Alzheimer Risk Factors

Background Among potential environmental risk factors for Alzheimer disease (AD), occupational exposures have received some attention, including extremely low frequency electromagnetic fields (ELF-EMF). A systematic review and meta-analysis of published epidemiological studies on this subject was carried out.

Methods The search was concluded in April 2006. Bibliographic databases consulted included PubMed, EMBASE, Cochrane Library and NIOSHTIC2. Pooled estimates wereobtained using random-effects meta-analysis. Sources of heterogeneity between studies were explored, as was publication bias.

Results Fourteen different studies (nine case-control and five cohort studies) accomplished inclusion criteria. All these studies followed standardized criteria for AD diagnosis and most of them obtained quantitative estimates of exposure. Pooled estimates suggest an increased risk of AD from case-control studies (ORpooled 2.03; 95% CI 1.38–3.00) and from cohort studies (RRpooled 1.62; 95% CI 1.16–2.27), with moderate to high statistical heterogeneity in both cases (respectively, I2 = 58% and I2 = 54%). Cohort studies showed consistently increased risks for exposed men (RRpooled 2.05; 95% CI 1.51–2.80, I2 = 0%). Evidence of dose–response relationship was not present. Test for publication bias suggests small study effects, mostly for case-control studies.

Conclusions Available epidemiological evidence suggests an association between occupational exposure to ELF-EMF and AD. However, some limitations affecting the results from this meta-analysis should be considered. More information on relevant duration and time windows of exposure, on biological mechanisms for this potential association and on interactions between electromagnetic fields exposure and established risk factors for AD is needed.

Keywords Alzheimer disease, electromagnetic fields, occupational exposure, review literature, meta-analysis

Source: International Journal of Epidemiology, OUP
IJE Advance Access originally published online on February 1, 2008
International Journal of Epidemiology 2008 37(2):329-340; doi:10.1093/ije/dym295



Exposure risks

In 2006 the worldwide prevalence of Alzheimer’s disease was estimated to be 26.6 million; and by 2050, Alzheimer’s disease prevalence is expected to quadruple because of the increasing life expectancy in many countries. Although the years of life lost per Alzheimer’s disease case are relatively small, the disease causes considerable distress for afflicted families. Moreover, Alzheimer’s disease patients need substantial care resulting in substantial costs for the health care system.

Many environmental, occupational or lifestyle risk factors have been hypothesized to increase the risk for Alzheimer’s disease, which include exposure to extremely low frequency magnetic fields (ELF-MF). Given the ubiquity of ELF-MF in our everyday environment, such an association would be of high public health relevance.

In view of the large statistical heterogeneity and the observed publication bias, pooled effect estimates of the risk for Alzheimer's disease due to exposure are not meaningful and they should be considered with caution. On the other hand, the approach of Garcı´a and colleagues in comparing pooled estimates according to different study characteristics is appreciated. This allows evaluation of whether the heterogeneity between studies is explained by different methodological approaches. Overall, Garcia et al. found little evidence that heterogeneity between studies was explained by study type, gender, or differences in the exposure level. Surprisingly, studies based on death certificates reported similar risk estimates like studies based on clinical examinations.

This finding contradicts a recent statement from the WHO in the Environmental Health Criteria Monograph 238. The statement concluded that ‘the evidence for an association between ELF-MF exposure and Alzheimer’s disease is inadequate because the higher quality studies that focused on Alzheimer morbidity rather than mortality do not indicate an association’.

International Journal of Epidemiology 2008;37:341–343, OUP
Commentary: Epidemiological research on extremely low frequency magnetic fields and
Alzheimer’s disease—biased or informative? Martin Roosli, Jan 2008
http://ije.oxfordjournals.org/cgi/reprint/37/2/341


In a case– control study nested within the Framingham cohort, the relative risk of dementia in stroke survivors compared with controls varied from 2.0 to 2.8, depending on the covariates considered for adjustment. Of the incident dementia cases, 51% were diagnosed as having VaD or mixed dementia, whereas this proportion was only 4% among the control subjects without a history of stroke.

source: http://hyper.ahajournals.org/cgi/reprint/49/3/389

Ivan CS, Seshadri S, Beiser A, Au R, Kase CS, Kelly-Hayes M, Wolf PA.
Dementia after stroke. The Framingham study. Stroke. 2004;35:
1264–1269.