A ‘mess’ of a diagnosis: Is it type 2 MI or a nonischemic imposter?

Role of the universal definitions

That challenge might in some ways be complicated by the universal definition, especially version 4, in which the definitions for type 1 MI, type 2 MI, and nonischemic myocardial injury are unified biochemically.

This version, published in 2018 in the European Heart Journal and Circulation, introduced a formal definition of myocardial injury, which was hailed as an innovation: cTn elevation to the 99th percentile of the upper limit of normal in a reference population.

It differentiates type 1 MI from type 2 MI by the separate pathophysiology of the ischemia – plaque rupture with intracoronary thrombosis and myocardial oxygen supply–demand mismatch, respectively. In both cases, however, there must be symptoms or objective evidence of ischemia. Absent signs of ischemia, the determination would be nonischemic myocardial injury.

Yet clinically and prognostically, type 2 MI and nonischemic myocardial injury in some ways are more similar to each other than either is to type 1 MI. Both occur secondary to other conditions across diverse clinical settings and can be a challenge to tell apart.

The universal definition’s perspective of the three events – so heavily dependent on cTn levels and myocardial ischemia – fails to account for the myriad complexities of individual patients in practice, some say, and so can muddle the process of risk assessment and therapy.

“Abnormal troponin identifies injury, but it doesn’t identify mechanism. Type 2 MI is highly prevalent, but there are other things that cause abnormal troponins,” Dr. Januzzi said. That’s why it’s important to explore and map out the clinical variables associated with the two conditions, to “understand who has a type 2 MI and who has cardiac injury. And believe it or not, it’s actually harder than it sounds to sort that out.”

“Practically speaking, the differentiation between these events is clinical,” Dr. Sandoval agreed. “There’s not always perfect agreement on what we’re calling what.”

Consequently, the universal definitions might categorize some events in ways that seem inconsistent from a management perspective. For example, they make a sharp distinction between coronary atherothrombotic and coronary nonatherothrombotic MI etiologies. Some clinicians would group MI caused by coronary spasm, coronary embolism, or spontaneous coronary artery dissection along with MI from coronary plaque rupture and thrombosis. But, Dr. Sandoval said, “even though these are coronary issues, they would fall into the type 2 bin.”

Also, about half of cases identified as type 2 MI are caused by tachyarrhythmias, which can elevate troponin and cause ECG changes and possibly symptoms resembling angina, Dr. Newby observed. “But that is completely different from other types of myocardial infarction, which are much more serious.”

So, “it’s a real mess of a diagnosis – acute myocardial injury, type 2 and type 1 MI – and it can be quite difficult to disentangle,” he said. “I think that the definition certainly has let us down.”

The diversity of type 2 MI clinical settings might also be a challenge. Myocardial injury according to cTn, with or without ischemia, occurs widely during critical illnesses and acute conditions, including respiratory distress, sepsis, internal bleeding, stroke, and pulmonary embolism.

Early in the COVID-19 pandemic, much was made of elevated troponin levels and myocarditis as an apparently frequent complication among hospitalized patients. “I raised my hand and said, we’ve been seeing abnormal troponins in people with influenza for 20 years,” Dr. Januzzi said. “Critical illness, infection, toxicity from drugs, from chemotherapy, from alcohol – there are all sorts of potential triggers of myocardial injury.”