Hypersensitivity myocarditis

Article written and reviewed by Dr Luca Moretti

Hypersensitivity myocarditis is an immunoallergic reaction of the myocardium, associated with a drug in most cases. Histologically, it shows an interstitial and perivascular infiltrate composed of lymphocytes, macrophages and variable numbers of eosinophils, often with limited necrosis; the condition may be clinically silent, present as an infarct-like syndrome or progress to heart failure and shock.

The term should not be used as a synonym for every eosinophilic myocarditis; myocardial eosinophils also occur in EGPA, clonal or idiopathic hypereosinophilic syndromes, parasitic infections and necrotizing eosinophilic myocarditis. Hypersensitivity is defined by the relationship with the agent and the immunoallergic context, not by the cell count alone.

Numerous drugs have been associated with it, including antimicrobials, anticonvulsants, antipsychotics, antidepressants, anti-inflammatory drugs and other therapies. The long list does not permit reliable prediction and makes it hazardous to attribute causality to the best-known drug; the entire chronology of prescriptions, self-medication, supplements and dose changes must be reconstructed.

Myocarditis may be a manifestation of DRESS, a systemic syndrome characterized by a cutaneous reaction, fever, hematologic abnormalities and visceral involvement. The heart may be affected at onset or weeks after the rash has improved and may sometimes show an aggressive necrotizing form; surveillance therefore does not end with dermatologic discharge.

Early diagnosis has high therapeutic value because discontinuing the culprit removes the stimulus. However, the immune response may continue after dechallenge and require corticosteroids or other immunomodulators; cardiology, allergology, pharmacology, dermatology and intensive care share the management of multiorgan cases.

Etiology, Pathogenesis and Pathophysiology

The drug or one of its metabolites may bind to proteins and create an antigen, interact directly with immune receptors or modify antigen presentation. Specific T lymphocytes release cytokines that recruit eosinophils and other cells; HLA predisposition, metabolism, herpesvirus reactivation in DRESS and concomitant drugs modulate the response.

Eosinophils release major basic protein, cationic protein, peroxidase, leukotrienes and reactive species that injure cardiomyocytes and endothelium. Degranulation and necrosis are not necessarily proportional to the number of circulating eosinophils; a patient may therefore have severe cardiac injury with modest or late peripheral eosinophilia.

In the classic histologic form, the infiltrate is often perivascular and interstitial with limited myocyte necrosis; this explains the incidental finding in some autopsies of drug-exposed patients without a recognized cardiac syndrome. By contrast, the necrotizing form shows extensive necrosis, a dense eosinophilic infiltrate and rapid hemodynamic deterioration.

DRESS typically has a longer latency than immediate reactions, often several weeks, and may worsen despite drug withdrawal. Reactivation of HHV-6 and other herpesviruses is associated with the syndrome, but does not justify attributing myocarditis to a virus without evidence; the immunopathology is dynamic and involves different organs at different times.

Clozapine represents a particularly well-studied drug phenotype, with risk concentrated in the first weeks of titration; eosinophils are not always present and mechanisms may include hypersensitivity, catecholamines and metabolic predisposition. Its therapeutic value in psychiatry requires monitoring protocols that recognize toxicity without discontinuing the drug for isolated benign tachycardia.

Aromatic anticonvulsants, sulfonamides and some antibiotics are classic DRESS triggers, but any attribution must consider the start date and latency. A drug started after the first symptoms cannot be the initial cause, whereas a therapy discontinued weeks earlier may remain plausible; chronology reduces errors more effectively than a memorized list.

A reaction confined to the myocardium may occur without rash or liver injury. Conversely, eosinophilia and rash during an infection or neoplasm do not prove hypersensitivity; the causal model integrates temporal compatibility, known phenotype, alternatives, dechallenge and, when available, histology.

Healing occurs through resolution of the infiltrate, but necrosis and eosinophil-associated thrombi may leave fibrosis and damaged endocardium. In prolonged forms, thrombotic organization and fibrotic endomyocardium lead to restrictive physiology and valvular regurgitation; removal of the trigger does not automatically reverse this structural stage.

The endothelium is not merely a passive target. Eosinophil mediators increase permeability, activate coagulation and promote mural thrombi; microvascular spasm and endothelial injury add ischemia to direct necrosis. This mechanism explains why regional abnormalities or pain may occur with normal epicardial coronary arteries and why the thrombotic phase requires separate assessment.

