Eosinophilic myocarditis: causes, symptoms, diagnosis and treatment

The British Heart Foundation describes myocarditis as inflammation of the heart muscle, which is most commonly caused by a virus. In most people myocarditis settles with rest and treatment, but in rare cases severe inflammation can scar the muscle, leaving the heart enlarged and weaker, or can disturb its electrical system and cause irregular heartbeats.

Eosinophilic myocarditis is a rare subtype. A published concise review describes it as characterised by eosinophilic infiltration into cardiac tissue leading to inflammation and cardiac dysfunction. Eosinophils are white blood cells that normally help fight parasites and take part in allergic reactions. When large numbers collect in the heart they release substances that damage the muscle. The review describes three overlapping stages: an acute necrosis stage, a thrombosis stage in which the blood becomes more prone to clotting, and a final fibrotic stage with scar tissue formation that can involve the heart valves.

The review explains that the cause is often not found. A recent systematic review it cites found that the most common category was idiopathic (28.8 percent), followed by eosinophilic granulomatosis with polyangiitis (19.3 percent), drug-induced hypersensitivity (13.1 percent) and hypereosinophilic syndrome (12.8 percent), with the remainder linked to parasitic infection, cancer and vaccine reactions.

Eosinophilic granulomatosis with polyangiitis (EGPA) is a rare vasculitis that affects small and medium blood vessels, and the review says cardiac involvement occurs in about 60 percent of patients. Hypereosinophilic syndrome (HES) is defined in the review by an absolute eosinophil count above 1.5 x 10 to the 9 per litre for more than six months with evidence of organ damage. Drug triggers listed include certain antibiotics (minocycline and beta-lactams), clozapine, carbamazepine, indomethacin and diuretics. Very rare reports followed some vaccines, including mRNA COVID-19 vaccination, mostly in young males after a second dose.

These percentages come from case-based reviews, not population studies, and they should be read as a rough guide to the range of causes rather than the risk of any single person.

The BHF lists the common symptoms of myocarditis as chest pain or tightness, shortness of breath, unusual tiredness, palpitations, an irregular heartbeat, feeling light-headed or fainting, flu-like symptoms and, rarely, swelling of the hands, legs, ankles or feet. Some people have no symptoms and the condition is picked up at a routine appointment.

The eosinophilic review adds features that can point towards the eosinophilic form: other allergic or atopic problems such as asthma, chronic sinus disease and nasal polyps, and skin involvement, particularly with EGPA or HES. It also notes that even people without these features or without a raised eosinophil count can have eosinophilic cardiac infiltration, which is why diagnosis can be delayed.

The BHF says you should call 999 immediately if chest pain makes your chest feel tight or heavy, lasts more than 15 minutes, spreads to the arms, back, neck or jaw, or comes with shortness of breath or feeling sick. If you have symptoms of myocarditis, contact your doctor straight away or call NHS 111.

The BHF says doctors will examine you and ask about recent illnesses, a history of autoimmune problems, exposure to toxins or drugs and other risk factors, and that tests may include an ECG, an echocardiogram, a chest X-ray and blood tests, with cardiac MRI and coronary angiogram used to rule out other common cardiac causes. In very rare cases a heart biopsy may be considered.

In eosinophilic myocarditis the review explains that blood tests can show raised troponin, natriuretic peptide and inflammatory markers, often with a high eosinophil count, though not always. Echocardiography can show reduced pumping, fluid around the heart, clots in the ventricles or, in the later fibrotic stage, a stiff restrictive pattern. Cardiac MRI can show a characteristic subendocardial pattern of enhancement and has become a useful non-invasive tool, but no large study has validated MRI criteria on their own. The gold standard remains endomyocardial biopsy, which shows the eosinophils in the tissue.

Because the underlying cause shapes treatment, the work-up often also looks for EGPA, hypereosinophilic syndrome, drug exposure, parasite infection and blood disorders, which may involve tests such as ANCA antibodies and specialist haematology or rheumatology review.

The review states that there is no consensus on any eosinophilic myocarditis-specific therapy because of the lack of large-scale trials, that treatment is chosen according to the underlying cause if known and that steroids are often used in the acute setting for their immunosuppressant effect. Observational studies have reported clinical benefit from biologic drugs targeting interleukin-5 or its receptor (mepolizumab and benralizumab) in refractory disease and in people who depend on corticosteroids.

Supportive heart care, such as medicines for heart failure or rhythm problems and measures to prevent clots, is part of treatment where needed. The BHF describes general myocarditis treatment as depending on symptoms and cause, and including painkillers, medicines to calm inflammation, antibiotics if a bacterial infection is the cause and rest. If a drug is the trigger, the implicated medicine is stopped under medical direction; do not stop a prescribed medicine on your own.

The review notes that, although rare, eosinophilic myocarditis has a significant in-hospital mortality rate in published case series, which reflect the sickest patients who reached hospital. Many people respond well, particularly when a trigger is found and removed or the underlying eosinophilic disease is controlled, but the outcome varies with the cause and with how much scarring has occurred.

The BHF says that most people with myocarditis will have one episode and get better with rest, medicine and avoiding high-intensity exercise while their heart recovers, with check-ups that may be yearly, or at least every few months if the heart has been damaged. Questions it suggests asking include what the follow-up plan is, and how and when you can exercise and return to work. Heart muscle scarring can lead to dilated cardiomyopathy or heart failure, so follow-up appointments matter even when you feel well.

Suspected myocarditis is assessed urgently in NHS hospitals, usually by cardiology with input from rheumatology, haematology or respiratory teams for the underlying cause. Because the condition is rare, care often involves a specialist centre. A private cardiologist can arrange scans, but the biopsy, immunosuppression and monitoring that eosinophilic disease needs are usually delivered through hospital teams, so make sure results are shared with your GP and treating team.