Stroke, High BP Don't Mediate Pollution's Link to Dementia

TOPLINE:

Contrary to what the authors had hypothesized, stroke and hypertension are not important mediators of the association between fine particulate matter (PM_2.5) and dementia, findings from a nationally representative cohort study show, suggesting more investigation is needed into how exposure to air pollution can affect dementia risk.

METHODOLOGY:

For this study, the authors hypothesized vascular dysfunction (stroke and hypertension) may act as a mediator of the association of PM_2.5 with dementia, given the association of PM_2.5 with vascular conditions, and of vascular conditions with dementia.
The study included 27,857 participants from the Health Retirement Study (HRS), a nationally representative cohort of older Americans, mean age 61 years, who were without dementia at baseline and had at least 1 follow-up interview between 1998 and 2016.
Researchers estimated PM_2.5 concentrations on the basis of residential addresses during the 10 years prior to baseline and used causal mediation modeling to quantify the degree to which stroke and hypertension could be mediators and/or modifiers of the association of PM_2.5 with incident dementia, which they classified using a standardized approach from survey respondents or their proxies.

TAKEAWAY:

After a mean follow-up of 10.2 years, 14.7% of study participants developed dementia; of these, 53.7% had a history of hypertension at baseline and 9.4% received a diagnosis of hypertension during the follow-up, while 9.2% had a history of stroke at baseline and 16.4% had a stroke over the follow-up period.
With adjustment for covariates, higher levels of PM_2.5 were not associated with increased risk of stroke (odds ratio [OR] per interquartile range (IQR) difference in PM_2.5, 1.08; 95% CI, 0.91 - 1.29).
There was no association of PM_2.5 with prevalent hypertension (OR per IQR increment in PM_2.5, 0.99; 95% CI, 0.92 - 1.07).
Sensitivity analyses found consistent results when the study was limited to younger participants and time periods with higher quality exposure data, and a suggested association of PM_2.5 with incident dementia among those with hypertension at baseline, with no similar interaction for stroke.

IN PRACTICE:

As the study didn't show evidence that hypertension or stroke mediated or modified the association of PM_2.5with incident dementia, "additional investigation of the pathways underlying the association of air pollution with dementia is needed to understand disease etiology and identify populations who might benefit most from pollution reduction strategies," the authors write.

SOURCE:

The study was carried out by Boya Zhang, PhD, Department of Epidemiology, University of Michigan School of Public Health, Ann Arbor, and colleagues. It was published online September 20 in JAMA Network Open.

LIMITATIONS:

The study used the 10-year average exposure before baseline, which may have resulted in an underestimation of the association of PM_2.5 with dementia and reduced power to capture mediation associations. Estimating PM_2.5 levels from residential history rather than real-time locations may have led to measurement error, and use of self-reported mediators may have introduced misclassification of hypertension and stroke.

DISCLOSURES:

The Health and Retirement Study (HRS) was funded by the National Institute on Aging (NIA) and the Social Security Administration, and the study was supported by the National Institutes for Environmental Health Sciences and NIA. Zhang has disclosed no relevant financial relationships; see paper for disclosures of other authors.

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