Nocturnal heat exposure and stroke risk

Abstract

Background and aims: In recent decades, nighttime temperatures have increased faster than daytime temperatures. The increasing prevalence of nocturnal heat exposure may pose a significant risk to cardiovascular health. This study investigated the association between nighttime heat exposure and stroke risk in the region of Augsburg, Germany, and examined its temporal variations over 15 years.

Methods: Hourly meteorological parameters, including mean temperature, relative humidity, and barometric pressure, were acquired from a local meteorological station. A data set was obtained consisting of 11 037 clinical stroke cases diagnosed during warmer months (May to October) between the years 2006 and 2020. The average age of cases was 71.3 years. Among these cases, 642 were identified as haemorrhagic strokes, 7430 were classified as ischaemic strokes, and 2947 were transient ischaemic attacks. A time-stratified case-crossover analysis with a distributed lag non-linear model was used to estimate the stroke risk associated with extreme nighttime heat, as measured by the hot night excess (HNE) index after controlling for the potential confounding effects of daily maximum temperature and other climatic variables. Subgroup analyses by age group, sex, stroke subtype, and stroke severity were performed to identify variations in susceptibility to nighttime heat.

Results: Results suggested a significant increase in stroke risk on days with extreme nighttime heat (97.5% percentile of HNE) (odds ratio 1.07, 95% confidence interval 1.01-1.15) during the full study period. When comparing the results for 2013-20 with the results for 2006-12, there was a significant increase (P < .05) in HNE-related risk for all strokes and specifically for ischaemic strokes during the more recent period. Furthermore, older individuals, females, and patients with mild stroke symptoms exhibited a significantly increased vulnerability to nighttime heat.

Conclusions: This study found nocturnal heat exposure to be related to elevated stroke risk after controlling for maximum daytime temperature, with increasing susceptibility between 2006 and 2020. These results underscore the importance of considering nocturnal heat as a critical trigger of stroke events in a warming climate.

Keywords: Climate change; Hot night; Ischaemic strokes; Stroke.

Structured Graphical Abstract

This 15-year time-stratified case-crossover study provides robust evidence that nighttime heat exposure is associated with increased stroke risk. Urgent action is needed through targeted prevention and urban planning interventions to mitigate its impact, particularly among vulnerable populations. OR, odds ratio.

Figure 1

Cumulative exposure–response relationships between hot night excess (HNE) and stroke risk over Lag 0–6 days for 2006–12 and 2013–20 with corresponding 95% confidence intervals (shaded areas). The vertical dashed line represents the 97.5th percentiles of the HNE distribution. Hot night excess represents the excess sum of high temperatures during the night when hourly temperatures are higher than the 95th percentile of daily minimum temperature during the whole study period. For instance, HNE = 0°C indicates non-hot night days during the warm months. HNE = 40°C represents the cumulative sum of hourly nighttime temperatures exceeding the threshold with values accumulating up to 40°C

Figure 2

Cumulative odd ratio estimated for daily stroke cases (95% confidence interval) associated with extreme heat exposure during the night [97.5th percentile of hot night excess (HNE) distribution] predicted for 2006–12 and 2013–20 stratified by three different subgroups. Asterisks and coloured groups indicate statistical significance for differences in odd ratio estimates between 2006–12 and 2013–20 (P < .05)