Seasonal variation in myocardial infarction is limited to patients with ST-elevations on admission

Previous studies have demonstrated seasonal variation in the incidence of acute myocardial infarction (AMI) with an increase in cases during the winter months. However, they did not assess whether ST-elevation MI (STEMI) and non-ST-elevation MI (NSTEMI) exhibit similar changes. The object of this study was to compare the seasonal variation of STEMI and NSTEMI. All patients who presented with AMI and underwent coronary angiography within seven days of admission were identified via the institutional database. STEMI diagnosis required admission ECG demonstrating ST elevation in at least two continguous leads. All AMIs not meeting criteria for STEMI were defined as NSTEMI. Patients were divided into monthly and seasonal groups based on the date of admission with MI. A total of 784 patients were included: 549 patients with STEMI and 235 with NSTEMI. When STEMI patients were analyzed by season, there were 170 patients (31%) in the winter months, a statistically significant difference of excess MI (p<0.005). When NSTEMI patients were analyzed, there were 62 patients (26%) in the winter with no statistically significant difference in the seasonal variation. Our findings suggest that the previously noted seasonal variation in the incidence of AMI is limited to patients presenting with STEMI, and that there are important physiological differences between STEMI and NSTEMI, the nature of which remains to be elucidated.