Diphtheritic myocarditis

Article written and reviewed by Dr Luca Moretti

Diphtheritic myocarditis is a systemic complication of toxigenic diphtheria and represents one of the clearest models of toxin-mediated cardiac injury. The bacterium generally remains at the respiratory or cutaneous site, while the toxin enters the circulation, binds to cells and interrupts essential protein-synthesis processes. The heart may therefore deteriorate when the pseudomembrane appears to be improving and cultures are becoming negative.

Corynebacterium diphtheriae is the classic cause, but toxigenic strains of other related species may produce similar syndromes; the ability to cause systemic disease depends on the presence and expression of the toxin gene, not simply on isolation of the species. Toxoid vaccination protects against toxic effects, although it does not absolutely prevent colonization, and declining coverage creates populations susceptible to outbreaks.

The frequency of myocarditis varies with definition, severity and early access to antitoxin; clinically evident forms are associated with substantial mortality, especially when complex conduction block, ventricular tachycardia or shock occurs. Series from outbreaks and intensive care units overrepresent the most severe cases, but consistently identify progressive electrocardiographic abnormalities as an adverse prognostic sign.

The most effective intervention against cardiac injury is early neutralization of toxin that is still free in the circulation. Antitoxin does not detach toxin that is already bound to the myocardium, so clinically suspected respiratory diphtheria is treated without waiting for microbiological confirmation; antibiotics, isolation and airway management complete the intervention but do not replace antitoxin.

The heart is protected mainly before myocarditis becomes clinically evident. Once toxin has already been internalized, antitoxin and antibiotics prevent further exposure but cannot immediately restore protein synthesis in injured cells. Prompt treatment of the pharyngeal infection and subsequent monitoring are therefore parts of the same strategy, not independent phases.

Toxigenicity is a property of the strain and cannot be inferred from local severity, while the amount absorbed depends on the extent and duration of ongoing toxin production. A patient with an apparently limited lesion may already have absorbed toxin and requires systemic assessment, whereas isolation of a nontoxigenic strain calls for a different public-health pathway and does not explain toxin-mediated cardiac disease.

Etiology, Pathogenesis and Pathophysiology

Diphtheria toxin is encoded by a bacteriophage and is produced by lysogenized strains under favorable conditions. After binding to its cellular receptor, the active subunit enters the cytoplasm and ADP-ribosylates elongation factor 2, interrupting protein synthesis. Cells with high metabolic requirements, such as cardiomyocytes and neurons, are particularly vulnerable.

Within the myocardium, degeneration, necrosis, edema, a variable infiltrate and subsequent fibrosis are observed, with a diffuse rather than coronary distribution. Mitochondria and fatty-acid metabolism are impaired, reducing energy production and contractility; injury to the conduction system explains a sequence of ECG abnormalities that may precede echocardiographic dysfunction.

The amount of toxin absorbed increases with the extent of the pseudomembrane, duration without treatment and respiratory localization; extensive pharyngeal or laryngeal disease therefore represents a greater risk than limited colonization. Equine antitoxin neutralizes the circulating fraction, and its benefit declines with delay because an increasing proportion has already been internalized.

Antibiotics stop bacterial replication, reduce further toxin production and eliminate transmission, but do not neutralize existing toxin. This distinction explains why rapid culture conversion to negative does not guarantee cardiac protection; likewise, antitoxin does not eradicate the bacterium and must be combined with an appropriate antimicrobial regimen.

Autonomic neuropathy may alter heart rate and blood pressure and add to direct injury; paralysis of the palate, cranial nerves and respiratory muscles appears at different times and may complicate ventilation and weaning. A bradycardia is therefore not necessarily only nodal in origin, and assessment should include the neurologic context.

Cardiac dysfunction results from cell loss, inefficient metabolism and arrhythmias, while hypoxia from airway obstruction and systemic infection increase the workload. High catecholamine levels may worsen electrical instability in a toxin-injured myocardium; support should achieve perfusion with the least proarrhythmic stimulation compatible with survival.

Cardiac severity is not a simple function of the troponin level; progressive conduction disease may reflect injury at a strategically important site despite modest biomarker elevation, while diffuse injury may produce a marked rise. The trajectory of ECG findings, function and perfusion provides a more robust assessment than a single value.

