Do solar cycles influence giant cell arteritis and rheumatoid arthritis incidence?

Abstract

Objective: To examine the influence of solar cycle and geomagnetic effects on the incidence of giant cell arteritis (GCA) and rheumatoid arthritis (RA).

Methods: We used data from patients with GCA (1950-2004) and RA (1955-2007) obtained from population-based cohorts. Yearly trends in age-adjusted and sex-adjusted incidence were correlated with the F10.7 index (solar radiation at 10.7 cm wavelength, a proxy for the solar extreme ultraviolet radiation) and AL index (a proxy for the westward auroral electrojet and a measure of geomagnetic activity). Fourier analysis was performed on AL, F10.7, and GCA and RA incidence rates.

Results: The correlation of GCA incidence with AL is highly significant: GCA incidence peaks 0-1 year after the AL reaches its minimum (ie, auroral electrojet reaches a maximum). The correlation of RA incidence with AL is also highly significant. RA incidence rates are lowest 5-7 years after AL reaches maximum. AL, GCA and RA incidence power spectra are similar: they have a main peak (periodicity) at about 10 years and a minor peak at 4-5 years. However, the RA incidence power spectrum main peak is broader (8-11 years), which partly explains the lower correlation between RA onset and AL. The auroral electrojets may be linked to the decline of RA incidence more strongly than the onset of RA. The incidences of RA and GCA are aligned in geomagnetic latitude.

Conclusions: AL and the incidences of GCA and RA all have a major periodicity of about 10 years and a secondary periodicity at 4-5 years. Geomagnetic activity may explain the temporal and spatial variations, including east-west skewness in geographic coordinates, in GCA and RA incidence, although the mechanism is unknown. The link with solar, geospace and atmospheric parameters need to be investigated. These novel findings warrant examination in other populations and with other autoimmune diseases.

Keywords: EPIDEMIOLOGY; RHEUMATOLOGY.

Figure 1

Giant cell arteritis (GCA) incidence (1950–2004) and lagged correlations with F10.7 and AL. Yearly average and yearly 3-year moving average of GCA incidence rates are plotted as dotted and solid lines, respectively, in panels (A and C). Their scales are given on the left y-axis label. F10.7 and AL are plotted as dashed lines, and their scales are indicated on the right y-axis in panels (A and C), respectively. The lagged correlations between (yearly average and yearly 3-year moving average GCA incidence rates) and (F10.7 and AL) are plotted as dotted and solid lines in panels (B and D), respectively. The grey lines in panels (B and D) indicate r_o such that p<0.05 for |r|>|r_o| (the threshold for significant correlation). The dashed horizontal grey line in panels B and D indicate r=0.

Figure 2

Rheumatoid arthritis (RA) incidence (1955–2007) and lagged correlation with F10.7 and AL plotted in the same format as in figure 1. Yearly average and yearly 3-year moving average of RA incidence rates are plotted as dotted and solid lines, respectively, in panels (A and C). Their scales are given on the left y-axis. F10.7 and AL are plotted as dashed lines, and their scales are indicated on the right y-axis in panels (A and C), respectively. The lagged correlations between (yearly average and yearly 3-year moving average RA incidence rates) and (F10.7 and AL) are plotted as dotted and solid lines in panels B and D, respectively. The grey lines in panels (B and D) indicate r_o such that p<0.05 for |r|>|r_o| (the threshold for significant correlation). The dashed horizontal grey line in panels (B and D) indicate r=0.

Figure 3

The power spectra of F10.7 (A), AL (B), giant cell arteritis (GCA) incidence rate (C), and rheumatoid arthritis (RA) incidence rate (D). The x-axis shows the cycle period. Each spectrum of the power spectra has been normalised so that the total power is one. The F10.7 power spectrum peaks at 10 years, suggesting that F10.7 has a cycle with a 10-year period (A). AL and GCA incidence rates have a period of about 10 years, whereas RA incidence rate has an 8–11-year period (B–D). AL, GCA and RA incidence rate power spectra show a secondary peak (periodicities) at about 4–5 years, but F10.7 does not.

Figure 4

(A) Altitude Adjusted Corrected Geo-Magnetic (AACGM) coordinates in grey lines overlaying Earth's continents with country boundaries drawn. The numbers in red indicate the latitude and longitude of AACGM. The geographic latitudes (eg, green lines) and longitudes are parallel to the x and y-axes, respectively. USA shares the same geographic latitudes as southern Europe and north Africa, but northern USA shares the same geomagnetic latitudes as northern Europe. (B) Inset showing AACGM latitudes (solid black lines) overlaying spatial variation in RA risk in the USA (adapted from Vieira et al7); reproduced with permission from Environmental Health Perspectives). The geographic latitudes are parallel to the x-axis, for example, the dashed green horizontal lines. The OR increases with increasing geographic latitude, but there is an east-west skewness. OR>2 (red) is found at higher geographic latitude on the west coast than in the eastern part of the Midwest and the east coast. However, in geomagnetic coordinates, these regions are generally within the same latitudes (poleward of 52.5°), except for the region in West Virginia, Kentucky and Ohio.