The dose-response relationship is weak in immunologic reactions; minimal therapeutic doses may cause disease after sensitization, whereas a patient may have tolerated previous courses. Renal and hepatic function remain relevant because they modify metabolites and exposure, but a normal concentration does not exclude hypersensitivity.

Cross-reactivity and chemical structure are assessed within each drug class; a reaction to a beta-lactam or anticonvulsant does not automatically justify prohibiting every related drug, but severe forms require caution. The allergist distinguishes theoretical risk, evidence and clinical need without using dangerous provocation tests.

Clinical Manifestations

Cardiac presentation includes chest pain, dyspnea, palpitations, syncope, tachycardia and heart failure. ECG and troponin may be abnormal while function is still preserved; severe forms may cause hypotension, biventricular congestion, ventricular tachycardia, conduction block and shock.

Fever, morbilliform rash, facial edema, lymphadenopathy and pruritus support a systemic reaction; eosinophilia, atypical lymphocytes, hepatitis, nephritis and pneumonitis complete the DRESS picture. No feature is mandatory, and previous corticosteroid use may attenuate skin manifestations, fever and eosinophilia.

Cardiac injury may appear after weeks of cutaneous symptoms or even during apparent recovery. New tachycardia, pain, dyspnea or syncope in a patient with DRESS requires immediate ECG and troponin testing; attributing fatigue to convalescence may delay recognition of a fulminant form.

The infarct-like form presents with pain, ST-T abnormalities and increased troponin. Allergic vasospasm and Kounis syndrome enter the differential diagnosis but represent a coronary mechanism distinct from myocarditis; CMR and coronary anatomy separate, when possible, interstitial injury from ischemia.

Arrhythmias and conduction block may be disproportionate to global dysfunction if the infiltrate involves the septum; syncope is a warning sign and requires telemetry. A normal initial ECG does not guarantee stability because the infiltrate may progress after the first assessment.

In clozapine-associated myocarditis, tachycardia, fever, malaise and elevated C-reactive protein are common during the first weeks, symptoms that overlap with titration effects and infections. A significant troponin rise or echocardiographic dysfunction makes the picture more specific; eosinophilia may appear after the troponin peak and is not sufficient as a screening test.

Multiorgan severity does not always parallel the rash; an extensive eruption may coexist with an unaffected heart, whereas improving skin may coexist with progressive myocarditis. Liver, kidney, lung, hematologic system and heart are assessed separately and then integrated into the same syndrome.

In children, recognition is complicated by nonspecific symptoms and exposure to anticonvulsants or antibiotics. Persistent tachycardia, reduced feeding and abdominal pain may be signs of low cardiac output; therapeutic dosing and pediatric DRESS require dedicated expertise.

Hypotension may be cardiogenic, distributive because of anaphylaxis or sepsis, or mixed; urticaria, bronchospasm and immediate onset suggest an acute reaction different from late DRESS, but phenotypes may overlap. Epinephrine is lifesaving in anaphylaxis and is used with monitoring even in the presence of myocardial injury, because failure to treat vasoplegia is more dangerous.

The nervous system may be involved indirectly through hypoperfusion, emboli or drugs, while DRESS may rarely be associated with encephalitis. Confusion and weakness are not automatically attributed to corticosteroids; blood-gas analysis, electrolytes, imaging and medication review seek reversible causes that also affect arrhythmic risk.

Investigations and Diagnosis

The medication chronology includes date of initiation, dose, increase, discontinuation, restart and previous exposures. Drugs administered for the first symptoms are also recorded because they may be falsely accused; the relationship is assessed for each substance rather than for the entire regimen as an undifferentiated block.

Serial complete blood count with differential, blood smear, liver and renal function, urinalysis, electrolytes, C-reactive protein and other tests look for DRESS and alternative diagnoses. Peripheral eosinophilia supports but does not confirm cardiac involvement; a normal count, especially early or after corticosteroids, does not allow it to be excluded.

ECG, troponin and natriuretic peptides are repeated according to the course; echocardiography assesses function, strain, effusion and hemodynamic profile. A normal ejection fraction does not exclude injury; rhythm abnormalities and biomarkers may precede dysfunction.

