Correlation of multiple sclerosis (MS) incidence trends with solar and geomagnetic indices: Time to revise the method of reporting MS epidemiological data

Abstract

Background: Recently, we introduced solar related geomagnetic disturbances (GMD) as a potential environmental risk factor for multiple sclerosis (MS). The aim of this study was to test probable correlation between solar activities and GMD with long-term variations of MS incidence.

Methods: After a systematic review, we studied the association betwee n alterations in the solar wind velocity (VSW) and planetary A index (AP, a GMD index) with MS incidence in Tehran and western Greece, during the 23(rd) solar cycle (1996-2008), by an ecological-correlational study.

Results: We found moderate to strong correlations among MS incidence of Tehran with VSW (rS = 0.665, P = 0.013), with 1 year delay, and also with AP (rS = 0.864, P = 0.001) with 2 year delay. There were very strong correlations among MS incidence data of Greece with VSW (r = 0.906, P < 0.001) and with AP (r = 0.844, P = 0.001), both with 1 year lag.

Conclusion: It is the first time that a hypothesis has introduced an environmental factor that may describe MS incidence alterations; however, it should be reminded that correlation does not mean necessarily the existence of a causal relationship. Important message of these findings for researchers is to provide MS incidence reports with higher resolution for consecutive years, based on the time of disease onset and relapses, not just the time of diagnosis. Then, it would be possible to further investigate the validity of GMD hypothesis or any other probable environmental risk factors.

Keywords: Environmental Risk Factor; Geomagnetic Disturbance; Incidence; Multiple Sclerosis; Solar Wind Velocity.

Figure 1

Age-adjusted multiple sclerosis incidence trends of both sexes during 1996–2008  (reconstructed from data of Tehran⁵ and western Greece,⁶ with the kind permission of publisher, copyright ^© 2008 and 2011 Karger Publishers, Basel, Switzerland), and annual solar wind velocity and A_P index from OMNIWeb interface (public domain).⁴

Figure 2

Scatter-plots of correlation of both sex multiple sclerosis incidence data with Solar and geomagnetic indices. Scatter-plots have been constructed after the execution of the mentioned delays on the solar and geomagnetic data sets (please see the text). *As non-parametric Spearman’s correlation analysis was used for Tehran data, scatter-plot was constructed by means of the ranked order of mentioned data to show the non-parametric correlation.

Figure 3

Distance of Tehran (white star) and western Greece area (black star) from geomagnetic 60  latitude. 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.