Geomagnetic disturbances may be environmental risk factor for multiple sclerosis: an ecological study of 111 locations in 24 countries

Abstract

Background: We noticed that a hypothesis based on the effect of geomagnetic disturbances (GMD) has the ability to explain special features of multiple sclerosis (MS). Areas around geomagnetic 60 degree latitude (GM60L) experience the greatest amount of GMD. The easiest way to evaluate our hypothesis was to test the association of MS prevalence (MSP) with angular distance to geomagnetic 60 degree latitude (AMAG60) and compare it with the known association of MS with geographical latitude (GL). We did the same with angular distance to geographic 60 degree latitude (AGRAPH60) as a control.

Methods: English written papers with MSP keywords, done in Europe (EUR), North America (NA) or Australasia (AUS) were retrieved from the PubMed. Geomagnetic coordinates were determined for each location and AMAG60 was calculated as absolute value of numerical difference between its geomagnetic latitude from GM60L. By an ecological study with using meta-regression analyses, the relationship of MSP with GL, AMAG60 and AGRAPH60 were evaluated separately. MSP data were weighted by square root of number of prevalent cases. Models were compared by their adjusted R square (AR2) and standard error of estimate (SEE).

Results: 111 MSP data were entered in the study. In each continent, AMAG60 had the best correlation with MSP, the largest AR2 (0.47, 0.42 and 0.84 for EUR, NA and AUS, respectively) and the least SEE. Merging both hemispheres data, AMAG60 explained 56% of MSP variations with the least SEE (R = 0.75, AR2 = 0.56, SEE = 57), while GL explained 17% (R = 0.41, AR2 = 0.17, SEE = 78.5) and AGRAPH60 explained 12% of that variations with the highest SEE (R = 0.35, AR2 = 0.12, SEE = 80.5).

Conclusions: Our results confirmed that AMAG60 is the best describer of MSP variations and has the strongest association with MSP distribution. They clarified that the well-known latitudinal gradient of MSP may be actually a gradient related to GM60L. Moreover, the location of GM60L can elucidate why MSP has parabolic and linear gradient in the north and south hemisphere, respectively. This preliminary evaluation supported that GMD can be the mysterious environmental risk factor for MS. We believe that this hypothesis deserves to be considered for further validation studies.

Figure 1

Geomagnetic coordinates in comparison to geographic coordinates. Note: curve lines indicate geomagnetic latitudes and longitudes. Straight lines illustrate geographic latitudes and longitudes. Reproduced by the kind permission of National Geophysical Data Center [13].

Figure 2

Cycle average of number of days that has been spent in each space-weather situation during solar cycle 17 to 23. Note: Cycle average was calculated based on daily Ap index, but because the location of auroral oval can be determined by Kp, measurements were interpreted as equal amounts of Kp. Constructed based on the Kp and Ap data from National Oceanic and Atmospheric Association (NOAA) [12] by the kind permission of NOAA.

Figure 3

Flow diagram of study selection.

Figure 4

MS Prevalence estimates by AMAG60, geographical latitude, geomagnetic latitude and AGARPH60. AMAG60: Angular distance to geomagnetic 60° latitude; AGRAPH60: Angular distance to geographic 60° latitude; N. America: North America; North hemis: North hemisphere; Both hemis: Both hemispheres. † Australasia comprises Australia and New Zealand. Dotted lines indicate line of regression.

Figure 5

Solar cycles and sunspot numbers. Note: number in the stars indicates to the solar cycle number. Reproduced by the kind permission of National Geophysical Data Center [125].