CMR may demonstrate edema and late gadolinium enhancement with variable distribution, including subendocardial patterns, and helps with differential diagnosis, but identifies neither the eosinophil nor the drug; a negative CMR after treatment or in focal disease must be interpreted together with the clinical probability.

Endomyocardial biopsy is the reference test in severe, rapidly progressive, arrhythmic or uncertain cases. Multiple samples increase yield and make it possible to distinguish classic hypersensitivity, necrotizing eosinophilic myocarditis, giant-cell myocarditis, vasculitis and infection; the pathologist must know the medication history, eosinophil count and timing.

DRESS is diagnosed using structured criteria, including RegiSCAR, which integrate fever, lymph nodes, eosinophils, rash, organ involvement and exclusion of alternatives. The score classifies the systemic syndrome but does not prove myocardial involvement, which requires its own clinical, instrumental or histologic evidence.

Parasitic diseases, EGPA, hypereosinophilic syndrome, myeloid neoplasms, infections and other immune-mediated forms are excluded according to phenotype. Extreme persistence of eosinophilia, splenomegaly, cytopenias or blood smear abnormalities require hematologic assessment; asthma and neuropathy increase suspicion of EGPA.

Patch testing, lymphocyte transformation testing and other allergologic investigations have variable availability and accuracy and are not performed during the acute phase as definitive proof. Because drug provocation is contraindicated after a severe reaction, causality remains a documented and graded clinical synthesis.

Troponin is selected and interpreted carefully when skeletal muscle injury, renal failure or sepsis is present; a dynamic curve associated with new ECG abnormalities is more informative than an isolated value. Natriuretic peptides reflect wall stress but also increase with renal dysfunction, age and tachycardia, and do not distinguish hypersensitivity from other mechanisms.

In marked eosinophilia, blood smear, vitamin B12, tryptase, molecular tests and bone marrow studies are selected to exclude a myeloid neoplasm. This distinction is urgent because a tyrosine-kinase inhibitor can transform the prognosis of some clonal forms; a recent prescription should not interrupt the investigation when the hematologic phenotype is atypical.

Infectious testing is guided by travel, food exposure, immune status and organs involved. Eosinophilia with myocarditis may result from parasitic infection; corticosteroids without appropriate coverage can cause Strongyloides hyperinfection. Screening before immunosuppression is particularly important in people from endemic areas.

Treatment and Prognosis

The immediate measure is to stop all plausible nonessential drugs, prioritizing the one with the most compatible chronology and phenotype. In polypharmacy, indiscriminately stopping every treatment may be harmful; lifesaving antimicrobials and anticonvulsants are replaced with safe alternatives through specialist decision-making.

Shock, arrhythmias and conduction block are treated without waiting for histologic certainty; ventilation, vasoactive drugs and circulatory support may sustain the patient while the trigger is removed and immunity is controlled. Drug selection considers QT interval, renal and hepatic function and interactions with DRESS therapy.

Systemic corticosteroids are commonly used for significant cardiac involvement, with high doses or pulse therapy in fulminant forms. Large specific trials do not exist, so intensity and taper derive from severity, response and organ involvement; a slow reduction is often necessary in DRESS to prevent recrudescence.

Other immunomodulators have been described in refractory forms, but before escalation the diagnosis, complete trigger removal, infections and histologic subtype are reassessed. Inflammation that does not respond may represent a necrotizing form, EGPA or a clonal syndrome; repeat biopsy is considered if it would change strategy.

Treatment of heart failure and arrhythmias continues according to guidelines and is adapted to reversibility. A temporary pacemaker may provide protection during septal edema; when possible, permanent device implantation is deferred until recovery can be defined, while intense exercise remains suspended throughout the active phase.

Clozapine is discontinued when myocarditis is probable or definite and the patient receives an alternative psychiatric strategy. In exceptional cases of extreme treatment resistance, re-exposure has been described under intensive monitoring, but this is not routine practice and requires multidisciplinary agreement. Psychiatric benefit does not eliminate the risk of recurrence.

Prognosis is often favorable with early recognition and limited necrosis; delayed withdrawal, late cardiac DRESS, necrotizing disease, arrhythmias, biventricular dysfunction and shock worsen outcome. Clinical recovery may precede resolution of edema, and follow-up must detect scar and recurrence.

