Kate Hanneman, Matteo Fronza and Paaladinesh Thavendiranathan
A study by UHN researchers, (L to R), Drs. Kate Hanneman, Matteo Fronza and Paaladinesh Thavendiranathan, found patients with myocarditis after COVID-19 vaccination have less severe cardiac injury than those who develop it through other causes. (Photo: UHN)

Vaccine-associated myocarditis shows a similar injury pattern on cardiac MRI compared to other causes of myocarditis, but abnormalities are less severe, according to a new study from UHN researchers.

Myocarditis, or inflammation of the heart muscle, often occurs as a result of a viral infection, including COVID-19. It can affect the heart's rhythm, its ability to pump blood, and may cause lasting damage. Myocarditis has also been reported as a rare complication of the mRNA-based COVID-19 vaccines.

Cardiac MRI is crucial in assessing patients with myocarditis. Understanding the pattern and extent of myocardial injury and its implications will allow for improved care of these patients and may help to address vaccine hesitancy.

Dr. Kate Hanneman, cardiac radiologist, Peter Munk Cardiac Centre (PMCC) and Joint Department Medical Imaging, UHN, set out to determine the pattern of cardiac MRI findings in myocarditis associated with COVID-19 vaccination, and to compare these findings to other causes of myocarditis, including COVID-19.

"We know that the risk of myocarditis following COVID-19 vaccination is very low," explains Dr. Hanneman. "However, there is limited data on the extent of injury in the heart in comparison to other causes of myocarditis."

Patients with myocarditis after vaccination have less severe cardiac injury

For this retrospective study, researchers analyzed data from 92 adult patients with myocarditis and abnormalities on cardiac MRI between 2019 and 2021. Patients were classified into one of three groups: myocarditis following COVID-19 vaccination, myocarditis following COVID-19 illness, and myocarditis not associated with COVID-19 vaccination or illness.

Of the 92 patients, 22 per cent had myocarditis following COVID-19 vaccination, 11 per cent had myocarditis following COVID-19 illness, and 66 per cent had other myocarditis. Patients with myocarditis following COVID-19 vaccination were younger and more frequently male compared to the other groups.

All patients with vaccine-associated myocarditis had chest pain, starting one to seven days after vaccination. The pain lasted between one and six days. Sixty-seven per cent per cent of these patients were admitted to hospital with an average stay of three days. No patients were admitted to the intensive care unit.

Cardiac MRI showed that, compared with other causes of myocarditis, patients with vaccine-associated myocarditis had less functional impairment and less extensive myocardial abnormalities. Patients with COVID-19 illness and other myocarditis had a higher prevalence of abnormalities involving the interventricular septum and worse myocardial injury.

Vaccine-related symptoms resolved quickly

At short-term follow-up, all patients with vaccine-associated myocarditis were asymptomatic with no adverse events.

"These results tell us that myocarditis following COVID-19 vaccination tends to be relatively mild and resolves quickly," says Dr. Hanneman.

Dr. Matteo Fronza, first author and cardiac imaging fellow, says that resolution of all symptoms and lack of adverse events at follow-up is reassuring, but longer-term follow-up is needed.

"COVID-19 illness can result in myocardial injury that is associated with adverse outcomes in hospitalized patients," says Dr. Hanneman. "This consideration should be balanced against the low risk of vaccine-related complications.

"Not only is myocarditis following COVID-19 vaccination very rare, it also tends to be less severe than myocarditis following SARS-CoV2 infection."

Dr. Paaladinesh Thavendiranathan, co-investigator and cardiologist at PMCC, agrees.

"Patients should not avoid COVID-19 vaccination for fear of myocarditis," he says. "The risk of myocarditis seems to be much higher following COVID-19 illness.

"However, if patients do experience symptoms – especially chest pain – early after COVID-19 vaccination, they should seek medical attention."

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