteries, Takotsubo syndrome, genetic cardiomyopathies, sarcoidosis, giant cell myocarditis, eosinophilic myocarditis and toxic injury; a lymphocytic infiltrate can appear adjacent to ischemic necrosis, making lesion distribution and morphology decisive. Multidisciplinary review helps identify when the lymphocytic pattern is the primary disease, a secondary response or only one component of a more complex process.

Treatment and Prognosis

Initial treatment depends on the phenotype, does not necessarily await histological confirmation and includes rest, restriction of exertion, monitoring and management of complications. In stable heart failure, recommended therapies for ventricular dysfunction are applied when hemodynamically tolerated; in shock, diuretics, vasopressors, inotropes and mechanical support are selected according to the hemodynamic profile. Beta-blockers and other drugs that reduce cardiac output are not forced during acute instability.

Intense physical activity increases adrenergic and mechanical stress on an electrically unstable myocardium. Temporary withdrawal from sports is maintained until symptoms, troponin, function and arrhythmias have resolved or stabilized, with reassessment by exercise testing and rhythm monitoring when appropriate. Duration is not an automatic fixed number for everyone: LGE extent, etiology, recurrence and type of activity modify the degree of caution required.

There is no justification for administering corticosteroids to every patient with lymphocytic myocarditis. The Myocarditis Treatment Trial did not demonstrate a general benefit from immunosuppressive therapy in historical populations in which viral and immunopathological characterization was limited; systematic reviews confirm that the evidence does not support routine use. The decision should distinguish self-limited acute disease, active infection and persistent immune disease, avoiding both indiscriminate treatment and rejection of indicated therapy.

In chronic lymphocytic inflammatory cardiomyopathy that is biopsy-documented and virus-negative, the TIMIC study showed functional improvement with prednisone and azathioprine compared with control, confirmed in prolonged follow-up of the cohort. These results apply to selected patients with persistent dysfunction and tissue inflammation, not to every episode of pain with a positive CMR or every small infiltrate; PCR quality, exclusion of specific causes and center expertise are conditions for translating the evidence into practice.

Checkpoint inhibitor myocarditis requires discontinuation of immunotherapy and prompt immunosuppression according to severity, often with high-dose corticosteroids and additional strategies in refractory cases. Forms associated with autoimmune diseases follow the mechanism and systemic activity, whereas an identified infection may require specific antimicrobial treatment; the term lymphocytic does not flatten these differences: etiologic therapy arises from integration of histological subtype, exposure and microbiology.

Arrhythmias and conduction blocks are treated according to risk and reversibility. A wearable defibrillator may be considered in selected situations during a potentially recoverable phase, whereas permanent implantation is assessed after stabilization when possible; sustained arrhythmias, extensive scar or a genetic substrate may require earlier decisions. Recovery of ejection fraction does not automatically negate an indication if major ventricular events have already occurred.

Prognosis is favorable in many patients with an infarct-like presentation, preserved function and no arrhythmias, but it cannot be inferred from the histological subtype alone. Shock, biventricular dysfunction, advanced block, sustained tachycardia and failure of biomarkers to decline identify a more complex course. Initial recovery in fulminant lymphocytic forms can be substantial, whereas nonfulminant disease with prolonged remodeling may leave chronic dysfunction.

On CMR, the extent and location of LGE add prognostic information. Septal involvement or persistent scar is associated with higher risk than complete absence of residual injury, although these findings do not determine a decision by themselves. Biventricular function, arrhythmias, genotype, recurrences and temporal course complete the assessment; repeating imaging after the acute phase helps distinguish reversible edema from stable fibrosis.

Follow-up includes clinical review, ECG, echocardiography, biomarkers and rhythm monitoring, tailored to severity; a new troponin elevation, syncope, worsening function or arrhythmias require etiologic reassessment and sometimes a new biopsy. The patient is not considered recovered on the basis of pain relief alone, because clinical, biological, mechanical and electrical recovery proceed on different timelines.

Complications

Cardiogenic shock is the most dramatic acute complication and results from the combination of contractile depression, edema, arrhythmias and myocyte loss; prolonged hypotension causes renal, hepatic and neurological failure and reduces the possibility of recovery even when inflammation is reversible. Early identification of reduced pulse pressure, rising lactate, oliguria and cold extremities allows transfer and support before multiorgan collapse.

Persistent heart failure can progress to dilation, functional mitral regurgitation and chronic inflammatory cardiomyopathy; progression may continue even after the infiltrate has diminished, because scar, wall stress and neurohormonal activation become autonomous. Heart failure therapy is therefore not automatically stopped when troponin normalizes; any de-escalation requires prolonged stability and specialist assessment.

Ventricular arrhythmias may emerge during the edematous phase or months later, when LGE represents scar. Frequent ectopy, nonsustained tachycardia and abnormalities during exercise help define risk, but ventricular fibrillation and sustained tachycardia are major events regardless of ejection fraction. Ablation can control selected scar-related circuits without treating any underlying inflammatory activity.

Conduction disturbances reflect edema or septal injury and may resolve, persist or progress. Advanced block requires temporary or permanent pacing according to stability and recovery, but should also prompt exclusion of giant cell disease, sarcoidosis and checkpoint inhibitor toxicity. Labeling the condition as lymphocytic on the basis of CMR alone can be dangerous when the phenotype suggests a more aggressive histological subtype that has not yet been sampled.

Recurrence produces repeated episodes of necrosis and accumulation of fibrosis, with progressive increase in arrhythmic risk even if each individual attack appears mild. Autoimmunity, drug re-exposure and genetic cardiomyopathies should be investigated in recurrent cases. Family counseling and genetic testing become particularly relevant when sudden deaths, cardiomyopathy, a typical scar pattern or similar episodes in relatives coexist.

Severe dysfunction promotes stasis, ventricular thrombi and embolism, while atrial fibrillation adds a separate risk. Contrast echocardiography or CMR can identify thrombi not visible on standard imaging; anticoagulation is guided by thrombus, arrhythmia and individual risk and is not prescribed routinely for the lymphocytic infiltrate alone. Bleeding, renal function and biopsy procedures must be coordinated within the plan.

Immunosuppression increases the risk of opportunistic infections, cytopenias, hepatic and metabolic toxicity, while failure to exclude a pathogen may allow it to progress. Screening, appropriate vaccinations, prophylaxis and laboratory monitoring are part of therapy and should be proportionate to the drugs and duration; deterioration during treatment is not always immune refractoriness: it may reflect infection, toxicity, arrhythmia or an incomplete histological diagnosis.

The most important diagnostic complication is confusing an uncertain infiltrate with active myocarditis or, conversely, considering a biopsy negative when it failed to sample the lesion. In the first case the patient receives risky therapy without a demonstrated target; in the second, a therapeutic window is lost. Specialist review, sample adequacy, correlation with imaging and willingness to repeat or redirect sampling are essential tools for limiting both errors.