BackgroundPatterns of disease incidence can identify new risk factors for the disease or
provide insight into the etiology. For example, allergies and infectious diseases
have been shown to follow periodic temporal patterns due to seasonal changes in
environmental or infectious agents. Previous work searching for seasonal or other
temporal patterns in disease diagnosis rates has been limited both in the scope of
the diseases examined and in the ability to distinguish unexpected seasonal
patterns. Electronic Health Records (EHR) compile extensive longitudinal clinical
information, constituting a unique source for discovery of trends in occurrence of
disease. However, the data suffer from inherent biases that preclude a
identification of temporal trends.MethodsMotivated by observation of the biases in this data source, we developed a method
(Lomb-Scargle periodograms in detrended data, LSP-detrend) to find periodic
patterns by adjusting the temporal information for broad trends in incidence, as
well as seasonal changes in total hospitalizations. LSP-detrend can sensitively
uncover periodic temporal patterns in the corrected data and identify the
significance of the trend. We apply LSP-detrend to a compilation of records from
1.5 million patients encoded by ICD-9-CM (International Classification of
Diseases, Ninth Revision, Clinical Modification), including 2,805 disorders with
more than 500 occurrences across a 12 year period, recorded from 1.5 million
patients.Results and conclusionsAlthough EHR data, and ICD-9 coded records in particular, were not created with
the intention of aggregated use for research, these data can in fact be mined for
periodic patterns in incidence of disease, if confounders are properly removed. Of
all diagnoses, around 10% are identified as seasonal by LSP-detrend, including
many known phenomena. We robustly reproduce previous findings, even for relatively
rare diseases. For instance, Kawasaki disease, a rare childhood disease that has
been associated with weather patterns, is detected as strongly linked with winter
months. Among the novel results, we find a bi-annual increase in exacerbations of
myasthenia gravis, a potentially life threatening complication of an autoimmune
disease. We dissect the causes of this seasonal incidence and propose that factors
predisposing patients to this event vary through the year.