Myocarditis Rare, Macrophage Infiltration Common at COVID Autopsy

Debra L. Beck

November 12, 2020

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

An international autopsy study of 21 patients who died from COVID-19 has shown the presence of multifocal lymphocytic myocarditis in three patients (14%). In an additional six patients, focally increased interstitial T-lymphocytes within the myocardium were noted, with only focal or no myocyte injury.

However, increased interstitial macrophage infiltration, possibly related to cytokine infiltration, was seen in 86% of patients.

"One way to think about this is that if these patients were having biopsies and not autopsies, there would be myocardial injury in the patients with myocarditis, even after they recovered. But with interstitial macrophages, there may or may not be any injury," said cardiovascular pathologist James R. Stone, MD, PhD, Massachusetts General Hospital, Boston.

Stone and colleagues from Mass General, two hospitals in Italy, the University of Amsterdam, and the Mayo Clinic in Rochester, Minnesota, conducted the autopsies in March and April. The results were published in the October 14 issue of the European Heart Journal.

Their technique was rigorous: a median of 20 full-thickness blocks of myocardium were examined histologically (range, 5 to 29 blocks).

The presence of myocarditis, defined by the presence of multiple foci of inflammation with associated myocyte injury, was determined, and the inflammatory cell composition analyzed by immunohistochemistry.

"I think one of the take-homes from this study is that you have to do a thorough sampling of the heart in order to exclude myocardial injury. You cannot exclude myocarditis with just a biopsy or two," said Stone to theheart.org | Medscape Cardiology.

"We looked at multiple different sections of tissue preserved in paraffin for every case and found only 14% had myocarditis. The vast majority of autopsies done on patients dying from COVID-19 have short-changed the autopsy and not been done in a way to exclude myocarditis," he added.

For all patients, COVID-19 was the underlying cause of death, but the mechanisms of death were acute respiratory distress syndrome in 15, viral pneumonia in four, cardiogenic shock in one, and cardiac arrest in one.

Seven patients had a history of cardiovascular disease, including atrial fibrillation in four, coronary artery disease in three, left ventricular hypertrophy in one, and previous valve replacement in one. Sixteen had hypertension, seven had diabetes mellitus, and one had chronic obstructive pulmonary disease.

In four cases, mild pericarditis was present. Acute myocyte injury in the right ventricle, most probably due to strain or overload, was also present in four cases.

A nonsignificant trend was seen toward higher serum troponin levels in the patients with myocarditis compared with those without myocarditis. There were no reports of disrupted coronary artery plaques, coronary artery aneurysms, or large pulmonary emboli.

Macrophage Infiltration Rather Than Myocarditis, Myocardial Injury?

The study sheds more light on previous cardiac magnetic resonance (CMR) imaging findings that have suggested that many patients who recover from COVID-19 show signs suggestive of myocarditis. These earlier studies include a recent one in competitive athletes and the earlier Puntmann et al study of relatively young COVID-19 patients, which showed ongoing myocardial involvement in a majority of patients.

"It would not surprise me if some or all of the cardiac MR changes seen in some of these recent imaging studies are due to the macrophages," said Stone.

"What we saw was not a routine pathology by any means. It was a huge amount of macrophages, higher that what we saw in SARS and more similar to a study published in 2007 that looked at patients with bacterial sepsis," said Stone.

In an older study of SARS patients, 35% had the virus detected in myocardial tissue by PCR. In that subset, the degree of myocardial macrophage infiltrate was comparable to that seen in 86% of the COVID-19 cases described in this series.

Another possibility is that the macrophage infiltration reflects underlying disease rather than COVID-19. All but one of the patients had known underlying medical conditions associated with cardiac remodeling, said Nikolaos G. Frangogiannis, MD, a cardiologist who studies the mechanisms of cardiac injury, repair, and remodeling.

Frangogiannis, from Albert Einstein College of Medicine, Bronx, New York, wrote an editorial that accompanied the autopsy study.

"The problem with this finding of increased macrophage infiltration is that it's very hard to interpret, because as we age, and especially in a less healthy population, the numbers and the density of macrophages in the heart increase, so it's impossible to interpret as an effect of the infection itself unless you have an appropriate control population that matches the same characteristics, which is almost impossible to ask for," he said.

"I've observed since the beginning of the pandemic that there seemed to be some people who wanted every single case to be myocarditis and others who had a bias toward not wanting COVID-19 to be a cause of myocarditis. I think what we're seeing is it's not either/or for anything with this virus, it's a bit of everything," said Stone.

Stone and Frangogiannis reported no conflict of interest.

Eur Heart J. 2020;41:3827-3835, 3836–3838. Full text, Editorial

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