Severity depends on toxin dose, duration of absorption and host susceptibility, so an extensive pseudomembrane and treatment delay increase risk without predicting it perfectly. A partially immunized person may have attenuated local disease but should not be considered protected in the absence of documentation; vaccination history is checked in registries whenever possible rather than relying on memory alone.

Blockade of protein synthesis progressively impairs enzymatic systems and membranes, which is why function may worsen after free toxin has been neutralized. This biological latency should be explained to the family, who might otherwise interpret deterioration as failure or error of antitoxin therapy; early treatment limits further injury, but tissue already exposed requires time to recover or progresses to necrosis.

Clinical Manifestations

Respiratory diphtheria begins with sore throat, often low-grade fever, malaise, cervical lymphadenopathy and an adherent grayish pseudomembrane that bleeds if removed. Neck edema and laryngeal extension indicate severity and airway threat; specimen collection should neither manipulate the membrane aggressively nor delay treatment and isolation.

Cardiac injury often appears after the first week, but may occur early in severe toxemia or present later. Disproportionate tachycardia, a gallop rhythm, hypotension, hepatomegaly and dyspnea sometimes precede overt congestion; improvement in pharyngeal symptoms does not reduce the need for monitoring during the risk window.

ECG may show ST-T abnormalities, PR prolongation, bundle branch block and atrioventricular block; progression from minor disturbances to complex block identifies more extensive injury and a worse prognosis. Rhythm may change rapidly and requires telemetry in a setting capable of pacing and resuscitation.

Ventricular tachycardia, fibrillation, idioventricular rhythms and dissociation may complicate severe disease; pacing protects against bradycardia but not against tachyarrhythmias or diffuse dysfunction. Coexistence of multiple electrical abnormalities is particularly unfavorable and requires intensive management.

Heart failure includes low output, congestion, biventricular dysfunction and shock; ejection fraction may deteriorate after the first ECG abnormalities, and strain may detect earlier impairment. Ventilation, airway status and respiratory neuropathy influence hemodynamics and must be coordinated with cardiac treatment.

Myocarditis may be accompanied by renal failure, thrombocytopenia, neuropathy and secondary infections; in children, irritability, reduced feeding and signs of shock may precede precise symptom descriptions. Care in a pediatric intensive care center is indicated when there is extensive pharyngeal disease or early cardiac involvement.

Cutaneous diphtheria generally causes less toxin absorption, but toxigenic strains and extensive lesions require individual assessment. An asymptomatic carrier does not have the same likelihood of myocarditis as a patient with a pseudomembrane, but remains relevant for transmission; clinical and microbiological classification guides antitoxin, isolation and contact management.

The cardiac course may be biphasic, with initial nonspecific abnormalities followed by more complex block and arrhythmias while the respiratory focus regresses. This interval creates false reassurance if surveillance and rest are stopped too early; serial ECGs and biomarkers should therefore be scheduled according to disease extent and toxemia, not solely according to the cardiac symptoms present on a given day.

Tachycardia disproportionate to fever may precede structural signs and is recorded together with rhythm, perfusion and biomarkers. Subsequent bradycardia does not necessarily represent improvement, because it may indicate progression of conduction-system injury; interpretation of heart rate therefore requires the complete trajectory rather than comparison with a single normal value.

Investigations and Diagnosis

The diagnosis of respiratory diphtheria is initially clinical; nasal, pharyngeal and pseudomembrane swabs are collected for culture before antibiotics if possible, notifying the laboratory so that appropriate media and procedures are used. Identification of C. diphtheriae should be completed by demonstrating toxin production through a phenotypic test or methods coordinated with reference laboratories.

PCR for the tox gene may accelerate suspicion but does not necessarily prove toxin expression; culture and toxigenicity testing remain important for confirmation and public health. The case is reported immediately and managed with public-health authorities without waiting for the result; absence of a positive culture after antibiotics does not invalidate a compatible clinical presentation.

