Methodological limitations in experimental studies on symptom development in individuals with idiopathic environmental intolerance attributed to electromagnetic fields (IEI-EMF) - a systematic review

Abstract

Background: Hypersensitivity to electromagnetic fields (EMF) is a controversial condition. While individuals with idiopathic environmental intolerance attributed to electromagnetic fields (IEI-EMF) claim to experience health complaints upon EMF exposure, many experimental studies have found no convincing evidence for a physical relation. The aim of this systematic review was to evaluate methodological limitations in experimental studies on symptom development in IEI-EMF individuals that might have fostered false positive or false negative results. Furthermore, we compared the profiles of these limitations between studies with positive and negative results.

Methods: The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guided the methodological conduct and reporting. Eligible were blinded experimental studies that exposed individuals with IEI-EMF to different EMF exposure levels and queried the development of symptoms during or after each exposure trial. Strengths and limitations in design, conduct and analysis of individual studies were assessed using a customized rating tool.

Results: Twenty-eight studies met the eligibility criteria and were included in this review. In many studies, both with positive and negative results, we identified methodological limitations that might have either fostered false or masked real effects of exposure. The most common limitations were related to the selection of study participants, the counterbalancing of the exposure sequence and the effectiveness of blinding. Many studies further lacked statistical power estimates. Methodically sound studies indicated that an effect of exposure is unlikely.

Conclusion: Overall, the evidence points towards no effect of exposure. If physical effects exist, previous findings suggest that they must be very weak or affect only few individuals with IEI-EMF. Given the evidence that the nocebo effect or medical/mental disorders may explain the symptoms in many individuals with IEI-EMF, additional research is required to identify the various factors that may be important for developing IEI-EMF and for provoking the symptoms. We recommend the identification of subgroups and exploring IEI-EMF in the context of other idiopathic environmental intolerances. If further experimental studies are conducted, they should preferably be performed at the individual level. In particular, to increase the likelihood of detecting hypersensitive individuals, if they exist, we encourage researchers to achieve a high credibility of the results by minimizing sources of risk of bias and imprecision.

Keywords: Electromagnetic field; Electromagnetic hypersensitivity; Idiopathic environmental intolerance; Methodological limitation; Provocation study; Symptom.

Fig. 1

Flow diagram of literature search, eligibility and inclusion process. Adapted from Moher et al. [45]

Fig. 2

Key questions judged to be at high risk of bias or judged to have concern regarding precision. The ratings are depicted for individual studies. Studies in blue: indicated an effect of exposure; studies in black: indicated no effect of exposure. Augner et al. [56] and Trimmel and Schweiger [55] reported effects of exposure for the combined group of individuals with IEI-EMF and healthy controls. The remaining studies with positive results reported effects of exposure for IEI-EMF individuals only

Fig. 3

Comparison of the profiles of limitations between studies with positive and negative results. The distributions are depicted in percentages and are sorted by study outcome. a Distributions for judgements about risk of bias. Upper figure: total number of key questions; lower figures: numbers of key questions according to direction of bias. See Table 1 for specification which key questions are grouped under the various directions of bias. b Distributions for judgements about imprecision. Upper figure: total number of key questions; lower figures: left – concern regarding statistical power, right – concern regarding missing adjustment for multiple comparisons