Ambient temperature and stroke hospitalization: causal analysis with empirical dynamic modelling

Abstract

Background: In the climate change context, the role of ambient temperature in stroke attack has drawn much research interest. However, prior epidemiologic evidence from mechanistic model-based regressions remains mixed. Inferring causation on these findings is a big challenge. Methods: Here we applied a novel approach, empirical dynamic modeling (EDM), to explore the causality between air temperature and stroke onset in Hong Kong from 2004-2011. EDM, a data-driven analysis scheme using convergent cross mapping (CCM) to detect causal pairs and multivariate S-map to quantify effect strength, is based on state space reconstruction for nonlinear dynamical systems where climate-health relations may accommodate.Results: By comparing with surrogate data to exclude seasonality, CCM determined that daily mean temperature and its deviation from previous week were potential drivers for hemorrhagic stroke (HS) hospitalization. Yet, no significant causal signal was identified for ischemic stroke (IS). S-map further found that daily mean temperatures ranging from 8.8°C to 31.8°C drove HS counts in a negative manner with the protective effect being stronger on warmer days. By contrast, temperature swings within a week showed a positive forcing in HS and a cooling trend may be more adverse than warming scenario. Also, we traced that mean temperature drove HS with delayed effects of 2 to 17 lagged days, while the impact of short-run temperature swings on HS could persist over the lagged period from 6 to 15 days. The strength of temperature-HS link peaked on the closest lag day of exposure. The effect strengths varied through time and by exposure levels, implying the nonlinearities internal to the system.Conclusion and Significance: Our analyses support that HS attack may be more sensitive to temperature condition than IS. We envision this EDM method could offer insights for epidemiologic studies to move beyond correlation when assessing health effects of climatic exposure.