Baseline and serial ECGs, telemetry, troponin, CK-MB, natriuretic peptides, electrolytes, renal and liver function monitor the heart. Testing frequency increases with toxemia, disease extent and abnormalities; a normal initial ECG does not exclude a late complication and does not justify shortened surveillance in severe cases.

Echocardiography assesses biventricular function, strain, effusion and hemodynamic profile; repeated examinations are more informative than a single ejection fraction. CMR and biopsy are not priorities in typical, already established diphtheria because the mechanism is known and the patient may be unstable or contagious.

The differential diagnosis of a pseudomembrane includes mononucleosis, streptococcal infection, candidiasis, Vincent angina, and traumatic or neoplastic causes; removal may cause bleeding and obstruction. The cardiac differential diagnosis includes sepsis, hypoxia, electrolyte disturbances and other forms of myocarditis, but the temporal association with toxigenic diphtheria makes the mechanism highly plausible.

Prognostic monitoring is based on electrical as well as hemodynamic evolution; progressive conduction delay, bifascicular or complete block, and tachyarrhythmias indicate high risk even if blood pressure is still preserved. Transferring the patient only after shock develops means losing a window for preparation for pacing and support.

Assessment of contacts includes symptoms, cultures according to protocol, antibiotic prophylaxis and vaccination status. Eradication of the toxigenic strain is documented according to public-health guidance; the patient also requires vaccination or completion of the series because natural infection does not guarantee protective antitoxin levels.

The laboratory should be notified of the suspicion before receiving the specimen, because identification, preservation and referral to a reference center require dedicated procedures. Demonstration of the tox gene does not guarantee toxin expression, while phenotypic testing completes classification; these distinctions serve public health but should not delay antitoxin in a compatible respiratory presentation.

Airway management takes precedence over nonurgent cardiac procedures when pseudomembrane and edema threaten obstruction, but intubation and sedation alter hemodynamics and may destabilize a vulnerable heart. Anesthesiology, otolaryngology, intensive care and cardiology jointly plan access, medications and pacing availability; an uncoordinated procedure may turn two manageable risks into a simultaneous crisis.

Treatment and Prognosis

Diphtheria antitoxin is administered as soon as possible in suspected respiratory diphtheria, after the assessment and precautions required for an equine product, without waiting for culture or toxigenicity results. Dose and method of administration follow severity, extent and duration according to WHO guidance and national procedures; availability may require urgent coordination with authorities and reference centers.

An effective antibiotic, generally an appropriate macrolide in contemporary WHO guidance according to context and susceptibility, eradicates the bacterium, stops further production and reduces transmission. The regimen is adapted to resistance, age and tolerance and followed by microbiological verification when required; antibiotic and antitoxin are not alternatives because they act on different components.

Isolation with appropriate precautions, strict rest, airway management and nutrition are essential. Forced removal of the pseudomembrane is avoided; edema and obstruction may require intubation or tracheostomy by an experienced team. Early exertion increases cardiac demand and is prevented during the vulnerable phase.

Myocarditis requires intensive monitoring, cautious fluid balance and support of perfusion; inotropes and vasopressors are titrated carefully because of proarrhythmic effects, while ventilation corrects hypoxia and respiratory work. The effectiveness of mechanical support in extreme cases is documented mainly by limited experience, and selection depends on reversibility and available resources.

Conduction block with instability may require temporary pacing, preferably prepared before predictable deterioration. Observational studies have suggested possible benefit in selected patients, but pacing does not correct diffuse necrosis and does not guarantee survival when tachyarrhythmias and shock occur. Permanent implantation is considered only after the trajectory and recovery are known.

Prognosis worsens with delayed antitoxin, extensive pseudomembrane, complex conduction block, ventricular tachycardia and dysfunction; patients who survive the toxic phase may recover function and conduction over weeks or months. Sufficiently prolonged monitoring is necessary because electrical recovery may be slower than clinical improvement suggests.

After the acute phase, rehabilitation and resumption of activity are gradual and based on ECG, function and capacity; vaccination is completed before or after discharge according to clinical condition and schedule. Contacts and exposed healthcare workers follow public-health guidance, because treatment of the individual case alone does not interrupt a chain of transmission.