Reporting to pharmacovigilance documents the active ingredient, dose, latency, diagnostic criteria, biopsy and outcome. The medical record should specify the suspected molecule and severity, avoiding the vague label “drug allergy”; the patient and clinicians receive written guidance on classes to avoid.

In DRESS, replacement of the drug should minimize cross-reactive structures and metabolic burden on the liver and kidney. Skin improvement is not the only endpoint: temperature, eosinophils, troponin, transaminases, creatinine and respiratory function are followed on their own schedules; a flare during taper may involve an organ different from the one initially affected.

Nutrition and thromboembolic prevention become important during prolonged hospitalization. Edema, catabolism and corticosteroid myopathy slow recovery and ventilation, while immobility and inflammation increase thrombosis; prophylactic anticoagulation is balanced against platelet count, biopsies and bleeding.

Complications

Necrotizing eosinophilic myocarditis can progress to refractory shock and death within a short period; rising troponin, progression of conduction abnormalities and ventricular deterioration require immediate escalation. Early biopsy can transform a generic diagnosis into more aggressive and appropriate therapy.

DRESS may simultaneously involve the liver, kidney, lung, pancreas, thyroid and hematologic system. Multiorgan failure limits drugs and support options, while myocarditis may emerge when other organs seem to be recovering; a post-acute surveillance schedule reduces the risk of unrecognized late presentations.

Arrhythmias and conduction block may persist because of residual fibrosis, so Holter monitoring, echocardiography and CMR guide follow-up, and recovery of function does not automatically eliminate electrical risk. Return to sport requires absence of active disease and rhythm stability.

Intracavitary thrombi and progression to endomyocardial fibrosis are more likely in prolonged eosinophilic forms. Anticoagulation is decided according to the presence of thrombus and overall risk rather than isolated eosinophilia; advanced restrictive physiology may require specialist heart-failure management.

Opportunistic infections, viral reactivations and metabolic complications result from immunosuppression; fever during taper does not always mean DRESS recurrence. Cultures, imaging and organ assessment precede an automatic increase in corticosteroids.

Accidental re-exposure may cause a more rapid reaction; medication reconciliation and alert systems should include brand names and combinations. Entire drug families are not prohibited without evidence of cross-reactivity, because excessive restrictions may compromise future treatment.

The disease leaves psychological and therapeutic consequences, especially when the responsible drug was essential for epilepsy, psychosis or infection. A shared alternative plan reduces interruptions and self-medication; cardiac recovery is more stable when the underlying disease also remains controlled.

After the acute phase, autoimmune thyroiditis, diabetes or other immune sequelae of DRESS may appear; late symptoms are not always cardiac, but endocrine abnormalities may mimic palpitations, fatigue and heart failure. Programmed follow-up prevents normalization of the rash from being interpreted as the definitive end of the syndrome.

The quality of communication directly affects prevention because the patient receives the generic and brand name of the culprit, possible related agents, a description of the reaction and contacts for evaluating alternatives. A photograph of the rash and retained histologic reports preserve information that would otherwise disappear once the reaction resolves.

Pharmacogenomics identifies robust HLA associations for some severe reactions, but there is no universal screening for hypersensitivity myocarditis. A test validated in one population is not automatically transferred to every ancestry; prevention remains based on history, drug exposure, latency and rapid identification of early signs.

Herpesvirus reactivation in DRESS is associated with immune dysregulation and may correlate with a prolonged course. A positive PCR does not prove that the virus directly causes myocarditis and does not automatically lead to antiviral therapy; viral load, involved organs, immune status and evidence of viral disease are interpreted before adding treatment.

Follow-up extends beyond normalization of troponin when DRESS, dysfunction or scar are present. ECG, echocardiography, Holter monitoring and CMR are repeated according to risk, while liver, kidney, blood count and endocrine assessments follow the systemic syndrome; this continuity makes it possible to distinguish recurrence from a sequela and to taper immunosuppression gradually.

Return to work and sport is graded according to function, arrhythmias and scar, not simply disappearance of the rash. Early return may make it difficult to distinguish physiologic palpitations from recurrence, whereas indefinite restriction promotes deconditioning; functional tests and monitoring provide objective criteria for safe progression.