Non-ST Elevation Myocardial Infarction.

Non-ST-elevation myocardial infarction (NSTEMI) is the most common acute coronary syndrome presentation and represents a heterogeneous clinical entity with distinct mechanisms and distinct diagnostic and therapeutic implications. This narrative review summarizes contemporary concepts in NSTEMI pathophysiology, classification, angiographic phenotypes, and treatment strategies, with an emphasis on differentiating Type 1 from Type 2 myocardial infarction (MI) and the implications for revascularization decisions. The PubMed/MEDLINE database was searched for English-language publications addressing NSTEMI pathophysiology, classification, diagnosis, and management, supplemented by the most recent American and European acute coronary syndrome guidelines, the Universal Definition of Myocardial Infarction, landmark randomized controlled trials, and pivotal mechanistic and intravascular imaging studies, with priority given to contemporary and high-impact sources. NSTEMI emerges as a heterogeneous syndrome rather than a single disease, and optimal care depends on identifying the underlying mechanism. This requires distinguishing myocardial infarction from myocardial injury, separating Type 1 atherothrombosis from Type 2 supply-demand mismatch, and recognizing entities such as myocardial infarction with non-obstructive coronary arteries and spontaneous coronary artery dissection. A mechanism-specific approach should guide antithrombotic therapy and revascularization decisions, and the transition from a syndromic label to an individualized, mechanism-based diagnosis is essential for optimizing patient outcomes.