Transfer to intensive care is anticipated when progressively longer PR intervals, bundle branch block, complex ectopy or signs of low output appear, because instability may develop rapidly. Preparing access, defibrillation and pacing expertise before shock reduces response time; the device remains support for conduction and is not treatment for the toxin or diffuse necrosis.

Rest is maintained beyond improvement of throat symptoms because adrenergic demand and exertion may aggravate a still unstable cardiac function. Mobilization and rehabilitation begin only after a favorable electrical and contractile trajectory, with slow progression in patients who have had block or heart failure. Early discharge without a monitoring plan may miss complications belonging to the late phase of toxemia.

Contact prophylaxis does not replace observation for symptoms and verification of vaccination status, because an antibiotic may prevent or eradicate colonization without correcting future susceptibility to toxin. Close contacts and exposed healthcare workers undergo cultures, prophylaxis and boosters according to public-health authorities; the index case remains isolated until the required microbiological criteria are met.

Complications

Complete heart block and asystole may occur after apparently slow ECG progression or suddenly. Availability of pacing is essential, but stimulation of a severely necrotic myocardium may fail to produce effective contraction; perfusion and rhythm must be assessed simultaneously.

Ventricular tachycardia and fibrillation are associated with a very poor prognosis and may emerge after bradycardic phases. Antiarrhythmic drugs, defibrillation and electrolyte correction address the event while already bound toxin continues to cause injury; drugs that further depress conduction require particular caution.

Cardiogenic shock progresses to renal, hepatic and cerebral failure, superimposed on hypoxia from obstruction and infection. Excessive fluids and catecholamines may worsen congestion and arrhythmias; serial echocardiography and perfusion indicators guide dynamic support.

Neuropathy may cause palatal paralysis, cranial nerve deficits, peripheral weakness and respiratory failure weeks after pharyngitis. Cardiac recovery therefore does not eliminate the need for neurologic surveillance; aspiration and prolonged ventilation increase the risk of pneumonia and intensive-care complications.

Equine antitoxin may cause immediate anaphylaxis or delayed serum sickness and should be administered in a setting prepared to recognize and treat them. The risk of reaction is balanced against the lethality of the toxin and does not justify inappropriate delay; guidance defines preparation, administration and monitoring.

Delayed diagnosis exposes contacts and personnel and may generate an outbreak; isolation, notification, contact tracing, prophylaxis and vaccination are therefore part of preventable complications. An initially negative swab does not terminate these measures if clinical probability remains high.

Residual fibrosis, conduction block and dysfunction may persist and require a device or heart failure therapy, but the need for permanent treatment is assessed after an adequate period. Cardiology and infectious-disease follow-up should document both functional recovery and eradication; vaccine prevention remains the most effective strategy for avoiding renewed toxin exposure.

Serum sickness may appear after several days with fever, rash, arthralgia and systemic abnormalities, overlapping with convalescence and raising concern for renewed toxemia. Chronology, cultures, ECG and allergy assessment distinguish the processes, while any treatment is adapted to severity without abandoning cardiac surveillance; this possible reaction is explained before discharge so that the patient neither ignores symptoms nor independently stops other therapies.

Respiratory and neurologic rehabilitation may continue for months after cardiac function has recovered, with the need to prevent aspiration, falls and deconditioning. A unified program coordinates Holter or ECG monitoring, physiotherapy, swallowing assessment and vaccine boosters, avoiding a situation in which each specialist considers their phase complete while another toxin-mediated consequence persists. In children, school and physical activity are resumed gradually according to shared recommendations.

Every case is a signal of vulnerability in the vaccination system and requires review of community coverage, access to antitoxin and notification times. Prevention of myocarditis does not depend on mass cardiac screening, but on preventing toxin exposure through immunization, early diagnosis and treatment of the infectious focus. Maintaining stocks, protocols and contact pathways reduces delays when the disease is rare and clinicians have little direct experience.

In resource-limited settings, frequent ECGs, structured clinical observation and early transfer of patients with signs of toxemia may provide benefit even when CMR and advanced support are unavailable. Rarity should not generate overly complex protocols that delay antitoxin and antibiotics; priorities, referral pathways and product availability are defined before outbreaks and adapted to local capacity.