Influenza-like Illness As a Trigger for Ischemic Stroke

Objective: We hypothesized that ILI is associated with risk of incident stroke, and that the risk would be highest closest in time to the event.

Methods: This case-crossover analysis utilized data obtained from the California State Inpatient Database of the Healthcare Cost and Utilization Project (HCUP). The outcome of interest was ischemic stroke. Exposure was defined as a visit to the emergency department or hospitalization for influenza-like illness (ILI) 365, 180, 90, 30, or 15 days before stroke (risk period) or similar time intervals exactly 1 or 2 years before stroke (control period). Conditional logistic regression was used to calculate the odds ratio and 95% confidence interval (OR, 95% CI).

Results: In 2009, 36,975 hospitalized ischemic strokes met inclusion criteria, and of these strokes, 554 (1.5%) had at least 1 episode of ILI in the 365-day risk period prior to their stroke. Using non-overlapping time intervals from ILI to stroke, the odds of ischemic stroke was greatest in the first 15 days post ILI (OR: 2.88, 95% CI: 1.86-4.47). The strength of the relationship decreased as the time from ILI increased, and was no longer significant after 60 days. There was a significant interaction ( = 0.017) with age and ILI; the odds of stroke associated increased 7% with each 10-year decrease in age (OR per 10-year age decrease 1.07, 95% CI: 1.03-1.35).

Interpretation: We found that ILI increases short-term risk of stroke, particularly in people under the age of 45, and therefore may be considered to act as a trigger for stroke.