Immune-mediated and toxic forms

Article written and reviewed by Dr Luca Moretti

Immune-mediated and toxic forms comprise myocarditis in which the injury is not attributed primarily to a replicating microorganism, but to loss of tolerance, a drug reaction, iatrogenic lymphocyte activation or toxic exposure. The category is useful for structuring clinical reasoning, but it does not constitute a single diagnosis: mechanism, histology, urgency and therapeutic response differ substantially.

Autoimmune myocarditis may be organ-specific or may accompany lupus, systemic sclerosis, inflammatory myopathies, vasculitides and other systemic diseases. Hypersensitivity myocarditis, by contrast, is an immunoallergic response, frequently drug-related and often eosinophilic; the two conditions may overlap phenotypically, but require different causal reconstruction.

Drug-induced myocarditis is a broader clinical and etiologic category than hypersensitivity: it includes immune activation, direct injury with secondary inflammation and specific syndromes such as that associated with clozapine. Immune checkpoint inhibitor myocarditis represents a distinct model in which cancer therapy removes physiologic brakes on T lymphocytes and can cause rapidly fatal toxicity.

The fundamental problem is to demonstrate that myocardial inflammation exists and establish its cause without confusing association with causation. A patient with autoimmune disease may have an acute coronary syndrome; a person taking multiple drugs may develop a viral infection; during cancer immunotherapy, elevated troponin may result from ischemia, sepsis or myositis. The etiologic label must therefore be built from converging evidence rather than simply presumed.

These forms share a practical consequence: treatment may require discontinuing an essential therapy or suppressing the immune system. Both decisions carry substantial costs and must be based on severity, causal probability and alternatives; collaboration among cardiology, immunology, pharmacology, oncology and pathology is an integral part of safety.

Etiology, Pathogenesis and Pathophysiology

In autoimmunity, cardiac antigens are recognized by lymphocytes and antibodies after loss of central or peripheral tolerance; infection, cellular injury and genetic predisposition can expose previously sequestered epitopes and promote molecular mimicry. The initial response may continue after the trigger has disappeared and transform an acute episode into inflammatory cardiomyopathy.

Systemic diseases involve the heart through multiple mechanisms: small-vessel vasculitis, complement, autoantibodies, eosinophils, thrombosis and interstitial inflammation. Elevated troponin in lupus, for example, may reflect myocarditis, thrombotic ischemia, microangiopathy or renal failure; etiologic classification must describe the dominant mechanism and concomitant alternatives.

Drug hypersensitivity activates T lymphocytes and mediators that recruit eosinophils to perivascular and interstitial spaces; rash, fever and peripheral eosinophilia increase plausibility but are not mandatory. Necrosis may be limited in the classic form, whereas necrotizing eosinophilic myocarditis has a fulminant course and should not be regarded merely as a quantitative variant.

Drug toxicity may be immunologic, metabolic, catecholaminergic, mitochondrial or ischemic; not all cardiotoxicity is myocarditis: anthracyclines, fluoropyrimidines, stimulants and other substances produce different phenotypes. Demonstration of edema or troponin elevation, without a coherent inflammatory context, does not justify a change in terminology.

CTLA-4, PD-1 and PD-L1 inhibitors enhance the antitumor response by removing inhibitory signals; a population of activated lymphocytes may recognize antigens shared by the tumor, myocardium and skeletal muscle. The infiltrate is composed predominantly of T cells and macrophages, may affect the conduction system and respiratory muscles, and can explain the frequent overlap of myocarditis, myositis and myasthenia.

Immune injury is not measured solely by ejection fraction; small septal lesions may cause complete heart block, whereas preserved global function may coexist with lethal arrhythmias. Edema, necrosis, scar distribution and associated organ involvement must be considered separately when estimating risk.

Individual susceptibility includes cardiomyopathy genetics, pre-existing autoimmunity, prior cardiac injury and therapeutic combinations. These factors do not allow the event to be predicted with certainty and do not justify broad exclusion from effective therapies; instead, they are used to modulate baseline assessment and the threshold for surveillance.

Resolution may be complete after trigger removal, or persistent lymphocytes and fibrosis may remain. A drug with a short half-life can initiate a response that continues after discontinuation; conversely, concentration-dependent toxic injury may regress rapidly. The biologic time course of the agent and that of the immune response must be distinguished during follow-up.

