Seasonal distribution of relapse onset in rheumatoid arthritis and spondyloarthropathy: the possible effect of the solar factor.

BACKGROUND AND OBJECTIVE The seasonal effect on the relapse of rheumatoid arthritis and spondyloarthropathies is still unclear. To assess the seasonal distribution of relapse onset in rheumatoid arthritis (RA) and spondyloarthropathy (SpA) and its association with solar factors. METHODS The monthly distribution of relapse onsets during the years 1998-2000 was retrospectively chart reviewed in 364 patients. In 1998 a total of 131 patients were studied; 60 with seropositive (sp) RA, 30 with seronegative (sn) RA and 41 with SpA; 113 patients in 1999: 44 with spRA, 38 with snRA and 31 with SpA; 120 patients in 2000: 56 with spRA, 38 with snRA and 26 with SpA. All of them were treated in the Department of Rheumatology, which serves the population of northwestern Israel. Solar activity was analyzed according to the "Solar Terrestrial Activity Report Charts 1998-2000". The Central Israel Bureau of Statistics provided the sun global radiation data. Data was assessed during the summer (April-September) and winter (January-March, October-December). The correlation between the monthly distribution of disease relapses and solar factors was measured (SPSS-10 for WIN). RESULTS Relapses in spRA patients occurred mostly during the summer months with peak activity during the month of July 2000. Single monthly peaks of spRA relapse onset were noted in January 1998-1999 and April 1998 and for snRA in January 1998 and June 2000, but there were no seasonal differences for spRA, snRA and SpA in 1998-1999 and for snRA and SpA in 2000. Relapses in spRA patients were associated with a summer bias of increased solar activity and global solar radiation in 2000 compared with lower peak solar activity in 1998-1999. Furthermore, in 2000 we found a significant correlation of the spRA monthly relapse count to solar activity (p = 0.005) and global sun radiation (p = 0.048) unlike snRA and SpA. No above-mentioned association and correlation was noted in 1998-1999. We revealed mild negative correlation (p = 0.046) of SpA relapse count only to peak solar flux (PSF) by analysis of data for 1998-2000 as one united group. CONCLUSIONS Relapses were more frequent during the summer of 2000 (May-June-July) in spRA but not in snRA and SpA. The reasons are still unclear. No seasonal differences were observed in 1998-1999. Enhanced solar activity in summer-2000 compared with 1998-1999 may be inferred to be the proposed cause but coincidence may occur as well. Outbreak in RA and SpA was not registered despite increased peak solar activity in 2000. We observed mild evidence of reciprocal relation between SpA relapsing and solar activity during 1998-2000. Solar and any other possible contributory factors remain still to be elucidated.