A new study suggests that geomagnetic activity from the sun may contribute to autoimmune disease. Simon Wing, senior staff physicist at Johns Hopkins University in Laurel, Md., and colleagues investigated whether measures of solar cycles correlated with the incidence of giant cell arteritis (GCA) and rheumatoid arthritis (RA) in a single county in southern Minnesota. The investigators found that disease incidence rates vary with the solar cycle; GCA incidence peaks one year after solar maximum, and RA incidence peaks approximately six years after solar maximum. The results of the data analysis were published in BMJ Open.1
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Physicists measured the geomagnetic effects of the sun on the Earth in multiple ways. Two measures of this activity are the AL index and F10.7. The AL index reflects geomagnetic activity and can serve as a proxy for the westward auroral electrojet. AL has a major periodicity of approximately 10 years and a secondary periodicity of 4–5 years. The F10.7 index is a measure of solar radiation at 10.7 cm and, thus, serves as a proxy for solar extreme ultraviolet radiation.
Prior to the study, investigators noted that the major and minor AL periodicities coincide with incidences of GCA and RA. The coincidence led researchers to suggest that geomagnetic activity may underlie both the temporal and spatial variations in disease incidence. In their study, Mr. Wing and colleagues reported a highly significant correlation between GCA incidence rate and AL. In contrast, the correlation between incidence of GCA and F10.7 was statistically significant, but not strong. The researchers noted a minimum of 0–1-year lag time in the correlation between the GCA incidence rate and AL, and concluded that additional factors may be influencing the incidence of GCA.
When the investigators evaluated the relationship between RA and the two geomagnetic parameters, they found that it was more complex than the relationship of GCA to the parameters. Although there was a highly significant correlation between RA incidence and AL, the lag time was 5–7 years. The researchers also raised the possibility that peaks of geometric activity that occur in the spring and fall might play a role in the onset of GCA and RA.
After establishing a temporal association between geomagnetic activity and disease incidence, the research team investigated whether geomagnetic activities, such as auroral electrojets, play a role in the spatial distribution of GCA and RA. They did this in response to epidemiological data, noting that the GCA and RA incidences have an east-west skew in geographic coordinates—with disease incidence increasing with increasing geographic latitude. Although some have hypothesized that Vitamin D deficiency may result in a higher incidence of RA in populations at a distance from the equator, the Vitamin D hypothesis does not account for the reported east-west skewing.
The investigators calculated the altitude-adjusted corrected geomagnetic (AACGM) coordinates and identified the aurororal oval, in which auroral electrojets are commonly found. They then overlaid the spatial variation of RA risk and found that regions with a >2 odds ratio tended to be found at higher geographic latitudes on the West Coast than on the East Coast. The auroral oval also appeared to explain why the GCA incidences in Minnesota and northern Europe are higher than the GCA incidences in southern Europe and Israel. Thus, the analysis by Mr. Wing and colleagues offers insight into a possible mechanism behind the east-west distribution, which, up until this point, has remained unexplained. Moreover, the investigators suggest that such geomagnetic disturbances as substorms might further induce temporal variations in the electric and magnetic fields in the vicinity of the auroral oval, which might result in a weak, rapidly fluctuating magnetic field that might influence the immune system.
Although the study was not designed to determine a biological mechanism behind the relationship between geomagnetic activity and autoimmune disease, the authors suggest that geomagnetic disturbances might result in reduced production of the antiinflammatory agent melatonin. They conclude by calling for further investigation of the link between solar, geospace, atmospheric parameters and autoimmune disease.
Lara C. Pullen, PhD, is a medical writer based in the Chicago area.
- Wing S, Rider LG, Johnson JR, et al. Do solar cycles influence giant cell arteritis and rheumatoid arthritis incidence? BMJ Open. 15 May 2015. 5(5):e006636. doi: 10.1136/bmjopen-2014-006636.