Pregnancy is a particular setting: immune physiology, volume changes and pharmacologic limitations modify diagnosis and treatment; peripartum cardiomyopathy, preeclampsia, embolism and autoimmune disease may overlap. The choice among corticosteroids, immunosuppressants and heart-failure therapy must consider the placenta, breastfeeding and maternal stability without delaying lifesaving support.

In children and adolescents, autoinflammatory diseases, drugs, infections and vaccinations enter the differential with probabilities that differ from those in adults. Abdominal pain, reduced feeding or irritability may replace chest pain; biomarker thresholds, cardiac dimensions and treatment tolerability require pediatric reference values.

Clinical Manifestations

Chest pain, dyspnea, palpitations, fatigue, syncope and heart failure are common manifestations; an infarct-like presentation predominates in some acute forms, whereas chronic dysfunction may be insidious. Fever, rash, facial edema, lymphadenopathy, myalgia, weakness or diplopia suggest a systemic process and should not be separated from the cardiac history.

Autoimmune myocarditis may accompany a systemic flare or present with few extracardiac signs; Raynaud phenomenon, arthritis, photosensitivity, ulcers, neuropathy, eosinophilic asthma and proximal weakness guide the diagnostic panel. The absence of autoantibodies does not exclude an organ-specific process, and isolated positivity does not prove cardiac involvement.

Hypersensitivity may begin a few days or many weeks after a drug. Rash, fever and eosinophilia may precede or follow myocardial injury; in DRESS, the liver, kidney, lung and lymph nodes may also be involved. Late onset after withdrawal is biologically possible when immune activation has already become autonomous.

Checkpoint inhibitor myocarditis often appears during the first weeks or doses, although late events have been documented. Arrhythmias, conduction block and elevated troponin may precede severe reduction in function; ptosis, dysphagia, neck weakness or dyspnea disproportionate to congestion signal neuromuscular overlap and risk of respiratory failure.

Fulminant shock requires simultaneous recognition of mechanism and severity; hypotension may result from biventricular dysfunction, vasoplegia, tamponade or immune-mediated endocrinopathy. Echocardiography, lactate and volume assessment guide support while the etiologic diagnosis proceeds.

Arrhythmias include ventricular tachycardia, ventricular fibrillation, atrial fibrillation and conduction block; risk is not proportional solely to ejection fraction, especially in ICI-related forms. Syncope, worsening conduction and a dynamic rise in troponin require intensive monitoring even if the patient initially appears stable.

A subclinical form is sometimes detected during biomarker surveillance; isolated troponin elevation is not enough: exercise, renal failure, sepsis, pulmonary embolism and myositis enter the differential diagnosis. Repeat testing, ECG and clinical assessment determine whether the signal represents noise, noninflammatory injury or early myocarditis.

The temporal trajectory provides valuable information, because improvement after withdrawal supports causality but does not prove it, whereas recurrence after re-exposure is highly informative but is not intentionally provoked in a dangerous disease. Medication history, dosing dates and symptoms are reconstructed precisely.

The cutaneous phenotype provides mechanistic information: immediate urticaria suggests a process different from a late morbilliform rash, vasculitic purpura or DRESS. Skin biopsy and serial photography may support causality when the heart is not biopsied; however, the skin does not determine cardiac severity, and a mild eruption may accompany aggressive myocarditis.

Respiratory function deserves attention when myositis, myasthenia, pneumonitis or corticosteroid-related weakness coexist; dyspnea disproportionate to congestion requires vital-capacity testing, blood-gas analysis and chest imaging. Attributing it to ejection fraction may delay ventilation or treatment of a parallel immune toxicity.

Investigations and Diagnosis

ECG and high-sensitivity troponin are initial tools, followed by natriuretic peptides, complete blood count with differential, renal and hepatic function, electrolytes and inflammatory markers. A normal ECG does not exclude myocarditis; normal troponin lowers the probability of significant necrosis but does not eliminate all forms, and results acquire meaning especially when interpreted as a coherent time series.

Echocardiography assesses biventricular function, strain, effusion, valves and hemodynamic profile; CMR applies criteria for edema and nonischemic injury and provides prognostic information through late gadolinium enhancement. Sensitivity and patterns vary with timing and mechanism, and an early negative CMR does not reliably exclude checkpoint inhibitor myocarditis.

