Purpose: The etiology of pericarditis, myocarditis, and endocarditis is predominantly infectious, and infections often show seasonal variation. Little is known, however, about seasonal patterns in these cardiopathies. Methods: Using Danish health care registries, we identified all patients with a first-time hospital-based diagnosis of pericarditis, myocarditis, or endocarditis between 1994 and 2016. We estimated peak-to-trough ratios from fitted sine curves to measure the intensity of seasonal variation in occurrence during the study period. Because randomness will lead to small apparent seasonal patterns, we also conducted a plasmode simulation to assess the degree of seasonality that randomness would produce. Results: Crude peak-to-trough ratios of monthly frequencies summarized over a year were small. We estimated a peak-to-trough ratio of 1.10 (95% confidence interval [CI], 1.05–1.14) for pericarditis, 1.11 (95% CI, 1.02–1.21) for myocarditis, and 1.01 (95% CI, 1.00–1.07) for endocarditis. The simulated mean peak-to-trough ratios found after randomly reassigning the monthly frequencies within each year were 1.04 (95% CI, 1.00–1.09) for pericarditis, 1.04 (95% CI, 1.00–1.13) for myocarditis, and 1.04 (95% CI, 1.00–1.10), for endocarditis. Conclusion: The data indicate no important seasonal variation in the occurrence of pericarditis, myocarditis, and endocarditis in Denmark between 1994 and 2016.
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