Journal Article

A retrospective cohort study of stroke onset: implications for characterizing short term effects from ambient air pollution

Villeneuve, Paul J, Johnson, Julie YM, Rowe, Brian H, Pasichnyk, Dion

2011

Overview

Background Case-crossover studies used to investigate associations between an environmental exposure and an acute health response, such as stroke, will often use the day an individual presents to an emergency department (ED) or is admitted to hospital to infer when the stroke occurred. Similarly, they will use patient's place of residence to assign exposure. The validity of using these two data elements, typically extracted from administrative databases or patient charts, to define the time of stroke onset and to assign exposure are critical in this field of research as air pollutant concentrations are temporally and spatially variable. Our a priori hypotheses were that date of presentation differs from the date of stroke onset for a substantial number of patients, and that assigning exposure to ambient pollution using place of residence introduces an important source of exposure measurement error. The objective of this study was to improve our understanding on how these sources of errors influence risk estimates derived using a case-crossover study design. Methods We sought to collect survey data from stroke patients presenting to hospital EDs in Edmonton, Canada on the date, time, location and nature of activities at onset of stroke symptoms. The daily mean ambient concentrations of NO 2 and PM 2.5 on the self-reported day of stroke onset was estimated from continuous fixed-site monitoring stations. Results Of the 336 participating patients, 241 were able to recall when their stroke started and 72.6% (95% confidence interval [CI]: 66.9 - 78.3%) experienced stroke onset the same day they presented to the ED. For subjects whose day of stroke onset differed from the day of presentation to the ED, this difference ranged from 1 to 12 days (mean = 1.8; median = 1). In these subjects, there were no systematic differences in assigned pollution levels for either NO 2 or PM 2.5 when day of presentation rather than day of stroke onset was used. At the time of stroke onset, 89.9% (95% CI: 86.6 - 93.1%) reported that they were inside, while 84.5% (95% CI: 80.6 - 88.4%) reported that for most of the day they were within a 15 minute drive from home. We estimated that due to the mis-specification of the day of stroke onset, the risk of hospitalization for stroke would be understated by 15% and 20%, for NO 2 and PM 2.5 , respectively. Conclusions Our data suggest that day of presentation and residential location data obtained from administrative records reasonably captures the time and location of stroke onset for most patients. Under these conditions, any associated errors are unlikely to be an important source of bias when estimating air pollution risks in this population.

• Publisher: BioMed Central,BioMed Central Ltd,Springer Nature B.V,BMC
• Subject: Aged / Aged, 80 and over / Air Pollutants - analysis / Air Pollutants - toxicity / Air pollution / Alberta - epidemiology / Analysis / Case studies / Cities / Cohort Studies / Cross-Over Studies / Earth and Environmental Science / Emergency medical care / Emergency medical services / emergency service / Environment / Environmental aspects / Environmental Exposure - adverse effects / Environmental Health / Environmental risk / Exposure / Female / Health aspects / Humans / Ischemic Attack, Transient - epidemiology / Ischemic Attack, Transient - etiology / Male / Medical research / Middle Aged / Nitrogen dioxide / Nitrogen Dioxide - analysis / Nitrogen Dioxide - toxicity / Occupational Medicine/Industrial Medicine / Particulate matter / Particulate Matter - analysis / Particulate Matter - toxicity / Pollution levels / Public Health / Retrospective Studies / Risk Assessment / Risk factors / Stroke / Stroke (Disease) / Stroke - epidemiology / Stroke - etiology / Studies / survey / Time Factors