Medication reconstruction includes prescriptions, self-medication, supplements, vaccines, substances and oncologic infusions; dose, titration, latency, dechallenge, previous exposures and alternative causes feed into a structured assessment. Causality algorithms improve transparency, but do not replace clinical judgment or transform probability into histologic certainty.

Antinuclear and extractable nuclear antigen antibodies, complement, ANCA, myositis-specific antibodies and other tests are requested according to phenotype rather than as an indiscriminate panel. Anti-heart autoantibodies may support an immune process in specialist settings but have limited standardization and predictive value; incidental positivity alone does not guide immunosuppression.

Eosinophils, IgE, blood smear, tryptase and hematologic assessment help in eosinophilic forms, whereas CK, aldolase and neuromuscular autoantibodies are relevant in ICI overlap syndromes. Troponin T may be influenced by skeletal-muscle injury; troponin I may improve cardiac specificity in this setting, without being infallible.

Coronary angiography or coronary CT is used when ischemic probability and presentation require it; pulmonary embolism, Takotsubo syndrome, sepsis, tachycardia-induced cardiomyopathy and cancer progression are concrete alternatives. A correct diagnosis may include more than one mechanism, for example atherothrombosis and immune toxicity in the same patient.

Endomyocardial biopsy is decisive in fulminant or arrhythmic presentations, conduction block, lack of response or major therapeutic uncertainty. Histology and immunohistochemistry distinguish lymphocytic or eosinophilic infiltrates, giant cells and necrosis; molecular analyses look for infections before selected immunosuppression. Multiple samples reduce, without eliminating, sampling error caused by focal disease.

The diagnosis is expressed with a level of certainty, avoiding concealment of uncertainty behind a definitive name. This allows urgent therapy to be started when necessary and corrected as new data emerge; documentation is particularly important for pharmacovigilance, oncology and future re-exposure decisions.

FDG-PET may demonstrate inflammatory metabolism, but preparation, glucose, corticosteroids and physiologic uptake affect interpretability. It is more useful for specific questions, distribution and extracardiac sites than as a universal screening tool; reduced uptake after treatment may indicate response without quantifying residual scar.

Tissue analysis includes cell density and phenotype, necrosis, fibrosis, vasculitis and deposits; a biopsy that is “positive for inflammation” is not an etiologic conclusion: eosinophils, lymphocytes and macrophages acquire meaning only when interpreted with drugs, infections and systemic disease. Central review is useful when an irreversible oncologic or immunosuppressive decision depends on a few tissue fragments.

Treatment and Prognosis

The first intervention is to stop the suspected trigger when cardiac risk outweighs the benefit of continuing it. In autoimmune diseases there is no external agent to remove, and systemic activity is controlled instead; withdrawal must not delay support for shock, arrhythmias, conduction block or respiratory failure.

Corticosteroids are used in severe immune-mediated forms at a dose and route proportionate to urgency; response is measured through clinical status, troponin, ECG, function and associated organ involvement, not C-reactive protein alone. Tapering too quickly may promote recurrence, whereas excessive exposure increases infections, myopathy and metabolic complications.

Azathioprine, mycophenolate, cyclosporine, rituximab, cyclophosphamide, abatacept and other immunomodulators have different roles according to etiology and do not constitute a universal therapeutic ladder: histologic subtype, systemic disease, latent infections, organ function and oncologic objective determine the choice, often on evidence from cohorts or consensus rather than large trials.

Heart-failure therapy follows function and stability, while arrhythmias and conduction block receive specific treatment and intense exercise is avoided during the active phase. Temporary mechanical support may allow recovery in reversible fulminant myocarditis, but candidacy takes account of multiorgan prognosis and oncologic disease.

Infection control is part of immunosuppressive therapy; screening for tuberculosis, hepatitis and selected parasites, opportunistic prophylaxis and vaccination are adapted to the regimen. New fever is not automatically attributed to the immune disease, especially after high-dose corticosteroids.

Prognosis is markedly heterogeneous, because hypersensitivity forms recognized early may resolve after withdrawal; severe ICI myocarditis retains a high risk; chronic autoimmune cardiomyopathy may improve with selected immunosuppression but leave scar. Shock, arrhythmias, conduction block, right ventricular dysfunction and treatment delay worsen outcome.

Rechallenge is not used as a diagnostic procedure and, after a mild reaction to an indispensable drug, may be discussed only if benefits, alternatives and monitoring justify it; after severe myocarditis it is generally avoided or considered only exceptionally by a multidisciplinary team. Informed consent includes uncertainty and the possibility of a more rapid recurrence.

Follow-up verifies biomarker normalization, electrical stability, functional recovery and resolution of extracardiac involvement; residual CMR scar may prolong surveillance even in the absence of symptoms. Resumption of physical activity and suspended therapies is decided according to actual risk rather than a single interval for everyone.

De-escalation is planned from induction onward, establishing the objective, reliable biomarkers, minimum duration, prophylaxis and failure criteria. Without a plan, mildly persistent troponin may prolong corticosteroids indefinitely, or early clinical improvement may lead to an excessively rapid taper; treatment is a clinical experiment controlled by predefined measurements.

Rehabilitation considers deconditioning, steroid myopathy, neuropathy and fear of exercise; a graded program begins only after inflammatory and rhythm stability, but is not postponed longer than necessary. Functional capacity is an independent outcome and may remain reduced even after the echocardiogram has normalized.

Complications

Cardiogenic shock and malignant arrhythmias can develop within hours and require a network with intensive care, electrophysiology and circulatory support. The effect of immunotherapy is not immediate, so organ support acts as a biologic bridge; frequent reassessment avoids both delayed escalation and disproportionate procedures.

Atrioventricular block may be transient during edema or permanent after septal necrosis; temporary pacing protects during the unstable phase, whereas a permanent device is evaluated after considering the trajectory and ventricular risk. In some forms, a defibrillator is preferable to a pacemaker alone because of coexisting arrhythmias.

Residual fibrosis sustains ventricular tachycardia, recurrence and cardiomyopathy even when the trigger is no longer present, so recovery of function does not imply zero risk; Holter monitoring, exercise testing and CMR are selected according to the initial presentation and desired activity.

Multiorgan complications include hepatitis, nephritis, pneumonitis, colitis, endocrinopathies, DRESS, myositis and myasthenia. Some require treatments incompatible with heart failure or increase infectious risk; multidisciplinary coordination prevents fragmented prescribing and duplicate immunosuppression.

Opportunistic infections and reactivations can mimic an inflammatory relapse. Rising troponin during fever after immunosuppression requires cultures, imaging and etiologic assessment before corticosteroids are automatically increased; risk rises with combinations, duration and lymphopenia.

Permanent discontinuation of an effective oncologic, psychiatric or immunologic therapy may worsen the underlying disease. This indirect harm must be included in prognosis and in the choice of alternatives; cardiac safety is not pursued by isolating the heart from the overall therapeutic objective.

Because incomplete documentation exposes patients to accidental re-exposure, the active ingredient, class, severity, level of certainty and alternative drugs are reported in the discharge letter and pharmacovigilance systems. A generic “allergy” label without description may both fail to protect and unnecessarily deprive the patient of entire drug classes.

Future decisions are recorded in an accessible document: suspected drug or mechanism, certainty, histologic subtype, immunosuppression dose, infections and criteria for any re-exposure. This clinical memory is essential when the patient changes center or the tumor requires new treatment lines; years later, a generic diagnosis without evidence may become either an unjustified contraindication or inadequate protection.

The prognosis communicated to the patient distinguishes immediate risk, possibility of recovery and causal uncertainty; saying that a form is “immune” does not mean it will last forever, just as stopping the drug does not guarantee immediate resolution. A mechanistic explanation reduces both fear of every future therapy and underestimation of signs of recurrence.

Quality of care depends on measurable timelines: access to ECG and troponin, CMR reporting, availability of biopsy, contact with pathology and initiation of immunotherapy. Audits of severe cases identify delays and alternative diagnoses; a formalized network is particularly useful because no single department sees enough rare forms to maintain every required skill.

Future studies must separate histologic subtypes and severity, use shared definitions and report infections, recurrences and control of the underlying disease. Improvement in ejection fraction does not exhaust relevant outcomes; survival, rhythm, quality of life and the possibility of continuing the responsible therapy are necessary dimensions for judging a strategy.